Broadband Study of the Source Characteristics of the Earthquake

Abstract

We have determined the source characteristics of the 1989 Loma Prieta earthquake, using teleseismic data. The solution from body waves gives a mechanism with a strike of 128°, a dip of 70° a rake of 138°, and a seismic moment of 3x10^(26)dyne-cm (M_w=6.9). This solution is similar to those obtained from long-period Rayleigh and Love waves, PnJ waves, and first-motion data. The body-wave solution suggests a depth of about 15 km. The effective duration of the source is 6 s, suggesting lengths of 30 and 15 km for bilateral and unilateral faulting, respectively. Considering the extent of the aftershock zones, we estimate a total rupture length of 35 km. The strike-slip and thrust components of coseismic slip are 177 and 159 cm, respectively. The large thrust component raises an important question regarding the recurrence pattern. If the 1989 Loma Prieta earthquake is a characteristic earthquake with a recurrence interval of about 100 yr, the 159-cm displacement implies a long-term uplift rate of about 1 cm/yr, which appears too high for this region. Three hypotheses for reconciling this apparent conflict are that (1) the geometry of plate motion along the Santa Cruz Mountains section of the San Andreas fault changes on a time scale of several thousand years, and so the coseismic displacement has not accumulated enough to produce high topographic relief; (2) the coseismic-slip direction varies from event to event; and (3) the slip plane of the 1989 Loma Prieta earthquake is distinct from the Pacific-North America plate boundary-if so, then this earthquake is a rather rare, noncharacteristic event. The surface slip of about 1 m for the 1906 San Francisco earthquake is one of the key data in long-term forecasting. No surface slip was observed in the 1989 Loma Prieta earthquake, even if the horizontal slip at depth was as large as 1.8 m. This discrepancy points to a risk of relying too heavily on surface observations for long-term seismic-risk analysis.

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