Abstract

Following the 18 October 1989 Loma Prieta earthquake, an array of 22 three-component IRIS/PASSCAL instruments was deployed in the Santa Cruz Mountains. We use an empirical Green's function method to study source properties of aftershocks as well as attenuation in the epicentral area. Using pairs of closely spaced events, we show that the largest aftershocks are well modelled by an ω2 source spectrum, and we obtain estimates of the P-wave corner frequency for a total of 15 events. Brune stress drops are observed to have no systematic variation over the range of moments spanned by the data set, down to 1019 dyne * cm. There is a suggestion that aftershocks that occurred outside the mainshock rupture have higher stress drops than those that occurred within the mainshock rupture. A ml = 3.4 foreshock in the subsequence associated with the 18 April 1990 ml = 5.4 Watsonville earthquake yields a stress drop estimate of 820 bars. This value is well outside the stress drops obtained for all other events (6 to 266 bars). Site-specific attenuation parameters suggest large lateral and vertical heterogeneity in P-wave Q structure. Station HOLY, in a valley near the surface trace of the San Andreas Fault, is characterized by low Q values, while stations to the north of the aftershock zone are characterized by the highest Q values.

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