| Summary: | 碩士 === 國立成功大學 === 地球科學系碩博士班 === 92 === We analyze earthquake data in southwestern Taiwan from Central Weather Bureau between January 1991 and September 2002 and relocate earthquake locations by using 3D velocity model and double difference location method base on the 1D velocity model of our study area. Using 3D earthquake relocations to determine 283 focal mechanisms (ML≧3.5) and corresponding stress inversions. Finally we estimate earthquake potentials of those seismogenic structures in southwestern Taiwan.
We suggest that: (1) the stress patterns inverted from focal mechanisms exhibit that distributions of σ2 and σ3 axes are random within a shape of belt (girdle) in Chiayi and Rueyli area before Chi-Chi earthquake. The permutations between principal stress axes σ2/σ3 occurred because the horizontal stress decreased after Chi-Chi earthquake. (2) The results of stress inversions were divided into north and south earthquake clusters by N23.4o. The azimuth of principal stress axis σ1 in north cluster is 130o and the σ1 in south cluster is 110o. It appears that the maximum principal stress axes are perpendicular to strikes of local structures. (3) A 30o west-dipping reverse fault in depth of 4~17 km is in south of Chiayi, estimate of fault area is 128 km2 and the largest magnitude is Mw 6.2; a 80o south-dipping strike-slip fault is in north of Chiayi area, estimate of fault area is 500 km2 and the largest magnitude is Mw 6.7. (4) The northern segment of Chukou fault is a 40o east-dipping reverse fault in depth of 0~10 km, estimate of fault area is 216 km2 and the largest magnitude is Mw 6.4. (5), A 35o west-dipping reverse fault is in Dapu area, estimate of fault area is 160 km2 and the largest magnitude is Mw 6.3. (6) A 40o west-dipping reverse fault in depth of 15~20 km is in Southeast area, estimate of fault area is 36 km2 and the largest magnitude is Mw 5.7.
Hypocenteral depths of most earthquakes in southwestern Taiwna are deeper than 10 km according to the relocation results. West-dipping reverse faults are caused by the reactivations of pre-existing normal fault. Therefore, we need to clarify the structures beneath the detachment further, besides monitoring active faults in the upper crust.
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