Wei-Hau Wang

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.

Static Stress Transferred by the 1999 Chi-Chi, Taiwan, Earthquake: Effects on the Stability of the Surrounding Fault Systems and Aftershock Triggering with a 3D Fault-Slip Model

We investigated the effect of changes in static stress caused by the 1999 M L 7.3 Chi-Chi earthquake on the surrounding fault systems in Taiwan. We examined this problem from two perspectives. First, we applied a statistical test for the hypothesis of static-stress triggering by comparing the distributions of the Coulomb failure stress changes on the focal mechanism nodal planes both before and after the mainshock. Since preshocks could obviously not have been affected by the mainshock, the deviation of these distributions determined the significance of static-stress triggering. Second, we calculated the Coulomb failure stress changes on the Chukou and Meishan faults in specified slip directions to evaluate the stability of these two active faults after the Chi-Chi earthquake. The static-stress changes were obtained by applying the theory of dislocation in an elastic half-space to a 3D rupture model derived from the surface displacements associated with the Chi-Chi earthquake. Our results showed that the distributions of the Coulomb failure stress changes for events preceding and following the Chi-Chi earthquake were significantly different, yet consistent with the hypothesis of static-stress triggering. We also found that both the Chukou and the Meishan faults were in stress shadows at depths shallower than 10 km. However, because a number of aftershocks fell in these stress shadows, it is possible that other mechanisms, such as dynamic stress, were responsible for triggering these events. Manuscript received 6 November 2000.

Fault Slip Inverted from Surface Displacements during the 1999 Chi-Chi, Taiwan, Earthquake

We use coseismic surface displacement data to determine the slip distribution on the Chelungpu fault during the 1999 Chi-Chi, Taiwan, earthquake. The surface displacements were obtained from both GPS surveys and integrations of near-field strong ground motions. The fault rupture is taken as a series of dislocations along the Chelungpu fault in a homogeneous, elastic half-space. Using this assumption, we are able to find both a least-squares solution and optimal solutions from genetic-algorithm inversion with a step-wise rake constraint. Our results show that with the rake constraint, the resulting rupture features are more reasonable and more consistent with geologic observations than with the least-squares solution. We also evaluate the influence of fault geometry by calculating slip distributions on a simplified 2D fault plane and a more sophisticated 3D fault surface, noting that the 3D rupture model can greatly reduce model misfit when compared with the 2D rupture model. Both models do show, however, that slip dramatically increases from the hypocenter toward the northern end of the Chelungpu fault and that right-lateral slip is found at a large asperity near the southern end of the Chelungpu fault at depths below 10 km. Our models predict a much larger average slip than others derived from body-wave inversions. We interpret this inconsistency to be a result of partitioning of aseismic sliding due to mineral composition, state of stress, and/or frictional heat.

Effects of reaction kinetics and fluid drainage on the development of pore pressure excess in a dehydrating system

Abstract Fluid is released by dehydration reactions during prograde metamorphism. If the Claperyron slope for the dehydrating reaction is positive, then there is a net decrease in the total solid volume, which implies an irreversible increase in porosity. If the dilation of the pore space is insufficient to provide storage for all the released fluid, then pore pressure excess is generated, and if it becomes sufficiently high, it may lead to brittle fracturing. The time scale for pressure generation and the pore pressure excess can be maintained over long duration hinge on the interplay of reaction kinetics and fluid drainage. Motivated by experimental and microstructural observations, a hydrological model is developed that incorporates dehydration kinetics and its pressure dependence. Analytic solutions were derived for the undrained development of pore pressure. Whether lithostatic pressure may be exceeded hinges on magnitude of the overstep in temperature and corresponding equilibrium pressure. The time scale for development of pore pressure depends on the trade-off between poroelasticity and the pressure sensitivity of reaction rate. A finite difference model was also developed to simulate the progressive development of pore pressure excess, dehydration and porosity development. The model captures the experimental observation in gypsum of a reaction front that progressively propagates from the drained end toward the undrained end of a laboratory sample. It is also in reasonable agreement with experimental data on fluid drainage and porosity production.

Lithospheric flexure under a critically tapered mountain belt: a new technique to study the evolution of the Tertiary Taiwan orogeny

Abstract A joint analysis of both the lithospheric flexure of an underlying plate and mountain-building mechanics is developed based on the critical wedge theory and the flexure response of an elastic plate over an inviscid asthenosphere. According to the analysis, the amount of deflection beneath a critically tapered mountain belt strongly depends upon the internal and basal friction coefficients of that belt and the flexural rigidity of the underlying plate rather than the horizontal stress induced by plate convergence. An increase in the internal friction of a mountain belt or a decrease in the friction along its basal decollement tends to enlarge the deflection of the underlying plate. The tendency toward an increased deflection of the underlying plate due to a decrease in flexural rigidity is suppressed by the development of a more narrowly tapered mountain belt. The newly developed technique presented here is used to analyze the evolution of the Tertiary Taiwan orogeny and shows that the optimal flexural rigidity of the Eurasian Plate in Taiwan is about 3×10 22 Nm. It also demonstrates that the collision between the Eurasian Plate and the Luzon Arc might have initiated 7 Myr ago, a finding that would account for the burial history of the foreland basin in south-central Taiwan.

A Possible Causal Relationship Between the 1998 Ruey-Li Sequence and the 1999 Chi-Chi Earthquake in Taiwan

We have investigated the causal relationship between two recent Taiwan events, the 1998 M 6.2 Ruey-Li and the 1999 M 7.6 Chi-Chi earthquakes. It is found that both earthquakes are generated by ruptures essentially along nearby sections of a large through-going fault zone along the Western Foothills of Taiwan. The rapid termination (in 10 days) of the M 6.2 Ruey-Li sequence is interpreted by the presence of an asperity in terms of the Peikang basement high, which dominates the tectonic deformation in central western Taiwan. We have carried out the relocation of the aftershocks, their fault-plane solutions, plus stress inversion, waveform source slip inversion, Coulomb stress change calculations, and regional strain energy calculations for the Ruey-Li sequence, so as to examine its causal relationship from different angles. We have concluded that as both events are results of the same regional stress accumulation due to the plate convergence, the Ruey-Li earthquake sequence would likely have escalated into a much larger rupture including the Chelungpu fault had it not been for the presence of the Peikang basement high.

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

Our study seeks to obtain a more detailed 3D velocity structure for the region surrounding the Chi-Chi source volume, both before and after the Chi-Chi earthquake. Through tomographic inversion, we have computed 3D V p and V p/ V s models using the high-quality data set from local earthquakes as recorded by the Taiwan Central Weather Bureau Seismographic Network (CWBSN). We have selected 760 Chi-Chi aftershocks with 12,568 P - and 9,915 S -wave arrival times that are used in this study. In order to examine whether the large Chi-Chi earthquake sequence has changed the crustal properties, we have used another data set selected from the CWBSN database from 1993 to 1999. This includes arrival times of 22,500 P - and 14,460 S -wave arrival times from 975 events …