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1.
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.  相似文献   

2.
In order to understand the fault zone architecture and mechanisms that caused the Chi-Chi earthquake, the Chelungpu drilling project was conducted during April 2000 through a collaborative project between Japan and Taiwan. In this study, chemical and mineralogical variations within the overall Chelungpu fault zone, including variations between less damaged host rocks, damaged zones, and fault cores caused by the Chi-Chi earthquake were examined. Slopes of TiO2 immobile isocons were consistently > 1 for analyses comparing host rocks with rocks from damaged zones or with gouges from fault cores, indicating that volume loss occurred in damaged zones and the fault cores. These results strongly imply that pervasive fluid infiltration occurred within the fault zone. Volume loss within the damaged zone and fault core is interpreted to result from a two-stage process involving: (i) coseismic mechanical wearing and/or dissolution in the fault core, and (ii) fluid infiltration within the fault zone during postseismic and interseismic periods along cracks caused by seismic failure. Semi-quantitative XRD analysis indicates that the kaolinite content consistently increases from the less damaged host rocks to the damaged zone and gouges in each fault core. Mineralogic changes indicate that pervasive acidic fluid infiltration occurred within the fault zones and reacted with the feldspars or muscovite to form kaolinite. Enrichment of kaolinite and illite found in the fault zones of southern drilling site could play some role on the slipping behavior of the southern part of the Chelungpu fault. Greater volume loss in the fault core may have resulted from moderate permeability, combined with the very fine grain nature of pulverized material in the fault core, which enhanced chemical reactions including transformation of feldspars and muscovite to clay minerals. The study results indicate that pervasive fluid infiltration occurred and changed the mineralogical and chemical architecture of fault zones caused by the cyclic earthquakes.  相似文献   

3.
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.  相似文献   

4.
In this study, we report the characteristics of clay minerals present in the Chelung-pu fault in Taiwan. In the 1999 Chi-Chi earthquake, different slip motions were recorded along the Chelung-pu fault in the northern and southern parts of the fault. The characteristics of clay minerals present in the fault zones can be attributed to the differences in motion. We analyzed the shallow drill core samples obtained from the northern and southern sites penetrating the fault. The clay minerals identified in most of the samples from both the sites are smectite, illite, and chlorite. There are illite–smectite mixed layers with a high illite content and no chlorite–smectite mixed layer. In some samples, no smectite is detected. We also examined the iron content and symmetry of iron and magnesium in the silicate and hydroxide layers in chlorite. At the northern site, the total iron content in chlorite of gouge is larger than that of the host rocks. On the other hand, at the southern site, the total iron content varies widely. It is hypothesized that the smectite consumption and differences in the characteristics of chlorite may be controlled by the differences in the lithology, fluid chemistry, fluid temperature, or fault activities (heating or breakage) between the northern and southern sites.  相似文献   

5.
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.  相似文献   

6.
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.  相似文献   

7.
Yih-Min Wu  Chien-chih Chen   《Tectonophysics》2007,429(1-2):125-132
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.  相似文献   

8.
Cheng-Horng Lin   《Tectonophysics》2007,443(3-4):271
In 1999, a large earthquake (Mw = 7.6) occurred along the Chelungpu fault in the fold-and-thrust belt of western Taiwan. To shed more light on the subsurface structures and the seismogenic layers, three-dimensional velocity structures were inverted by using the travel times of both P- and S-waves from 2391 aftershocks recorded by the Central Weather Bureau during the 15 months that followed. From tomography, a typical image of the large-scale thrusting structures in the upper crust across the Chelungpu fault was obtained. In general, high velocities beneath the Western Foothills and Central Ranges are separated from low velocities beneath the Coastal Plain by an east-dipping boundary that is roughly consistent with the Chelungpu fault on the surface. The contrast in velocity on either side of the Chelungpu fault is indicative of about a 7- to 9-km vertical offset in the upper crust. The relocated hypocenter for the Chi-Chi earthquake shifts by 2.2 km toward the northwest, and its focal depth decreases by 0.7 km. A plot of focal depths versus rock velocities where the aftershocks occurred shows earthquakes are more inclined to occur in rock with a velocity of around 5.6 km/s. This strongly suggests the seismogenic layer in the fold-and-thrust belt of Taiwan is more structure-dependent than depth-dependent.  相似文献   

9.
Following the 1999 Mw 7.6 Chi-Chi earthquake, a large amount of seismicity occurred in the Nantou region of central Taiwan. Among the seismic activities, eight Mw ⩾ 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.  相似文献   

10.
The landslide area along the Tachia River catchment of central Taiwan was investigated using the remote sensing images of various typhoon and earthquake events taken from 1996 to 2004 and the sediment discharge measured at hydrometric stations. Our findings indicate that 88% of the coseismic landslides triggered by the 1999 Chi-Chi earthquake were first-time occurrences. After the Chi-Chi earthquake, 59% of the landslide area was reactivated during typhoon Toraji and 66% during typhoon Mindulle. The landslides prone to reactivation were on the formations with closely spaced discontinuities. It is suggested that further rock-mass defects in the formations were added or opened by the earthquake, and that the landslide areas continued to extend until the end of 2004. Post-1999 rainstorms delivered large amounts of colluvial sediment into the main channel, leading to a 2-fold increase in post-seismic sediment discharge while the precipitation was only half that of the pre-earthquake rate.  相似文献   

11.
This paper is aimed at creating an empirical model for assessing failure potential of highway slopes, with a special attention to the failure characteristics of the highway slopes in the Alishan, Taiwan area prior to, and post, the 1999 Chi-Chi, Taiwan earthquake. The basis of the study is a large database of 955 slope records from four highways in the Alishan area. Artificial neural network (ANN) is utilized to “learn” from this database. The developed ANN model is then used to study the effect of the Chi-Chi earthquake on the slope failure characteristics in the Alishan area. Significant changes in the degrees of influence of several factors (variables) are found and possible reasons for such changes are discussed. The novelty of this paper lies in the fact that the developed ANN models are used as a tool to investigate the slope failure characteristics before and after the Chi-Chi earthquake.  相似文献   

12.
Typhoon Herb in 1996 caused widespread debris flows in central Taiwan. The 7.3 Chi-Chi earthquake on September 21, 1999, which also took place in central Taiwan, induced many landslides in the region. These landslides turned into debris flows when Typhoon Toraji struck Taiwan in 2001. This research selects three regions which suffered a ground motion class of 5, 6, and 7 on the Richter scale during the Chi-Chi earthquake as study areas. Air photos from 1997 and 2001 of these regions are used to map the gully-type debris flows that took place after Typhoons Herb and Toraji, respectively. The gullies adjacent to the debris flow, but without a trace of debris flows, are also mapped as the non-debris flow data. The topography, hydrogeology, and rainfall factors – where debris flow occurred and in which there was no occurrence of debris flows in these gullies were retrieved from DTM, geological maps, and iso-countour maps, and of rainfall through GIS processing. These characteristic are introduced into a probabilistic neural network to build a predicting model for the probability of the occurrence of debris flows. Three series of cross analyses are conducted to compare the probability of the occurrence of debris flows of the same dataset predicted by different prediction models. The results reveal that the susceptibility of debris flows was elevated after the Chi-Chi earthquake struck. The upsurge of susceptibility was more obvious for the regions that received a higher class of ground motion.  相似文献   

13.
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.  相似文献   

14.
This study looks at 102 typhoons that passed nearby or traversed Taiwan from 1995 to 2011 and their potential association with ordinary earthquakes. The study found an overall association of 63.75?%. Interestingly, prior to the September 21, 1999, M w ?=?7.6 Chi-Chi earthquake, only 4 of 24 typhoons (16.67?%) were potentially associated with the earthquakes. This figure increased substantially after the Chi-Chi earthquake to 78 typhoons being possibly associated with earthquakes (78.21?%). From the results of the chi-square test, both correlations between the typhoons and their possible triggered earthquakes before and after Chi-Chi earthquake have significant difference. The results are discussed in terms of changes in crustal conditions after the Chi-Chi earthquake and potential mechanisms, for example, heavy rainfall and atmospheric pressure causing the ordinary earthquakes. The atmospheric pressure effect predominates over the rainfall effect during the typhoon time period by statistical multivariate approach. However, to test rainfall effect is a non-neglected mechanism; seven small earthquakes without typhoon occurring near a region experiencing heavy rainfall and earthquake activity related to accumulated rainfall values from January 1995 to July 2012 are examined.  相似文献   

15.
A disastrous earthquake rocked Taiwan on September 21, 1999, with magnitude ML=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.  相似文献   


16.
We use coseismic GPS data from the 1999 Chi-Chi, Taiwan earthquake to estimate the subsurface shape of the Chelungpu fault that ruptured during the earthquake. Studies prior to the earthquake suggest a ramp–décollement geometry for the Chelungpu fault, yet many finite source inversions using GPS and seismic data assume slip occurred on the down-dip extension of the Chelungpu ramp, rather than on a sub-horizontal décollement. We test whether slip occurred on the décollement or the down-dip extension of the ramp using well-established methods of inverting GPS data for geometry and slip on faults represented as elastic dislocations. We find that a significant portion of the coseismic slip did indeed occur on a sub-horizontal décollement located at 8 km depth. The slip on the décollement contributes 21% of the total modeled moment release. We estimate the fault geometry assuming several different models for the distribution of elastic properties in the earth: homogeneous, layered, and layered with lateral material contrast across the fault. It is shown, however, that heterogeneity has little influence on our estimated fault geometry. We also investigate several competing interpretations of deformation within the E/W trending rupture zone at the northern end of the 1999 ground ruptures. We demonstrate that the GPS data require a 22- to 35-km-long lateral ramp at the northern end, contradicting other investigations that propose deformation is concentrated within 10 km of the Chelungpu fault. Lastly, we propose a simple tectonic model for the development of the lateral ramp.  相似文献   

17.
Reservoir sedimentation data and sediment yields from Taiwanese rivers show increased soil erosion in response to both 20th century changes in land use and a more recent increase in typhoon frequency and intensity. Decadal variations of up to 5- to 20-fold in suspended-sediment rating curves demonstrate supply-limited transport and correspond to increased sediment delivery from hillslopes due to changes in land use, regional ground shaking during the Chi-Chi earthquake, and post-2000 changes in typhoon frequency and intensity. While accelerated erosion in central Taiwan after the Chi-Chi earthquake has been documented previously, our results show that periods of increased upland erosion also occurred earlier, in response to 20th century changes in land use. Analyses of rainfall records and typhoon frequency for the period 1900–2009 further point to an island-wide increase in erosion rates corresponding to increased typhoon frequency and intensity after 1990.  相似文献   

18.
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.  相似文献   

19.
张蕾  李海兵  孙知明  曹勇 《地球学报》2019,40(1):157-172
断裂岩的岩石磁学研究可以揭示地震断裂作用的物理和化学环境,对于探讨地震断裂作用机制具有重要作用。本文在断裂岩岩石磁学最新文献的基础上,结合笔者及所在研究团队在龙门山断裂带获得的研究成果,综述了断裂岩的岩石磁学研究进展。大量研究发现断层泥和假玄武玻璃通常具有磁化率值或剩磁强度异常特征。顺磁性矿物在摩擦热或流体作用下形成新的铁磁性矿物是断层泥和假玄武玻璃高磁化率值或高剩磁强度的主要原因;地震断裂摩擦熔融作用中形成的单质铁是假玄武玻璃中高磁化率值或高剩磁强度异常的另一个重要原因。蠕滑断裂和出露于浅地表的断裂带中可见一些具有低磁化率值异常的断层泥,原因可能是流体作用或断裂带未经历高温摩擦热。断裂岩的岩石磁学研究为地震断裂带的应力应变、形成温度、摩擦热效应、流体作用、形成深度和氧化还原特征等提供了重要信息,可用于分析地震断裂的孕震和发震环境。综合岩石磁学测试和微米至纳米尺度的超显微学研究,并辅助地震断裂岩的摩擦实验、高温热模拟实验等研究可以更好地获得断裂岩的岩石磁学信息。  相似文献   

20.
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.  相似文献   

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