Chau-Huei Chen

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.

A fast velocity anomaly to the west of the Australian‐Antarctic discordance

To examine the hypothesis that the Australian-Antarctic Discordance overlays an anomalously cold upper mantle, we have measured Rayleigh wave group velocities along paths between the Southeast Indian Ridge and the Australian station NWAO. The group velocity peaks for events in the vicinity of 117°E, rather than for those within the discordance zone between 120 and 127°E. This variation is resolved at least for periods of 35–45 s. To model the cause, assumed velocity anomalies with simple geometry were tested using the surface wave Gaussian beam method in a forward sense. The preferred model consists of an elongated structure centered 300–500 km west of the discordance and stretching northward for at least 1000 km. The position of the anomalous structure is consistent with recent global tomographic models in this region.

Ten-second Love-wave propagation and strong ground motions in Taiwan

Applying the surface-wave Gaussian beam method to a three-dimensional (3-D) structure of Taiwan, we have studied the generation and propagation of short-period (10-s) surface waves in a region of pronounced crustal heterogeneity, especially in a region defined to be at an epicentral distance larger than a few focal depths away, where high-frequency near-field motions are largely attenuated and surface waves are adequately developed. By perturbing the source model as well as the crustal model in an iterative inversion process, we have achieved an excellent fit to the recent strong-motion observations recorded during the Tung-Ao earthquake (June 5, 1994, ML = 6.0) in Taiwan. This allows us to (1) obtain a refined 3-D crustal model of Taiwan from an initial model that was derived from recent tomographic results and, more important, (2) explain the distribution patterns of the strong shaking reported in terms of intensity maps for historical large earthquakes, and predict the long-period strong-motion distribution patterns for future large earthquakes in Taiwan. With an improved 3-D structure, we have gained better insight into short-period surface-wave propagations as they are modified by crustal lateral heterogeneity. Focusing and defocusing are clearly demonstrated. Of particular interest is the Central Mountain Range, a massive Tertiary metamorphic body forming the backbone of Taiwan, that seems to serve as a “divergent lens” for the propagating surface waves, whereas the large sedimentary basin in southwestern Taiwan seems to focus the wavefronts instead. The computed distortions of the propagating surface-wave field due to lateral heterogeneity are correctly reflected by the observations.

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 …

KẾT QUẢ BƯỚC ĐẦU QUAN TRẮC ĐỘNG ĐẤT NHỎ BẰNG MẠNG MÁY ĐỊA PHƯƠNG GURALP-6TD BỐ TRÍ TẠI VÙNG THANH HOÁ

The preliminary results of earthquake observation by deployment of the local seismic network Guralp-6TD in the region of Thanh Hoa province Ma River and Son La - Bim Son active faults predicted as the strongest seismogenic zones in Vietnam with the maximum earthquake can be reached 7.1 degree on Richter scale. The seismicity of these fault zones at several segments in Northwest regions is much more active than that at the southeast segments of the Thanh Hoa province. The low seismicity in these segments may be related to the sparse network of the instruments in these areas. A large region of Thanh Hoa and Nghe An provinces is distributed only three instruments, thus it is very difficult for us to sufficiently record small earthquakes. Since November 2009, in the framework of the cooperation between the Institute of Geological Sciences - VAST and the Chung Cheng National University - Taiwan, we connected a local seismic network Guralp-6TD with 12 instruments along the south-eastern of Ma River fault in Thanh Hoa Province to study more detail the seismic activity of the Ma River and Son La - Bim Son Faults. Our equipment recorded about 300 earthquakes during the past three years, and located near 190 earthquakes among them, the greatest part of them is small earthquakes. According to the study the segments of Ma River and Son La - Bim Son faults in Thanh Hoa province are not quite by seismic activity. The correlation to the tectonic fault distribution is revealed the small earthquakes happened in all the branches of the Ma River fault and the density of the earthquakes in the southwest branches is higher than in the northeast ones. The high density of the small earthquakes is detected in the section from Thuong Xuan district to the sea bank, where accepted as a stable section in the previous studies. A not small portion of the earthquakes is occured in the Son La - Bim Son, Hieu River faults, etc... Much more interesting result is the detection of a large number of small earthquakes inside the young submeridian tectonic faults Thuong Xuan - Ba Thuoc and Thach Thanh - Nong Cong. These faults are sill not well studied up to now by both tectonics and earthquake observation.