Search

In Taiwan, the main purpose of earthquake fault zone legislation is to prevent earthquake-related disasters around the surface traces of active faults, particularly in urban areas. Here, the Geologically Sensitive Area (GSA) of the Milun Fault (Milun Earthquake Fault Zone) is used as an example to reveal the importance of such legislation. Field data collected along the Milun Fault before and after the 2018 Hualien Earthquake were used to reveal the reappearance of damages within the GSA. The 2018 Hualien Earthquake represents one of the shortest recurrence intervals (67 years) among all major faults in Taiwan. Most of the surface ruptures and damaged buildings in Hualien City were within the Milun Fault GSA and concentrated on the hanging wall of the fault. Moreover, 61% (91/148) of the damaged buildings and 83% (692/835) of the surface ruptures occurred within 100 m of the fault line. The results of this study demonstrate the importance of defining GSAs of active faults for mitigating earthquake hazards.

Not only direct fault ruptures but also later mass movement may result in complicated frontal deformation of the faults. Consequently, the deformation front or the contacts between the unconsolidated materials from the hanging wall and footwall of the thrust fault may indicate the toe of the mass movement instead of the actual fault zone. In this study, we used a combination of surface electrical resistivity imaging methods and borehole records in order to investigate the geometries of the structures in the frontal deformation zone of the Chihshang Fault at the Dapo elementary school. From the cores, we observed three different geological components at the Dapo site: the conductive Lichi melange of the hanging wall, the colluvial gravels and the underlying fluvial-gravel layer at the footwall. The resistivity images from two parallel survey lines reveal that the position where the fault trace was thought to be is actually the toe of the slumping body's surface ruptures consisting of materials from the Lichi melange. On the basis of the resistivity images, we also found that the actual fault plane is located on the southeastern side of the resistivity survey line near the hilltop. As a result, we conclude that mass movement induced by the inter-seismic creeping, not direct faulting, is the main factor affecting the frontal deformation zone of the Chihshang fault at the Dapo site.

We describe and analyze the surface and subsurface deformation of the 1999 Chi-Chi earthquake thrust termination of the Chelungpu fault in and near the Chushan excavation site, central Taiwan. In order to minimize damage to engineering structures within a deformation zone and formulate regulations for earthquake fault zones and fault setbacks, one needs to know the characteristics of the deformation zone and the connection between surface and subsurface deformation. The surface deformation zone of high strain induced by the earthquake faulting ranges from 15 to 70 m in width, and characterized by a 0.5 to 2 m high escarpment is much wider in the hanging wall. Exposures in the trench, 40 m long and 10 m deep, excavated across the earthquake ground rupture, show on the one hand the heterogeneous structure of a steep, monocline-like fold. On the other hand, strikingly different surface deformation profiles on either side of the 14 m wide trench, i.e., semiparabola-like and monoclinal, do not reflect in any obvious way the subsurface structure. The analysis of our detailed mapping results along with well logs suggests that the significant width variation of the surface deformation zone is attributed to secondary faults branching from the dominant fault at several tens of meters depth. The surface profile of the escarpment is controlled by the depth of the relevant fault tip, and the subsurface heterogeneous structure formed during at least four earthquakes.

A trenching site was excavated for the purpose of paleoseismic exploration and slip-rate acquisition along the Chelungpu fault, the source fault for the 1999 Chi-Chi earthquake (Mw 7.3) in central Taiwan. In previous studies that used conventional and AMS radiocarbon dating, sample ages clustered in three unevenly spaced groups: 30000 BP. Those prior studies also yielded questionable long-term slip rates: the rate for the last 2 ka was six times higher than the average for the past 40 ka. In order to reevaluate the time frame for this trench, optically stimulated luminescence (OSL) dating was applied. The large- and small-aliquot dose distributions were used to investigate the completeness of bleaching on nearby collected modern samples. Results indicate that the 5% minimum ages derived from the small-aliquot dose distributions are preferable for dating young samples, which may include unbleached sediment grains associated with the incompletely bleached sediments. The final OSL dates indicate that the previously published radiocarbon ages include some incorrect ages resulting from contamination and reworking of deposits, and a revised time frame is presented. Cumulative vertical slip independently derived from trench log restoration is 1.8 m in the past 2000 years, which is consistent with the vertical slip rate, i.e., 0.91 m/ka, calculated from the depositional rate in the footwall. This result inversely suggests that the time frame presented in this study is reliable.

The geomorphic expression of the frontal Western Foothills in central Taiwan is usually defined by a late Holocene scarp that ranges from tens to hundreds of meters in height. This scarp is the product of displacement on a near-surface 20- 30 east-dipping thrust fault, the Chelungpu fault, which ruptured during the Chichi earthquake. The large scarp height may correspond directly to the accumulation of successive surface ruptures. The Chushan site is located on the southern part of this fault where the Chichi earthquake produced a scarp with a 1.7-m vertical offset for the total vertical separation. Based on core-boring estimates, the vertical displace- ment on both sides is 7 m along a 24 east-dipping thrust fault. The results from our paleoseismic analysis indicate that five large paleoearthquake events have caused the large offsets during the past 2 ka. The radiocarbon age constraints of the paleoearth- quakes suggest a clustering of 540-790 cal yr B.P. (E2), 710-950 cal yr B.P. (E3), 1380-1700 cal yr B.P. (E4), 1710-1930 cal yr B.P. (E5), and the 1999 Chichi earth- quake. Events E3 and E4 have not been reported in previous studies and we did not observe event E1 (300-430 cal yr B.P.) at the site. Based on displacement and fault segmentation from the geologic features, we argue that the two new events may have occurred along the northern part of the Chelungpu fault. The vertical slip rate is estimated to be at least 3.9 0.2 mm/yr over the past 2 ka, which is similar to the long-term estimation through a calculation of late Pleistocene-Holocene terrace el- evations on the hanging wall.

The surface rupture of the 1999 Chi-Chi earthquake ( M w 7.6) trendsmore than 100 km in a north–south direction. Surface deformation at the northernend stops abruptly at an area between the Tachia River and the Taan River where abroad pop-up structure with east to northeast strike can be found that has a trenddifferent from the north–south-striking main thrust. We combine the absolute ele-vation data before and after the Chi-Chi earthquake to obtain the regional verticaldisplacement and the magnitude of the pop-up structure. The greatest uplift couldreach as high as 15–16 m. Using deformation magnitude and the area-balancingmethod, we measure the depth of the detachment to show the subsurface geometryof the Chelungpu fault at its northern end. This shows that the geometry of theChelungpu fault controls termination of the surface rupture and the depth of thedetachment controls the amount of deformation.Introduction Perhaps the most unexpected geological feature asso-ciated with the Chi-Chi earthquake (

Lee et al. (2001) applied the line-length balance method on the deformed soil to calculate the shortening of the surface rupture on Chelungpu Fault at Wufeng, Western Taiwan. Their results showed a shortening of 3.3 m and a total slip of 4 m. We, on the other hand, calculated the slip vector of the surface rupture by restoring the deformed concrete fence and found that the horizontal slip is 1.77 m and the slip direction is 279°. We then combined the horizontal slip vector with the vertical slip (2.1 m) to get a net slip of 2.75 m. The shortening we had in hand was 1.76 m, which was only half of Lee et al. 9s (2001) data. We thus applied the area-balance method to check whether the data measured by line-balance method are reasonable.

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.