AFTERSHOCKS AND FREE ARTHQU AKE SEISMICITY By CARL E. JOHNSON, U.S. GEOLOGICAL STRVLY; and L. K. HLTTON, CALIFORNIA INSTITUTE OFTECHNOLOCY CONTENTS seismicity of a moderate earthquake in a detail never before possible. During the next few years we expect that the tens of thousands of digital seismograms for Page Abstract _______________________________ 59 several thousand aftershocks, together with a large Introduction ___________ ______ ___________ __ 59 body of other geophysical data, will provide the basis for Analysis _ _________________________ _________ 59 many inquiries into the physical processes attending Aftershock distribution __________ _______ ________ 60 major earthquakes. Here we provide only a preliminary Historical perspective ___. 63 and incomplete picture of the most conspicuous of these Preearthquake seismicity 69 phenomena. Conclusions __________ 73 Acknowledgments 73 Our ideas are not presented chronologically. We first References cited _____ . 76 discuss gross aspects of the aftershock distribution and then place it within a context of seismicity during the preceding years. Having discussed what appears to ABSTRACT have been normal background seismicity for the central Although primary surface faulting was mapped for nearly 30 km, Imperial Valley, we can then consider possibly unusual aftershocks extended in a complex pattern more than 100 km along aspects of activity during the weeks immediately before the trend of the Imperial fault. A first-motion focal mechanism for the the main shock. main shock is consistent with right-lateral motion on a vertical fault striking N. 42° W., in agreement with the strike of the Imperial fault within the limits of resolution. There is evidence that conjugate fault­ ing on a buried complementary northeast-trending structure occurred ANALYSIS at the north limit of displacement on the Imperial fault near Brawley, Calif. This faulting was apparently initiated at the time of a mag­ Our data are from 150 short-period vertical instru­ nitude 5.8 aftershock 8 hours after the main shock. A line of epicenters ments in the southern California seismic network oper­ extending along the trend of the San Andreas fault nearly 100 km into ated jointly by the California Institute of Technology the eastern Imperial Valley was noted during the aftershock se­ quence, in an area recognized as notably aseismic during the preced­ (CIT) and the U.S. Geological Survey (USGS). These ing 5 years. The main shock was preceded by a 3-month period of data were telemetered in analog form to CIT, where significantly reduced seismicity affecting the central Imperial Valley. they were digitized at 50 Hz. A real-time event detector Although three small events near the incipient epicenter during this recorded selected intervals of record on magnetic tape interval may be deemed foreshocks, no distinct foreshocks im­ for subsequent offline analysis on an interactive mediately before the main shock were observed. cathode-ray-tube (CRT) display terminal. Both the event locations and magnitudes reported INTRODUCTION here (table 7) are preliminary results from the first The Imperial Valley earthquake of October 15, 1979, stage of routine processing, using the CEDAR system was the largest in California since the installation of described by Johnson (1979). Final values will not be dense seismic networks and thus provides a unique op­ available until much more editing and analysis of the portunity for studying both the aftershocks and prior data have been completed. Processing of the more than 2,000 aftershocks that occurred during the first 20 days after the earthquake required the accurate timing of 'Contribution No. 3459, Division of Geological and Planetary Sciences, California Institute more than 40,000 discrete arrivals. Hypocenters were of Technology, Pasadena, CA 91125. calculated using an unpublished program (QED1) de- 59 THE IMPERIAL VALLEY, CALIFORNIA, EARTHQUAKE OF OCTOBER 15, 1979 TABLE 7. Preliminary origin times, epicentral coordinates, and local We selected events for analysis and graphic presenta­ magnitudes for aftershocks of M] 3=4.0 tion solely on the basis of location quality (epicentral Local error, less than 5 km), a criterion that generally favors Origin time Epicentrjil coordinates magnitude Date (G.m.t.) Latitude N. Longitude W. (M, > the largest events during a given period. Some events of magnitude larger than 4.0 have been ignored either 10,15 2316:53.44 32°36.82' 1153 19.09' 6.6 2319:29.98 32°45.94' 115°26.45' 5.0 because they were immediately preceded by others 2325 4.0 2355:03 32=55' 115~°31' H.2 large enough to make accurate timing difficult or be­ 10/16 0022:14.20 32 57.46' 115"31.19' 4.2 cause they occurred during an interval for which digital 0100:13.86 32°57' 115°29' >4.6 0114:21.29 32°55.54' 115°31.38' 4.3 data were not available. 0139:04 32°56.45' 115°31.45' 4.0 0310 4.5 0316:25.43 32°56~73' 115°32.56' 4.1 0339 4.5 AFTERSHOCK DISTRIBUTION 0549:10.18 32°56~63' 115°32.38' 5.0 0604:39.03 32°54.79' 115°32.07' 4.1 0611:59.96 32-56.05' 115°30.88' 4.2 Figure 32 illustrates all well-located aftershocks that 0613:13.41 32°55.45' 115C31.60' 4.2 0619:48.68 32°55.71' 115°32.36' 5.1 occurred during the first 20 days of the aftershock se­ 0655 4.6 0658:42.69 32°59~83' 11J.34.41' 5.8 quence (October 15 through November 5). The main 0723:24.21 32°53.92' 115"31.12' 4.2 0749 4.0 shock, denoted by the star south of the United States- 0936:41.14 32°56.31' 115°30.87' 4.0 1051:27.11 32°56.34' 115=33.02' 4.0 Mexican border, lies within a zone that remained sur­ 1126:27.40 32°57.83' H5335.19' 4.0 1146 4.8 prisingly aseismic throughout the sequence. A more 1201:44.96 32°52.31' 115°31.07' 4.0 1500 4.0 accurate epicenter for the main shock, obtained by 2316 ---- ---- 4.9 Chavez and others (this volume) using a balanced suite 10/17 1914:37.72 32°54.39' 115°35.00' 4.2 2052 4.2 of U.S. and Mexican data, plots about 2 km to the north­ 2245 --- ____ 4.6 east, midway between the location shown on the map 10/19 1222 - - 4.1 (fig. 32) and the United States-Mexican border. 'Epicentral location in error by 5 km or more. The focal mechanism for the main shock (lower right, fig. 32) is consistent with right-lateral motion on a ver­ tical fault striking N. 42° W., in general agreement with veloped at CIT, based on the generalized inverse method the trend of the southern section of the Imperial fault. formalized by Wiggins (1972). The crustal model used Although surface breaks were limited to a zone 30 km for all hypocentral calculations (table 8) was obtained long (heavy lines, fig. 33), aftershocks occurred within graphically from the models presented by Mooney and an area 110 km long, from the Cerro Prieto geothermal McMechan (this volume), using data obtained during area to the Salton Sea. Except for one small group of the Imperial Valley seismic-refraction experiment dis­ events near the south terminus of surface rupture, most cussed by Fuis and others (this volume). This model is aftershocks were clustered within about 15 km of representative of the central Imperial Valley south of Brawley, particularly during the first 8 hours after the Brawley, Calif. Most magnitudes greater than 3.0 are main shock (fig. 33). standard M/ 's from peak amplitudes on Wood-Anderson A significant change in the aftershock distribution torsion seismometers; the remaining magnitudes are occurred after the largest aftershock (Af/_ = 5.8) at 0658 Mt.a's calculated from coda amplitudes, using the G.m.t. October 16, approximately 8 hours after the main method of Johnson (1979). Focal mechanisms were ob­ shock and immediately after the interval plotted in tained using the computer program described by Whit- figure 33. The location of this event, here referred to as comb (1973). the Brawley aftershock, is marked by its focal mecha­ nism, plotted west of Brawley on the map (center, fig. 32). The most distinctive change in the aftershock pat­ TABLE 8. Crustal model used for locating after­ shocks tern was a strong northeast-trending line of epicenters from west of Brawley to just south of Wiest Lake (WLK, Depth to top Layer of layer velocity figs. 32, 33), in agreement with the left-lateral plane of (km) (km/s) the focal mechanism. Tectonically the local increase in 2.15 strain at the north end of the Imperial fault break was 2.75 3.60 apparently accommodated by left-lateral motion on a 3 ______________________ 4.30 conjugate fault propagating from southwest to north­ 4 _____________________________ 5.05 5 ________________________ 5.50 east. Independent support for this conclusion is pro­ 5.5 _________________________ 5.70 vided by a coincident linear zone of ground disturbance, 10.0 ______________________ 5.80 13.0 ____ ____________ 6.95 liquefaction, and cracking mapped by T. H. Heaton, 14.0 _____________________ 7.20 J. G. Anderson, and P. T. German (unpub. data, 1980). 25.0 ________________________ 7.80 Unfortunately, fieldwork was complicated by slumping AFTERSHOCKS AND PREEARTHQUAKE SEISMICITY 61 116°00' 30' 15' 115W 33°30' EXPLANATION Local magnitude (M,) of aftershock > c 3 < 3 Fault Dashed where approximately located Focal mechanism + FNK Station location UNITED STATES ^" 32°30' Cerro Pricto geothermal area FIGURE 32. Well-located aftershocks (epicentral error, less than 5 km) from October 15 through November 5,1979.