dc.description.abstract | The Central Weather Bureau (CWB) starts the modern digital earthquake monitoring from 1990. The CWB seismic network (CWBSN) currently includes 70 weak motion and 86 strong motion real time stations, and 685 free-field strong motion stations. In the past ten years, a large amount digital waveforms were recorded by the CWBSN. It offers a rich data set for seismological study.
Four different earthquake locating programs are evaluated: 1) three-dimensions velocity structure location (3DLOC), 2) layered model joint hypocenters determination (JHD), 3) double-difference earthquake location algorithm (hypoDD), and 4) three-dimensions velocity structure with stations corrections location (3DCOR). The 1997 Rueyli earthquake sequence were used to test the performance of different methods. We found that the 3DCOR can greatly improve the location precision. The Rueyli mainshock was relocated to 120.660ºE, 23.510ºN at the depth of 5.6 km, deeper than the original CWBSN value of 2.8 km. Using this new location, a first-motion fault-plane solution shows that this earthquake is a thrust event with strike 53º, dip 42º, and rake 121º. Based on the hypocentral distribution pattern and the focal mechanism, the Rueyli earthquake was probably caused by faulting in the Tachienshan fault.
The 1999 Chi-Chi earthquake in Taiwan represented the island’’s largest inland seismic event of the last century. Abundant high-resolution digital seismic data were recorded by CWBSN. In this study, I relocated the mainshock to be 23.853ºN, 120.816ºE at the depth of 8 km. The focal mechanism was of a thrust type with strike 5º, dip 34º and rake 65º based on the new location and the first motion polarities. I also analyzed the 1-hr aftershocks. They were mixed up in the records and not easy to isolate individual ones. The 1-hr aftershocks show good correlation with the tectonic pattern predicted by the thin-skinned thrust model. Two significant aftershocks: Chiayi and Changhua earthquake sequences are also discussed. They reveal interesting relations with structures of the Peikang-High. Some fault systems claim the responses for the aftershock sequence distribution patterns.
We relocated 6446 shallow earthquakes and determined focal mechanism solutions of thirty-eight ML ≥ 5.0 events in the eastern Taiwan region (bounded by 22.7ºN, 120.8ºE ; 24.5ºN, 121.5ºE; 24.3ºN, 122.3ºE and 22.5ºN, 121.6ºE). The results suggest that the distribution of the relocated earthquakes generally agree with the fault plane determined from the focal mechanism solutions. The Longitudinal Valley (LV) is an east-dipping seismic suture zone that separates two different tectonic regions. On the left-hand side, shallow earthquakes and normal fault-type focal mechanism were found in the Central Range region. But on the right-hand side, most events occurred in the Philippine Sea Plate (PSP) were of the thrust-type, especially for the south portion and a few normal and strike-slip focal mechanism events were observed in the north portion. The tectonics of the eastern Taiwan can be classified from south and north into two regions, and there may exist a structure or boundary at about latitude 23.5°N with a strike of N110°E. In the northern portion of the eastern Taiwan, the plate boundary appears to be an east-dipping plane defined by the earthquake distribution. The focal mechanism solutions determined in this zone show thrust type faulting and agreed with this boundary plane (strike N20°E and dip about 50°). On the other hand, the southern part of the eastern Taiwan is a simple tectonics condition. The EP subducts to the east under the PSP. The LV is a suture zone dipping to the east, and most events occurred on the right-hand side of the LV. Focal mechanism solutions of these events were thrust-type with strike about N25°E and dip 54° to the east. | en_US |