' d y THE PARKFIELD, CALIFORNIA, EARTHQUAKE OF JUNE 27, 1966 ^tA£NTo/t> Sc '*NCE StRM^ S U.S. DEPARTMENT OF COMMERCE ENVIRONMENTAL SCIENCE SERVICES ADMINISTRATION COAST AND GEODETIC SURVEY U.S. DEPARTMENT OF COMMERCE John T. Connor, Secretary ENVIRONMENTAL SCIENCE SERVICES ADMINISTRATION Robert M. White, Administrator COAST AND GEODETIC SURVEY James C. Tison, Jr., Director U.S. GOVERNMENT PRINTING OFFICE WASHINGTON : 1966 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price 45 cents THE PARKFIELD, CALIFORNIA, EARTHQUAKE OF JUNE 27, 1966 PRELIMINARY SEISMOLOGICAL REPORT By Samuel T. Harding and Wilbur Rinehart PRELIMINARY ENGINEERING SEISMOLOGICAL REPORT By William K. Cloud Edited by Jerry L. C off man Si e co ^3 8 TABLE OF CONTENTS Page Preliminary Seismological Report 1 Introduction 1 Geologic Setting 1 Seismicity 2 Hypocenters, Origin Times, and Magnitudes of Foreshocks, Main Shock, and Aftershocks 5 Focal Mechanism Studies 10 Conclusions 11 References 15 Preliminary Engineering Seismological Report 17 Introduction 17 Intensity 17 Modified Mercalli Intensity Scale of 1931 28 Area of Major Interest 35 Array Instrumentation 35 Instrumental Results 35 Summary 35 Conclusions 36 Acknowledgments 36 References 36 LIST OF TABLES Table 1 Earthquakes in the Parkfield area, 1769-June 1966 4 2 Chronological listings of foreshock and aftershock epicenters, June 28- July 2, 1966 9 3 List of hypocenter computations for the main shock at 04:26:14.4 G.M.T_ 9 4 Magnitude calculations 10 5 Character of first motion for main shock at 04:26:14.4 G.M.T 11 6 Accelerations recorded by AR-240 accelerographs 40 7 Comparison of shaking table accelerations recorded by an AR-240 ac- celerometer and a Statham accelerometer 41 8 Maximum velocity response of seismoscopes relative to the ground during main shock of Parkfield earthquake series 41 LIST OF FIGURES Figure View looking northward along San Andreas fault. 1 Earthquakes (intensity V or greater) in the Cholame-Parkfield, California area 3 2 Location of epicenter of June 28, 1966 (G.M.T.) earthquake, foreshocks, and aftershocks 6 in iv CONTENTS Figure Page 3 Time difference hyperbola plot of main earthquake occurring at 04 : 26 : 14.4 G.M.T., June 28, 1966 7 4 Time difference hyperbola plot of major aftershock occurring at 04 : 39 : 08.9 G.M.T., June 28, 1966 8 5 Stereographic projection showing the first motion and direction of nodal planes 12 6 Comparison of nodal plane solutions of June 28, 1966 (G.M.T.) earth- quake to regional faulting B plane 13 7 Comparison of nodal plane solutions of June 28, 1966 (G.M.T.) earth- quake to regional faulting A plane 14 8 Map showing Modified Mercalli intensity distribution of the June 27, 1966 (20 : 26 : 14.4 P.S.T.) , Parkfield earthquake 18 9 Crack across pavement of Highway 466 about 270 feet east of Strong- motion Station No. 2 19 10 En echelon cracks across road about % mile south of Strong-motion Sta- tion No. 2 20 11 Ground surface fractures about 270 feet east of Strong-motion Station No. 2 20 12 Displacement of pipeline located about 200 yards southeast of Strong- motion Station No. 2 21 13 Ground surface fractures about 2 miles northwest of Strong-motion Station No. 2 21 14 Crack between concrete slab front of Cholame Cafe and older wood floor on wood pier section of cafe 24 15 Bridge on Highway 466 about 200 feet west of Strong-motion Station No. 2 24 16 Crack in east abutment of bridge shown in figure 15 25 17 Crack in concrete pier of bridge shown in figure 15 25 18 Indication of motion at bottom of bridge pier shown in figure 17 26 19 Dead trees knocked down by earthquake, about 1 mile west of Jack Ranch 26 20 Tombstone at Parkfield, Todd Cemetery 29 21 Same as figure 20 29 22 Same as figure 20 29 23 Parkfield Meeting Hall 30 24 Close-up of chimney damage shown in figure 23 30 25 Parkfield Fire Station 30 26 Interior view of cracked west wall of Parkfield Fire Station 31 27 Bridge about 5 miles south of Parkfield 31 28 Buckled X -bracing of bridge shown in figure 27 32 29 Same as figure 28 32 30 Cracked swimming pool about 2.3 miles south of Parkfield 33 31 Acetylene and oxygen cylinders in Cochrum Garage storage shed reported to have been knocked over by earthquake 33 32 Union Oil Co., Shandon Pumping Station 33 33 Indication of earthquake-caused motion at base of tank in foreground of figure 32 34 34 Standard Pipeline Co., Shandon Pumping Station 34 CONTENTS v Figure Page 35 Parkfield-Cholame section of epicentral region 37 36 Location of the Cholame-Shandon strong-motion instrument array 38 37 AR-240 strong-motion accelerograph 39 38 Photocopy of Station 8 accelerogram 42 39 Photocopy of Station 5 accelerogram 43 40 Photocopy of Station 2 accelerogram 44 41 Photocopy of Station Temblor accelerogram 45 42 Station 8 46 43 Station 5 47 44 Temblor Station 48 45 Coast and Geodetic Survey seismoscope 49 46 Seismoscope record — Station 12 50 47 Seismoscope record — Station 11 51 48 Seismoscope record — Station 10 52 49 Seismoscope record — Station 9 53 50 Seismoscope record — Station 8 54 51 Seismoscope record — Station 7 55 52 Seismoscope record — Station 6 56 53 Seismoscope record — Station 5 57 54 Seismoscope record — Station 4 58 55 Seismoscope record — Station 3 59 56 Seismoscope record — Station 2 60 57 Seismoscope record — Station 1 61 58 Seismoscope record — Station A-l 62 59 Seismoscope record — Station A-2 63 60 Seismoscope record — Antelope Pumping Station 64 61 Seismoscope record — A venal Gap Station 65 Digitized by the Internet Archive in 2012 with funding from LYRASIS Members and Sloan Foundation http://archive.org/details/parkfieldcaliforOOhard PRELIMINARY SEISMOLOGICAL REPORT by Samuel T. Harding' and Wilbur Rinehart INTRODUCTION On June 27, 1966, at 8 :26 p.m., Pacific Standard Time, the Parkfield-Cholame, California, region was shaken by a series of earthquakes, similar in many respects to the June 7, 1934, Parkfield series. This report is a preliminary interpreta- tion of the seismological data obtained from permanent seismograph stations throughout the world. Seismological data available were un- usually abundant. This can be attrib- uted to the high density of seismograph stations operated in California by the University of California, California In- stitute of Technology, and the Coast and Geodetic Survey. Excellent records were written by five strong-motion seis- mographs, all located within 35 km of the epicenter. A number of seis- moscopes were also located in the imme- diate area. GEOLOGIC SETTING The area encompassing the epicentral region of the Parkfield earthquake is structurally and lithologically complex. This area is part of the Southern Coast Ranges. Reed (1933) defines the South- ern Coast Ranges as "that part of the Coast Ranges from San Francisco south- ward to the Transverse Ranges." This portion of the Southern Coast Ranges trends N.35-40°W., is subparallel to the trend of the shore of the Pacific Ocean, and lies between the Great Valley and the coast. The Southern Coast Ranges are di- vided longitudinally into three struc- tural subdivisions on the basis of the character of the basement terrain visible beneath unmetamorphosed Cretaceous and Tertiary strata. The basement beneath the middle division, composing the ranges that flank the Salinas River Valley, is coarsely crystalline rock, com- posed of two intermingled parts — gra- nitic rocks of Late Cretaceous age, and enclosing schists and gneisses, mostly of sedimentary origin (Dickinson, 1966). In the flanking divisions, the basement is a strongly deformed and mildly met- amorphosed rock of the Franciscan formation. The Lower Mesozoic Fran- ciscan formation includes igneous, sedi- mentary, and metamorphic rock of eu- geosynclinal assemblage (Reed, 1933). Structurally, the Franciscan complex is an incompetent mass of rocks with a minor proportion of competent beds in- terspersed through the formation. The incompetence of the Franciscan forma- tion, coupled with a highly resistant su- perjacent formation, tends to make the structure in the flanking segments com- plex with many diaper-type folds and faults. The plutonic basement of the middle division is sharply bounded on the east by the active San Andreas fault zone, and on the west by the inactive Sur fault zone. The extension of the Sur COAST AND GEODETIC SURVEY fault to the southeast, as the Nacimiento fault, is poorly located because its posi- tion in the basement is masked by over- lying strata. The region east of the San Andreas fault zone is a group of en echelon anti- clinal and hogback ranges, known collec- tively as the Diablo Range. The Diablo Range is considered a geologic unit be- cause all of its ridges and valleys were formed by erosion of the members of a system of en echelon faulted folds, each trending N.45-75°W. at an acute angle to the San Andreas fault zone. Each major fold, or system of folds, is termi- nated against the fault zone on the northwest and plunges southeastward into the Great Valley. Eardley (1962) points out, "that the most perplexing problem about the San Andreas fault in the Southern Coast Ranges is its set- ting in a typical compressional structure running parallel, or at an acute angle, to the fault zone. The great fault seems to be at odds with the geomorphic provinces." Paleocene, Eocene, Oligocene, and Early Miocene times were generally characterized by subsidence of the San Joaquin basin. The Tertiary strata of this region reflect times of mild, recur- rent deformation throughout the Terti- ary age. The deformation of poorly dated, Pliocene and Pleistocene gravel sequences probably records further evolution of the fold system during the Quaternary period. SEISMICITY The Parkfield-Cholame region is part of the active zone of seismicity extend- ing from north of San Francisco to the Murray fracture zone, part of which can be directly, or indirectly, attributed to the active San Andreas fault. The epi- center of the June 27, 1966, Parkfield earthquake falls in the San Andreas fault zone between the epicenters of the two largest earthquakes ever reported in California — the southern California earthquake in 1857, and the 1906 San Francisco earthquake. The Parkfield earthquake epicenter lies southeast of the 1906 earthquake location, and prob- ably not far from the northern limit of faulting during the 1857 earthquake. Richter (1958) points out that there is a comparative lack of minor shocks along sectors of the San Andreas fault which were displaced by the earthquakes of 1857 and 1906. This may be an indi- cation of why the seismicity of the im- mediate region is slightly higher than the portions northwest and southeast, along the San Andreas fault. Eppley (1966) lists 399 earthquakes in California from 1769 through 1963 with intensity V or greater. The earth- quakes are distributed in intensity as follows : Intensity XI, 4 ; intensity X, 6 ; intensity IX, 13 ; intensity VIII, 32 ; in- tensity VII, 87; intensity VI, 145; in- tensity V, 36 ; and 76 additional shocks believed to have been intensity V or greater. Historically, the Parkfield-Cholame area has been subjected to frequent earthquakes of moderate intensity. Table 1 lists the earthquakes with Modi- fied Mercalli intensity of V or greater since 1769. Figure 1 shows the loca- tions, where possible, of the earthquakes listed in table 1. The largest earth- quake in this region occurred on March 10, 1922. Its epicenter was located 18 miles southeast of the 1966 earthquake, near the town of Cholame. The Modi- fied Mercalli intensity was IX, according to Eppley (1966), and it was felt over 100,000 square miles. Pipelines cross- ing the Cholame Valley were ruptured by this shock, and considerable damage was done to existing structures in the Valley. The March 2, 1901, earthquake was assigned a maximum intensity of VII- THE PARKFIELD, CALIFORNIA, EARTHQUAKE Figure 1. — Earthquakes (intensity V or greater) in the Cholame-Parkfield, California area. 231-020 o- COAST AND GEODETIC SURVEY TABLE 1.— Earthquakes in the Parkfield area, 1769-June 1966 Year Date Origin time N. lat. W. long. Depth Felt area Intensity Mag- Au- thor- ity** P.S.T. M.M. nitude h m s deg deg km sq. mi. 1901 Mar. 2 23 45 36.0 120.5 40,000 VII-VIII 1 1922 Mar. 10^__ *03 21 20.0 35.8 120.3 100,000 IX 6.5 1 Mar. 16_„_ 15 10 35.8 120.3 V 2 Aug. 17___ 21 12 35.5 120.5 25,000 VI 2 1931 Feb. 23___ 02 01 35.8 120.6 5,000 V 3 Feb. 23___ 02 33 35.8 120.6 5,000 V 3 1934 June 5 13 49 35.8 120.3 11,000 About V 5.0 3 June 7 20 30 36.0 120.5 34,000 VI-VII 5.0 3 June 7 20 48 35.9 120.5 34,000 VIII 6.0 3 June 7 21 43 35.8 120.3 V 4.5 3 June 10_^_ 00 03 35.8 120.3 V 4.5 3 1939 Dec. 28„__ *04 16 38.0 35.8 120.3 14,000 V 5.0 3 1947 Feb. 4 *22 14 23.0 36.2 120.6 VI 5.0 3 1956 Nov. 15... *19 23 09.0 35.9 120.5 8,000 VI 5.0 3 1958 Oct. 10__- *05 05 16.0 35.9 120.5 3,500 V 4.5 3 1961 July 30___ *16 07 07.0 35.7 120.4 5,000 V 4.5 3 1966 June 27___ *20 08 57.0 35.54 120.54 (V) 5.0 4 June 27___ *20 26 14.4 35.54 120.54 5 20,000 VII 5.3 4 ( )-Estimated intensity of V or greater. ♦-Instrumental epicenter. ♦♦-Authorities are as follows: 1. Eppley, R. A., Earthquake History of the United States, Part II, Stronger Earthquakes of California and Western Nevada, Environmental Science Services Administration, Coast and Geodetic Survey, 1966. 2. Townley, S. D. and Allen, M., "Descriptive Catalog of Earthquakes of the Pacific Coast of the United States, 1769-1928," Bull. Seis. Soc. Am., vol. 29, pp. 21-252, 1939. 3. United Stales Earthquakes, 1928-196!,, Environmental Science Services Administration, Coast and Geodetic Survey. 4. "Preliminary Determination of Epicenters, June 1966," cards distributed by the Environmental Science Services Admini- stration, Coast and Geodetic Survey. VIII, and was felt over an area of 40,000 square miles or more. Surface cracks, 6-12 inches in width, were reported along the trace of the San Andreas fault zone. In some places vertical motion of 12 inches was reported (Townley and Allen, 1939). The epicenter of the June 7, 1934, earthquake lies within a few kilometers of the 1966 shock. (The seismological aspects of the 1934 earthquake are discussed by Wilson, 1936.) It was pre- ceded by three foreshocks, and was followed by a series of aftershocks. The first foreshock had a maximum intensity of V and was felt over about 11,000 square miles. The largest foreshock, which preceded the main shock by 18 minutes, had a maximum intensity of VI-VII and was felt over an area of about 34,000 square miles. The main shock was felt over an area of 34,000 square miles and was assigned an in- tensity of VIII. The damage caused by this earthquake was moderate to existing structures, and surface crack- ing was reported in the Middle Mountain area northwest of Parkfield (Byerly and Wilson, 1935) . There were two zones of cracks which were close to the surface trace of two faults of the San Andreas fault zone. The zones of cracks were about 25 feet wide and arranged en echelon. The largest single crack was about 55 feet long, 9 inches wide, and 18 inches deep. The sparsity of population in this por- tion of California prior to 1901 may be a THE PARKFIELD, CALIFORNIA, EARTHQUAKE contributing factor to the lack of re- ported seismic activity in the Cholame- Parkfield region. This is evidenced by the following population figures for Monterey County: 1850, 1,872; 1900, 19,380 ; and 1966, 224,316. This factor has also been documented for the Alaska region in "The Prince William Sound, Alaska, Earthquake of 1964 and After- shocks," Department of Commerce, Coast and Geodetic Survey Publication 10-3, vol 1, p. 21, 1966. HYPOCENTERS, ORIGIN TIMES, AND MAGNITUDES OF FORESHOCKS, MAIN SHOCK, AND AFTERSHOCKS It has been pointed out by Eaton (1963),McEvilly (1966),Healy (1963), and Hamilton, et al. (1964) that the crustal velocity on the west side of the San Andreas fault is approximately 0.6 km/sec higher than the 5.6 km/sec veloc- ity reported for the east side of the fault by Byerly (1938) and Byerly and Wil- son (1935), and confirmed by Eaton (1963). This is probably, as pointed out by Eaton (1963), the effect of the large strike-slip movement which has occurred in Cenozoic time. A terrain with granitic basement rock on the west has been brought in contact with ter- rain with a Franciscan basement on the east, producing different basement con- ditions in the areas on opposite sides of the fault. The hypocenter and origin time of the main earthquake, the foreshocks, and series of aftershocks were computed us- ing two methods, both taking into con- sideration the effects of local geology. A digital computer program, described by Rinehart (in press) using local travel-time curves obtained from seismic refraction work and station corrections to compensate for the geologic effect on the velocities between the refraction line and the station, processed the arrival- time data reported by 27 local and re- gional stations. The digital computer program, described by Engdahl and Gunst (1966), which is used regularly by the Coast and Geodetic Survey for epicenter computation, processed the arrival-time data reported by 54 sta- tions located at distances greater than 25°. This program uses Jeffreys-Bullen travel-time curves. The epicenter computed using the sta- tions less than 400 km distant is con- sidered to be closest to the actual epicen- ter, since the velocity data used were from local seismic refraction studies. In the case of Healy's work, the explo- sive source was located at Camp Roberts, about 30 km to the southwest of the epi- center of the Parkfield earthquake, and would be representative of the velocities for seismic waves leaving the focus of the Parkfield earthquake traveling to stations on the west side of the San Andreas fault. The travel-time curve used for stations east of the San An- dreas fault (Eaton, 1963) was derived from an explosive source in the San Francisco area. This confirmed the travel-time curves which Byerly and Wilson (1935) used for this area for locating the 1934 Parkfield earthquake. Figures 3 and 4 are time difference hyperbola plots of the arrival times for the first nine-station pairs reporting the main shock and a major aftershock. The origin of the coordinates of the plot is 36° N., 120° W. The scale is in kilo- meters and the plus signs are the inter- sections of the geographic meridians and parallels. The plots were made on the cathode ray oscilloscope plotter attached to the IBM 7030 computer during the preliminary data pass. The main event, figure 3, clearly shows intersection of numerous hyperbola at about x= — 40 COAST AND GEODETIC SURVEY 120°15' Figure 2. — Location of epicenter of June 28, 1966 (G.M.T.) earthquake, foreshocks, and aftershocks. THE PARKFIELD, CALIFORNIA, EARTHQUAKE ( -, » I | I I I I | I I I I | I I I I | I I I I | I II I | I I I I | I I I I | I I I I | I I I I | I I I I | I I M | l I I I | I M I | I I I I | I I I I | I I I I I I I 800 /CO .1)00 -500 400 300 200 100 O 100 200 300 400 500 fcOO ',00 800 15.41 19 Figure 3. — Time difference hyperbola plot of main earthquake occurring at 0&:26 :14.A G.M.T. June 28, 1966. and y=— 10 (approximately 35.9° N., 120.5° W.). It also shows that the ar- rival time at station J AS (Jamestown) associated with hyperbola E, L, R, W, 1, 5, 6, and 7 does not fit the solution. This station's data were removed from the hypocenter computation during the final data pass. The plot of the June 28 aftershock (04:39:08.9 G.M.T.) shows the intersection of all hyperbola at about x=-40, y=-10 (approximately 35.9° N., 120.5° W.). There are no apparent errors in the input data. Time differ- ence hyperbola plots were made for each event to assist in the removal of arrival times which were in error and to sug- gest a first approximation of epicenter location to the least-squares iterative solution used to determine the epicenter and origin time. Table 2 is a list of these earthquakes, three of which are considered foreshocks, and twenty-two are aftershocks. These earthquakes are plotted in figure 2, with the exceptions of one aftershock on June 28 and three aftershocks on July 2, which fall outside the coordinates of the map. The epicenter, using stations greater than 25° distant, is listed in table 3. The focal depth was constrained to 5 km since there is practically no focal-depth resolution possible when all data are in this distance range. This solution is considered less reliable than the previ- ously discussed solution since most of the readings used were of emergent na- COAST AND GEODETIC SURVEY . i00 t 1 1 i i i i I i i i i | 1 1 i 1 1 i i i i | i i 1 1 | i i 1 1 | i i i 1 1 1 1 1 1 j 1 1 i i 1 1 1 i i 1 1 1 1 1 1 1 i i i 1 1 i 1 1 1 1 i i 1 1 1 i i i 1 1 1 1 1 1 -800 -"/DO -fcOO -WO -400 - JOO -200 -100 O 100 200 300 400 500 600 'iOd 900 15.05^1 FIGURE h. — Time difference hyperbola plot of major aftershock occurring at 0^:39:08.9 G.M.T., June 28, 1966. ture, and there is some question as to what portion of the wave train was read for the P arrival. The depths assigned to these earth- quakes are placed somewhere between 5 and 10 km. In computing the depths, the closest station to the epicenter was Priest (25 km distant). This should give a good depth determination if the velocity model used is correct. But if an increasing with depth velocity is the case, such as proposed by Udias (1965) or McEvilly (1966), the P waves arriv- ing at Priest would be leaving the source at close to 80° from the vertical, allowing little or no depth resolution. The magnitude of the main shock has been reported to be 5.3 by the Coast and Geodetic Survey ; 5.8, by Pasadena ; 5.5, by Berkeley; and 61/2- by Palisades. The surface wave magnitude of 6.2 was computed from the horizontal compo- nent seismographs using the method of Gutenberg (1945). The amplitudes were read at approximately the same time, within the same cycle on each of the horizontal components, and the vec- tor sum was used in the computations. In all cases, records were used where it was possible to read amplitudes for periods between 17 and 23 seconds (Bath, 1956). For the seven stations reported here, the periods of the surface waves used ranged from 19 to 22 seconds. The results of these computa- tions are shown in table 4. THE PARKFIELD, CALIFORNIA, EARTHQUAKE 9 TABLE 2.— Chronological listings of foreshock and aftershock epicenters, June 28-July 2, 1966 Date Origin time G.M.T. N. lat. W. long. Magnitude 1966 Brk Pas h m s deg deg June 28 01 00 33.2 35.97 120.49 3.0 3.5 04 08 57.0 35.94 120.54 5.3 4.8 04 18 35.8 35.98 120.51 2.5 3.2 04 26 14.4 35.94 120.54 5.5 5.8 *04 31 59.3 35.81 120.13 3.5 04 32 51.6 34.74 120.34 3.5 4.0 04 35 02.3 35.84 120.39 3.5 3.1 04 39 08.9 35.91 120.34 3.0 3.5 04 46 23.0 35.90 120.33 2.5 3.0 05 01 02.9 35.96 120.48 3.0 3.6 05 46 02.2 35.86 120.38 3.0-3.5 3.7 06 32 18.9 35.97 120.48 3.5 3.9 07 45 50.8 36.00 120.29 2.5-3.0 3.0 11 51 13.3 35.79 120.41 13 48 21.1 35.78 120.24 3.0 3.3 20 46 58.9 35.91 120.30 3.0 3.4 23 57 24.2 35.88 120.29 3.0 3. June 29 02 19 41.1 35.96 120.48 3.5-4.0 4.1 08 55 55.9 35.87 120.47 3.0 3.2 13 12 00.7 35.80 120.31 3.0-3.5 3.8 19 53 26.8 35.99 120.48 5.0 4.8 June 30 01 17 36.9 35.88 120.36 4.0-4.5 4.3 July 1 09 41 23.1 35.96 120.44 3.0 3.6 July 2 *12 08 35.8 36.02 120.06 3.0-3.5 3.9 *12 16 23.8 35.72 119.60 3.0 3.4 *12 25 24.3 36.08 120.08 2.5 3.1 NOTE: Depths are assumed to be between 5-10 km. The asterisk (*) indicates those epicenters not plotted in figure 2. TABLE 3. — List of hypocenter computations for the main shock at 04:26:14.4 G.M.T. Hypocenter computation no. Origin time G.M.T. N. lat. W. long. Depth No. of sta- tions used Comments 1 h m s 04 26 12. 4 04 26 15. 8 04 26 14. 4 deg 35.9 35. 99 35. 94 deg 120. 5 120. 36 120. 54 km 4 *5 5 48 54 27 C&GS Preliminary Deter- 2 3 mination of Epicenter Card No. 45-66. Station data A > 25° only. Station data A< 5° only. *Depth constrained. 10 COAST AND GEODETIC SURVEY TABLE 4. — Magnitude calculations Station Bogota Bermuda Caracas Copenhagen Istanbul Quito M. 6.4 6.3 6.4 6. 1 6. 6. 1 FOCAL MECHANISM STUDIES A nodal plane solution was made using Byerly's (1938) extended distance tech- nique for the main shock. Tables de- veloped by Hodgson and Storey (1953), based on the Jeffreys-Bullen (1940) travel-time curves, were used to com- pute the extended distance in the dis- tance range of approximately 12° and greater. For the distance range in which Pn is the first arrival, a crustal velocity of 5.6 km /sec and an upper mantle velocity of 7.9 km/sec (Eaton, 1963) were used to determine the critical circle (Romney, 1957). A velocity dis- tribution based on the refraction data of Eaton, 1963, and Healy, 1963, was tried with little or no change in the nodal plane orientation. Stations at which Pg is the first arrival were omitted from the plot because the extended distance tech- nique is sensitive to the depth of the earthquake at short epicentral distances, thus requiring assumptions which the available data do not warrant. For the solution of the main earth- quake, 38 first motions were used (see table 5). The seismograms from 14 of these stations were read by the authors, and the remaining 24 readings were ob- tained from stations that report direc- tions of first motions routinely. Two of the 38 first motions were found to be in- consistent with the solution, amounting to about 5 percent error. Figure 5 is an extended distance plot in stereographic projection of the direc- tion of first motions of the 38 stations used in the solution. The circles rep- resent the A and B planes found for this earthquake. The smaller circle in figure 5 represents the A plane and the larger circle, the B plane. The A plane has a strike of N.62°E. and dips 60°W. The B plane, restrained by the orthogonality criterion, strikes N.25°W. and dips 86° E. The strike of the San Andreas fault in this region is N.44°W. The B plane is oriented N.25°W., which is 19° east of the strike of the San Andreas fault. This nodal plane indicates right-lateral motion. The A plane is nearly trans- verse to the regional structure, figure 7. No attempt is made in this discussion to differentiate between the fault plane and the auxiliary plane, since this would imply that the earthquake is classified as a single couple. This cannot be deter- mined by the method used. Work is progressing on an S-wave solution which should enable the authors to better evaluate the nature of the focal mecha- nism. THE PARKFIELD, CALIFORNIA, EARTHQUAKE TABLE 5.— Character of first motion for main shock at 04:26:14.0 G.M.T. 11 Station First motion Station First motion *Albuquerque C Mould Bay D Areata c Mt. Hamilton C Barrett c Mineral D Byerly c Mount Wilson C *Blacksburg c *Oroville D *Bozeman c Pasadena C *Cedar Springs c Palomar C China Lake c Priest C Cottonwood c Riverside C Edmonton D Santa Barbara D *Eureka C *State College C * Flaming Gorge C *Salt Lake City c Fort Tejon c *San Luis Dam D *Glen Canyon c San Nicolas D Goldstone c Tinemaha C *Hungry Horse D *Tucson D Isabella C Tulsa C Jamestown D Ukiah D ♦Junction City C Woody C C-Compressional. D-Dilatational. ♦-Stations read by authors. CONCLUSIONS The Cholame-Parkfield area is part of an active zone of seismicity which is as- sociated with the San Andreas fault zone. Table 1 leads one to believe that the seismic activity in this region has greatly increased since 1901, but as ex- plained on pages 4-5, the sparse popula- tion previous to 1901 may account for the lack of reported earthquakes. The most severe earthquake in the Parkfield area occurred on June 7, 1934, within a few kilometers of the 1966 location. Surface cracks and the fore- and aftershock activity associated with this earthquake have a strong re- semblance to the June 27 earthquake effects. One very interesting similarity is that, in both instances, the largest foreshock preceded the main shock by 18 minutes (see table 1). The B nodal plane of the focal mecha- nism study is oriented 20° east of the strike of the San Andreas fault. 231-020 12 COAST AND GEODETIC SURVEY O Compression A Dilatation Figure 5. — Stereographic projection showing the first motion and direction of nodal planes. THE PARKFIELD, CALIFORNIA, EARTHQUAKE 13 120°15' 36°00' 120°15' Figure 6. — Comparison of nodal plane solutions of June 28, 1966 (G.M.T.) earthquake to regional faulting B plane. 14 COAST AND GEODETIC SURVEY 36° 00 Figure 7. — Comparison of nodal plane solutions of June 28, 1966 (G.M.T.) earthquake to regional faulting A plane. THE PARKFIELD, CALIFORNIA, EARTHQUAKE 15 REFERENCES Bath, M., "The Problem of Earth- quake Magnitude Determination," Tra- vaux Scientifiques, Series A, Fascicule 19, p. 5, 1956. Byerly, P., 'The Earthquake of July 6, 1934 : Amplitudes and First Motion," Bull. Seis. Soc. Amer., vol. 28, pp. 1-13, 1938. Byerly, P. and Wilson, J. T., "The Central California Earthquakes of May 16, 1933, and July 7, 1934, Bull. Seis. Soc. Amer., vol. 25, pp. 223-246, 1935. Crowell, J. C, "Displacement along the San Andreas Fault, California," Geological Society of America, Special Paper No. 71, 1962. Dickinson, W. R., "Table Mountain Serpentinite Extrusions in California Coast Ranges," Geological Society of America Bulletin, vol. 77, pp. 451-471, 1966. Eardley, A. J., Structural Geology of North America, Harper and Row, 743 pp., New York, 1962. Eaton, J. P., "Crustal Structure from San Francisco, California, to Eureka, Nevada, from Seismic-Refraction Meas- urements," Journal of Geophysical Re- search, vol. 68, pp. 5789-5806, 1963. Engdahl, E. R. and Gunst, R. H., "Use of a High Speed Computer for the Pre- liminary Determination of Earthquake Hypocenters," Bull. Seis. Soc. Amer., vol. 56, pp. 325-336, 1966. Eppley, R. A., Earthquake History of the United States, Part II, Environ- mental Science Services Administration, Coast and Geodetic Survey, 48 pp., 1966. Gutenberg, B., "Amplitudes of Sur- face Waves and Magnitudes of Shallow Earthquakes," Bull. Seis. Soc. Amer., vol. 35, pp. 3-12, 1945. Hamilton, R. M., Ryall, A., and Berg, E., "Crustal Structure Southwest of the San Andreas Fault from Quarry Blasts," Bull. Seis. Soc. Amer., vol. 54, No. 1, pp. 67-77, 1964. Healy, J. H., "Crustal Structure along the Coast of California from Seismic- Refraction Measurements," Journal of Geophysical Research, vol. 68, No. 20, pp. 5777-5787, 1963. Hodgson, J. H. and Storey, R. S., "Tables Extending Byerly's Fault-Plane Techniques to Earthquakes of any Focal Depth," Bull. Seis. Soc. Amer., vol. 43, pp. 49-61, 1953. Jeffreys, H. and Bullen, K. E., "Seis- mological Tables," British Association for the Advancement of Science, Lon- don, 1940. McEvilly, T. V., "The Earthquake Sequence of November 1964 near Corra- litos, California," Bull. Seis. Soc. Amer., vol. 56, p. 755, 1966. Jennings, C. W. and Strand, R. G., Geologic Map of California, Santa Cruz Sheet: Calif. Dept. Nat. Res., Div. of Mines, Scale 1 : 250,000, 1958. Jennings, C. W., Geologic Map of California, San Luis Obispo Sheet: Calif. Dept. Nat. Res., Div. of Mines, Scale 1:250,000, 1958. Pakiser, L. C, "Structure of the Crust and Upper Mantle in the Western United States," Journal of Geophysical Re- search, vol. 68, pp. 5747-5756, 1963. Reed, R. D., "Geology of California," The American Association of Petroleum Geologists, Tulsa, Oklahoma, 1933. Reed, R. D. and Hollister, J. S., "Struc- tural Evolution of Southern California," The American Association of Petroleum Geologists, Tulsa, Oklahoma, 1936. Richter, C. F., Elementary Seisynol- °9V, W. H. Freeman and Company, San Francisco, 1958. Romney, C, "Seismic Waves from the Dixie Valley-Fairview Peak Earth- quakes," Bull. Seis. Soc. Amer., vol. 47, No. 4, pp. 301-319, 1957. 16 COAST AND GEODETIC SURVEY Seismological Notes, Bull. Seis. Soc. Amer., vol. 14, pp. 169-172, 1924. Townley, S. D. and Allen, M. W., "Descriptive Catalog of Earthquakes of the Pacific Coast of the United States, 1769-1928," Bull. Seis. Soc. Amer., vol. 29, pp. 21-252, 1939. Udias, A., "A Study of the After- shocks and Focal Mechanism of the Salinas-Watsonville Earthquakes of Au- gust 31 and September 14, 1963," Bull. Seis. Soc. Amer., vol. 55, pp. 85-106, 1965. Wallace, R. E., "Structure of a Por- tion of the San Andreas Rift in Southern California," Bull. Geol. Soc. Amer., vol. 60, pp. 781-806, 1949. Wilson, J. T., "Foreshocks and After- shocks of the Nevada Earthquake of December 20, 1932, and the Parkfield Earthquake of June 7, 1934," Bull. Seis. Soc. Amer., vol. 26, pp. 189-194, 1936. PRELIMINARY ENGINEERING SEISMOLOGICAL REPORT by William K. Cloud INTRODUCTION In an effort to gain knowledge on earthquake motions of engineering in- terest at and within a few miles of a fault, a strong-motion instrument array was completed in June 1965 across the San Andreas fault zone near Cholame, California. Implementation of the pro- ject became possible as the result of a cooperative agreement between the Coast and Geodetic Survey and the State of California Department of Water Re- sources. On June 27, 1966, the hopes for ob- taining information from the array were fulfilled. The main shock of the Park- field-Cholame earthquake series, at 20 : 26 Pacific Standard Time, activated in- struments in the array and yielded some of the most interesting records ever ob- tained. Richter magnitude of the shock was reported as 5.3 by the Coast and Geodetic Survey, 5.5 by Berkeley, and 5.8 by Pasadena. Epicenter placement was difficult for seismologists due to the scarcity of teleseismic stations east and west of the area. However, field inspec- tion and past history of the region sug- gested the Middle Mountain area a few miles northwest of Parkfield. From this area, surface evidence of minor faulting was visible southward along the San Andreas fault zone to a few miles beyond Cholame, a distance of approximately 20 miles. Results from the strong-motion in- struments in the array are presented in this preliminary report in context with effects of the earthquake as determined by field investigation and questionnaire card coverage. INTENSITY 1 The main shock of the Parkfield, Cali- fornia, earthquake series occurred on June 27 at 20 : 26 Pacific Standard Time. It was felt over an area of approxi- mately 20,000 square miles. A map of the felt area showing Modified Mercalli intensities is displayed in figure 8. The map was prepared from 356 earthquake questionnaire card reports, press re- ports, and from field investigations con- ducted by the Coast and Geodetic Survey. A maximum intensity of VII has been assigned to a small area in and near the San Andreas fault zone, extending southeasterly from a few miles north of Parkfield to a few miles south of Cholame. This intensity is based pri- marily on the ground displacement and cracking which occurred in this area. (Figures 9-13 show pavement cracks on Highway 466, and ground surface frac- tures and displacement of pipeline near Strong-motion No. 2.) INTENSITY VII : Cholame and vicinity. — Water heater torn loose in kitchen. Furniture shifted about 1 foot from walls. Lamps overturned; desk drawers were opened; and a large amount of glassware was broken. "It was like riding a bucking bronco." At the Rafter E Ranch, plaster cracked and fell and furniture shifted. Water sloshed from the swimming pool. Dam- age moderate. At the Standard Oil Service 1 By Nina H. Scott, Seismological Field Survey. Coast and Geodetic Survey. 17 18 COAST AND GEODETIC SURVEY Station, one window cracked near anchoring of shelter ceiling beam, and possible slight buck- ling of metal facia. Coke machine shifted southeast. At the Cholame Cafe and Gro- cery Store, concrete slab floor of front of build- ing separated from older wood floor on wood piers (fig. 14) . Many objects fell from shelves. No apparent damage to wood-frame residences, wood barn, small wood-frame school, and water tanks. Cholame (about 1 mile northeast of; Strong- motion Station No. 2). — Bridge cracked and pavement buckled (figs. 9 and 15-18). Base- ground separation at power pole (with small MODIFIED MERCALLI INTENSITY MAP MAIN SHOCK OF THE PARKFIELD, CALIFORNIA EARTHQUAKES JUNE 27, 1966 20:26 PST FIGURE 8.— Map showing Modified Mercalli intensity distribution of the June 27, 1966 (20:26:144 P.S.T.), Park field earthquake. THE PARKFIELD, CALIFORNIA, EARTHQUAKE 19 3s 5* v. 3 Ss O 55 t^ o ° 231-020 o — oe 20 COAST AND GEODETIC SURVEY i^; PJB^v. A- :&'&' so § a* I. ii? 'm^^M^^ ■'¥.'', /'^ y~i © O s ° V *« B >o ■S *> CO e ^* -Si ■S £S (X, ••», ^ >1 a »* ^e & s s O 05 CO THE PARKFIELD, CALIFORNIA, EARTHQUAKE 21 lMfc- -1 %: • -~\;-%jl WH^-i Figure 12. — Displacement of pipeline located about 200 yards southeast of Strong- motion Station No. 2. Bent grass indicates original location of pipeline. View looking northeast. {Photo by B. J. Morrill) «* ' - Juw iBH - - 'if & ~ ■ ' '■ " 7m Figure 13. — Ground surface fractures about 2 miles northwest of Strong- motion Station No. 2. View looking northwest. (Photo by Ed Etheredge) 22 COAST AND GEODETIC SURVEY transformer) just east of the station indicated considerable shaking. Quart jar of water over- turned at Strong-motion Station No. 2. Cholame (north of; Jack Ranch, 6.5 miles north of Highway 466, on the Cholame-Park- field Road) . — Water pipes to stock tank, and irrigation pipe broke. Chimney on bunkhouse broke and slipped % inch, but did not fall. About 1 mile west of Jack Ranch, dead trees were knocked over (fig. 19). Cholame (6-7 miles east of; on U.S. High- way 466). — Observer reported old, poorly built house "tore apart in two places," but also re- ported damage was slight. Volume of spring water increased. "Our tank is now running over. Before the shock, we had hardly any water in the tank." Cholame (3.2 miles southeast of Strong-mo- tion Station No. 2; William Alley Ranch).— (VI) Stock water tank (not very full) showed indication of slight motion. Domestic water tank (volume about 4,960 gal and kept rather full) showed evidence of oscillation of not over 1 mm about one edge of base, direction about south-southeast. Some plaster cracked; ob- jects knocked from shelves. Observer stand- ing in yard sat down on ground in fear of being knocked down. Cholame (Everett Hatch Ranch on Ming Road, about 7 miles east-southeast of Strong- motion Station No. 2). — Tank tower moved about 3 inches off base. Kitchen cabinet doors opened and some dishes, glasses, etc., fell out; jars and canned goods knocked off shelves. The only furniture movement was a table, which slid 10 inches in a westward direction. Doors were difficult to close after the shock. Electricity and phone service disrupted. Cholame (about 8 miles southeast of). — (VI) Back porch slipped down about 6 inches from rest of house. Few dishes broke. Phone out for about 36 hours. Cholame (about 3 miles southwest of). — (VI) "The light went out so I couldn't see what was going on. I just sat there in the dark and listened to things crashing down inside the house and trees swishing outside. Texture paint shaken off one wall. No damage to brick chimney." Cholame (10 miles east of; on U.S. High- way 466). — (VI) Water increased in two springs. "The house seemed to roll from east- west; then at the last very severe shudder the dishes and supplies fell from shelves." Cholame (10 'miles northeast of; on Highiuay 41). — (VI) Plaster cracked. Water tank swayed. Small objects shifted, overturned, and fell. Cholame (about 6 miles south of). — (VI) Extensive minor plaster cracks. Power and phones out. Cholame (12 miles southeast of). — (V) Felt by all. Motion seemed like riding on small waves. No perceptible damage. Cholame (Bitterwater Road; close to Choice Valley School).— (V) Felt by all. Old crack in cement enlarged. Cholame (Bitterwater Road). — (V) Small objects overturned; picture fell from shelf. "Six small shocks and one large shock felt on June 27th." Parkfleld and vicinity. — (Press) Water pipes and windows broke at the Parkfleld Episcopal Church. Light fixtures broke at the Parkfleld School. Floor dropped out of a trailer house. Heavy damage to merchandise at the Parkfleld Store. There were numerous reports of minor damage to homes in the rest of the little Cho- lame Valley, and in Bear Valley which runs just to the west of Parkfleld. A 12,000-volt, PG&E power line was torn loose and fell across the phone line serving the forestry station. Fifteen of the 70 telephones in Parkfleld were out of service. The forestry service also reported live oak trees were toppled, pulling down wires. The forestry station was slightly damaged by fire touched off by a "hot" wire. It was also re- ported that several tombstones were overturned in the Parkfleld cemeteries (figs. 20-22). At the Parkfleld Meeting Hall, a chimney was damaged (figs. 23 and 24), and the west wall was cracked at the fire station (figs. 25 and 26). About five miles south of Parkfleld, the X-bracing of a bridge buckled as shown in figures 27-29. Parkfleld (report from Herbert H. Durham; Weather Bureau Observer) . — Some landslides. Several large limbs broken off trees. Walls cracked some; plaster cracked, broke, and fell. Furniture shifted. Much glassware breakage. "All my clocks broke." Parkfleld (report from Mrs. John O. Bunch). — Water muddy. Piano shifted 2 feet. Dishes and radio broke. Parkfleld (report from D. M. Garrett). — Water source murky for several days ; several water wells showed signs of disturbance. Chim- neys fell; windows cracked; plaster cracked, broke, and fell. Furniture shifted, overturned, and broke. Parkfleld (about 1 mile south of; Raymond Miller residence) . — Brick chimney fell. Dishes broke ; other minor damage. Parkfleld (1 mile south of; Robert H. Dur- ham residence). — Chimneys cracked; plaster cracked; one window cracked. Damage mod- THE PARKFIELD, CALIFORNIA, EARTHQUAKE 23 erate to new log house. Furniture shifted. Much china broken. Tombstones cracked. "For two nights we have had shocks all night long." Park field (2.3 miles south of; Henry Miller place on Parkfield to Cholame Road; report from Henry Miller). — Very violent shock. Ground crack, about 1.3 cm wide, was observed about 30 feet behind the house, running parallel with the house (N.25°W.) and through the swimming pool. Damage to the pool was esti- mated at about $1,000. Water sloshed from pool. Two water tanks, part of the filter sys- tem and about the size of a standard home hot water heater', were shifted about 18 cm in a northeast direction, toward the crack. The tanks are set on a concrete pad about 5 feet south of the crack (fig. 30). Several land- slides occurred along a stream behind and to the southeast of the house. Kitchen cabinet doors, facing N.15°E., opened and all dishes fell out, with about 80 percent breakage. No cracked windows or plaster, and no damage to large stone fireplace. Parkfield (about 5.8 miles south of ; Art Wil- son residence on Parkfield to Cholame Road) . — This house sustained the most damage. A brick chimney fell through the kitchen roof. Parkfield (1 mile north of; report from Mrs. William Thomason) . — Two brick chimneys damaged ; windows broke. Heavy upright deep- freeze (full of meat) overturned. Consider- able glassware breakage. Electricity dis- rupted. "The breaking of glass and buckling of walls blotted out any earth noises." Parkfield (2.8 miles north of). — Old wooden bridge, concrete abutments, with additional steel-supporting framework of I-beams. Road- bed separations indicated longitudinal shaking of possibly several centimeters. Diagonal cracks in south abutment were perhaps due to the earthquake. Longitudinal and lateral movements at wood and steel contacts. Parkfield (3.3 miles north of). — Roadbed cracks. Parkfield (4.1 miles north of). — Wooden bridge. Roadbed separations. Parkfield (abut 4.5 miles north of; Clyde F. Taylor Ranch). — Chimney snapped off at roof line; foundation cracked; water pipes broke; plaster cracked. Much dish breakage. Furni- ture shifted. Water disturbed. Parkfield (5 miles north of, on Coalinga Road; report from Zena Varian). — (VI) Ground cracked. Water disturbed. Hanging objects swung violently. Small objects shifted, overturned, and fell; furniture shifted. No plaster or windows cracked; no damage re- ported. INTENSITY VI : Annette (about 10 miles southeast of Cho- lame, and about 1 mile from San Andreas Fault; Old Annette School). — "I was walking down the hall and was pushed back and forth against the walls." Hanging objects swung violently north-south. Two bottles rolled from shelf. Annette Lookout. — ( V) Water splashed from elevated water tank. Power cut off. No visible damage. Avenal (about 23 miles northeast of Cho- lame). — Small objects and furniture shifted. Hanging objects swung violently. Damage slight (no details). No plaster or windows cracked. Bitterwater Pumping Station (about 18 miles southeast of Cholame). — Small objects shifted. Hanging objects swung violently west- east. Water disturbed. No damage. Bitterwater Valley (in area to east of Bitter- water Pumping Station). — Exterior hairline house cracks. Two small objects shifted. Bitterwater Valley (20 miles east of Shandoyi). — (V) Water disturbed. "Earth noises sounded like rocks grinding together." Coalinga (about 15 miles from San Andreas fault). — Plaster cracked. Merchandise fell from grocery store shelves. Water in swim- ming pool disturbed. Coalinga (about 16 miles %vest of; Los Gatos Canyon, 36"14' north, 120°34' west; Weather Bureau Observer). — (V) "There are some large cracks in our jeep road which we cannot remember seeing before. The soil is dry bog- type." Creston. — House swayed from side to side. Small objects shifted. Paso Robles. — Police Department reported they had received no reports of damage in the city. Few cans, bottles, etc., knocked from shelves in some stores. At cafe, few new cracks were observed in plaster ceiling. At fire station (second floor), coarse concrete spalled off along a previous crack near the ceiling along edge of an I-beam. Water sloshed from swimming pool at motel. Police officer reported telephone poles swayed. Paso Robles (Kiler Canyon). — (V) "We easily observed bushes shake and the wooden arbor shake." Paso Robles (12 miles northeast of). — Water tower rocked violently; water disturbed. Small objects shifted. Pendulum clock on north-south wall stopped, but clock on east-west wall did not. 24 COAST AND GEODETIC SURVEY Figure 14. — Crack between concrete slab front of Cholame Cafe and older wood floor on wood pier section of cafe. (Photo by George Murray) : : ir- : Figure 15. — Bridge on Highway 466 about 200 feet ivest of Strong-motion Station No. 2. (Photo by B. J. Morrill) THE PARKFIELD, CALIFORNIA, EARTHQUAKE 25 Figure 16. — Crack in east abutment of bridge shown in figure 15. (Photo by George Murray) Figure 17. — Crack in concrete pier of bridge shown in figure 15. (Photo by B. J. Morrill) 26 COAST AND GEODETIC SURVEY Figure 18. — Indication of motion at bottom of bridge pier shown in figure 17. {Photo by B. J. Morrill) Figure 19. — Dead trees knocked down by earthquake, about 1 mile ivest of Jack Ranch. {Photo by Ed Etheredge) THE PARKFIELD, CALIFORNIA, EARTHQUAKE 27 San Luis Obispo. — Press reported plaster cracked at the police station. San Miguel (ivest side of town, next to free- way). — Few dishes broke. Lamps and stands almost toppled. House has 12-foot ceiling. Walls seemed to twist. Shafter. — Minor plaster cracks. Lighting fixtures moved rather violently. Small objects shifted, overturned, and fell. Shandon and vicinity. — Plaster cracked. "Shock at 20:26 was much heavier than the one at 20:09." Shandon (O. V. Monett home; owner of Cholame ' Cafe and Grocery). — Cement-block house, slab floor. Little damage. Some cracks enlarged. Few dishes off shelves. Cabinet doors opened. Phone line to restaurant at Cholame was out. Shandon (Standard Oil Pumping Station on Highway 466, 1 V 2 miles east of Shandon; report from Mrs. 0. W. Lund). — Hanging objects swung violently. Forty-pound mirror fell off wall; kitchen cupboard doors popped open and items "flew" out; objects fell from window sills and off tables; pictures fell from walls; pepper shaker on stove turned completely upside down. Attic dust fell on beds from ceiling movement. Shandon (northeast of; Strong-motion Sta- tion No. 5, Cochrum Garage). — No damage. Several cylinders in the garage fell northeast; tank in the garage shifted northeast. Tall cylinders in an outside shed (raised off the ground on skids and alined approximately par- allel to the fault) fell to the southwest (fig. 31) . Many tires were thrown southeast out of an overhead rack in the garage. Ground contact to the concrete slab floor of the garage was gen- erally broken. Two soda-water fire extinguish- ers hanging on wall were set off, but did not fall. Steel-frame building. Shandon (about midway between Shandon and Cholame; Strong-motion Station No. 6, Shandon Pumping Station) . — Slight shifting of storage tank ladders on back fill. Evidence of shaking at base of small water tank (about 1,000 bbl) on a low mound as shown in figures 32 and 33. Shandon (northeast of; Strong-motion Sta- tion No. 8, Shandon Pumping Station). — Only damage was shifting of one of the eight boilers (fig. 34) . The two southwest boilers were filled with water; the end boiler shifted southwest, probably V2 cm, forcing out caulking around the southwest boiler fi-ont. Water in each boiler was roughly estimated to be 350 bbl or about 11,000 gal. Mr. D. R. Imhoof reported one window broke and one cracked on south side of his wood-frame house. A mirror, 24 by 30 231-020 0—66—5 inches, on dressing table fell to floor without disturbing small perfume bottles on dressing table. Shandon (soutluvest of; Strong-motion Sta- tion No. 12; report from Wilbur Thompson) . — Strong rolling motion. Kitchen cabinet doors opened and a few items spilled onto floor. No other damage reported. No damage to irriga- tion pipes. Slack Canyon (about 12 miles north by west of Parkfield; report from James B. Harri- son). — Ground cracks, landslides, water dis- turbances. Nothing shifted, overturned, or fell; no damage. Valleton (Indian Valley; report from Mrs. John Curtis). — Damage slight. Cracks around windows and in west walls of cinderblock house. Furniture shifted; small objects fell. Knocked needle off chart of Weather Bureau rain gage and bent the arm. Walti Ranch (about 10 miles northwest of Parkfield) . — Very little damage (no details; no windows or plaster cracked). Hanging objects swung violently. Small objects shifted. Work Ranch (about 8 miles northtvest of Shandon; Hog Canyon Road, near Ellis School). — Adobe house. Damage moderate in east portion; severe in west wing. Plaster cracked, broke, and fell. Furniture shifted slightly. Small objects shifted, overturned, and fell. INTENSITY V : Adelaida area (west of Paso Robles), Alpaugh, Arroyo Grande, Atascadero, Avila Beach, Bakersfield (one observer reported plaster cracked around bathtub, but other re- ports indicated only moderate intensity), Bay- wood Park, Bryson, Burrel, Buttonwillow and 1 mile west of, Earlimart, Fellows, Frazier Park and Cuddy Valley area west of Frazier Park, Greenfield, Harmony, Indian Valley (20 miles north of San Miguel), Kettleman City, King City, La Panza, Lost Hills, Maricopa, Mee Ranch (corner Highway 198 and Highway 25), Morro Bay, Moss Landing, Musick (about 15 miles southeast of San Luis Obispo), Nipomo, Oceano, Panoche (report indicated the shock was only moderately felt, but observer stated one irrigation pipe was broken), Pine Canyon (lower Burns Road), Pozo Guard Sta- tion (about 10 miles west of La Panza), Priest Valley (report indicated the shock was mod- erately felt; however, on August 7, observer reported: "We believe the June 27-28 earth- quakes made us loose our well. One of the streams has disappeared."), San Ardo, San Joaquin, San Lucas, San Simeon, Simmler 28 COAST AND GEODETIC SURVEY (about 12 miles southeast of La Panza, Carrisa Plains), Stratford, Templeton, and Vanden- berg Air Force Base. MODIFIED MERCALLI INTENSITY SCALE OF 1931 All intensities used by the Coast and Geodetic Survey refer to the Modified Mercalli Intensity Scale of 1931 (Wood and Neumann, 1931). The abridged ver- sion of this scale is given here with equiv- alent intensities according to the Rossi- Forel scale. (ABRIDGED) I. Not felt except by a very few under spe- cially favorable circumstances. (I Rossi-Forel scale.) II. Felt only by a few persons at rest, espe- cially on upper floors of buildings. Delicately suspended objects may swing. (I to II Rossi-Forel scale.) III. Felt quite noticeably indoors, especially on upper floors of building, but many people do not recognize it as an earth- quake. Standing motorcars may rock slightly. Vibration like passing of truck. Duration estimated. (Ill Rossi-Forel scale.) IV. During the day felt indoors by many, outdoors by few. At night some awakened. Dishes, windows, doors disturbed; walls make creaking sound. Sensation like heavy truck striking building. Standing motorcars rocked noticeably. (IV to V Rossi-Forel scale.) V. Felt by nearly everyone, many awakened. Some dishes, windows, etc., broken; a few instances of cracked plaster; unstable objects overturned. Disturb- ances of trees, poles, and other tall objects sometimes noticed. Pendulum clocks may stop. (V to VI Rossi-Forel scale.) VI. Felt by all, many frightened and run outdoors. Some heavy furniture moved; a few instances of fallen plaster or damaged chimneys. Damage slight. (VI to VII Rossi-Forel scale.) VII. Everybody runs outdoors. Damage neg- ligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; consid- erable in poorly built or badly designed structures; some chimneys broken. Noticed by persons driving motorcars. (VIII Rossi-Forel scale.) VIII. Damage slight in specially designed struc- tures; considerable in ordinary sub- stantial buildings with partial collapse; great in poorly built structures. Panel walls thrown out of frame structures. Fall of chimneys, factory stacks, col- umns, monuments, walls. Heavy fur- niture overturned. Sand and mud ejected in small amounts. Changes in well water. Persons driving motorcars disturbed. (VIII+ to IX- Rossi- Forel scale.) IX. Damage considerable in specially de- signed structures; well-designed frame structures thrown out of plumb; great, in substantial buildings, with partial collapse. Buildings shifted off founda- tions. Ground cracked conspicuously. Underground pipes broken. (IX + Rossi-Forel scale.) X. Some well-built wooden structures de- stroyed; most masonry and frame struc- tures destroyed with foundations; ground badly cracked. Rails bent. Landslides considerable from river- banks and steep slopes. Shifted sand and mud. Water splashed (slopped) over banks. (X Rossi-Forel scale.) XI. Few, if any, (masonry) structures re- main standing. Bridges destroyed. Broad fissures in ground. Under- ground pipelines completely out of service. Earth slumps and land slips in soft ground. Rails bent greatly. XII. Damage total. Waves seen on ground surfaces. Lines of sight and level dis- torted. Objects thrown upward into THE PARKFIELD, CALIFORNIA, EARTHQUAKE 29 Figure 20. — Tombstone at Parkficld, Todd Cemetery. Direction of fall, northwest. {Photo by Frank McClure) ■ . '*££. Figure 21. — Sarnc as figure 20. Figure 22. — Same as figure 20. 30 COAST AND GEODETIC SURVEY Figure 23. — Parkfield Meeting Hall. (Photo Figure 24. — Close-up of chimney damage by Frank McClure) shown in figure 23. >^5fcs»tot8sd«J2?SK Figure 25. — Parkfield Fire Station. (Photo by George Murray) THE PARKFIELD, CALIFORNIA, EARTHQUAKE 31 j .--• Figure 26. — Interior view of cracked west wall of Parkfield Fire Sta- tion. (Photo by George Murray) Figure 27.— Bridge about 5 miles south of Parkfield. Orientation N.3°E. (Photo by Frank McClure) 32 COAST AND GEODETIC SURVEY Figure 28. — Buckled X-bracing of bridge shown in figure 27. (Photo by Frank McClure) Figure 29. — Same as figure 28. THE PARKFIELD, CALIFORNIA, EARTHQUAKE 33 Figure 3i. — Acetylene and oxygen cylinders in Cochrum Garage storage shed reported to have been knocked over by earthquake. (Photo by George Murray) Figure 30. — Cracked swimming pool about 2.3 miles south of Parkfield. Adjacent home ivas undamaged. (Photo by Frank McClure) Figure 32. — Union Oil Co., Shandon Pumping Station. Strong-motion Station No. 6 is adjacent to pumping station. Note undamaged stack at right side of photo. (Photo by George Murray) 34 COAST AND GEODETIC SURVEY ;<■ 3$* fllwll V r . f THE PARKFIELD, CALIFORNIA, EARTHQUAKE 35 AREA OF MAJOR INTEREST The area of major interest from an en- gineering viewpoint was roughly from Parkfield to Cholame (fig. 35) and in the vicinity of the strong-motion instru- ment array (fig. 36). Effects of the main shock within this area were de- scribed in the preceding section under intensity VII. Additional details on the relatively moderate damage in the area are shown by the photographs (figs. 9-34). ARRAY INSTRUMENTATION The 16 strong-motion recording sta- tions, located relative to the fault as shown in figures 35 and 36, were instru- mented with Coast and Geodetic Sur- vey seismoscopes (Cloud and Hudson, 1961). In addition, for control, Sta- tions 12, 8, 5, 2, and Temblor were instrumented with Model AR-240 ac- celerographs (Halverson, 1964). Each accelerograph in the array, after being triggered into operation by the main shock through a pendulum starter switch, recorded the relative motion of three, mutually perpendicular pendu- lums (vertical, longitudinal, and trans- verse) against time. Period and damp- ing of the pendulum were such that recorded amplitudes could be assumed reasonable approximations of ground acceleration in the period range t e >t . Shaking table tests of a single produc- tion model pendulum made prior to the earthquake indicate the probable degree of approximation to be expected (see table 7). Each seismoscope in the array con- sisted of a magnetically damped, uni- filar suspended, free, conical pendulum which could move in any horizontal di- rection, angular deflection being re- corded by a scriber on a smoked spheri- cal watch glass. The purpose of the seismoscopes was to measure one point 23,1-020 O— S6 6 on the 10 percent-damped velocity re- sponse spectrum and to provide informa- tion on the sequence of motion during the shock. Complete spectra (not included in this report) result from analysis of accelerograph records. Pictures of an AR-240 accelerograph and a seismoscope appear in a later sec- tion of this report (figs. 37 and 45). INSTRUMENTAL RESULTS The five AR-240 accelerographs in the array, of the type shown in figure 37, recorded the accelerograms shown in fig- ures 38—41. Since the figures are precision photocopies of the original records they are suitable for analysis, provided the ratio of the measured diam- eter of the circle that appears on each record to 5 cm is used as a scale factor for adjusting sensitivity. To supplement the records, some of the more noticeable accelerations and the roughly estimated ground periods at which they occurred are given in table 6. To further supplement the records, re- sultant horizontal acceleration trajec- tories are shown in figures 42, 43, and 44. The fifteen C&GS seismoscopes in the array, plus the one at the Avenal Gap Station, all of the type shown in figure 45, produced the records shown in fig- ures 46-61. Each figure is a precision photocopy of the original 6.35-cm-diam- eter smoked watch glass record, and thus suitable for analysis. The maximum response of each seis- moscope, scaled from what was judged by eye to be the neutral or rest point of the scriber needle on the record, is given in table 8. SUMMARY The relatively shallow-focus, small- magnitude (5.3-5.8) main shock of the Parkfield-Cholame earthquake series produced new and provocative engineer- ing seismological data that will require 36 COAST AND GEODETIC SURVEY time, analysis, and thought to fully ex- plain. First, there was surface evidence of faulting for approximately 20 miles along the San Andreas fault zone, with rupture approximately proceeding southeasterly from the epicentral re- gion. Second, maximum, short-dura- tion, transient accelerations at several array stations were the highest ever re- corded, the extreme maximum being 0.5 gravity. (The highest acceleration pre- viously recorded was 0.3 gravity during the El Centro, California, earthquake of May 18, 1940.) Third, the recorded high acceleration caused only relatively minor damage to existing structures in the area. Fourth, the maximum re- sponse of seismoscopes near the fault was at approximately right angles to the strike of the fault. CONCLUSIONS Since strong motions at and adjacent to faulting have never before been in- strumentally measured, there is no way of actually knowing if data recorded during the main shock of the Parkfield- Cholame series are unusual or common- place. However, the explosionlike high accelerations, the directions of seismo- scope maximum response, and the direc- tion of tombstone fall suggest very local phenomena, such as progressive rupture along the fault, as the cause. If this is true, then high transient accelerations might be expected near any point where faulting reaches the surface regardless of earthquake magnitude. Damage po- tential of the high accelerations would depend on duration, and on period and damping of existing structures. ACKNOWLEDGMENTS The author is indebted to R. P. Maley who selected the location for the array of strong-motion stations ; to B. J. Mor- rill, George Murray, and Edwin Ethe- redge who retrieved and processed the records and investigated effects; and to C. F. Knudson who assisted with data reduction. REFERENCES Cloud, W. K. and D. E. Hudson, "A Simplified Instrument for Recording Strong-motion Earthquakes," Bull. Seis. Soc. Am., vol. 51, No. 2, April 1961. Halverson, Harry T., "The Strong- motion Accelerograph," Technical Bul- letin 13, Teledyne Industries, Inc., UED Earth Sciences Division, Pasadena, Cali- fornia, 1964. Wood, H. O. and Frank Neumann, "Modified Mercalli Scale of 1931," Bull. Seis. Soc. Amer., vol. 21, pp. 277-283, 1931. THE PARKFIELD, CALIFORNIA, EARTHQUAKE 37 36°00' 36°00' ome Figure 35. — Parkfield-Cholame section of epicentral region. Locations where photographs were taken south of Parkfield are noted by circled figure numbers. 38 COAST AND GEODETIC SURVEY CO m CO i»iqoj| osdj 01 ^ S0|iuj 9( XjejDuijxojddy £ F- =» © S e CO o O o •-a to W « P a fa THE PARKFIELD, CALIFORNIA, EARTHQUAKE Figure 37. — AR-2U0 strong-motion accelero graph. 40 COAST AND GEODETIC SURVEY TABLE 6. — Accelerations recorded by AR-240 accelerographs (See notes at end of table) Component Component Component Accelero- graph (9) Period (sec) Accelero- graph (9) Period (sec) Accelero- graph (9) Period (sec) Station 12-9.3 miles; 1,120 feet, elevation; alluvium N50°E Down Nl>0°W 0.06 0.3 0.05 0.1 0.03 0.3 .05 .1 .01 1.3 .06 .2 .01 1.3 .01 2.5 .03 .02 .1 1.8 Station 8-5.9 miles; 1,060 feet, elevation; alluvium N50 ( >E Down NiO°W 0.07 0.4 0.14 0.1 0.11 0.2 .27 .1 .03 1.0 .17 .3 .13 .2 .28 .2 .09 .5 .05 .6 Station 5-3.4 miles; 1,100 feet, elevation; alluvium N85°E Down N5°W 0.18 0.2 0.18 0.1 0.10 0.1 .46 .3 .01 .8 .17 .5 .08 .8 .40 .3 .03 1.3 .02 .9 Station 2 -0.05 mile; 1,140 feet, elevation; alluvium N65' "E Down N25°W 0.10 0.1 0.35 0.1 Not in operating condition at .11 1.0 .09 .8 time of earthquake. .50 .6 .02 1.4 .44 .4 .45 .8 Station Temblor-6.7 miles; 1,480 feet, elevation; rock N65°W Down S25°W 0.29 0.3 0.16 0.2 0.40 0.2 .12 .2 .04 .3 .15 .4 .03 .6 .09 .1 .24 .5 NOTES: Component direction given is pendulum motion for trace up on the record. Distance given is from station to crack across Highway 466 near Strong-motion Station 2. Period given is merely a crude timeguide based on half-cycle estimates. Photocopy of Station 12 accelerogram was not available in time to be included in this report, but may be obtained upon request to the Coast and Geodetic Survey. THE PARKFIELD, CALIFORNIA, EARTHQUAKE 41 TABLE 7. — Comparison of shaking table accelerations recorded by an AR-240 accelerometer and a Stat ham accelerometer Statham accelerometer ' AR-240 accelerometer 2 Shaking table period (Fraction of gravity) (Fraction of gravity) (sec) 0.157 0.161 0.515 .655 .645 .350 .445 .457 .270 .562 .550 .220 .515 .500 .183 .632 .632 .158 .925 .890 .133 1.29 1.25 .117 .511 .518 .100 .410 .438 .088 .374 .390 .078 .340 .352 .073 .328 .344 .067 .318 .312 .059 .292 .274 .054 .281 .250 .050 1 Statham Model A45-2-350, ±2g-natural period 0.01 sec. 2 AR-240 accelerometer #333-naturaI period 0.051 sec. TABLE 8. — Maximum velocity response of seismoscopes relative to the ground during main shock of Parktield earthquake series T h> S A Sv 2 Station Period Damping Sensitivity Max. trace R=A/S Velocity Distance 3 (sec) (percent critical) (in/radian) amplitude (inches) (radius) response (ft/sec) (miles) 12 0.78 20 2.39 0.08 0.033 0.186 9.3 11 .78 18 2.36 .12 .055 .295 8.0 10 .78 10 2.29 .19 .083 .332 7.5 9 .77 7.5 2.18 .36 .165 .564 6.8 8 .78 11 2.32 .28 .121 .507 5.9 7 .78 9.5 2.46 .39 .159 .616 5.0 6 .78 10 2.50 .34 .136 .544 4.0 5 .78 9 2.36 .48 .203 .770 3.4 4 .78 10 2.29 .51 .227 .907 2.0 3 .78 9 2.44 .92 .377 1.278 .9 2 .78 8.5 2.32 >1.4 >.603 >2.2 .05 1 .78 9 2.52 >1.4 >.556 >2.1 1.2 Al .75 9 2.17 .41 .189 .727 5.6 A2 .75 10 2.17 .29 .134 .515 6.5 Antelope .75 10 2.17 .38 .175 .638 7.1 Avenal Gap .78 10 2.44 .17 .070 .319 19.4 1 From damping curve for trace amplitude "A". 2jt * 10 3 Distance from station to crack across Highway 466 near Strong-motion Station 2. 42 COAST AND GEODETIC SURVEY 8 $2 a, Oh THE PARKFIELD, CALIFORNIA, EARTHQUAKE 43 D O Q_ O O O CN o o CN a> c 3 8, | c O E o u _c a o \_ o u < c o J L 3) a. 46 COAST AND GEODETIC SURVEY Figure U2. — Station 8. Resultant horizontal accelerations from 3.5 to 5.5 seconds after start of record. THE PARKFIELD, CALIFORNIA, EARTHQUAKE 47 Figure US. — Station 5. Resultant horizontal accelerations from 6.0 to 9.0 seconds after start of record. 48 COAST AND GEODETIC SURVEY 0.0 Olg. I . . . I Figure hU- — Temblor Station. Resultant horizontal accelerations from 3.0 to 5.0 seconds after start of record. THE PARKFIELD, CALIFORNIA, EARTHQUAKE 49 ^ Figure 45. — Coast and Geodetic Survey seismoscope. 50 COAST AND GEODETIC SURVEY Figure A6. — Seismoscope record - Station 12. THE PARKFIELD, CALIFORNIA, EARTHQUAKE 61 s^^~' h^ ^>v jr- / / /- N 1 T-*?-* t 1 ~*~ i # - 1* Ng CD // V n x.v^> ?*> s ^sFy ^y Figure 47. — Seismoscope record - Station 11. 62 COAST AND GEODETIC SURVEY Figure 48. — Seis7noscope record - Station 10. THE PARKFIELD, CALIFORNIA, EARTHQUAKE 53 Figure 49. — Seismoscope record - Station 9 54 COAST AND GEODETIC SURVEY Figure 50. — Seismoscope record - Station 8. THE PARKFIELD, CALIFORNIA, EARTHQUAKE 55 Figure 51. — Seismoscopc record - Station 7. 56 COAST AND GEODETIC SURVEY Figure 52. — Seismoscope record - Station 6. THE PARKFIELD, CALIFORNIA, EARTHQUAKE 57 Figure 53. — Seismo scope record - Station 5. 58 COAST AND GEODETIC SURVEY Figure 54. — Seismoscope record - Station 4- THE PARKFIELD, CALIFORNIA, EARTHQUAKE SB Figure 55. — Seismoscope record - Station 3. 60 COAST AND GEODETIC SURVEY FIGURE 56. — Seismoscope record - Station 2. Glass record plate fell off instrument during main shock. THE PARKFIELD, CALIFORNIA, EARTHQUAKE 61 Figure 57. — Seismoscope record - Station 1. 62 COAST AND GEODETIC SURVEY Figure 58. — Seismoscope record - Station A-l. THE PARKFIELD, CALIFORNIA, EARTHQUAKE Figure 59. — Seismoscope record - Station A-2. 64 COAST AND GEODETIC SURVEY Figure 60. — Seismoscope record - Antelope Pumping Station. THE PARKFIELD, CALIFORNIA, EARTHQUAKE 65 Figure 61. — Seismoscope record - Avenal Gap Statioyi. U.S. GOVERNMENT PRINTING OFFICE: 1966 O— 231-020 p ENN STATE UNIVERSITY LIBRARIES AQDDD7D^D1131 HI w > l-H w O n > m > s ill m O hi d w -4