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Wang, TH, Cochran ES, Agnew D, Oglesby DD.  2013.  Infrequent triggering of tremor along the San Jacinto Fault near Anza, California. Bulletin of the Seismological Society of America. 103:2482-2497.   10.1785/0120120284   AbstractWebsite

We examine the conditions necessary to trigger tremor along the San Jacinto fault (SJF) near Anza, California, where previous studies suggest triggered tremor occurs, but observations are sparse. We investigate the stress required to trigger tremor using continuous broadband seismograms from 11 stations located near Anza, California. We examine 44 M-w >= 7.4 teleseismic events between 2001 and 2011; these events occur at a wide range of back azimuths and hypocentral distances. In addition, we included one smaller-magnitude, regional event, the 2009 M-w 6.5 Gulf of California earthquake, because it induced extremely high strains at Anza. We find the only episode of triggered tremor occurred during the 3 November 2002 M-w 7.8 Denali earthquake. The tremor episode lasted 300 s, was composed of 12 tremor bursts, and was located along SJF at the northwestern edge of the Anza gap at approximately 13 km depth. The tremor episode started at the Love-wave arrival, when surface-wave particle motions are primarily in the transverse direction. We find that the Denali earthquake induced the second highest stress (similar to 35 kPa) among the 44 teleseismic events and 1 regional event. The dominant period of the Denali surface wave was 22.8 s, at the lower end of the range observed for all events (20-40 s), similar to periods shown to trigger tremor in other locations. The surface waves from the 2009 M-w 6.5 Gulf of California earthquake had the highest observed strain, yet a much shorter dominant period of 10 s and did not trigger tremor. This result suggests that not only the amplitude of the induced strain, but also the period of the incoming surface wave, may control triggering of tremors near Anza. In addition, we find that the transient-shear stress (17-35 kPa) required to trigger tremor along the SJF at Anza is distinctly higher than what has been reported for the well-studied San Andreas fault.

R
Gomberg, J, Wech A, Creager K, Obara K, Agnew D.  2016.  Reconsidering earthquake scaling. Geophysical Research Letters. 43:6243-6251.   10.1002/2016gl069967   AbstractWebsite

The relationship (scaling) between scalar moment, M-0, and duration, T, potentially provides key constraints on the physics governing fault slip. The prevailing interpretation of M-0-T observations proposes different scaling for fast (earthquakes) and slow (mostly aseismic) slip populations and thus fundamentally different driving mechanisms. We show that a single model of slip events within bounded slip zones may explain nearly all fast and slow slip M-0-T observations, and both slip populations have a change in scaling, where the slip area growth changes from 2-D when too small to sense the boundaries to 1-D when large enough to be bounded. We present new fast and slow slip M-0-T observations that sample the change in scaling in each population, which are consistent with our interpretation. We suggest that a continuous but bimodal distribution of slip modes exists and M-0-T observations alone may not imply a fundamental difference between fast and slow slip.