Physics of the Earth and Planetary Interiors, 75 (1993) 267-288 267 Elsevier Science Publishers B.V., Amsterdam Analysis of the 1986 Mt. Lewis, California, earthquake: preshock sequence-mainshock-aftershock sequence Yi Zhou, Karen C. McNally and Thorne Lay Institute of Tectonics and C\ F. Richter Seismological Laboratory, Unil.,ersity of California, Santa Cruz, CA 95064, USA (Received 27 April 1992; revision accepted 7 July 1992) ABSTRACT Zhou, Y., McNally, K.C. and Lay, T., 1993. Analysis of the 1986 Mr. Lewis, California, earthquake: preshock sequence- mainshock-aftershock sequence. Phys. Earth Planet. Inter.. 75: 267-288. The 1986 Mt. Lewis earthquake (M L = 5.7) occurred on a right-lateral fault northeast of and oblique to the Calaveras fault in a region that had not experienced significant seismicity since 1943. Data from the nearby Lawrence Livermore Seismic Network and selected U.S. Geological Survey stations are used to relocate events within 15 km of the mainshock epicenter during the period 1980-1987, using the master-event method. Beginning 17 months before the mainshock, 22 events ruptured in the depth range 5-9 km within 1.4 km and northwest of the mainshock epicenter, in an area subsequently almost devoid of aftershocks. This cluster of preshock activity is clearly separated both spatially and temporally from the background activity in the surrounding area. Composite focal mechanisms for the preshocks and for nearby aftershocks suggest that there are two slightly different focal mechanisms amongst the preshocks, one being similar to the mainshock and aftershocks and one being rotated in strike. Cross-correlations of digitally recorded short-period waveforms of 10 of the clustered preshocks (Mu. = 1.5-2.5) reveal that the average inter-event peak cross-correlation between seismograms is 0.62. Six nearby early aftershocks show an average inter-event peak cross-correlation between seismograms of 0.54. Only a few aftershocks have cross-correlations of 0.6 or higher, which implies that the events were slightly further apart or multiple mechanisms were active during the early aftershock period. No significant differences are observed between the spectra of the preshocks and aftershocks. The aftershock area expanded along the strike with time, ultimately defining a north-south fault plane, 11 km in length, extending from 3 to 10 km in depth. The mainshock appears to have originated at the base of the seismogenic zone and ruptured bilaterally along strike and updip. A forward modeling technique is used to model teleseismic body waveform data of the mainshock. The long-period data (P and SH) are consistent with a point source strike-slip earthquake with a teleseismic moment of 3.9 × 1017 N m. We infer that the mainshock involved the rupture of an asperity in the central portion of the aftershock zone. I. Introduction variation of seismicity patterns. One of the most promising procedures for understanding this The study of seismicity patterns is one of many complexity is to analyze preshock, mainshock, approaches that can be used to understand the and aftershock behavior (e.g. Doser, 1989), but physical mechanisms of earthquake failure pro- relatively few events with clear preshock se- cesses. Unfortunately, seismic behavior exhibits quences have been studied. Many investigators substantial complexity and regional variation. The have studied spatio-temporal variation of seismic- complex structure and stress heterogeneity of ity before large earthquakes, in attempts to un- fault zones may be responsible for the observed derstand the physical processes leading to rup- ture (e.g. McNally, 1977; Ishida and Kanamori, Correspondence to: T. Lay, Institute of Tectonics and C.F. 1977, 1980; Ishida and Ohtake, 1984; Smith and Richter Seismological Laboratory, University of California, Priestley, 1988). These investigations have often Santa Cruz, CA 95064, USA. found that seismic activity before the mainshock 0~)31-9201/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights re~rved 268 Y. ZIIOU E'I" AL. is located at or near the mainshock epicenter. plane that lacked both preshock and aftershock This can be interpreted as the result of stress activity. They inferred that before the mainshock increase in and around the rupture nucleation localized asperity regions wcrc locked and quies- region before the mainshock. It has also been cent whereas surrounding weaker areas on the observed that for some earthquakes with predom- fault slipped with small events or aseismically. inantly strike-slip focal mechanisms, the spatial Schwartz et al. (1989) examined the relationship distribution of well-located aftcrshocks has a dis- between aftershocks and mainshock fault slip for tinctive pattern. Reasenberg and Ellsworth (1982) other large intcrplatc events. They also found studied aftershocks of the Coyote Lake, Califor- that aftershock hypocenters tended to be absent nia, earthquake. They found that the larger after- from regions where mainshock slip was concen- shocks surround a quiet region whereas preshocks trated. Near-total relaxation of stress in the rup- concentrate within the central quiet portion of tured asperities is one possible explanation for the aftershock zone. The 1984 Morgan Hill earth- the sparsity of aftershocks in the asperity regions. quake and its aftershocks were studied by Cock- Hartzell et al. (1991) and Beroza (1991) found a erham and Eaton (1987). They showed that the similar tendency for aftershocks to lic outside most intriguing feature of the aftershock distribu- primary mainshock slip regions for the Loma tion is a central quiet zone surrounded and al- Pricta event. Thus, although exceptions certainly most completely outlined by aftershocks. The exist, it is often possible to infer where regions of mainshock hypocenter lies within this quiet zone principle coseismic slip arc located on the basis at its northwest end. of reduced aftershock activity. Tajima and Kanamori (1985a,b) investigated Although seismicity data are of great impor- the patterns of aftershock area expansion associ- tance, not all of the source physics information is ated with large subduction zone earthquakes on a included in hypocentral locations and time se- global scale. They argued that if the fault zone is quence. The actual seismograms contain more represented by relatively large asperities sepa- detailed information about the sources, but it is rated by small weak zones, then little expansion not always straightforward to extract that infor- of aftershock activity is expected. On the other mation. If the asperity model proposed by hand, if relatively small asperities are sparsely Kanamori (1981) is correct, foreshocks should bc distributed, significant expansion may occur. The concentrated along the edges of strong asperities. aftershock area expansion pattern may thus re- Foreshocks at stress concentrations may occur as flect the spatial variation of fault-zone properties. groups of events with very similar locations and Mendoza and Hartzeil (1988)and Hartzell and focal mechanisms and thus very similar wave- Iida (1990) analyzed aftershock patterns for sev- forms. Also, stress drops of preshocks should be eral moderate to large earthquakes. They found high on average if they represent the final stages that aftershocks tended to occur outside or near of stress accumulation around asperities (e.g. the edges of the regions of principal slip in the Zufiiga et al., 1987). mainshock. The pattern of aftershock activity Relatively few studies have investigated wave- concentrated around primary slip zones implies a forms of preshocks and aftershocks to test these redistribution of stress after the earthquake, as a ideas. Ishida and Kanamori (1978) observed five result of increased loading away from the area of events located in the epicentrai region of the greatest seismic moment release. Engdahl et al. 1971 San Fernando earthquake during the 2 years (1989) and Houston and Engdahl (1989) com- before the earthquake, and found that the wave- pared the spatial distribution of coseismic slip in forms were remarkably similar. Geller and the 1986 Andreanof Islands earthquake with relo- Mueiler (1980) studied four ME=2.7 earth- cated seismicity before and after the event. They quakes on the San Andreas fault in Central Cali- found that preshock and aftershock seismicity fornia. They hypothesized that earthquakes pro- coincided spatially, but the mainshock moment ducing nearly identical waveforms must have sim- release tended to occur in regions of the fault ilar focal mechanisms and hypocenters within THE 1986 MT. LEWIS, CALIFORNIA. EARTHOtJAKI~ 269 one-quarter of the shortest wavelength. To test Lawrence Livermore Seismic Network and a sub- this hypothesis, Thorbjarnardottir and Pechmann stantial number of preshocks and aftershocks (1987) studied cross-correlation of bandpass- were detected. Accurate relative locations can be filtered seismograms of open-pit mine blasts in obtained, and the digital recording capability of Utah with known locations. Their results sup- the network allows an examination of the wave- ported the one-quarter wavelength hypothesis and forms of many small preshocks and aftershocks. therefore increased the confidence in application This earthquake is somewhat unusual in that of this method to earthquake data. Pechmann most large California earthquakes have not had and Kanamori (1982) studied small-scale earth- pronounced foreshock activity. It occurred on a quake clustering before and after the (M~. --- 6.6) previously