A proposed rupture scenario for the 1925 M-w 6.5 Santa Barbara, California, earthquake

TitleA proposed rupture scenario for the 1925 M-w 6.5 Santa Barbara, California, earthquake
Publication TypeJournal Article
Year of Publication2018
AuthorsHough SE, Martin SS
Date Published11/2018

The 29 June 1925 Santa Barbara earthquake is among the largest 20th century earthquakes in southern California. The earthquake also predated the installation of strong motion and local monitoring instruments in southern California; some instrumental data are, however, available from long-period instruments at regional and teleseismic distances. The current catalog moment magnitude is M w 6.8. Initial intensity magnitudes (M-I) estimated from original Coast and Geodetic Survey intensity assignments were lower (M(I )6.3). In this study we assign modified Mercalli intensity values at 239 locations, including 144 specific locations within the city of Santa Barbara for which detailed damage information is available. Comparing the reinterpreted intensities with Did You Feel it? intensities for recent events in California, we estimate M-w = 6.5, with a plausible range of 6.3-6.6. We further consider reported instrumental amplitudes to estimate an instrumental moment magnitude of M-w = 6.6 +/- 0.5. Our preferred final estimate is M-w 6.5. Based on available constraints including aftershock locations inferred from data recorded on portable instruments, we propose that the earthquake nucleated east of the city of Santa Barbara, closer to the coast than previously estimated, and ruptured unilaterally similar to 30 km to the west, possibly along the south-dipping Mesa-Ftincon Creek, and the More Ranch fault systems. Contrary to suggestions made in earlier studies (e.g. Willis, 1925a), relatively high intensities similar to 50 km west of Santa Barbara can then be explained by directivity rather than involvement of the Santa Ynez fault. Finally, we discuss the possibility that the earthquake was triggered by the larger M-w = 6.6 Clarkston, Montana earthquake the previous day or induced by oil production in the Summerland oil field.