HR: 0800h
AN: S31A-0204    [Abstracts]
TI: Spatial Clustering and Repeating of Seismic Events Observed Along the 1976 Tangshan Fault, North China
AU: Li, L
EM: lile@seis.ac.cn
AF: Institute of Earthquake Science, China Earthquake Administration, P.O.Box 166 NO 63 Fuxing Rd., Beijing, 100036,
AU: Li, L
EM: lile@seis.ac.cn
AF: Department of Earth Science, Rice University, 6100 Main Street, Houston, TX 77005,
AU: Chen, Q
EM: chenqf@seis.ac.cn
AF: Institute of Earthquake Science, China Earthquake Administration, P.O.Box 166 NO 63 Fuxing Rd., Beijing, 100036,
AU: Cheng, X
EM: xcheng@rice.edu
AF: Department of Earth Science, Rice University, 6100 Main Street, Houston, TX 77005,
AU: * Niu, F
EM: niu@rice.edu
AF: Department of Earth Science, Rice University, 6100 Main Street, Houston, TX 77005,
AB: The magnitude 7.8 earthquake that struck the city of Tangshan,~160 km east of Beijing, on July 28, 1976 is one of the most destructive events in terms of the loss of life. The main shock occurred along NNE trending strike-slip fault system. We investigated the spatial and temporal distribution of microearthquakes occurring along this fault during 2001-2006 with waveform data recorded by the Beijing metropolitan digital Seismic Network. The relocated seismicity with the double difference method reveals a dextral bend in the middle of the fault that divides the fault into the southern and northern segments. More than 85 percent of the earthquakes were found in two clusters in the northern segment where relatively small coseismic slips were observed during the 1976 earthquake. The b values calculated from the seismicity occurring in the northern and southern segment are 1.03±0.02 and 0.85±0.03, respectively. The distinct seismicity and b values are probably the collective effect of the fault geometry and the regional stress field that has a NNE-SSW oriented compression. The southern segment thus experiences a lower shear stress but a higher normal stress compared to the northern segment, resulting a smaller number of earthquakes with relatively larger magnitude that explains the observed lower seismicity and b value. Using cross-correlation and fine relocation analyses, we also identified a total of 21 doublets and 25 multiplets that make up >50% of the total seismicity. Most of the sequences are aperiodic with recurrence intervals varying from a few minutes to hundreds of days. Based on a quasi-periodic sequence, we obtained a fault slip rate of ~2 mm/yr at ~15 km, which is consistent with surface GPS measurements. The high percentage of similar and repeating events suggests that the intraplate Tangshan fault is likely to have a well developed weak zone which is a characteristic of many mature interplate faults.
DE: 1207 Transient deformation (6924, 7230, 7240)
DE: 1217 Time variable gravity (7223, 7230)
DE: 1240 Satellite geodesy: results (6929, 7215, 7230, 7240)
DE: 1242 Seismic cycle related deformations (6924, 7209, 7223, 7230)
SC: Seismology [S]
MN: 2007 Fall Meeting