Asperities, barriers, characteristic earthquakes and strong motion prediction

Recent results from paleoseismology suggest that earthquakes recurrent on a given fault may often have the same characteristic length and amount of slip. Stable asperities and barriers, which survive many earthquakes, can explain these results. The activity of Volcano Usu, Japan, since 1977 which produced a dacite dome with a total upheaval of 180 m, offered a great opportunity to study the recurrence behavior of a fault. Seismologists at Hokkaido University revealed a family of repeated earthquakes with nearly the same amount of slip generated from the same fault plane on the crater wall, along which the dome moved upward. This family can be explained by the existence of a stable asperity on the crater wall and is called the “asperity-type” family. They found, however, another type of family of earthquakes, which share the same fault plane but show a great range of variation in the amount of slip. The behavior of the latter family can be explained by the presence of weak barriers over a fault plane defined by strong stable barriers, and this family is referred to as the “barrier-type” family. This type of earthquake model has been used successfully in the interpretation of observed strong motion acceleration spectra from California, and the results reveal the stability of local stress drop among different earthquakes. The agreement found between local stress drops estimated from strong motion data and those inferred from geological observation supports the possibility for predicting earthquake strong motion directly from geological observation of the causative fault.

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