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U.S. GEOLOGICAL SURVEY, BOX 25046, MS 966DENVER FEDERAL CENTER, DENVER, COLORADO 80225
PACIFIC GEOPHYSICS, INC., 170 SOUTH CHESTER AVENUE, SUITE 20, PASADENA, CALIFORNIA 91106
Abstract
Broadband, digital, teleseismic body waves (P and SH) are inverted to obtain the history of slip for the 17 October 1989, Loma Prieta earthquake. Comparison of finite-fault, waveform, inversion results using an L1 norm and an L2 norm reveal some basic uncertainties in the rupture history that are not usually appreciated in earthquake source studies. However, the more robust features of the source can be identified that are less dependent on the choice of minimization norm. From a consideration of first-motion data and forward modeling of body waves, the strike and dip of our model fault plane are set at 126° and 67°, respectively. The waveform inversion results indicate that the majority of the moment release occurs over a fault length of 35 to 40 km and a depth range of 2 to 18 km. The hypocenter (18 km depth) experienced a relatively small amount of slip. The rupture propagated upward and bilaterally with a peak slip of about 3 m occurring at a depth of 10 km. The average velocity of the rupture propagation is 2.5 km/sec. The details of the slip distribution depend on the minimization norm, but two main sources can be identified with the larger source above and to the southeast of the hypocenter. The rake vector varies considerably over the fault. The southern source is mostly strike slip, while the northern source is approximately equally partitioned between dip slip and strike slip. There is also a trend for more strike-slip motion at shallower depths. The majority of the moment release occurs in the first 7.5 sec with an integrated moment of 2.6 x 1026 dyne-cm. Including smaller, peripheral sources that are less well resolved raises the moment estimate to 3.0 x 1026 dyne-cm.
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