HR: 0800h
AN: T21A-0354    [Abstracts]
TI: A new hypocenter determination method using the summation of waveform cross- correlation
AU: * Ohta, K
EM: ohta@eps.s.u-tokyo.ac.jp
AF: Dept. EPS, Univ. Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo, 113-0033, Japan
AU: Ide, S
EM: ide@eps.s.u-tokyo.ac.jp
AF: Dept. EPS, Univ. Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo, 113-0033, Japan
AB: Deep low frequency tremor is a swarm activity of low-frequency earthquakes (LFEs) at least in the western Shikoku, Japan [Shelly et al., 2007]. LFEs are considered as shear slip on the plate interface because their focal mechanism has a low-angle thrust fault plane [Ide et al., 2007] and because precisely determined hypocenters are located on the plate interface [Shelly et al., 2006]. However, it is not obvious whether LFEs in other regions have the same characteristics. For example, in the Tokai region and the Kii Peninsula the hypocentral depths of LFEs determined by Japan Meteorological Agency are widely distributed from 20 to 50 km. Precise hypocenters of LFEs are essential for discussion on regional diversity of low frequency tremor. The low signal-to-noise (S/N) ratio of LFE records can be a source of larger estimated error in the hypocenter determination. Although waveform cross-correlation is a useful tool for precise hypocenter determination, it is not reliable in the case of the low S/N ratio because the value of cross-correlation is generally small. However, Shelly et al. [2007] showed that the summation of waveform cross-correlation for many stations can enhance event detectability. Similarly, using the summation of waveform cross-correlation, we develop a new hypocenter determination method that is less sensitive to noises. We apply this method to LFEs in the Tokai region and the Kii Peninsula. The data are 3-component velocity records from the NIED Hi-net, bandpass filtered between 2-8Hz. We first determine the relative hypocenral location between a pair of LFEs in a event list. Assuming a layered structure, we calculate theoretical differential travel times of body waves relative to the arrival times from the one event, After shifting the records of the other event using these differential travel times, we calculate the summation of the waveform cross-correlation coefficient between two events for all stations. The relative location is determined to maximize this summation by a grid search. For some pairs the maximum takes a statistically significant value that is not explained by a Gaussian distribution. Then we invert the relative locations to determine the hypocenter distribution using least square which is weighted according to a value of sums of waveform cross-correlation.
DE: 7209 Earthquake dynamics (1242)
DE: 7215 Earthquake source observations (1240)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting