A faulting model for the 1992 Petrolia earthquake: Can extreme ground acceleration be a source effect?

Abstract

The 1992 Petrolia earthquake (M=7.0) was quite remarkable in that an accelerometer at Cape Mendocino (CAP) recorded a high‐frequency pulse of almost 2 g, whereas the nearby (roughly 6 km away) Petrolia station (PET) recorded a maximum of only 0.6 g. In our model for this event, we have investigated the possibility that extreme ground acceleration with high spatial variability can be caused by a source effect, specifically the rupture of a barrier/asperity. This type of rupture feature can produce extreme ground acceleration provided that certain geometrical constraints are met. In the present study, we derive a combined high‐ and low‐frequency faulting model for the 1992 Petrolia earthquake that is consistent with the slip distribution on the fault and that produces the high‐frequency pulse at CAP. Our model consists of four parts: (1) A low‐ frequency (0.1–0.6 Hz) inversion that determines the slip, rupture time, and risetime distributions on the fault; (2) an interpolation and perturbation of the above distributions that produces synthetic accelerograms between 0.6 and 3.0 Hz; (3) random phase signal that is scaled to the spectral amplitude level of the data between 3.0 and 12.5 Hz; and (4) the rupture of a barrier‐like asperity that produces greatly amplified radiation at CAP but not at other stations. We find that our model produces a good fit to the near‐source records of the 1992 Petrolia event. The results emphasize the interaction between fault geometry and rupture propagation to produce ground motion, as well as the hazard associated with locations above the hanging walls of dipping faults, where the geometry permits the production of extreme ground acceleration.