BSSA tip sheet for June 2010 issue

Revised seismotectonic model for California Central Coast — More complex than previously thought

A new catalog of earthquake locations and focal mechanisms for the California Central Coast underscores the fault complexity of the region and identifies newly observed features offshore near San Luis Obispo.

The Central Coast is bounded on the east by the San Andreas Fault, the major plate boundary fault, and lies between the greater San Francisco and Los Angeles areas. This coastal region is not as densely instrumented or as tectonically well understood as the San Andreas Fault or the major urban areas.

The identification of new faults, and the reinterpretation of known faults, suggests that further work is necessary to better constrain the seismic hazards of the Central Coast. While the locations and focal mechanisms (the direction of slip in an earthquake and the orientation of the fault on which it occurs) for aftershocks of the 2003 M6.5 San Simeon and 2004 M6.0 Parkfield earthquakes are similar to those found in previous aftershock studies, the seismicity features in the offshore region near San Luis Obispo are sharpened considerably by this study.

The most prominent newly-observed feature is the Shoreline Fault, a ~25 km-long vertical strike-slip fault running parallel to the coastline just offshore of Point Buchon. Several smaller strike-slip seismicity lineations are also observed in Estero Bay, along with a deep reverse structure at the depth of the top of the remnant subducted slab. Strike-slip faulting is observed along the Hosgri-San Simeon Fault system, up to ~10-15 km inland from the Hosgri Fault in Estero Bay and near Point Buchon, and on the onshore Rinconada and West Huasna Faults.

The Shoreline Fault in particular requires further study to better constrain its geometry, how it may connect to the Hosgri Fault or other faults to its east, its slip rate and whether it has produced large earthquakes in the past.

“Seismotectonics and Fault Structure of the California Central Coast,” BSSA.

Author: Jeanne Hardebeck, US Geological Survey, [email protected], 650-329-4711

Large Earthquake Triggering, Clustering and the Synchronization of Faults.

The “signature of synchronicity” of large earthquakes — clusters of ruptures of several faults followed by periods of quiescence — found in the paleoseismic record also reflects the common observation that large earthquakes can trigger other large earthquakes on nearby faults. In this paper, author Christopher Scholz of Lamont-Doherty Earth Observatory at Columbia University, examines the mechanism by which the seismic cycles of nearby faults become synchronized.

Changes in stress caused by one earthquake can hasten the failure of neighboring faults. This paper looks at three case studies of large intraplate earthquakes that trigger even larger earthquakes on nearby faults. Because the large earthquakes that rupture these faults produce slip of a few meters, they have seismic cycle periods of several thousand years. The calculated stress transfer between faults suggests that the faults were near the end of their respective seismic cycles.

In the south Iceland seismic zone, the central Nevada seismic belt, and the eastern California shear zone, several synchronous clusters, that apparently act independently, can be recognized. This behavior is the three dimensional equivalent of the phase locking that results in the seismic cycle of individual faults being dominated by large ‘characteristic’ earthquakes, and for synchronization of fault segments along a given fault.

Rupture patterns of repeated individual earthquakes or earthquake clusters are not identical in either the two or three dimensional cases. The state of this system, which exhibits strong indications of synchrony without exact repetition, may be called fuzzy synchrony.

“Large Earthquake Triggering, Clustering and the Synchronization of Faults,” BSSA.

Author: Christopher Scholz, Lamont-Doherty Earth Observatory, Columbia University, [email protected], 845-368-5360.

Please cite the Bulletin of the Seismological Society of America (BSSA) as the source of this information.

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