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Frequent high seismic activities occur in Taiwan due to fast plate motions. According to the historical records the most destructive earthquakes in Taiwan were caused mainly by inland active faults. The Central Geological Survey (CGS) of Taiwan has published active fault maps in Taiwan since 1998. There are 33 active faults noted in the 2012 active fault map. After the Chi-Chi earthquake, CGS launched a series of projects to investigate the details to better understand each active fault in Taiwan. This article collected this data to develop active fault parameters and referred to certain experiences from Japan and the United States to establish a methodology for earthquake probability assessment via active faults. We consider the active faults in Central Taiwan as a good example to present the earthquake probability assessment process and results. The appropriate “probability model” was used to estimate the conditional probability where M ≥ 6.5 and M ≥ 7.0 earthquakes. Our result shows that the highest earthquake probability for M ≥ 6.5 earthquake occurring in 30, 50, and 100 years in Central Taiwan is the Tachia-Changhua fault system. Conversely, the lowest earthquake probability is the Chelungpu fault. The goal of our research is to calculate the earthquake probability of the 33 active faults in Taiwan. The active fault parameters are important information that can be applied in the following seismic hazard analysis and seismic simulation.

[1] Taiwan's 1999 Mw 7.6 earthquake generated over 85 km surface rupture along the Chelungpu thrust fault. Paleoseismic studies at the Shi-Jia site near Nantou city, reveal folding as the predominant form of deformation. Stratigraphic relations across the 1999 fold scarp show the style and degree of deformation caused by the penultimate event is similar to observed 1999 deformation. A boring transect across the fold scarp provides additional evidence of an earlier earthquake. Investigations at the Shi-Jia site revealed three prehistoric events; accelerator mass spectrometry (AMS) radiocarbon ages indicate that the penultimate earthquake occurred between 1160 and 1440 A.D. Paleoseismic studies north of the Shi-Jia site reveal much younger penultimate earthquakes, suggesting a 1999-type event may not be characteristic along the Tsaotun segment of the Chelungpu fault.

A trench and two boreholes located at the western margin of a young terrace, east of Fengyuan city, show the style and frequency of surface faulting on the northern part of the Chelungpu fault, central Taiwan. Both the 1999 earthquake fault and penultimate earthquake event are characterized by low-angle reverse faulting with overturning of younger strata. The penultimate event occurred between ca. 1400 yr BP (1340 - 1170 cal yr BP) and 2000 yr BP (2120 - 1800 cal yr BP). The age of the lower part of the terrace gravel is ca. 4000 yr BP; thus the two faulting events occurred during the past ca. 4000 years. The amount of horizontal shortening and vertical displacement since the penultimate event are significantly larger (3.7 - 3.9 m

The 1999 Chi-Chi earthquake, M w = 7.6, was associated with a near 100-km-long surface rupture along which the horizontal slip and vertical displacement reached to 9 m and 4 m, respectively, according to coseismic GPS slip vectors. After the Chi-Chi earthquake, drilling projects were conducted to clarify the characteristics of the earthquake rupture in depth. At the Fengyuan site, inclined-well drilling was performed at an angle of 50 degrees westward to a drilling depth of 455.3 m in the northern segment of the Chi-Chi earthquake rupture. Two possible earthquake rupture zones were observed at drilling depths of 225 m and 330 m according to the core structures and seismic profiling. We apply an area balancing method to determine the depth of the earthquake rupture in the cores; and we suggest that the 330-m drilling depth is the most likely candidate for the earthquake rupture.

The Chi-Chi earthquake rupture that mostly follows the mountain front in central Taiwan bends east/northeast into hills in the Tachia River valley. We found no major pre-existing fault systems in this hill area. However, based on river terrace morphology and sequences, the lower Tachia River valley appears to have been uplifted more rapidly than the lower Taan River valley in the past ten thousands of years, which is consistent with the bend of the earthquake rupture. The earthquake rupture (and associated uplift) also coincides with: (I) straight mountain-front escarpment south of the Tachia River valley; (2) two fault/fold-scarps on the Holocene terraces, and abnormal sideward inclination and downstream flattening of these terraces in the Tachia River valley; (3) local development of non-laterized terraces in the Taan River valley; (4) abnormal rise and narrowing of the Taan/Tachia drainage divide. These geomorphic anomalies, however, are either local or diffusive; features (1) to (3) might have been interpreted as of fluvial origin. Also, the link between these features and the movement of the Chelungpu-Sanyi fault, the causative fault of the earthquake, might have been uncertain. We thus argue that the pre-location of this portion of the Chi-Chi earthquake rupture would have been difficult, even though we could have roughly delineated a potential coseismic deformational zone in the region.