Authors

Kris Walker, IGPP/SIO/UCSD, walker -@- ucsd.edu
Debi Kilb, IGPP/SIO/UCSD, dkilb -@- ucsd.edu
Guoqing Lin, IGPP/SIO/UCSD, gulin -@- ucsd.edu

Project Description
Two M ~5 earthquakes occurred generally within the Anza seismic gap along the San Jacinto Fault zone during the last 4 years (M 5.1, October 31, 2001; M 5.2, June 12, 2005). The 2005 event occurred ~9 km southeast of the town of Anza, and the 2001 event was ~6 km farther southeast. These events have significantly different focal mechanisms, and it is unclear if they occurred on a northwest-striking fault parallel to the San Jacinto Fault or a conjugate northeast-striking fault. Both events were followed by productive aftershock sequences (Felzer, and Shearer et al., SCEC 2005). Significant post-seismic creep was recorded several days following the mainshock by strain meters near Anza (Agnew and Wyatt, SCEC 2005). In light of these observations, several questions arise regarding the focal mechanisms and spatial/temporal behavior of the mainshocks and associated aftershocks: (1) how similar are the two sequences; (2) does the data define a well-delineated fault system consistent with surface observations; and (3) is there a spatial/temporal evolution or clustering of the aftershock focal mechanisms? To investigate these questions we calculate focal mechanisms using polarity information from the SCEC catalog, relocate aftershocks using a waveform cross-correlation technique, and explore the data using 3D visualizations (Kilb et al., SCEC 2005). We use a clustering algorithm to identify similar focal mechanism types, and search for trends in the occurrences of these events as a function of space and time. The spatial distribution of the relocated aftershocks appear ‘cloud like’, not aligning with a narrow fault core. Similarly, the aftershock focal mechanisms are heterogeneous, in that the 2001 and 2005 sequences are only comprised of 42% and 64% strike-slip events, respectively. These values are reduced to 25% and 46% when we consider only strike-slip mechanisms that are consistent with the strike of the San Jacinto Fault. In addition, there is a relatively large proportion of normal-faulting aftershocks in the 2001 sequence (18%) relative to the 2005 sequence (7%). These results suggest that both aftershock zones are highly fractured and heterogeneous volumes.