We have carried out a comprehensive relocation of a total of 267,210 earthquakes in Taiwan that occurred during the past 15 yr. We based our relocation process on the earthquake catalog of the Taiwan Central Weather Bureau Seismic Network (CWBSN) and made improvements in three aspects. First, we incorporated a large dataset of the S-P times from 680 Taiwan Strong-Motion Instrumentation Program (TSMIP) stations distributed throughout the island of Taiwan to improve the coverage of earthquakes on the island. Secondly, we added 18 Japan Meteorological Agency (JMA) stations in the southern Ryukyu Island chain to enhance the station coverage for eastern offshore events, especially around the subduction zone northeast of Taiwan. Thirdly, we adopted 3D V P and V P =V S models in predicting the travel times of P and S waves. The effectiveness of these improvements in earthquake relocation can be seen in three aspects: (1) the reduction in the residuals of P-wave arrival times and S-P times, (2) a better understanding of the attenuation relationship between the peak-ground acceleration and epicentral distance, and (3) the geologically meaningful patterns of station corrections to P-wave arrival times and S-P times.
We determined the focal-mechanism solutions for earthquakes with magnitude M L ≥ 4:0 that occurred in the Taiwan region between 1991 and 2005. Firstmotion polarities of P waves recorded at over 700 seismic stations in Taiwan were used. Because of the large number of events and stations involved, we implemented the genetic algorithm in a nonlinear global search for the focal-mechanism solutions. The algorithm was tuned and validated through synthetic tests. We finally determined the focal mechanisms of 1635 events with good qualities among 4188 earthquakes. Focal-mechanism solutions for a majority of the earthquakes display a dominant pattern of thrust-fault type reflecting the compressive stress field due to the plate collision. Normal-fault events occurred at intermediate depths in subduction zones, which is likely the result of the bending of the subducting slabs. Strike-slip faults are also found within the Eurasia plate around the Peikang Basement High and in collision zones near Ilan where the geometry of the colliding plates is complex. Our study provides a database of focal mechanisms for studying seismogenic structures and plate tectonics. This database can also be used by structural seismologists to compute synthetics for waveform tomography studies.
The 1999 Chi-Chi earthqqake (ML =7.3) which occurred in central Taiwan marks the island's largest inland seismic event of the twentieth cen tury. The Chi-Chi earthquake had three notable characteristics: 1) its source depth was very shallow (8 km), with aftershocks distributed in a semi-cir cular belt surrounding the Peikang basement high; 2) it triggered a 90 km long, north-south trending, active fault (the Chelungpu fault), mostly of the thrust type. Its hanging wall side suffered much larger distortions than the foot wall side; and 3) the fault trace bent toward the northeast at its north ern tip 50 km away from the source, where the largest surface rupture occurred (9.8 m). We suggest that the thinskinned thrust model would be the most suit able to explain the behavior of the Chi-Chi earthquake. The overall decollement surface might not be too deep (20 km?), which would redis tribute the tectonic pressure to a weak near-surface layer, this could be the Chinshui Shale (10 km?) which activated the brittle layer-coupled fault even before the accumulation of earthquake stress in the near area was 'mature' or 'adequate'. By using the aftershock data and other observations, we at tempt to reconcile the different aspects of the Chi-Chi earthquake based on the thinskinned model. Many inferences emerge as remarkably reason able, that match the observations quite well. However, a deep seismic reflection on study or deep well drilling is definitely required to better evaluate the model.
[1] A Mw 6.1 earthquake occurred on April 1st, 2006 near Taitung, eastern Taiwan. It produced significant coseismic ground displacements and a large number of aftershocks in the ensuing month. This event provides an opportunity to diagnose the seismogenic structure in the southern Longitudinal Valley (LV) of eastern Taiwan, long viewed as one of the collision sutures between the Philippine Sea and the Eurasian plates. With precisely relocated main-and aftershock hypocenters, focal mechanisms for M ! 3.8 events, and coseismic ground displacements from strong motion records, we determine a main shock dislocation model. Our results indicate that the main shock occurred on a high angle fault (azimuth 198°, dip 77°). The model comprises a fault with two segments; the main shock and a large number of aftershocks are associated with the northern segment that exhibited predominantly left-lateral strike-slip motion, in agreement with P-wave first motions and waveform (USGS) solutions. The southern segment exhibits a slightly larger thrust component, in agreement with CMT solutions. Tectonically, this event highlights a NNE-trending fault on the west side of the LV, which is predominantly strike-slip. The aftershocks clustered to the east of the main shock, which exhibit mainly thrust mechanisms, indicate that shortening is still acting on the sedimentary materials deposited between the Coastal and Central ranges prior to collision. As a result, the southern LV is undergoing slip partitioning along different faults, which has never been specified before. Citation: Wu, Y.-M.,
The 1999 Chi-Chi earthquake in Taiwan represented the island's larg est inland seismic event of the last century. This earthquake severely struck the western-middle part of the island. Abundant high-resolution digital seis mic data were recorded by two seismic networks operated by the Central Weather Bureau (CWB). In this paper, we combined data from two CWB seismic networks to analyze the characteristics of the Chi-Chi earthquake sequence. The mainshock was relocated to 23.853°N, 120.816°E at the depth of 8 km. The focal mechanism was of a thrust type with strike 5°, dip 34° and rake 65° based on the new location and the first motion polarities. The distribution of the aftershocks revealed a special distribution pattern which marked out an aseismic dipping plane with the aftershocks mostly occur ring on its upper side. It is highly suggested that there is a close relationship between this aseismic dipping plane and the Chelungpu fault .
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