The Foreshock Activity of the 1971 San Fernando Earthquake, California by Mizuho Ishida and Hiroo Kanamori

All of the earthquakes which occurred in the epicentral area of the 1971 San Fernando earthquake during the period from 1960 to 1970 were relocated by using the master-event method. Five events from 1969 to 1970 are located within a small area around the main shock epicenter. This cluster of activity is clearly separated spatially from the activity in the surrounding area, so these five events are considered foreshocks. The wave forms of these foreshocks recorded at Pasadena are, without exception, very complex, yet they are remarkably similar from event to event. The events which occurred in the same area prior to 1969 have less complex wave forms with a greater variation among them. The complexity is most l ikely the effect of the propagation path. A well located aftershock which occurred in the immediate vicinity of the main shock of the San Fernando earthquake has a wave form similar to that of the foreshocks, which suggests that the foreshocks are also located very close to the main shock. This complexity is probably caused by a structural heterogeneity in the fault zone near the hypocenter. The seismic rays from the foreshocks in the inferred heterogeneous zone are interpreted as multiple-reflected near the source region which yielded the complex wave form. The mechanisms of the five foreshocks are similar to each other but different from either the main shock or the aftershocks, suggesting that the foreshocks originated from a small area of stress concentration where the stress field is locally distorted from the regional field. The number of small events with $-P times between 3.8 to 6 sec recorded at Mt. Wilson each month suggests only a slight increase in activity of small earthquakes near the epicentral area during the 2-month period immediately before the main shock. However, because of our inabil i ty to locate these events, the evidence is not definit ive. Since the change in the wave forms is definite the present result suggests that detailed analyses of wave forms, spectra, and mechanism can provide a powerful diagnostic method for identifying a foreshock sequence. INTRODUCTION Foreshock activity has long been considered to be potentially useful for earthquake prediction. In fact, pronounced foreshock activity was one of the key elements in the successful short-term prediction of the 1975 Haicheng, China earthquake (e.g., Haicheng Earthquake Study Delegation, 1977). However, not all earthquakes are preceded by observable foreshock activity. Although Jones and Molnar (1976) found that 44 per cent of all large shallow events in the world from 1950 to 1973 had foreshocks, most large California earthquakes did not have pronounced foreshock activity. In particular, the 1971 San Fernando earthquake was believed to be completely lacking in foreshocks. However, a recent study by Ishida and Kanamori (1977) suggested an increase in seismic activity near the hypocentral area of the 1971 San Fernando earthquake for the 2-year period just before the main shock. Although this activity can be considered to be foreshock activity in a broad sense, the conclusion was solely based on the spatiotemporal plot of the epicenters; it was not clear whether this activity was physically distinct from the background activity. 1265 1266 MIZUnO ISHIDA AND HIROO KANAMORI From the point of view of earthquake prediction, it is important to distinguish foreshock activity from swarm activity or ordinary background activity on a physical basis. To this end we made a more detailed study of the "foreshocks" of the San Fernando earthquake: (I) by relocating the events using the master-event method, (2) by studying the wave forms of these events recorded at Pasadena, (3) by investigating the mechanism, and (4) by examining the activity of smaller events. Ishida and Kanamori (1977) identified five events located within 15 km from the epicenter of the San Fernando earthquake during the period from January 1969 to February 9, 1971. Whether these events can be called foreshocks or not may be somewhat controversial. Although there is no established definition of foreshocks, usually foreshocks refer to an activity preceding the main shock by hours or days. The activity before the San Fernando earthquake does not belong to this category. However, as we will show later, since these events are clearly distinct in several important aspects from the events prior to 1965, in this paper we will call these events foreshocks. RELOCATION OF HYPOCENTERS All the events that occurred within 35 km from the epicenter of the San Fernando earthquake during the period from 1960 to 1971 were relocated by using the master event method (Johnson and Hadley, 1976). The master event employed is one of the best-constrained aftershocks used by Hadley and Kanamori (1978). The HYPO 71 location program (Lee and Lahr, 1975) were used, and the S-P times at stations within about 100 km from the epicenter were included in the analysis. The P times registered on the original card file at the Seismological Laboratory and the crustal structure employed by Hadley and Kanamori (1978) were used. Figure 1 shows the results for the three periods: period II (1961 to 1964), period III (1965 to 1968), and period IV (1969 to February 9, 1971). The events during period I (1932 to 1960) are taken from the Caltech Catalog (Hileman et al., 1973) without relocation. These periods are the same as in Ishida and Kanamori (1977). The overall pattern is the same as that found in the previous paper: a relatively high activity during period II, complete absence of activity within 15 km from the main-shock epicenter during period III, and clustering of activity around the main-shock epicenter during period IV. However, for period IV, the concentration of activity toward the main-shock epicenter is more remarkable than in the earlier study; the spread is about 7 km compared with about 20 km in the preliminary analysis. This cluster of activity is clearly separated spatially from the activity in the surrounding area. Table I lists the events which occurred within 15 km of the epicenter during period IV. Brady (1976) investigated the seismicity pattern prior to the San Fernando earthquake to test his inclusion theory of earthquakes. He interpreted the central part of the activity during period II as "the primary inclusion zone of the impending failure". In the present paper we will be primarily concerned with the foreshock activity during period IV rather than the activity during period II. WAVE FORM ANALYSIS All of the five foreshocks were recorded by a Wood-Anderson seismograph at Pasadena (h 40 km). The response of the Wood-Anderson seismograph is nearly flat over a period range from 0 to 0.6 sec, and is suitable for the wave form analysis of these small events. The seismograms of these five events are shown in Figures 2a and 3a. The wave forms recorded on the E-W component (Figure 2a) are very complex yet they are remarkably similar f rom event to event (see also Figure 6). FORESHOCK ACTIVITY OF THE 1971 SAN FERNANDO EARTHQUAKE 1267