Using nearby strong-motion recordings and a finite-source model, we determine the focal mechanisms and identify the activated fault planes of two earthquakes of seismotectonic interest in Taiwan: the 15 December 1993 Tapu (Mw 5.5) and the 17 July 1998 Rueyli (Mw 5.7). These events occurred about 35 km apart, in the same geological province, which is characterized mainly by north-northeast trending thrust faults as well as some strike-slip faults. They are moderate-sized events that did not produce surface ruptures. Therefore, the identification of fault planes appears a significant issue for this region accommodated in a complex tectonic environment and constitutes the main objective of this article. Both events were located in the vicinity of the Chukou-Tachienshan fault system, which is known to be dominated by east-dipping thrust faults, and both earthquakes resulted in almost pure reverse source mechanisms. However, we find that the two ruptures dip in opposite directions, the northwest-dipping fault of the Tapu earthquake being in disagreement with the nearby fault system. For the Rueyli event, we show that fault-plane discrimination becomes effective when more than three stations are used, and that certain stations contribute more than others. We infer that this latter event occurred on one of the secondary east-dipping thrust faults parallel to the Tachienshan fault.

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