Tsui-Yu Chang
1
, Bertrand Delouis
2
, Chung-Pai Chang
3
, Jiun-Yee Yen
3
, Chung-Han
Chang
1
, Kou-Fong Ma
1
, Yu-Ing Chen
4
, and Yi-Ben Tsai
1
( 張翠玉、
Bertrand Delouis
、張中白、顏君毅、詹忠翰、馬國鳳、陳玉英、蔡義本 )
1.
Institute of Geophysics, National Central Univ., Taiwan
2.
Geosciences Azur CNRS/UNSA, France
3.
Center for Space and Remote Sensing Research, National Central Univ., Taiwan
4.
Institute of Statistics, National Central Univ., Taiwan
In 1999, a series of severe earthquakes struck west Taiwan, which have caused many casualties and economic loss. Among these seismic events, two earthquake sequences were recognized as the most disastrous sources, they are the Chi-Chi earthquake (mainshock in M
L
7.3, 17:47 UTC, 20 September) and the Chiayi earthquake (mainshock in M
L
6.4, 04:38 UTC, 22 October). The former sequence, occurred at the central Taiwan, was well studies as the reactivation of the Chelungpu fault and was considered as the germ of the seismic disaster in 1999 for the Taiwan area; whereas in the latter one, source mechanism and rupture planes of the concerned seismic events were still controversial. Two major issues about the Chiayi earthquake sequence are kept in debate: (1) whether or not the Chiayi sequence has been triggered by the Chi-Chi sequence, or is it independently occurred hors the influence of the Chi-Chi sequence? (2) is the motion of the Chiayi sequence a thrusting event correlated with the Chiuchiungkeng fault to the east, or a lateral strike-slip related to the Meishan fault to the west? Previous examination on stress interaction showed that static stress change near the location of the Meishan fault is obscure after the Chi-Chi earthquake. The further study on probabilistic aftershock

hazard revealed that there was little spatiotemporal correlation between these two earthquake sequences.
In this study, we applied a waveform inversion from the near-field records to directly determine the geometry of fault plane in the Chiayi earthquake. The source parameters of earthquakes involved in the Chiayi sequence were refined and the possible fault geometries were identified. Seven parameters, including the strike, dip, rake, and dislocation, were exploited with a grid search and minima of the misfit error between the observed and calculated seismograms. It is worth remarking that there was little surface deformation observed in the Chiayi sequence, and their fault plane, with its blind fault character, may not be identified by other information.
Combining our inversion result with the spatial distribution of earthquakes in this sequence, we construct a distinct seismotectonic feature for the process of rupture propagation, which depicted the interaction between separate fault activities in the
Chiayi area and provide further information for the relationship between the Chiayi sequence and other seismic events.