HR: 17:30h
AN: S54A-07    [Abstracts]
TI: A High-Frequency Secondary Event During the 2004 M6.0 Parkfield Earthquake
AU: * Allmann, B P
EM: ballmann@ucsd.edu
AF: University of California, San Diego, Scripps Institution of Oceanography, IGPP, 9500 Gilman Drive, La Jolla, CA 92093-0225, United States
AU: Shearer, P M
EM: pshearer@ucsd.edu
AF: University of California, San Diego, Scripps Institution of Oceanography, IGPP, 9500 Gilman Drive, La Jolla, CA 92093-0225, United States
AB: We present an image of the rupture propagation of the 2004 M6.0 Parkfield earthquake using records from a dense network of local strong motion stations. We back-propagate high-frequency waveforms in 3D with a method, similar to reverse time migration, to obtain an estimate of the distribution of radiated high-frequency seismic energy in space and time. The image is forced to be coherent at the known hypocenter location and the quake origin time by applying small static time shifts obtained using waveform cross-correlation. We observe that the Parkfield earthquake radiated a distinct secondary high-frequency phase, which is located about 12.5~km northwest of the hypocenter with an onset of seismic radiation about 5~s after the rupture initiation. The time history of the back-projection suggests a rupture velocity of 2.5~km/s between hypocenter and subevent. The back-projection result is confirmed by inversion of picked arrival times of the secondary event clearly visible at some of the local and regional stations. By comparing our rupture image with a number of inverted finite slip models, we find that the secondary subevent had a higher moment release than the hypocenter while radiating less high-frequency seismic energy. From this we can infer that the northern subevent exhibited a lower stress drop than the hypocenter. This result is consistent with spatial stress-drop patterns observed from background seismicity and small-magnitude aftershocks. Furthermore, we find that the high- frequency subevent is located in an area of large slip gradient seen in most seismic and geodetic inversions, indicating that slip may have grown abruptly at this point.
DE: 7200 SEISMOLOGY
DE: 7209 Earthquake dynamics (1242)
DE: 7215 Earthquake source observations (1240)
SC: Seismology [S]
MN: 2007 Fall Meeting