HR: 1340h
AN: S23B-0245    [Abstracts]
TI: The 2001 and 2005 Anza Aftershock Sequences: Examination of Mainshock Rupture Direction, Post-Seismic Creep, Focal Mechanisms and Spatial/Temporal Behavior of Relocated Aftershocks
AU: * Kilb, D
EM: dkilb@ucsd.edu
AF: IGPP/SIO/UCSD, UCSD/IGPP-0225, La Jolla, CA 92093-0225 United States
AU: Agnew, D C
EM: dagnew@ucsd.edu
AF: IGPP/SIO/UCSD, UCSD/IGPP-0225, La Jolla, CA 92093-0225 United States
AU: Allmann, B
EM: ballmann@ucsd.edu
AF: IGPP/SIO/UCSD, UCSD/IGPP-0225, La Jolla, CA 92093-0225 United States
AU: Kane, D L
EM: dlkane@epicenter.ucsd.edu
AF: IGPP/SIO/UCSD, UCSD/IGPP-0225, La Jolla, CA 92093-0225 United States
AU: Lin, G
EM: gulin@ucsd.edu
AF: IGPP/SIO/UCSD, UCSD/IGPP-0225, La Jolla, CA 92093-0225 United States
AU: Prieto, G A
EM: gprieto@ucsd.edu
AF: IGPP/SIO/UCSD, UCSD/IGPP-0225, La Jolla, CA 92093-0225 United States
AU: Shearer, P M
EM: pshearer@ucsd.edu
AF: IGPP/SIO/UCSD, UCSD/IGPP-0225, La Jolla, CA 92093-0225 United States
AU: Vernon, F L
EM: flvernon@ucsd.edu
AF: IGPP/SIO/UCSD, UCSD/IGPP-0225, La Jolla, CA 92093-0225 United States
AU: Walker, K T
EM: walker@ucsd.edu
AF: IGPP/SIO/UCSD, UCSD/IGPP-0225, La Jolla, CA 92093-0225 United States
AB: The M 5.2 earthquake (June 12, 2005) in the Anza region was preceded by a M 5.1 event (October 31, 2001) about 7 km to the southeast. Both earthquakes occurred near 17 km depth in the San Jacinto fault zone within the Anza seismic gap, and were followed by productive aftershock sequences. It is unclear if they occurred on a northwest-striking fault parallel to the San Jacinto Fault or a conjugate northeast-striking fault. Following the 2005 shock, significant slow deformation was recorded by the strainmeters at Pinon Flat Observatory, suggesting aseismic slip at depth beyond the rupture zone. We apply empirical Greens function deconvolution to strong motion records from the Anza network to resolve the mainshock P and S displacement pulses. Both earthquakes have shorter pulses to the north, suggesting northward rupture propagation. We relocate 22,056 earthquakes between 1984 and June 2005 in the Anza region using source-specific station terms and waveform cross-correlation. Aftershocks for the 2001 earthquake are very compact and form an approximately 2 km cluster just south of the mainshock. The 2005 aftershocks are more distributed but lie mostly within a wedge of seismicity extending about 6 km southeast of the mainshock. A small number of aftershocks extend for tens of kilometers to the northwest along the San Jacinto fault. The aftershocks all occurred within regions of ongoing background seismicity. We calculate focal mechanisms using polarity information from the SCEC catalog. In general, the aftershock focal mechanisms are heterogeneous, in that the 2001 and 2005 sequences are only comprised of 41% and 62% strike-slip events, respectively. In addition, there is a relatively large proportion of normal-faulting aftershocks in the 2001 sequence (50%) relative to the 2005 sequence (28%). Our results suggest that both aftershock zones are highly fractured and heterogeneous volumes
UR: http://eqinfo.ucsd.edu/~dkilb/anza_2001_2005.html
DE: 1734 Seismology
DE: 7215 Earthquake source observations (1240)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7299 General or miscellaneous
SC: Seismology [S]
MN: Fall Meeting 2005