Analysis of the Chiufengershan landslide triggered by the 1999 Chi-Chi earthquake in Taiwan

A disastrous earthquake rocked Taiwan on September 21, 1999, with magnitude M_L=7.3 and an epicenter near the small town of Chi-Chi in central Taiwan. The Chi-Chi earthquake triggered landslide on the dip slope at the Chiufengershan. In this study, a review of the topography and geology of this area was followed by field investigations. Laboratory testing was applied to understand the geomaterial composing the slope. Then, based on a series of limit equilibrium analyses, the failure mechanism of this landslide and the risk of the residual slope were studied.

According to the stability analyses, the pre-quake slope is quite stable, with factor of safety of 1.77 (dry) to 1.35 (full groundwater level); explaining why there is no written record of a landslide here for the past 100 years. In contrast, a back analysis shows that the Chi-Chi earthquake-induced dynamic loading is far more than the dip slope can sustain, due in part to the short distance to the epicenter. A Monte Carlo type probability analysis suggests that the residual slope is more dangerous than the pre-quake slope and needs more attention.     

Delphi process applied to the 921 earthquake relief for damaged bridges in Taiwan

This paper presents panel-developed, expert opinion-based suggestions, and a Delphi process for the development of an earthquake damage assessment and recovery model. The model was specifically designed for the disaster relief of the damaged bridges of the 921 earthquake in Taiwan and uses four major indices: emergency handling ability, administrative capability, budget execution responsibility, and law enforcement reliability. The 921 earthquake, which occurred on September 21, 1999, forced the Taiwanese government into an unprecedented relief operation consisting of emergency rescue and reconstruction. However, some of the relief measures reflected possible inefficiency in intergovernmental cooperation. Regarded as one of the most decisive relief measures in Taichung County before it was consolidated and upgraded to a municipality on 25 December, 2010, the rescue and reconstruction of the majorly damaged bridges in the county were taken as an example to analyze the differences in the earthquake relief policies between the central and local governments. Based on experts’ experiences in government affairs and the four major indices of the damage assessment and recovery model, the ongoing governmental restructuring policy is considered retrospectively not only at the central government level but also at the local government level; this would serve to improve preparedness for catastrophes and to solve possible problems in earthquake-related intergovernmental cooperation.     

基于台湾集集地震数据的CPT与SPT液化判别方法比较

利用集集地震静力触探试验(CPT)数据,对基于CPT测试的Robertson液化判别方法和Olsen方法进行了检验,两个方法对液化点判别成功率分别为82.61%和80.43%,对非液化点判别成功率分别为31.82%和44.32%。CPT液化判别方法对液化点判别基本可靠,但对非液化点判别准确性较差。对集集地震标准贯入试验(SPT)数据,美国地震工程研究中心(NCEER)推荐的SPT液化判别方法对液化点和非液化点判别成功率分别达到92.41%和94.35%。SPT方法判别成功率非常高,整体准确性远高于CPT方法。另一方面,CPT的土分类图可以同时反映土的种类与强度,甚至可以对集集地震液化土与非液化土进行区分。对于细粒土的液化初判,CPT土分类图也优于SPT方法中的黏粒含量指标。因此,土分类图是CPT的优势所在。     

Significant resistivity changes in the fault zone associated with the 1999 Chi-Chi earthquake, west-central Taiwan

The purpose of this paper is to demonstrate the possible influence of an earthquake on the resistivity distribution in a fault zone. We collected resistivity image profiles across a proposed fault trace prior to the 7.3 magnitude 1999 shallow earthquake in the Chi-Chi area of Taiwan. Significant positive resistivity anomalies were observed in the hanging wall after the earthquake. However, there were negligible resistivity changes in the footwall. From an examination of geoelectric phenomena and surface rupture in the fault zone, it is believed that geoelectric anomalies are associated with abrupt displacement along the active Chelungpu fault. This result indicates a potential for resistivity methods to provide a basis for the monitoring of an active fault.     

Geological causes and geomorphological precursors of the Tsaoling landslide triggered by the 1999 Chi-Chi earthquake, Taiwan

The Tsaoling landslide, one of the largest landslide areas in Taiwan, has been affected by catastrophic events triggered by rain or earthquakes six times since 1862. These landslides, including that caused by the 1999 earthquake, have essentially not been reactivated old slides, but were sequential new ones that developed upslope, retrogressively. The landslide area is underlain by Pliocene sandstone and shale to form a dip slope with a bedding plane, dipping uniformly at 14°. The slip surface of the 1999 landslide was smooth and planar, parallel to the bedding plane with a slightly stepped profile; it formed within thinly alternated beds of fine sandstone and shale with ripple lamination or in a shale bed. The shale is weathered by slaking and probably by sulfuric acid, which is inferred to be one of the major causes of the intermittent retrogressive development of the landslides. The weathering was likely accelerated by the removal of overlying beds during earlier landslides in 1941 and 1942. The top margin of the 1999 landslide, in plan view, coincided with a V-shaped scarplet, which can be clearly recognized on aerial photographs taken before the landslide. This geomorphological feature indicates that this landslide had already moved slightly before its 1999 occurrence, providing precursory evidences.     

Seismic behavior in central Taiwan: Response to stress evolution following the 1999 Mw 7.6 Chi-Chi earthquake

Following the 1999 M_w 7.6 Chi-Chi earthquake, a large amount of seismicity occurred in the Nantou region of central Taiwan. Among the seismic activities, eight M_w ⩾ 5.8 earthquakes took place following the Chi-Chi earthquake, whereas only four earthquakes with comparable magnitudes took place from 1900 to 1998. Since the seismicity rate during the Chi-Chi postseismic period has never returned to the background level, such seismicity activation cannot simply be attributed to modified Omori’s Law decay. In this work, we attempted to associate seismic activities with stress evolution. Based on our work, it appears that the spatial distribution of the consequent seismicity can be associated with increasing coseismic stress. On the contrary, the stress changes imparted by the afterslip; lower crust–upper mantle viscoelastic relaxation; and sequent events resulted in a stress drop in most of the study region. Understanding seismogenic mechanisms in terms of stress evolution would be beneficial to seismic hazard mitigation.     

Impacts of September 21, 1999 Chi-Chi earthquake on the characteristics of gully-type debris flows in central Taiwan

Debris flows are more frequent in central Taiwan, because of its mountainous geography. For example, many debris flows were induced by Typhoon Herb in 1996. The Chi-Chi earthquake with a magnitude of 7.3, which took place in 1999 in central Taiwan, induced many landslides in this region. Some landslides turned into debris flows when Typhoon Toraji struck Taiwan in 2001. This study investigates the characteristics of the gullies where debris flows have occurred for a comparison. Aerial photos of these regions dated in 1997 (before the earthquake) and 2001 (after the earthquake) are used to identify the occurrence of gully-type debris flows. A Geographic Information System (GIS) is applied to acquire hydrological and geomorphic characteristics: stream gradient, stream length, catchment gradient, catchment area, form factor, and geology unit of these gullies. These characteristics in different study regions are presented in a statistical approach. The study of how strong ground motion affects the debris flows occurrence is conducted. The characteristics of the debris flow gullies triggered by typhoons before and after the Chi-Chi earthquake are quantitatively compared. The analysis results show that a significant transformation in the characteristics was induced by the Chi-Chi earthquake. In general, the transformation points out a lower hydrological and geomorphic threshold to trigger debris flows after the Chi-Chi earthquake. The susceptibility of rock units to strong ground motion is also examined. The analysis of debris flow density and accumulated rainfall in regions of different ground motion also reveal that the rainfall threshold decreases after the Chi-Chi earthquake.     

Characterization of slip zone associated with the 1999 Taiwan Chi-Chi earthquake: X-ray CT image analyses and microstructural observations of the Taiwan Chelungpu fault

To characterize the fault-related rocks within the Chelungpu fault, we performed X-ray computed tomography (CT) image analyses and microstructural observations of Hole B core samples from the Taiwan Chelungpu-fault Drilling Project. We identified the slip zone associated with the 1999 Chi-Chi earthquake, within the black gouge zone in the shallowest major fault zone, by comparison with previous reports. The slip zone was characterized by low CT number, cataclastic (or ultracataclastic) texture, and high possibility to have experienced a mechanically fluidized state. Taking these characteristics and previous reports of frictional heating in the slip zone into consideration, we suggested that thermal pressurization was the most likely dynamic weakening mechanism during the earthquake.     

Detailed surface co-seismic displacement of the 1999 Chi-Chi earthquake in western Taiwan and implication of fault geometry in the shallow subsurface

The northern segment of the Chelungpu Fault shows an unusually large co-seismic displacement from the event of the Mw 7.6 Chi-Chi earthquake in western Taiwan. Part of the northern segment near the Fengyuan City provides an excellent opportunity for characterizing active thrust-related structures due to a dense geodetic-benchmark network. We reproduced co-seismic deformation patterns of a small segment of this Chelungpu Fault using 924 geodetic benchmarks. According to the estimated displacement vectors, we identified secondary deformations, such as local rigid-block rotation and significant shortening within the hanging wall. The data set also allows us to determine accurately a 3D model of the thrust fault geometry in the shallow subsurface by assuming simple relations between the fault slip, and the horizontal and vertical displacements at the surface. The predicted thrust geometry is in good agreement with borehole data derived from two drilling sites close to the study area. The successful prediction supports our assumptions of rigid displacement and control of displacement in the hanging wall by the fault geometry being useful first approximations.     

Changes in hydrogeological properties of the River Choushui alluvial fan aquifer due to the 1999 Chi-Chi earthquake, Taiwan

Changes in hydrogeological properties of the River Choushui alluvial fan aquifer before and after the 1999 Chi-Chi earthquake, Taiwan, have been identified using pumping tests. Three wells, SH2, YL2 and SC2, located in a compressional zone with high coseismic groundwater levels, were tested. The threshold of the aquifer deformation with respect to transmissivity (T) is greater than that with respect to storage coefficient (S). Decreases in the post-earthquake S are approximately 60% at SH2 and SC2, indicating aquifer compression after the Chi-Chi earthquake. Changes in the post-earthquake T range from 61% increase to 0.8% decrease. Moreover, results from anisotropy analysis of T at SC2 further illustrate that normal stresses induced by the Chi-Chi earthquake have consolidated soil particles. Soil particles dilated laterally after the earthquake, resulting in an increase of the equivalent T. The changes in hydrogeological properties have a considerable influence on spatiotemporal fluid pressure and horizontal groundwater movement, resulting in different amounts of drawdown during post-earthquake pumping.     

Seismic reversal pattern for the 1999 Chi-Chi, Taiwan, MW 7.6 earthquake

We in this study have calculated the standard normal deviate Z-value to investigate the variations in seismicity patterns in the Taiwan region before and after the Chi-Chi earthquake. We have found that the areas with relatively high seismicity in the eastern Taiwan became abnormally quiet before the Chi-Chi earthquake while the area in the central Taiwan with relatively low seismicity showed unusually active. Such a spatially changing pattern in seismicity strikingly demonstrates the phenomenon of “seismic reversal,” and we here thus present a complete, representative cycle of “seismic reversal” embedding in the changes of seismicity patterns before and after the Chi-Chi earthquake.     

Geomagnetic anomaly at Lunping before the 1999 Chi-Chi earthquake (M w = 7.6) in Taiwan

A strong earthquake with a magnitude of 7.6 (M _L  = 7.3) occurred on September 21, 1999, in central Taiwan. In order to discern any potential precursors before this earthquake, geomagnetic data at Lunping (LNP), Taiwan, Geomagnetic Observatory situated 100 km from the epicenter are examined using two methods, i.e., the traditional induction arrows and complex demodulation. Our results show that the remarkable temporal variation of real induction arrows appear to be strong prior to the great earthquake over the previous 24 months. After the great earthquake, the magnitudes of induction arrows decreased to the normal (mean of 8 years) levels. In other words, the direction of real induction arrows of the periods 30 and 20 min rotated 85° and 40° anticlockwise, respectively, before the Chi-Chi earthquake and returned to mean direction of last 10 years after the earthquake. A horizontal source field model using the finite difference method for 3-D shows that the variation of the real induction arrows might be ascribed to the conductivity variation body, which is 5 km buried at the epicenter area of the Chi-Chi earthquake, changing its conductivity from 0.002S/m to 0.06 S/m. The ratios of modulus (demodulated by using the complex demodulation method) over a period 12, and 8 h relative to the period of 24 h reveal a remarkable change that appeared 4–5 months prior to this strong earthquake. They increased gradually from the beginning of 1999 to August 1999 and decreased again to a (8 years) mean level after the strong earthquake occurrence. We consider that the variation of the induction arrow might be ascribed to the conductivity anomaly, which is buried 5 km at the south-east side of LNP with a conductivity change of 0.06 S/m. We propose that this elevation might be related to the preparation process of the great earthquake.     

Electrical structures of the source area of the 1999 Chi-Chi, Taiwan, earthquake: Spatial correlation between crustal conductors and aftershocks

Wide-band magnetotelluric (MT) data were collected on an east–west profile, approximately perpendicular to the local strike of the Chelungpu thrust, through the hypocentral area of the Chi-Chi earthquake for imaging the seismogenic structure. MT data were then inverted for two-dimensional resistivity models plus best-fitting static shift parameters using a nonlinear conjugate gradient algorithm that minimizes the sum of the normalized data misfits and the smoothness of the model. As shown in the inverted 2D resistivity models presented in this paper, an electrical conductor beside the hypocenter of the Chi-Chi earthquake indicates that deep-crustal fluids may participate in the rupture process of the Chi-Chi earthquake. A striking spatial correlation between the crustal conductor and occurrence of aftershocks beneath the Chelungpu fault suggests a postseismic pore pressure adjustment ongoing after the mainshock. Additionally, the hypocenter exhibits an electrical resistive zone, consistent very well with a predicted compact zone from a crustal deformation and transient fluid flow modeling.     

Why does the co-seismic slip of the 1999 Chi-Chi (Taiwan) earthquake increase progressively northwestward on the plane of rupture?

The Chi-Chi 1999 earthquake ruptured the out-of-sequence Chelungpu Thrust Fault (CTF) in the fold-and-thrust belt in Western Central Taiwan. An important feature of this rupture is that the calculated slip increases approximately linearly in the SE–NW convergence plate direction from very little at its deeper edge to a maximum near the surface. We propose here a new explanation for this co-seismic slip distribution based on the study of both stress and displacement over the long-term as well as over a seismic cycle. Over the last 0.5 My, the convergence rate in the mountain front belt is accommodated by the frontal Changhua Fault (Ch.F), the CTF and the Shuangtung Fault (Sh.F). Based on previously published balanced cross sections, we estimate that the long-term slip of the Ch.F and of the CTF accommodate 5–30% and 30–55% of the convergence rate, respectively. This long-term partitioning of the convergence rate and the modeling of inter-seismic and post-seismic displacements suggest that the peculiar linear co-seismic slip distribution is accounted for by a combination of the effect of the obliquity of the CTF to the direction of inter-seismic loading, and of increasing aseismic creep on the deeper part of the Ch.F and CTF. Many previous interpretations of this slip distribution have been done including the effects of material properties, lubrication, site effect, fault geometry and dynamic waves. The importance of these processes with respect to the effects proposed here is still unknown. Taking into account the dip angle of the CTF, asperity dynamic models have been proposed to explain the general features of co-seismic slip distribution. In particular, recent works show the importance of heterogeneous spatial distribution of stress prior to the Chi-Chi earthquake. Our analysis of seismicity shows that previous large historic earthquakes cannot explain the amplitude of this heterogeneity. Based on our approach, we rather think that the high stress in the northern part of the CTF proposed by Oglesby and Day [Oglesby, D.D., Day, S.M., 2001. Fault geometry and the dynamics of the 1999 Chi-Chi (Taiwan) earthquake. Bull. Seismol. Soc. Am. 91, 1099–1111] reflects the latitudinal variation of inter-seismic coupling due to the obliquity of the CTF.     

Co-seismic displacements, folding and shortening structures along the Chelungpu surface rupture zone occurred during the 1999 Chi-Chi (Taiwan) earthquake

A nearly 100-km-long surface rupture zone, called Chelungpu surface rupture zone, occurred mostly along the pre-existing Chelungpu fault on the northwestern side of Taiwan, accompanying the 1999 Chi-Chi Ms 7.6 earthquake. The Chelungpu surface rupture zone can be divided into four segments based on the characteristics of co-seismic displacements, geometry of the surface ruptures and geological structures. These segments generally show a right-step en echelon form and strike NE–SW to N–S, and dip to the east with angles ranging from 50 to 85°. The co-seismic flexural-slip folding structures commonly occurred in or near the surface rupture zone from a few meters to a few hundreds of meters in width, which have an orientation in fold axes parallel or oblique to the surface rupture zone. The displacements measured in the southern three segments are approximately 1.0–3.0 m horizontally and 2.0–4.0 m vertically. The largest displacements were measured in the northern segment, 11.1 m horizontally and 7.5 m vertically, respectively. The amount of co-seismic horizontal shortening caused by flexural-slip folding and reverse faulting in the surface rupture zone is generally less than 3 m. It is evident that the co-seismic displacements of the surface rupture zone are a quantitative surface indicator of the faulting process in the earthquake source fault. The relations between the geometry and geomorphology of the surface rupture zone, dips of the co-seismic faulting planes and the striations on the main fault planes generated during the co-seismic displacement, show that the Chelungpu surface rupture zone is a reverse fault zone with a large left-lateral component.     

Integrated application of the analytic hierarchy process and the geographic information system for flood risk assessment and flood plain management in Taiwan

Flooding is one of the major natural hazards in Taiwan, and most of the low-lying areas in Taiwan are flood-prone areas. In order to minimize loss of life and economic losses, a detailed and comprehensive decision-making tool is necessary for both flood control planning and emergency service operations. The objectives of this research were (i) to develop a hierarchical structure through the analytic hierarchy process (AHP) to provide preferred options for flood risk analysis, (ii) to map the relative flood risk using the geographic information system (GIS), and (iii) to integrate these two methodologies and apply them to one urban and one semi-rural area in central Taiwan. Fushin Township and the floodplain of Fazih River (1 km on either side of the channel) in Taichung City were selected for this study. In this paper, the flood risk is defined as the relative flood risk due to broken dikes or the failure of stormwater drainage systems. Seven factors were considered in relation to the failure of stormwater drainage, and five to that of broken dikes. Following well-defined procedures, flood maps were drawn based on the data collected from expert responses to a questionnaire, the field survey, satellite images, and documents from flood management agencies. The relative values of flood risk are presented using a 200-m grid for the two study areas. It is concluded that integration of AHP and GIS in flood risk assessment can provide useful detailed information for flood risk management, and the method can be easily applied to most areas in Taiwan where required data sets are readily available.     

浅谈21世纪中国城市的防洪战略

通过现代城市与历史发展进程和欧洲的比较,说明根据我国的自然特点和社会经济状况,城市防洪是一项长期艰巨的任务,21世纪我国的城市防洪战略只能是人与洪水协调共处.     

Changes of PTSD Symptoms and School Reconstruction: A Two-year Prospective Study of Children and Adolescents after the Taiwan 921 Earthquake

Children and adolescents living in earthquake impacted areas may present various post-traumatic responses and adjustment along the post-traumatic reconstructive phases. Their surrounding environmental damage and reconstructive conditions may influence their adjustment as well. As part of a prospective and longitudinal research project aims to investigate the range and severity of post-traumatic stress disorder (PTSD) symptoms in children and adolescents residing near the epicenter after the Taiwan 921 Earthquake, this paper reports the changes of PTSD symptoms over a 2-year period. The study also examines the effects of school damage and reconstructive condition on PTSD symptoms. Earthquake Exposure Index for Youths and Child Posttraumatic Stress Disorder Reaction Index were administered to 2,028 and 2,077 youths at the first and second year, respectively, from two heavily impacted townships. Given that location, gender, individual and dwelling trauma exposure dose effect were controlled through a balanced sampling procedure and prior statistical confirmation, post-traumatic phase and school damage condition yield significant main effects. Students from the heavily damaged schools consistently displayed significantly more PTSD symptoms, including re-experiencing/avoidance and numbness/maladaptive symptoms, than their counterparts at the first and second year. Decline of prevalence of PTSD symptoms was noted from the first year to the second. Accordingly, there may be a need to implement a broad disaster recovery project with periodic screenings as well as school-specific and school-based mental health program.     

Variability of impact of earthquake on debris-flow triggering conditions: case study of Chen-Yu-Lan watershed,Taiwan

This study investigates the variations in the critical conditions for debris-flow occurrence before and after the Chi-Chi earthquake in the Chen-Yu-Lan watershed, Taiwan. Topographical and rainfall parameters such as the gully gradient, drainage area, rainfall intensity, cumulative rainfall, and rainfall duration in the Chen-Yu-Lan watershed were used to analyze the conditions of debris-flow occurrence over the past 25 years. A recovery equation was proposed on the basis of rainfall parameters and used to determine the variations in the critical line of rainfall that trigger debris flow after the earthquake and to evaluate the recovery period required for the rainfall threshold of debris-flow occurrence after the earthquake to return to that before the earthquake in the watershed. The critical line for the runoff parameter versus gully gradient in the watershed was also presented.     

Recognition of debris flow,debris flood and flood hazard through watershed morphometrics

Debris flows, debris floods and floods in mountainous areas are responsible for loss of life and damage to infrastructure, making it important to recognize these hazards in the early stage of planning land developments. Detailed terrain information is seldom available and basic watershed morphometrics must be used for hazard identification. An existing model uses watershed area and relief (the Melton ratio) to differentiate watersheds prone to flooding from those subject to debris flows and debris floods. However, the hazards related to debris flows and debris floods are not the same, requiring further differentiation. Here, we demonstrate that a model using watershed length combined with the Melton ratio can be used to differentiate debris-flow and debris-flood prone watersheds. This model was tested on 65 alluvial and colluvial fans in west central British Columbia, Canada, that were examined in the field. The model correctly identified 92% of the debris-flow, 83% of the debris-flood, and 88% of the flood watersheds. With adaptation for different regional conditions, the use of basic watershed morphometrics could assist land managers, scientists, and engineers with the identification of hydrogeomorphic hazards on fans elsewhere.