Chi-Chi Event Research Articles

Identification and Representation of Multi-Pulse Near-Fault Strong Ground Motion Using Adaptive Wavelet Transform

In order to identify the horizontal seismic motion owning the largest pulse energy, and represent the dominant pulse-like component embedded in this seismic motion, we used the adaptive wavelet transform algorithm in this paper. Fifteen candidate mother wavelets were evaluated to select the optimum wavelet based on the similarities between the candidate mother wavelet and the target seismic motion, evaluated by the minimum cross variance. This adaptive choosing algorithm for the optimum mother wavelet was invoked before identifying both the horizontal direction owning the largest pulse energy and every dominant pulse, which provides the optimum mother wavelet for the continuous wavelet transform. Each dominant pulse can be represented by its adaptively selected optimum mother wavelet. The results indicate that the identified multi-pulse component fits well with the seismic motion. In most cases, mother wavelets in one multi-pulse seismic motion were different from each other. For the Chi-Chi event (1999-Sep-20 17:47:16 UTC, Mw = 7.6), 62.26% of the qualified pulse-like earthquake motions lay in the horizontal direction ranging from ±15° to ±75°. The Daubechies 6 (db6) mother wavelet was the most frequently used type for both the first and second pulse components.

Experimental evidence that thrust earthquake ruptures might open faults.

Many of Earth's great earthquakes occur on thrust faults. These earthquakes predominantly occur within subduction zones, such as the 2011 moment magnitude 9.0 eathquake in Tohoku-Oki, Japan, or along large collision zones, such as the 1999 moment magnitude 7.7 earthquake in Chi-Chi, Taiwan. Notably, these two earthquakes had a maximum slip that was very close to the surface. This contributed to the destructive tsunami that occurred during the Tohoku-Oki event and to the large amount of structural damage caused by the Chi-Chi event. The mechanism that results in such large slip near the surface is poorly understood as shallow parts of thrust faults are considered to be frictionally stable. Here we use earthquake rupture experiments to reveal the existence of a torquing mechanism of thrust fault ruptures near the free surface that causes them to unclamp and slip large distances. Complementary numerical modelling of the experiments confirms that the hanging-wall wedge undergoes pronounced rotation in one direction as the earthquake rupture approaches the free surface, and this torque is released as soon as the rupture breaks the free surface, resulting in the unclamping and violent 'flapping' of the hanging-wall wedge. Our results imply that the shallow extent of the seismogenic zone of a subducting interface is not fixed and can extend up to the trench during great earthquakes through a torquing mechanism.

Seismic ground motion amplification in a 3D sedimentary basin: the effect of the vertical velocity gradient

Ground motion amplification in sedimentary basins has been observed in some moderate or large earthquakes, such as the 1994 Northridge and 1999 Chi-Chi event. Many numerical studies with simplified 2D models have shown significant effects of the vertical velocity gradient of sediment on basin amplification. However, we need to consider a more realistic 3D model and solve wave equations with 3D numerical methods in order to improve our understanding of basin amplification. In this study, we extend a 2D pseudospectral and finite difference hybrid method to a 3D case and investigate the effects of the vertical velocity gradient for a 3D basin model. Numerical simulations were performed for four basin models with increasing vertical velocity gradients on a PC cluster using 64 processors for 67 108 864 discretized grids. The results show that the vertical velocity gradient enhances basin amplification through strong secondary surface waves and basin trapped waves. The 3D geometry of the basin causes a wave-front focusing effect that contributes significantly to a localized strong amplification with the maximum peak ground velocity in the basin. The results of this study suggest that it is important to consider the detailed properties of sedimentary basins in seismic ground motion studies.

Testing a P-Wave Earthquake Early Warning System by Simulating the 1999 Chi-Chi, Taiwan, Mw 7.6 Earthquake

The earthquake early warning (EEW) system is becoming a key practical tool for mitigating loss due to seismic events. Depending on the distance to the earthquake, it provides a few seconds’ to a few tens of seconds’ warning for people or automated facilities. Currently, many countries have an online operating or experimental EEW system, such as Japan (Nakamura 1988; Odaka et al. 2003; Horiuchi et al. 2005), Taiwan (Wu et al. 1998; Wu et al. 1999; Wu and Teng 2002; Hsiao et al. 2009; Hsiao et al. 2011), Mexico (Espinosa-Aranda et al. 1995; Espinosa-Aranda et al. 2009), the United States (Allen and Kanamori 2003; Wu et al. 2007; Allen et al. 2009; Bose, Hauksson, et al. 2009), Italy (Zollo et al. 2006; Zollo et al. 2009), Turkey (Alicik et al. 2011), Beijing (Peng et al. 2011), and Romania (Bose, Sokolov, and Wenzel 2009). Taiwan is situated at two converging plates, the Eurasian plate and the Philippine Sea plate. Located in this seismic active region, with a convergence rate of about 8 cm/year (Yu et al. 1997; Hsu et al. 2009), Taiwan has experienced many destructive earthquakes in the past century; the 1999 Chi-Chi earthquake is the largest such event. At that time, there was no real-time, online EEW system in Taiwan; however, a rapid reporting system had been operating since 1995 (Wu et al. 1997). Fortunately, during the 1999 Chi-Chi event the earthquake rapid reporting system had good estimates of hypocenter (23.87°N, 120.75°E, depth = 10 km) and magnitude ( ML = 7.3) and provided an intensity map in 102 seconds after the earthquake occurrence (Wu et al. 2000). The first EEW system in Taiwan was tested in 1998 in the Hualien area (Wu et al. …

Observations of changes in waveform character induced by the 1999 Mw7.6 Chi-Chi earthquake

[1] We observe changes in the waveforms of repeating earthquakes in eastern Taiwan following the 1999 Mw7.6 Chi-Chi earthquake, while their recurrence intervals appear to be unaffected. There is a clear reduction in waveform similarity and velocity changes indicated by delayed phases at the time of the Chi-Chi event. These changes are limited to stations in and paths that cross the 70 × 100 km region surrounding the Chi-Chi source area, the area where seismic intensity and co-seismic surface displacements were largest. This suggests that damage at the near-surface is responsible for the observed waveform changes. Delays are largest in the late S-wave coda, reaching approximately 120 ms. This corresponds to a path averagedSwave velocity reduction of approximately 1%. There is also evidence that damage in the fault-zone caused changes in waveform character at sites in the footwall, where source-receiver paths propagate either along or across the rupture. The reduction in waveform similarity persists through the most recent repeating event in our study (November 15, 2007), indicating that the subsurface damage induced by the Chi-Chi earthquake did not fully heal within the first 8 years following the Chi-Chi earthquake.

Analyses of pseudotachylyte from Hole-B of Taiwan Chelungpu Fault Drilling Project (TCDP); their implications for seismic slip behaviors during the 1999 Chi-Chi earthquake

The seismic slip behavior during the 1999 Chi-Chi, Taiwan, earthquake ( M w 7.6) was contrastive between the northern and southern segments of the activated Chelungpu fault; large, fast and smooth slips with large stress drop in the north, while smaller, slower and irregular slips with smaller stress drop in the south. We analyzed the pseudotachylyte samples recovered from 1194 m, 1243 m and 1314 m depths of Hole-B of Taiwan Chelungpu fault Drilling Project (TCDP) to reveal the spatial difference in friction mechanism. All pseudotachylyte layers are thin (0.7–2.8 cm), the volume fraction of protoliths is very large (more than 63%), and the estimated temperature distribution is very heterogeneous from ca. 750–1750 °C. These observations suggest that these pseudotachylyte melts were in the partial melting regime of Montgomery [Montgomery, R.S., 1976. Friction and wear at high sliding speeds. Wear 36, 275–298] where friction coefficient is abnormally large. Similar pseudotachylyte was found already in the core sample from 175 m depth of the Nanto borehole penetrating the southern fault. Since both pseudotachylyte samples from the two boreholes are older than the 1999 Chi-Chi event and have been uplifted from depths farther down-dip of their current locations, it is likely that recent seismic ruptures also would have encountered these mechanical barriers of viscous melt patches at deeper parts in the north than in the south. Elastohydrodynamic lubrication of clayey gouge worked effectively at the shallower parts of the northern segment, however there is no evidence that it played an important role in the south. These differences are the plausible causes of the contrastive local slip behaviors during the Chi-Chi earthquake.

Ruptures in a Highly Fractured Upper Crust

The 1999 Chiayi M 6.4 earthquake sequence that occurred about one month after the 1999 Chi-Chi M 7.6 earthquake is remarkably similar to the Ruey-Li M 6.2 earthquake sequence which occurred one year before the big Chi-Chi event. Both sequences were energetic and short in duration; with events confined in small source volumes, and the fault-plane orientation of individual events in these sequences varied abruptly during the development of the dynamic rupture. Both occurred on the southern extension of the large Chelungpu rupture causal to the Chi-Chi event, and in an area of highly fractured upper crust. We consider that this faulting behavior is due to the rupture-induced dynamic stress that interacts with the pre-existing highly fractured upper crust and continually perturbs the local stress field. This phenomenon appears to be typical of particularly active plate boundary regions with a highly fractured upper crust under intense tectonic loading.

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.

Variations in Tectonic Activities of the Central and Southwestern Foothills, Taiwan, Inferred from River Hack Profiles

A longitudinal profile of a river under static equilibrium shows no degradation or aggradation and can be ideally described as a straight line on a semi-logarithmic graph. This type of profile is called a ”Hack profile”. If a river runs across uprising active structure systems, its Hack profile becomes convex. Accumulated tectonic strain varies positively with the intensity of the up warping in Hack-profile convexity. In this paper, we compare curvature changes in Hack profiles of a series of rivers running through faults in the central and southwestern Foothills of Taiwan. Longitudinal profiles of these rivers were derived from two versions of topographic maps (1904 and 1985) and recent DTM data (2000). Prior to comparisons, we calibrated the 1904 topographic map, named ”Taiwan Bautu”, by ”offsetting” horizontal coordinates north and westward approximately 440 m and then ”linear transforming” the elevation values. The Tungtzchiau fault of the central Foothills has remained inactive since 1935. Here relatively high uplift activity near the Wu River is indicated by significantly convex Hack profiles. This strain accumulation can be attributed to a lack of small magnitude earthquakes along the fault over the past 70 years. In the southwestern Foothills, relatively high uplift activity of similar intensity to the central Foothills is indicted near the Neocho River. Significant profiles with concave segments below the ideal graded profiles, at the lower reaches of rivers where continuous small magnitude strain release events have occurred, can only be found along the Sandieh, Neocho and Bazhang rivers in the southwestern Foothills. All these findings indicate that fault systems in the central Foothills tend to be locked and these faults could yield large earthquakes similar to the Chi-Chi event.

Liquefaction in the Chi-Chi Earthquake-Effect of Fines and Capping Non-Liquefiable Layers

ABSTRACT The 1999 Chi-Chi, Taiwan, earthquake caused great destruction to buildings, bridges, and other facilities, and a death toll of 2470. Of particular interest to geotechnical engineers in this event is the evidence of widespread liquefaction that caused significant damage to buildings, lifelines and harbor facilities. Shortly after the earthquake, an extensive field investigation in the Yuanlin area, including in situ tests and laboratory tests, was conducted by Moh and Associates, Inc. These in situ and laboratory tests along with ground performance observations offer a chance to examine the existing methods of liquefaction potential evaluation. In this paper, 22 cases of ground performance in the Yuanlin area that experienced extensive soil liquefaction are studied. These are case histories that were well documented with field observations of ground performance in the Chi-Chi event. Attempt is made to analyze liquefaction potential using the Standard Penetration Test (SPT) data that are associated with each case history. Results of the analyses show that in general, surface manifestation of liquefaction can be fairly accurately predicted using simplified methods. In many cases where the subsurface deposit consists of alternate layers of silty clay and silty sand, however, the effect of capping clay layers must be considered in order to correctly account for the presence or lack of surface manifestation of liquefaction.

3D Velocity Structure around the Source Area of the 1999 Chi-Chi, Taiwan, Earthquake: Before and After the Mainshock

3D V P and V P / V S models for the source area before and after the Chi-Chi earthquake are derived from a tomographic inversion of seismic travel-time data from the Taiwan Central Weather Bureau Seismic Network (CWBSN). Resulting 3D velocity models reveal a strong lateral variation in the crust, particularly at a depth range of 5 to 25 km along the trend of the dipping Chelungpu thrust. In general, low V P and high V P / V S anomalies are found in and around the source volume of the Chi-Chi earthquake, a volume centered around the entire 100 km × 40 km Chelungpu rupture surface. We pay special attention to the crustal properties and their changes due to the Chi-Chi event and the association of the crustal anomalies with seismicity. Combining the results with those of other geological and geophysical surveys, the crustal anomalies are interpreted as images of a highly fractured and fluid-filled Chi-Chi source region. Fluid overpressure is known to reduce the strength of the fault zone and in this case may have initiated the nucleation of the Chi-Chi earthquake. The V P models obtained before and after the Chi-Chi event show some interesting heterogeneities. Regions of low V P anomaly appear to have further expanded with the occurrence of the Chi-Chi earthquake. The V P / V S model shows high anomalies near the source rupture, especially near where the rupture is believed to have nucleated. After the Chi-Chi event, the V P / V S model shows a substantial change that may reflect an outward shift of the fluid-filled fractured source region. This shift is shown by a general expansion in V P / V S anomalies to a broader region (e.g., where two intensive aftershock clusters occurred) and a significant V P / V S reduction to more normal values near the Chelungpu rupture. We attribute these changes in V P and V P / V S anomalies, as well as the expansion and shift of the anomalous regions, to be a response to the localized stress change caused by the slip of the Chi-Chi rupture. The location of the seismic events is correlated with that of the crustal V P and V P / V S anomalies. Large events appear to occur in the transition zones of both V P and V P / V S anomalies. This finding may be of use in the identification of weakened crustal regions in a seismic zone, thus suggesting where impending large earthquakes are likely to occur. All input data and initial and final models of this article are given as electronic files in the CD attached to the end of this issue.

Stochastic Finite-Fault Modeling of Ground Motions from the 1999 Chi-Chi, Taiwan, Earthquake: Application to Rock and Soil Sites with Implications for Nonlinear Site Response

The stochastic method for simulating strong ground motions from finite faults is applied to the records of the 1999 Chi-Chi, Taiwan, earthquake. The method involves discretization of the fault plane into smaller subfaults, each of which is assigned an x 2 spectrum. The contributions from all subfaults are empirically atten- uated to the observation site and summed to produce the synthetic acceleration time history. The method is initially calibrated against the data recorded at 24 rock sites, located within 7-120 km from the mainshock hypocenter and providing a broad azimuthal coverage of the fault plane. The accuracy of the simulations is quantified through the model bias, defined as the logarithm of the ratio of the observed to simulated spectrum, averaged over all stations. The calibrated model for the Chi-Chi event has a near-zero average bias in reproducing the ground motions at rock sites in the frequency range from 0.1 to 20 Hz. An unusually low value is found for the radiation- strength factor s, controlling the high-frequency radiation level and directly related to the maximum slip velocity on the fault, compared with the mean value found for North American earthquakes. This result reflects the observed low peak ground ac- celerations of the Chi-Chi mainshock and, physically, its lower-than-usual slip ve- locities. The calibrated model is then used to simulate soil-site (site class D) records using the linear-response assumption. The simulated soil-site input motions are amplified by the weak-motion amplification functions, estimated by the spectral-ratio technique from available aftershock records. This analysis reveals an average reduction in strong-motion amplification to about 0.5-0.6 of that in weak motions, with an ac- celeration threshold for detectable nonlinearity near 200-300 cm/sec 2 . However, the derivation of soil-site specific weak-motion amplification was limited by the amount of aftershock data available; further improvement in the quantification of nonlinear soil response during the Chi-Chi earthquake may be possible with the release of additional aftershock datasets.

Liquefaction-induced ground failure: a study of the Chi-Chi earthquake cases

The 1999 Chi-Chi, Taiwan earthquake caused great destruction to buildings, bridges, and other facilities, and a death toll of more than 2400. Lessons should be learned from this event, which, albeit disastrous, represents a rare full-scale experimentation in geotechnical engineering and related fields. Of particular interest to geotechnical engineers is the phenomenon of soil liquefaction that caused significant damage to buildings, lifelines and harbor facilities. These liquefaction phenomena, in the forms of sand boils, lateral spreads, tilting and settlement of buildings, and ground settlement, were observed in the Chi-Chi event. Shortly after the earthquake, an extensive field investigation in the Yuanlin area, including in situ tests and laboratory tests, was conducted by Moh and Associates and other parties. These in situ and laboratory tests along with ground performance observations form the basis for the present study. In this paper, 24 cases of ground performance in the areas that suffered the most from liquefaction are studied. The ground performance in selected cases of the 24 cases is accounted for through a series of liquefaction analyses. The analysis of the potential of liquefaction and ground failure is performed using Cone Penetration Test (CPT) data. The results of the analyses show that the field observations of ground performance in the Yuanlin area in the Chi-Chi event can be accounted for satisfactorily using the evaluation procedure by Ishihara (Ishihara, K., 1985. Stability of natural deposits during earthquakes. Proc., 11th Int. Conf. on Soil Mechanics and Foundation Engineering, San Francisco, CA, vol. 1, pp. 321–376.).

Hypocentral Determination using the Simulated Annealing Method

AbstractBecause of the strong trade‐off between assumed origin time and focal depth and nonlinear effect of inversion problems, earthquake location is a basic issue in seismology which is not settled thoroughly. Aimed at the problem, this study presents a new earthquake location method named the SAMS method. The objective function is taken as the absolute values of the remnants of travel times together with the arrival times, and the Fast Simulated Annealing method is used to solve the inverse problem. Applied to the Chi‐Chi event relocation of Taiwan occurred on Sept. 21, 2000, the results show that the SAMS method can reduce the effect of the trade‐off between the origin time and focal depth, and get a better stability and a higher resolution.

The Chichi earthquake sequence, Taiwan: results from source parameter and stress tensor inversions

Abstract The main shock of the Chichi earthquake took place along the north–south-trending Chelungpu fault between the coastal plain and western foothills of Taiwan. It consisted of five major sub-events with the rupture propagating from south to north. The largest seismic moment is located 30–35 km north of the initial rupture. Waveform inversion results indicate a low-angle thrust fault dipping to the east. The centroid depth is 11–12 km. The seismogenic patterns of aftershocks indicate that the Chichi earthquake sequence can be explained by westward block thrusting on the Chelungpu fault, with strike-slip motions at the northern and southern tips. A deeper seismogenic zone lies 15 km directly beneath the Chichi main fault plane, indicating that the deformation associated with the Chichi event involves deep crust and thus cannot be entirely explained by detachment at shallow crust levels. The result of stress tensor inversion is remarkably consistent with that derived from regional background seismic data before the Chichi sequence, and with the palaeostress pattern reconstructed from geological evidences. It is inferred that the stress regime corresponding to the regional collision in Taiwan is the same as that responsible for the occurrence of the Chichi earthquake sequence. Consequently, we propose to consider the Chichi earthquake sequence as a typical manifestation of the mountain-building process in Taiwan.