Chi-Chi Earthquake In Taiwan Research Articles

Optical dating of a sedimentary sequence in a trenching site on the source fault of the 1999 Chi-Chi earthquake, Taiwan

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: <400, 1700–2200, and >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 identified origin of a linear slope near Chi–Chi earthquake rupture combining 2D, 3D resistivity image profiling and geological data

A slope on the west border of the foothill near 921 surface rupture (caused by the 1999 Chi-Chi earthquake) in central Taiwan shows distinctive topographic expression that was prone to be considered as a fault scarp formed by a preexisting active fault. The 2D and 3D resistivity images clearly delineate rock surfaces which show steep, deep, gentle, and subvertical displacement beneath the slope, the toe of slope, the non-lateritic terrace, and 921 surface rupture, respectively, which can be attributed to the significant contrast of resistivity between gravel and rock. The horizontal sand bed and clast-supported gravel were deposited in a fluvial environment, whereas wedge-shaped gravel and colluvium were scarp-induced colluvial deposits in the trench profile. The layers shown in the depth of excavation, except for rock, has no offset or disturbance by fault ever since at least 2,480 ± 50 year B.P., based on carbon 14 dating of charcoal sample at the bottom of trench profile. According to information from two boreholes close to the slope, an over 20-m-thick marker bed with transported shell fragments, was found for correlation. This correlation further implies the slope was not formed by fault. On the other hand, two boreholes which are far from the slope and located on the flat non-lateritic terrace frequently show fractured and sheared features. By comparison, the locations around these two boreholes indicate a reverse fault or faults that occurred before the deposition of gravel. Later on, the paleostream was developed along the foot of fault scarp that was subjected to erosion and led to subsequent retrogression or retreat of the slope. Consequently, the incision of paleostream is believed to be responsible for the high relief of rock surface around the slope. Furthermore, from resistivity and borehole data, the rock surface underlying terrace is gentle where no faults occur after the deposition of gravel. The result of RIP crossing the 921 surface rupture displays about 10 m difference in elevation of rock surface on both sides, which is greater than that of 3–4 m caused by Chi-Chi earthquake. This indicates that the 921 surface rupture is a preexisting thrust fault that resulted from several thrusting events since terrace gravel was deposited. So it is not necessary to establish an extra restricted zone for construction in study area, except close to the 921 surface rupture.

Two-dimensional Co-Seismic Surface Displacements Field of the Chi-Chi Earthquake Inferred from SAR Image Matching.

The Mw=7.6 Chi-Chi earthquake in Taiwan occurred in 1999 over the Chelungpu fault and caused a great surface rupture and severe damage. Differential Synthetic Aperture Radar Interferometry (DInSAR) has been applied previously to study the co-seismic ground displacements. There have however been significant limitations in the studies. First, only one-dimensional displacements along the Line-of-Sight (LOS) direction have been measured. The large horizontal displacements along the Chelungpu fault are largely missing from the measurements as the fault is nearly perpendicular to the LOS direction. Second, due to severe signal decorrelation on the hangling wall of the fault, the displacements in that area are un-measurable by differential InSAR method. We estimate the co-seismic displacements in both the azimuth and range directions with the method of SAR amplitude image matching. GPS observations at the 10 GPS stations are used to correct for the orbital ramp in the amplitude matching and to create the two-dimensional (2D) co-seismic surface displacements field using the descending ERS-2 SAR image pair. The results show that the co-seismic displacements range from about -2.0 m to 0.7 m in the azimuth direction (with the positive direction pointing to the flight direction), with the footwall side of the fault moving mainly southwards and the hanging wall side northwards. The displacements in the LOS direction range from about -0.5 m to 1.0 m, with the largest displacement occuring in the northeastern part of the hanging wall (the positive direction points to the satellite from ground). Comparing the results from amplitude matching with those from DInSAR, we can see that while only a very small fraction of the LOS displacement has been recovered by the DInSAR mehtod, the azimuth displacements cannot be well detected with the DInSAR measurements as they are almost perpendicular to the LOS. Therefore, the amplitude matching method is obviously more advantageous than the DInSAR in studying the Chi-Chi earthquake. Another advantage of the method is that the displacement in the hanging wall of the fault that is un-measurable with DInSAR due to severe signal decorrelation can almost completely retrieved in this research. This makes the whole co-seismic displacements field clearly visible and the location of the rupture identifiable. Using displacements measured at 15 independent GPS stations for validation, we found that the RMS values of the differences between the two types of results were 6.9 cm and 5.7 cm respectively in the azimuth and the range directions.

Dynamic discrete analysis of an earthquake-induced large-scale landslide

The Chiu-fen-erh-shan landslide, estimated to have a volume of 36 million cubic meters, was triggered by the M w 7.6 Taiwan Chi-Chi earthquake of 1999. The debris covered an area of 1.95 km 2 and claimed 36 lives. During the sliding, the landslide mass disintegrated with its debris acquiring high speeds to reach over 1 km of travel distance. The main objectives of the study were to investigate if it was possible to numerically model the landslide progression, including slope disintegration, and to reproduce the post-failure configuration. Toward these goals, the study employed a discrete element method called discontinuous deformation analysis for the dynamic analysis. By incorporating the spacing and orientation information of the joints and bedding planes, the slope was modeled as an assemblage of discrete deformable blocks. Horizontal and vertical ground motions were simultaneously applied. The near-field digital strong records were affected by the large uplift of the hanging wall of the reverse thrust fault and corrections were made to address this issue. The study showed that the methodology adopted was able to capture essential characteristics of the landslide and gave a post-failure configuration resembling that observed in the field.

Frictional strength of fault gouge in Taiwan Chelungpu fault obtained from TCDP Hole B

Three fault zones (FZB1136, FZB1194 and FZB1243) are found in core samples in Hole B borehole penetrating through the Chelungpu fault, central Taiwan that activated at the 1999 Taiwan Chi-Chi earthquake. Each fault zone is composed of a black gouge zone (BGZ) and a gray gouge zone (GGZ) surrounded by a fault breccia zone (BZ) and/or fracture-damaged zone (FZ). We conducted tri-axial friction experiments on samples collected from subzones of the above three fault zones under the 1 km depth ambient condition with axial shortening rates of 0.1 to 10 µm/s to investigate frictional strength and its velocity dependence. We found that the coefficient of friction is 0.3 for the BGZ at FZB1136 (1136BGZ), which is lower than those at other fault zones. This result indicates that fault slip at the Chi-Chi earthquake occurred at the 1136BGZ. The estimated frictional strength profile across FZB1136 was that the frictional strength was the lowest (the coefficient of friction was 0.3) for the BGZ and increased up to 0.5 for undeformed host rock with increasing distance from the BGZ. Presence of the intermediate frictional strength zone consisting of the GGZ and the BZ suggests a development of off-fault damage zone where deformation occurred not only in the BGZ but also in the intermediate zone. This feature should be important to estimate total fracture energy released during earthquakes. We also suggest that coefficients of friction and the velocity dependences of the samples are closely related to localization of shear deformation and clay content, respectively.

Core-log integration studies in hole-A of Taiwan Chelungpu-fault Drilling Project

SUMMARY Taiwan Chelungpu-fault Drilling Project (TCDP) was initiated to understand the physical mechanisms involved in the large displacements of the 1999 Taiwan Chi-Chi earthquake. Continuous measurements of cores (including laboratory work) and a suite of geophysical downhole logs, including P- and S-wave sonic velocity, gamma ray, electrical resistivity, density, temperature, electrical borehole images and dipole-shear sonic imager, were acquired in Hole-A over the depth of 500–2003 m. Integrated studies of cores and logs facilitate qualitative and quantitative comparison of subsurface structures and physical properties of rocks. A total of 10 subunits were divided on the basis of geophysical characteristics. Generally, formation velocity and temperature increase with depth as a result of the overburden and thermal gradient, respectively. Gamma ray, resistivity, formation density, shear velocity anisotropy and density-derived porosity are primarily dependent on the lithology. Zones with changes of percentage of shear wave anisotropy and the fast shear polarization azimuth deduced from Dipole Shear-Imager (DSI) are associated with the appearance of fractures, steep bedding and shear zones. The fast shear wave azimuth is in good agreement with overall dip of the bedding (approximately 30° towards SE) and maximum horizontal compressional direction, particularly in the Kueichulin Formation showing strong shear wave velocity anisotropy. Bedding-parallel fractures are prevalent within cores, whereas minor sets of high-angle, NNW–SSE trending with N- and S-dipping fractures are sporadically distributed. The fault zone at depth 1111 m (FZA1111) is the Chi-Chi earthquake slip zone and could be a fluid conduit after the earthquake. The drastic change in fast shear wave polarization direction across the underlying, non-active Sanyi thrust at depth 1710 m reflects changes in stratigraphy, physical properties and structural geometry.

Modification effects of coping on post-traumatic morbidity among earthquake rescuers

This study aims to investigate the modification effects of coping strategies on the relationships between rescue effort and psychiatric morbidity (i.e. general psychiatric morbidity and post-traumatic morbidity) in earthquake rescue workers. Firefighters ( n = 193) who were involved in the rescue effort after the Taiwan Chi-Chi earthquake were invited to complete a questionnaire which contained questions on demographics, exposure to rescue work, general psychiatric morbidity, post-traumatic morbidity, and coping strategies. Multivariate regression models with interaction terms were carried out to investigate the modification effect of coping strategies on the relationships between rescue effort and psychiatric morbidities. Older age and longer job experiences (> 3 years) were associated with both general psychiatric and post-traumatic morbidities. Coping strategies such as confrontive coping, distancing, seeking social support, accepting responsibility, escape-avoidance, planful problem solving, and positive appraisal significantly modified the effect of exposure to dead bodies on general psychiatric morbidity. Furthermore, confrontive coping, distancing, and planned problem solving significantly modified the effect of exposure to direct rescue involvement on general psychiatric morbidity. However, coping strategies were not observed to buffer the effect of rescue involvement or contact with dead bodies on post-traumatic morbidity. More frequent use of coping strategies could reduce the effect that exposure to rescue efforts has on the incidence of general psychiatric morbidity in rescue workers. However, coping strategies do not seem to reduce the influence of such exposure on trauma-related morbidities. This suggests that coping strategies can be used to prevent general psychiatric morbidity but not trauma-related morbidities.

Correction to “A chemical kinetic approach to estimate dynamic shear stress during the 1999 Taiwan Chi-Chi earthquake”

[1] In the paper ‘‘A chemical kinetic approach to estimate dynamic shear stress during the 1999 Taiwan Chi-Chi earthquake’’ by T. Hirono et al. (Geophysical Research Letters, 34, L19308, doi:10.1029/2007GL030743), the equation for estimation of dynamic shear stress, the calculations provided for stresses normal to the fault, and the resultant shear stresses and frictional coefficients were incorrect. [2] The fourth sentence of paragraph 1 should read as follows. [3] The dynamic shear stress during the Chi-Chi earthquake was deduced to be 1.37 MPa, and the frictional coefficient to be 0.03–0.08. [4] The second sentence of paragraph 11 including equation (9) should read as follows. [5] From equations (2), (4), (8), and f(a) = (1 a), a in the center of the fault zone (x = 0) is expressed as

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.

Non-catastrophic landslides induced by the Mw 7.6 Chi-Chi earthquake in central Taiwan as revealed by PIV analysis

The Chi-Chi earthquake occurred in the western foothills of central Taiwan, which triggered coseismic and catastrophic Chiufengershan dip-slope landslides, Tsaoling rockslides and surface stripping of the Jiujiufeng area. After a few days of the Chi-Chi event, the Hongtsaiping area in central Taiwan was reported to have several meters of slide in a small scale area. The reported landslides are dissimilar to the catastrophic events. To better understand the characteristics of such non-catastrophic and unobvious landslide, we try to characterize in detail the sliding directions, their magnitudes and region of the landslide. We analyzed three orthorectified aerial photographs of the Hongtsaiping area, which were taken in 1998/06/13, 1999/11/18 and 2002/10/26, using the Particle Image Velocimetry (PIV) technique. Through the method we correlated the image pixels of the two aerial photographs in order to derive the displacement vector field. The results of the PIV analysis have helped us not only find the areas where sliding had happened, but also evaluate the azimuth and magnitudes of the sliding event. The PIV analysis in the Hongtsaiping area covers a dimension of about 3228 × 2448 pixels in an area of about 1.2 km 2. Our results show that the maximum horizontal displacement of the landslide is about 24 m towards NW in the study area. This unobvious but wide-coverage landslide may need further investigation in mechanisms combining with available or new geological data.

Determining fault slip distribution of the Chi-Chi Taiwan earthquake with GPS and InSAR data using triangular dislocation elements

The reactivation of the Chelungpu fault triggered the 20 September 1999 Chi-Chi Taiwan earthquake (M-w = 7.6) which caused a 100-km long surface rupture that trends north-south. We reconstruct the fault geometry using 1068 planar triangular dislocation elements that approximate more realistically the curved three-dimensional fault surface. The fault slip distribution is then determined with the observed GPS coseismic displacements as well as interferometric synthetic aperture radar (InSAR) data. The results show that our smooth 31) fault slip model has improved the fit to the geodetic data by 44% compared with the previously published inversions. The slip distribution obtained both by inversion of GPS data only and by joint inversion of GPS and InSAR data indicates that notable slips occur on the sub-horizontal decollement at the depth of 6.1-8.9 km. (C) 2007 Elsevier Ltd. All rights reserved.

Ground strain estimation for lifeline earthquake engineering

Current seismic design guidelines in Japan are diverse in the seismic ground strain estimates, because the concepts on a horizontally propagating wave model are not consistent in various seismic design guidelines including gas, water and other underground structures. The purpose of this study is (a) to derive the analytical methods to estimate the ground strains for incident seismic waves, (b) to develop a statistical estimation technique of the ground strains, and finally (c) to compare the theoretical estimation with the observed data which was measured at 441 sites in the 1999 Chi-Chi Earthquake in Taiwan.

Study of landslides caused by the 1999 Chi-Chi earthquake, Taiwan, with multitemporal SPOT images

This paper presents the results of a study of landslides and other landscape changes caused by the 1999 Taiwan Chi-Chi earthquake using multitemporal Satellite pour l'observation de la terre (SPOT) images. An innovative method for landslide detection is proposed based on the normalized difference vegetation index (NDVI) using image differencing coupled with an automated change threshold calculation. A vegetation recovery index and a land-cover spatial distribution index are also developed to assess vegetation recovery. Landslide extraction by an autodetection technique was also undertaken, yielding Kappa values over 84.16% when compared with those from existing maps. Recovery since the earthquake, despite a typhoon event in 2002, was 66.25% for vegetation in the denuded areas but less on ridge and slope surfaces. The results were verified by field surveys. The methods presented in this paper can be used by government agencies to aid in landslide-area recovery and establish effective land-use policies.

Liquefaction-Induced Lateral Spreading in Near-Fault Regions during the 1999 Chi-Chi, Taiwan Earthquake

We document and analyze incidents of liquefaction-induced lateral ground deformation at five sites located in the near-fault region of the 1999 Chi-Chi Taiwan earthquake. Each of the lateral spreads involved cyclic mobility of young alluvial soils towards a free face at creek channels. In each case, the lateral spreading produced relatively modest lateral displacements (approximately 10–200 cm ) in parts of the spreads not immediately adjacent to channel slopes. For each site, we present displacement vectors across the spread features, which are based on mapping performed within three weeks of the earthquake. We review the results of detailed subsurface exploration conducted at each site, including cone penetration test soundings, borings with standard penetration testing, and laboratory index tests. We back-analyze the field displacements using recent empirical and semiempirical models and find that the models generally overestimate the observed ground displacements. Possible causes of the models’ overpr...

Disaster Management following the Chi-Chi Earthquake in Taiwan

The earthquake that occurred in Taiwan on 21 September 1999 killed >2,000 people and severely injured many survivors. Despite the large scale and sizeable impact of the event, a complete overview of its consequences and the causes of the inadequate rescue and treatment efforts is limited in the literature. This review examines the way different groups coped with the tragedy and points out the major mistakes made during the process. The effectiveness of Taiwan's emergency preparedness and disaster response system after the earthquake was analyzed. Problems encountered included: (1) an ineffective command center; (2) poor communication; (3) lack of cooperation between the civil government and the military; (4) delayed prehospital care; (5) overloading of hospitals beyond capacity; (6) inadequate staffing; and (7) mismanaged public health measures. The Taiwan Chi-Chi Earthquake experience demonstrates that precise disaster planning, the establishment of one designated central command, improved cooperation between central and local authorities, modern rescue equipment used by trained disaster specialists, rapid prehospital care, and medical personnel availability, as well earthquake-resistant buildings and infrastructure, are all necessary in order to improve disaster responses.

Slope stabilization and landslide size on Mt. 99 Peaks after Chichi Earthquake in Taiwan

The study focuses on the landslide characteristics of Mt. 99 Peaks in Nantou County, the most serious landslide prone area caused by Chichi Earthquake in Taiwan. Several investigations and field surveys were made on Mt. 99 Peaks for 5 years to research the landslide area and depth, rainfall trend, and slope stabilization. The total landslide volume caused by the earthquake on Mt. 99 Peaks was about 1.47×106 m3 and the mean landslide thickness was about 0.22 m. Gravel layers with a volume of more than 80% of total soil profile dominated Mt. 99 Peaks. The landslide on Mt. 99 Peaks was induced by heavy rainfall from July to September because the rainfall on Mt. 99 Peaks had a nonuniform distribution in time. Although the vegetation recovery on Mt. 99 Peaks was in progress, the soil slope had remained unstable. As a result, Typhoon Mindulle occurred in July 2004 collapsed the hillslope again after 5 years of Chichi Earthquake. This study suggests that vegetation recovery on Mt. 99 Peaks for 5 years was insufficient to stabilize the landslide affected area.

Model modifications in Schumann resonance intensity caused by a localized ionosphere disturbance over the earthquake epicenter

Abstract. This paper is a further extension of our latest observations and modeling by Hayakawa et al. (2005a), in which we discovered the anomalous behavior of Schumann resonance observed in Japan, in possible association with the Chi-chi earthquake in Taiwan. Schumann resonance intensity changes associated with a localized decrease in the lower ionospheric height over the earthquake epicenter are modeled. The knee model of the vertical conductivity profile of the ionosphere describes the regular Earth-ionosphere cavity, and the modified knee model is introduced for the disturbance. The localized ionosphere modification is of a Gaussian radial dependence; it has a 1-Mm radius, and the decrease reaches 20 km in the lower ionosphere height over the epicenter of the earthquake (Taiwan). The diffraction problem in the Earth-ionosphere cavity with a localized disturbance is resolved by using the Stratton-Chu integral equation. This solution is constructed for the case of natural resonance oscillations driven by independent random sources distributed worldwide. The data of the Optical Transient Detector (OTD) are used to introduce the source distribution. A pronounced increase in the intensity of the Schumann resonance is obtained around the fourth mode frequency (up to 20%) when thunderstorms are concentrated in Central America. The worldwide distribution of lightning strokes blurs and slightly reduces the effect (15% increase in intensity) for the observer in Japan and the localized nonuniformity positioned over Taiwan. A clear qualitative similarity is obtained in relation to the experimental data, indicating that records collected in Japan may be explained by the impact of a localized decrease in the lower ionosphere over the epicenter of the earthquake. It is admitted that the assumed conductivity decrease could only be caused by a severe change in the ionization in the middle atmosphere. It is not in the scope of this paper to discuss the possible mechanism, but rather to show that a closer and quantitative agreement with the experiment yields information about the form and size of the ionospheric modification and about the distribution of global thunderstorm activity during measurements.

Large-scale seismic thermal anomaly linked to hot fluid expulsion from a deep aquifer

A 200 km-wide thermal anomaly was recorded by thermal infrared satellite data in a heavily faulted region of the South China Sea, at the time of the 1999 Chi-Chi earthquake in Taiwan. Subsurface data, including vitrinite reflectance data, indicate anomalously hot fluid flow in porous aquifers in the geological record for the same region. These observations link a seismic thermal anomaly to the long-term expulsion of hot fluid from a deep, overpressured aquifer.

Precursory phenomena associated with the 1999 Chi-Chi earthquake in Taiwan as identified under the iSTEP program

This paper presents a brief summary of several types of precursors in the lithosphere, atmosphere and ionosphere that have been positively identified in relation to the 20 September 1999 M w 7.6 Chi-Chi earthquake and its aftershocks under the iSTEP Program.

Nonlinear soil behavior during the Chi-Chi (Taiwan) earthquake of 1999 from data of the Dahan vertical array

Records of the Chi-Chi (Taiwan) earthquake of 1999 (Mw = 7.7) obtained at the Dahan vertical array (a part of the SMART-2 array) in NE Taiwan are analyzed. The Dahan array was located at a distance of ∼80 km from the fault plane of the earthquake. Using the method previously applied to the analysis of soil behavior during the Japan earthquakes in Kobe (1995) and Tottori (2000), nonlinear stress-strain relations are estimated in soil layers at depths of 0–200 m at the Dahan site for successive time intervals. The resulting soil behavior model is used for estimating variations in shear moduli and for the identification of nonlinear behavior of soil layers during an earthquake. According to the model estimates, the earthquake-related decrease in shear moduli did not exceed ∼5% and the soil response was nearly linear (the amount of its nonlinear components was also no more than ∼5%). The stress-strain relations describing the soil responses to the earthquakes in Kobe (1995), Tottori (2000), and Chi-Chi (1999) are similar, implying that, in principle, it is possible to describe the behavior of soils of the same type by the same stress-strain relations and predict the soil response to a future earthquake.