Study of Triggered Tremor Characteristics and Triggering Threshold in Anza Region, Southern California

Non-volcanic tremor has been widely observed at subduction zones (e.g. SE Japan, Nankai and Cascadia) and, recently, along strike-slip faults in California (e.g. central to southern San Andreas and San Jacinto fault). In southern California, both ambient tremor and tremor triggered by distant large earthquakes have been observed close to well-developed faults. Tremor triggered by passing telesesimic waves suggests that tremor can be triggered by low amplitude stress changes. We examine data collected by the Anza seismic network to determine the characteristics of tremor along crustal faults. We examined continuous broadband waveforms from the Southern California Seismic Network (SCSN), in addition to 5 borehole stations around Anza, from 2002 to 2007. The borehole seismometers provide important high signal-to-noise data to assist in tremor detection and analysis. Before utilizing the borehole data for tremor detection, we determined the orientation of the horizontal components. Using 25 regional earthquakes in 2007 we found the best-fit P-onset polarization azimuths. To systematically search and document tremor locations we implemented an automatic detection program. In this preliminary work, by examining 41 potential triggering wave passages (Mw > 7.0) we found several episodes of triggered tremor on the SCSN stations near Anza. The most obvious triggered tremor episode occurs during passage of surface waves from the Denali earthquake (Mw 7.9), 2002. With the aid of borehole data we observed that, overall, tremor found in southern California is less periodic and more impulsive than subduction zone tremor. By surveying teleseismic events for triggered tremor, we will determine whether tremor only occurs when a critical stress threshold is exceeded. Or if additional factors, such as the dominant frequency of the surface wave, are important. If tremor is triggered at a critical stress level, we can better determine the underlying physical processes driving tremor. The section of San Jacinto fault where we detect tremor is described as Anza seismic gap due to an absence of large (M>6) earthquakes since 1790. If tremor is constrained to the Anza Gap section of the San Jacinto Fault, this might imply unusual frictional properties that favors tremor generation.