| Summary: | 博士 === 臺灣大學 === 地質科學研究所 === 95 === Abstract
Taiwan is located on active plate boundary, and earthquakes are therefore part of people’s everyday life. To clarify the potential effect area and to mitigate possible hazards are always the critical issue for scientists in Taiwan. During the past century, large earthquakes occurred five times and each time caused severe casualties as well as property losses. In our study, we used geodetic method such as Global Positioning System (GPS) and leveling to monitor the surficial 3D deformation of active structures, established short-term slip-rate monitoring system, to calculate the geometry, slip rate, creeping/locking behavior, recurrence interval and prospective earthquake magnitudes of major active structures, and thereby evaluated possible disaster area of each fault. Because of the double subduction system, different strain patterns are observed in different tectonic subdivisions: the western Foothills are compression-dominated; northern Taiwan is the combination of both compression and extension; northeastern Taiwan is subduction-related back-arc extension. We used Taichung Domain, Taipei Domain and Ilan Domain to discuss these three deformation behaviors.
The Taichung Domain was moving northwestwards before the 1999 Chi-Chi earthquake: in the footwall of Chelungpu Fault the slip rate was ~10 mm/yr, while in the hanging wall the rate increased eastwards up to 10-30 mm/yr. Two years prior to the earthquake, in the footwall of Changhua Fault the deformation direction changed from WNW to almost due N, and velocity gradient changed from increasing northwards to increasing eastwards, suggesting possible precursor of a large earthquake. As for coseismic deformation of Chi-Chi earthquake, the footwall of Chelungpu Fault moved southeastwards 0.3-1.5 m, and the value was increasing toward the fault; the hanging wall moved northwestwards 1.4-7.0 m, but the value was smaller south of the Choshui River. After the Chi-Chi earthquake, the hanging wall of Chelungpu Fault was still moving west-northwestwards and kept uplifting, while the footwall gradually changed direction from ESE to WSW or even WNW, clockwise resuming to inter-seismic deformation pattern. It is interesting to notice that despite the largest coseismic deformation in the northernmost segment of Chelungpu Fault, the post-seismic deformation peaked around the epicenter of Chi-Chi earthquake. We believe during the six years after the earthquake, most area in Taichung Domain are still undergoing strata compaction and stress reset, and therefore this domain has not yet fully recovered to inter-seismic stage.
In the Ilan Domain, we focused on the southwestern margin of Okinawa Trough. Previous studies proposed this margin as the transition between lateral collision and lateral extension, where crustal rotation and tectonic escape could be observed. Our study showed that maximum extension changes direction counterclockwise, from N-S in Okinawa Trough to NW-SE in northeastern Taiwan, and its value also decreases rapidly landwards. The Ilan Plain itself shows NW-SE extension in the middle and southern part, and the extension value rapidly increases southwards and southeastwards. When it gets close to mountain range, the extension gradually diminishes. This result is consistent to most measurements during the past decade, indicating a steadily ongoing back-arc extension from Okinawa Trough to Ilan Plain.
The Taipei Domain basically moved east-southeastwards or southeastwards relative to Penghu, at a rate of <5 mm/yr from 1992 to 1998 November. The only exception is the southwest the Linkou Tableland, which moved southwestwards. Two area with larger velocities are distributed roughly sub-parallel to the river mouth of Tanshui River, while the area in between has smaller slip rate. Along the Shanchiao Fault and Chinshan Fault, deformation pattern varies from hanging wall to footwall but is different from general inter-seismic pattern, probably due to the influence of large earthquake in the offshore of Ilan. Strain rate analysis from 1992 to 2000 shows extension in the direction of E-W or WNW-ESE, and some compression in the direction of N-S. Strain rate from 2000 to 2002 strengthens this accumulated pattern, and the compression direction slightly changed to NNE-SSW. The hanging wall of Shanchiao Fault kep subsiding, which could have resulted from not only stratigraphic compaction, but also the normal faulting of Shanchiao Fault. An antithetic normal fault may exist in the east of subsiding area.
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