HR: 1340h
AN: S43A-1054    [Abstracts]
TI: Repeat microearthquakes observed in western Nagano, Japan and implications to rupture dynamics
AU: * Cheng, X
EM: xcheng@rice.edu
AF: Department of Earth Science, Rice University, 6100 Main Street, Houston, TX 77005 United States
AU: Niu, F
EM: niu@rice.edu
AF: Department of Earth Science, Rice University, 6100 Main Street, Houston, TX 77005 United States
AU: Silver, P
EM: silver@dtm.ciw.edu
AF: Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington, DC 20015 United States
AU: Horiuchi, S
EM: horiuchi@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1 Tennodai, Tsukuba, Ibaraki, 305-0006 Japan
AU: Takai, K
EM: takai@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1 Tennodai, Tsukuba, Ibaraki, 305-0006 Japan
AB: Repeat earthquakes appear to be pervasive in many fault systems, and have been used to detect temporal variations, either coseismic or post seismic near fault zones as well as to understand dynamic rupture processes. For example, Rubin and Gillard (2000) found no evidence of "immediate repeaters" in the San Juan Bautista section of the San Andreas Fault. Consecutive repeat earthquakes occurred no closer than a distance equal to the radius of the first rupture, which is estimated by a stress drop of 10 MPa. Here we reported similar characteristics of repeat microearthquakes from a very different environment, a complicated intraplate fault system in Western Nagao, Central Japan. A magnitude 6.8 shallow earthquake (roughly right lateral strike slip) occurred in the study area in 1984. Very high level of seismicity continues since then. A very dense seismic network with 56 stations including two borehole seismometers has been set up to monitor the high level seismic activity in 1995. Continuous data have been recorded at a very high sampling rate 10 KHz. We have searched repeat events from a catalog of more than 20,000 microearthquakes with a magnitude between 0 and 4.5. We calculated the cross-correlation of all the possible pairs of events and found less than 1% of the events can be categorized as repeat events, which is extremely lower compared to those observed at San Andreas Fault. More than 80% of the repeaters are actually aftershocks with the second events occurred within one day after the first ones. To avoid the tradeoff between origin time and event depth, we use relative S-P travel times to determine the relative locations of these consecutive repeat events. Based on the signal-to-noise ratio, we were able to estimate S-P time to an accuracy of 0.01 - 0.1 ms (1/10 to 1 sample interval). The corresponding errors in relative location are estimated to be a few tenths to a few meters. We also found that the second events occurred at least one radius away from the first rupture, (i.e., no intermediate repeaters), similar to those observed by Rubin and Gillard (2000). Stress drop seems to be independent to earthquake size, and is estimated to be a few tens of mega-Pascal, which is slightly higher than those observed at the San Andreas Fault.
DE: 7209 Earthquake dynamics (1242)
DE: 7215 Earthquake source observations (1240)
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
SC: Seismology [S]
MN: Fall Meeting 2005