dc.description.abstract | The high precision Global Positioning System (GPS) geodetic survey technique provides an efficient tool to study active tectonics and geodynamics. The southwestern Taiwan is an active tectonic area, approximately half of the 80 mm/yr plate convergence rate is accommodated on the fold and thrust belt in the area. We use the GPS derived 1993~2012 interseismic velocity field and 1994-2011 earthquake data to study the crustal deformation and seismic potential in southwestern Taiwan. The observed GPS time series can be described by some model parameters such as linear rate, annual and semi-annual periodic motions, coseismic offsets, postseismic rate change, and exponential decay after earthquakes. Stacking of power spectral densities from continuous GPS data in Taiwan, we found the slopes of spectra (spectral index) are -0.71, -0.72, and -0.76 for the E, N, U components, respectively. It indicates the errors of continuous GPS data can be described as a combination of white noise and flicker noise. A more realistic noise model can give a better estimate of full covariance matrix and model parameters in GPS position time series. The common errors are removed by stacking data from 26 continuous GPS sites with data period more than 10 years. By removing the common errors, the precision of GPS data has been further improved to 2.3 mm, 1.7 mm, and 4.1 mm in the E, N, U components, respectively.
By estimating the seismic activity indicators, we can assess the seismic potential in southwestern Taiwan. The minimum magnitude of completeness (Mc) of this area is 1.5 from the least squares estimation and maximum likelihood method. Based on a 17-year earthquake catalog from 1994 to 2011, we derive the earthquake activity indicators, a, b values to be 2.68 and 0.72, respectively in southwestern Taiwan. It reveals relatively lower seismic activity than previous studies. The 2010-2011 earthquake activity relative to the background (Z value) indicates a lower seismicity near the Jiuchiunken Fault, Liuchia Fault, Zuozhen Fault, Shinhwua Fault and Chishan Fault, which may imply a higher seismic potential in those areas. The earthquake locating precision, ERH, ERZ and RMS are significantly reduced from 0.71, 3.19, and 0.25 to 0.33, 1.82, and 0.18 by earthquake events relocation using Wu et al., (2007)’s 3D velocity model . The results of summation of the moment tensor focal mechanisms from 1994 to 2010 indicate the stress pattern of the area is mainly associated with plate motion. The stress pattern of in the Chiayi-Tainan area was slightly changed by the impact of 1999 Chi-Chi earthquake during the period from 1999 to 2005. However, it seems to be slowly recovering to the state before earthquake after 2006.
Based on the 2004~2012 interseismic velocity field derived from GPS time series analysis, we use a new method (Hsu, 2004; Hsu et al., 2009) to estimate the crustal strain rate in southwestern Taiwan. The maximum dilatation rates of about -0.75~-0.9 μstrain/yr in the direction WNW-ESE are found at the frontal thrust faults belt in the Chiayi-Tainan area. In the Kaoshiung-Pingtung area, there is a shorting rate of about -1.4~-1.55 μstrain/yr at the triangle region bounded by Zuozhen Fault, Chishan Fault and Siaogangshan Fault. The maximum shear strain rate of about 0.8~0.9 μstrain/yr is detected along a NE-SW trending linear zone, almost parallel to the strike of Chishan Fault. It is presumably related to the tectonic escape. Extension in the E-W direction is observed to the east of Chaochou Fault (i.e., the southern section of the Central Range). The strain rate is strongly correlating with the variations of GPS velocity gradient and the subsurface fault geometry. Thus the high strain rate in southwestern Taiwan could be caused by the change of fault geometry or partly due to aseismicslip.
The intersesmic velocity field of southwestern Taiwan reveals very different characteristics in the Chiayi-Tainan area and Kaoshung-Pingtung area. With respect to Paisha (S01R) of Penghu, the horizontal velocities in the Chiayi-Tainan area are mostly in the westward direction and show remarkably decreasing from the west to the east. In the Central Range, the horizontal velocities are 33~44 mm/yr and decrease to 0~5 mm/yr near the coast. There is a velocity gradient of about 15~20 mm/yr across the Jiuchiunken-Muchiliao-Liuchia fault system (JMLF). To the south of Shinhwua Fault, the velocities increase southward with directions become southwesterly. It reaches 50~55 mm/yr in the Kaoshiung-Pingtung area and deflects about 30° counterclockwise in the direction near the Kaoshiung-Pingtung coast. This implies the complexity of tectonic structures in southwestern Taiwan.
To invert for fault geometries and slip rates from GPS velocities in the Chiayi-Tainan area, we consider three different approaches: (1) a block model, (2) a buried dislocation model, and (3) a two-dimensional fault model. The basis of model fault geometry is the thin-skinned model that proposed a décollement underneath the fold-thrust belt in western Taiwan. The modeling results of three different models all indicate the existence of a nearly horizontal décollement with depth of about 8~13 km and long-term slip rate of 42 mm/yr. All the thrust faults extend from the décollement are almost fully locked. The JMLF is a thrust fault with a locking depth of about 8~13 km and dips 23°~25°E. The average and maximum fault slip rates of JMLF are 27 mm/yr and 38 mm/yr, respectively. The Chiayi Fault is a blind thrust fault with the fault tip at 1 km depth and the dip of 20° to the east. It is difficult to resolve the slip rate and fault geometry of Chukou Fault because it is too close to the JMLF.
The BLOCKS software developed by Meade and Loveless (2009) of Harvard University is utilized to study the interseismic deformation of southwestern Taiwan with multi-blocks. The multi-blocks model can well resolve and explain more than 90% of the interseismic GPS data. It is found that Shinhwua Fault is a right-lateral strike-slip fault with creeping rate of 10 mm/yr. The Meishan Fault is also a right-lateral strike-slip fault with slip rate of only 2~3 mm/yr. The Chaochou Fault is a left-lateral strike-slip fault with small slip rate of about 6~8 mm/yr. Although the fault geometry of Zuozhen Fault is not well resolved, we can still obtain a left-lateral fault slip rate of about 12 mm/yr. The Chishan Fault is a thrust fault with minor strike-slip component, dips 55°~60°E. The results of multi-block models indicate a different tectonic motion on both sides of the Chishan Fault. In the eastern side, there could be a décollement at depth of 8~10 km, different from that in the Chiayi-Tainan area. In contrast, the tectonic escape of western block may be enhanced due to the compression from thrust movement of Chishan Fault and lateral motion by Shinhwua Fault and Zuozhen Fault. The Fengshan transform fault zone (FTFZ) is not a major seismogenic structure in the Kaoshiung-Pingtung area, as the seismic activity is quite low and its impact on the strain accumulation is not significant. The observed velocity changes at the eastern side of Chishan Fault may come from the slip on the faults and folds extending from the basal décollement, and tectonic escape occurs only in the offshore area with thick sediments.
All the BLOCKS modeling results indicate the JMLF is a thrust fault with dip angle 23°E~25°E, long-term slip rate of 28~31 mm/yr and slip deficit of 25~26 mm/yr, consistent with the results from previous modeling studies. With the highest fault slip rate in southwestern Taiwan, the JMLF can produce a major earthquake with maximum magnitude of 7.3 and 7.6 for 100 and 300 years period, respectively. The potential seismic hazard should not be overlooked. The widespread soft sediments and mudstone resulting in plastic deformation in the Kaoshiung-Pingtung area could be the main reason that there is active crustal deformation but low seismic activity in the area. | en_US |