The Oroville Earthquakes: A study of source characteristics and site effects

This is the authors' abstract. We don't add key points for this paper.

Half‐space modeling, in the time domain, of broadband seismic records from the Oroville aftershock sequence demonstrates that these events have simple sources. Systematic scaling between the far‐field source duration and seismic moment for the aftershocks studied here and other California earthquakes confirms Brune's (1970) source model and shows that stress drops for California earthquakes range between 10 and 100 bars. These events also show no trend in stress drop as a function of size for 3.0 ≤ ML ≤ 6.4. Analysis of static deformation associated with the main shock shows that slip occurred on a fault 81 km2 in area corresponding to the zone defined by the first 3 days of aftershocks. Teleseismic studies indicate that the seismic source area was 50 km2, corresponding to a zone completely free of aftershocks in the middle of the aftershock zone. Both results imply a stress drop of 29–39 bars and seismic moment between 5.7 and 8.7×1024 dyn cm. The evidence suggests that faulting occurred seismically on the central portion of the fault and propagated out slowly, enlarging the fault plane. Complicated waveforms recorded for the same events at nearby sites on alluvium indicate the critical role that propagation through complex structure plays in determining the character of the observed seismic signal at close‐in stations. Modeling of these records shows that dipping structure may account for the observed waveform complexities.