Recognition of Paleoseismicity along Creeping Faults: Examples from the Dry Lake Valley Site on the central San Andreas

Nathan A. Toke, Nicole M. Abueg, James Anderson, Lawrence T. Kellum, Jeff Selck, Tsurue Sato, James B. Salisbury, & J Ramon R. Arrowsmith

Published December 2012, SCEC Contribution #1842

Fault segmentation and probabilistic seismic hazard models rely heavily on data from paleoseismic investigations, however, relatively little paleoseismic data emanate from actively creeping faults such as the central San Andreas Fault (SAF). Geodetic studies have shown that this part of the SAF has been creeping at nearly 30 mm/yr during the historical period. Yet, only a handful of paleoseismic studies have been conducted. None of these have revealed unequivocal evidence of fault rupture. However, this does not mean that this portion of the SAF has not generated large earthquakes over the timespan breached by paleoseismology. Many pieces of data, such as displaced stratigraphy, unconformities, and folds, that would be utilized as evidence for a paleoearthquake on faults that are not historically creeping have been found in these trenches. These observations cannot be interpreted as earthquakes in light of historical aseismic creep. Generally, when facing fault creep, the best evidence for paleoearthquakes are observations of large ground failures such as filled fissures and buried fault scarp-derived colluvial wedges. At the 2012 Dry Lake Valley paleoseismic site, along the central creeping section of the SAF (DLV site; 36.46791, -121.05564), we exposed a sub-vertical, matrix-supported, package of large clasts (pebbles to boulders) within the main fault zone. Despite the apparent fissure shape and a capping horizon, discussions with independent trench reviewers led to several models for emplacing the gravel package within the fault zone at this orientation and without large earthquakes. Fault-parallel trenching revealed a source for the large clasts nearby the original excavation, however interpretation of a paleoearthquake remains ambiguous. Here, we present conceptual models and observations from the Dry Lake Valley paleoseismic site depicting how creep can create structures that are similar to paleoearthquakes and how creep may overprint paleoearthquake evidence, making it ambiguous. Finally, we present strategies for recognizing paleoseismicity in the face of creep.

Toke, N. A., Abueg, N. M., Anderson, J., Kellum, L. T., Selck, J., Sato, T., Salisbury, J. B., & Arrowsmith, J. R. (2012). Recognition of Paleoseismicity along Creeping Faults: Examples from the Dry Lake Valley Site on the central San Andreas. Eos, Transactions, American Geophysical Union,.