We carried out various stress measurements in boreholes penetrating the northern segment of the Chelungpu fault, drilled about 5 years after the 1999 Chi-Chi, Taiwan, earthquake. In the possible depth range of the Chelungpu fault, three major fault zones were encountered. Clearly recognizable principal stress rotations in the vicinity of the shallowest major fault zone, at 1133 m depth in hole B, suggest that this fault zone ruptured during the 1999 earthquake. Moreover, the fault's rupturing altered the stress state in the area surrounding this fault zone. In this paper, we constrain the possible magnitudes of the current principal horizontal stresses around the fault zone and show that the current stress state belongs to a normal or strike-slip fault regime. Therefore, the stress state was changed from that of a reverse fault regime by the rupturing.
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