EVIDENCE FOR LATERALLY DISTRIBUTED TREMOR ACTIVITY
ACROSS THE SAN ANDREAS FAULT ZONE AT CHOLAME CA. 

Nadeau, R.M. and A. GUILHEM, U.C., Berkeley Seismological Laboratory

Understanding the relationship of nonvolcanic tremor (NVT) activity to the fault zones on which they occur is crucial for a clearer picture of the conditions and mechanisms responsible for their generation.  Comparisons between the spatial and temporal behavior of ambient NVTs on convergent and transform plate boundaries may also provide additional constraints on the NVT generation problem.

Obtaining a clear picture of the spatial relationship of NVTs with the core of fault zones (in particular inclined/subducting faults) has been difficult due to depth control limitations and to demanding computational requirements (Kao et al., 2005).  Computational demands also make assessment of the longer-term evolution of NVT activity at high resolution difficult (Shelly et al., 2006).

Vertically oriented NVT bearing fault zones (e.g., the San Andreas Fault, SAF) may provide a better opportunity for understanding the fault-lateral (i.e., across-fault) and temporal behavior of NVT activity about the fault core, since since depth control issues are less critical in with this fault orientation and less computationally expensive location techniques can be applied.

In this presentation we report on the spatial and temporal relationships of several hundred NVT episodes detected along the SAF near Cholame and the Monarch Peak areas of California during 2006 and 2007 (inclusive).  Data from 5 different seismic networks (NC,BP,TA,BK,CI, ~ 74 stations) are used in the analysis, and locations of the NVTs are determined using a highly automated envelope cross-correlation approach similar to Obara, 2002, with particular attention paid to the identification and exclusion of inconsistent station-pair alignments.  Uncertainties in the relative positions of the NVTs are estimated using a bootstrap approach (Shearer, 1997) and yield estimates of (+/- 3.5, 3.0, and 5.0 km in the across- and along-fault directions and in depth, resp.)

Preliminary results indicate that the Cholame NVTs occurring during the 2006-2007 period continue to occur at subseismogenic depths (generally between 15 and 30 km), that they are distributed over an ~ 15 km wide zone in the fault-normal direction, and that they are concentrated in at least 3 and possibly as many as 5 subregions and with significant differences in the evolutionary pattern of NVT activity among subregions.  Also apparent from the bootstrap analysis are clear indications that the non-uniform station coverage in the Cholame area places significant limits on the resolvability of the spatial distribution of the NVTs in this region.