Seismotectonics of northeastern Taiwan: Structural characteristics of a transitional area from waning collision to subduction and post-collisional extension

• Main Authors: Hsin-Hua Huang, 黃信樺
• Other Authors: Yih-Min Wu
• Format: Others
• Language: zh-TW
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/80700649473319189064

碩士 === 國立臺灣大學 === 地質科學研究所 === 95 === Northeastern Taiwan (121.0-123.0°E，23.5-25.5°N) is a tectonically complex area acted by combined effects of slab subduction, backarc opening, and continental-arc collision. According to the previous studies, this area is usually divided into two tectonic entities, the Ilan basin and its sourthern collision zone. The Ilan basin related to Okinawa Trough is a extension environment dominated by normal faults and the collision zone is still a compressive environment because of the arc-continental collision. But the detail structure of the transition area between them has not been studied. Therefore, this study tried to explore the tectonic characteristics of the transition area from waning collision, to subduction and then post-collisional extension. A total of 12030 earquakes recorded by Central Weather Bureau Seismic Network（CWBSN） from January 1994 to July 2005 were relocated and studied in this study. These events were relocated by using three-dimensional velocity model. By combination the first motion polarities from Taiwan Strong Motion Instrumentation Program（TSMIP）and CWBSN, totally 169 focal mechanism solutions were determined for tectonic analysis. Base on the results of the relocated seismicity and focal mechanisms, in the study region, the spatial distribution of the focal mechanisms sequentially switches from thrust, strike-slip, and normal fault northward. It is obvious that the transition area between back-arc extension and continental-arc collision is dominated by left lateral strike-slip fault systematically. Furthermore, based on the seismicity profiles, right-lateral focal mechanisms were determined in Nanao earthquake sequence and a concentrated seismicity zone parallel to the subduction zone. It is possible that Coastal Range（CR）has already subducted along with Phillipine Sea Plate and the transition area is affected by Okinawa Trough more than CR. In addition, two active seismicity zones found in latitude 24.3-24.5ºN and 24.8-24.9ºN are corresponding to the normal and left-lateral strike-slip zone, respectively. It might imply that these two zones were located at the tectonic weak zone and thus with high seismicity. In conclusion, the southward opening of the Okinawa trough is a main driving force in northeastern Taiwan and bending the north Taiwan mountain belt. The left-lateral fault system appears at transition area was caused by the bedding-slip motion for considering the results from relocated seismicity, focal mechanisms and geological settings.