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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy (S) - IASPEI - International Association of Seismology and Physics of the Earth's Interior JSS014 Poster presentation 2311 Plate convergence at the Westernmost Philippine Sea plate Mr. Wen-Nan Wu Institute of Geophysics National Central Universit Ph.D Candidate IASPEI To understand the convergent characteristics of the westernmost plate boundary between the Philippine Sea Plate and Eurasian Plate, we have calculated the stress states of plate motion by focal mechanisms. Cataloged by the Harvard centroid moment tensor solutions (Harvard CMT) and the Broadband Array in Taiwan (BATS) moment tensor, 251 focal mechanisms are used to determine the azimuths of the principal stress axes. We first used all the data to derive the mean stress tensor of the study area. The inversion result shows that the stress regime has a maximum compression along the direction of azimuth N299. This result is consistent with the general direction of the rigid plate motion between the PSP and EP in the study area. In order to understand the spatial variation of the regional stress pattern, we divided the study area into six sub-areas (blocks A to F) based on the feature of the free-air gravity anomaly. We compare the compressive directions obtained from the stress inversion with the plate motions calculated by the Euler pole and the Global Positioning System (GPS) analysis. As a result, the azimuth of the maximum stress axis, σ1, generally agrees with the directions of the theoretical plate motion and GPS velocity vectors except block C (Lanhsu region) and block F (Ilan plain region). The discrepancy of convergent direction near the Ilan plain region is probably caused by the rifting of the Okinawa Trough. The deviation of the σ1 azimuth in the Lanhsu region could be attributed to a southwestward extrusion of the Luzon Arc block between 21N and 22N whose northern boundary may be associated with the right-lateral NE-SW trending fault (i.e. Huatung Fault, HF) along the Taitung Canyon. Comparing the σ1 stress patterns between block C and block D, great strain energy along HF may not be completely released yet. Alternatively, the upper crust of block C may significantly have decoupled from its lower crust or uppermost mantle. Keywords: stressinversion, philippineseaplate, focalmechanism

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy (S) - IASPEI - International Association of Seismology and Physics of the Earth's Interior JSS014 Poster presentation 2312 Temporal variation of the compressive direction in the hypocentral region after the 1999 Chi-Chi earthquake, Taiwan Mr. Wen-Nan Wu Institute of Geophysics National Central Universit Ph.D Candidate IASPEI The state of tectonic stress variations in the hypocentral region of the 1999 Chi-Chi earthquake (Mw7.6) is investigated by applying the stress inversion to the focal mechanisms reported from the Broadband Array in for Seismology data center. First, the entire data sets before and after the Chi-Chi main shock are separately to perform the stress inversion. The result indicates that the direction of the maximum compressive axes, σ1, before the main shock is N1220 and the after becomes N3020, which both are consistent with the relative plate motion direction between the Philippine Sea and Eurasian plates. The stress inversion is then carried out by accumulating the focal mechanisms of aftershock sequences as a function of time. In general, the direction of σ1 changes instantly and returns to its regional background stress state of prior to the Chi-Chi main shock and becomes stable less than two year. The recovery time of stress anomaly comparing to the recurrent interval of the Chelungpu fault is very short. Moreover, the 245 focal mechanisms of aftershocks were divided into 11 equal number of event windows with non-overlapping scheme to implement the stress inversion and concluded that the temporal variation of the direction of σ1 is significantly oscillatory, rather than smoothly as reported by previous study. Keywords: stressinversion, chi chiearthquake, focalmechanism

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