Yih-Hsiung Yeh

Two‐dimensional crustal structures of Taiwan from gravity data

To delineate the tectonic character of Taiwan, an island-wide gravity survey of Taiwan was conducted between 1980 and 1987. The Bouguer anomaly map shows that, in general, isogals trend NNE in consonance with the overall structural trend of the island. With seismic and other geophysical data as constraints, the subsurface density structures were modeled along three profiles across the major structural trends. The gravity data are consistent with average continental Moho depths of 26 km in the Coastal Plain and the Western Foothills, 28 km underneath the Coastal Range in eastern Taiwan, and 33 km under the Central Range. A lack of significant correlation of the Bouguer anomalies with the topography implies dynamic, rather than isostatic, support of topography in the Taiwan region.

Geomagnetic fluctuations during the 1999 Chi-Chi earthquake in Taiwan

On 21 September 1999 (Taiwan local time), a major earthquake measuring M7.3 occurred near the town of Chi-Chi in central Taiwan. After the Chi-Chi earthquake, geomagnetic data recorded by a network of 8 stations equipped with continuous recording systems was analyzed. The results revealed that the total geomagnetic intensity of the Liyutan station, about 8 kilometers from the northern end of the Chelungpu fault (considered to be related to the earthquake), fluctuated significantly for more than a month prior to the earthquake. The fluctuation features continued and then stopped after the Chia-Yi earthquake (M6.2) occurred near the southern end of the Chelungpu fault on 22 October 1999. The variation of intensity reached 200 nTs. Geomagnetic fluctuations were also found at the Tsengwen station, located about 42 kilometers from the southern end of the Chelungpu fault and 30 kilometers from the Chia-Yi earthquake. These geomagnetic disruptions with highly anomalous amplitudes associated with the Chi-Chi and Chia-Yi earthquakes appear to have been the result of the accumulation and release of crustal stress that led to the subsequently severe surface rupture at the time of the earthquakes.

Three-dimensional elastic wave velocity structure of the Hualien region of Taiwan: Evidence of active crustal exhumation

The Hualien region of Taiwan is located at a complex transition of the boundary between the Eurasian and Philippine Sea Plates. To the southwest, the mountains of Taiwan are uplifting rapidly as a consequence of an ongoing arc-continent collision, while to the east the oceanic Philippine Sea Plate is subducting northward beneath Eurasia. We investigated the structure and dynamics of this region by analyzing seismograms of local earthquakes recorded during a deployment of the Portable Array for Numerical Data Acquisition II network. P and S wave velocity structures deduced from travel time tomography analysis show that the collisional suture to the south of Hualien is characterized by a narrow (< 10 km width), near vertically dipping zone of low velocities that extends to depths in excess of 20 km. Velocities in the Eastern Central Range west of the suture zone are significantly higher and define a feature 10–15 km wide that appears to be continuous from the near surface to depths as great as 40 km. Farther to the west beneath the Western Central Range, the velocities again decrease. Focal mechanisms of local earthquakes show that while thrust faulting is the predominate mode of deformation throughout the region, normal faulting occurs as well beneath the Eastern Central Range. Thus the rapid uplift of the mountains of Taiwan may be a result not only of compressional shortening but also of an excess of positive buoyancy. We suggest that the higher velocities and extensional mechanisms in the Eastern Central Range are caused by the ongoing exhumation of previously subducted continental crust, while the lower velocities to the west reflect continued underthrusting of the crust beneath the Eastern Central Range.

Variations of radon content in groundwaters and possible correlation with seismic activities in northern Taiwan

Radon (222Rn) concentration in geothermal waters and CO2-rich cold springwaters collected weekly in duplicate samples from four stations in northern Taiwan were measured from July 1980 to December 1983. Seven spike-like radon anomalies (increases of 2 to 3 times the standard deviation above the mean) were observed at three stations. Following every anomaly except one, an earthquake ofML above 4.6 occurred within 4 to 51 days, at an epicentral distance 14 to 45 km, and at a focal depth of less than 10 km. The distribution of the earthquakes preceded by radon anomalies is skewed in certain directions from the radon stations; the radon stations seem to be insensitive to earthquakes occurring in the other directions. At the fourth station, near a volcanic area, much gas (mainly CO2) is discharged from the well, together with hot water. A very high concentration of radon was detected in the discharged gas; therefore trapping of gas in the water can result in anomalously high radon contents. According to limited measurements, the radon concentration in water appears to be undersaturated with respect to that in gas. This suggests that hot water is very susceptible to radon loss, and monitoring of radon in gas is more desirable.

A Simple Algorithm for Local Earthquake Location Using 3D VP and VS Models: Test Examples in the Central United States and in Central Eastern Taiwan

Traditional local-earthquake location using a horizontally layered homogeneous velocity model is limited in its resolution and reliability due to the existence of frequently overlooked 3D complexity of the real Earth. During traditional 3D seismic tomography, simultaneous earthquake relocation using the resultant 3D velocity model has produced reliable earthquake locations; however, only a small subset of events are typically used and thus relocated in the inversion. The rest of the events in a catalog must then be relocated using the 3D models. The repeated calculation of travel times across 3D V P and V S models is also not efficient and not practical for a routine network earthquake location when the very time-consuming exact 3D raytracing is used. Because high-resolution earthquake data are now available from many modern seismic networks, representative high-resolution 3D V P and V S models for a region can be better determined. By taking advantage of recently available high-speed computer technology and large disk space, we implemented a simple algorithm to efficiently locate every local earthquake using the best available regional 3D V P and V S models. Once the V P and V S information for all cubic cells in a 3D grid model are determined, P and S travel times from each grid point to all seismic stations can be calculated and stored on disk files for later usage. During the iteration process for earthquake location, travel times from a trial hypocenter to all recording stations can be determined simply by a linear interpolation from those of the adjacent eight grid points available in the previously stored disk files without the need for raytracing. The iterations continue until the hypocenter adjustments at the end of the last iteration are below the given criteria and the travel-time residual, or the difference between the observed and the calculated travel times, is a minimum. Therefore, any local earthquake can be efficiently and reliably located using the available 3D velocity models. This simple location program has been applied to relocate earthquakes in the New Madrid Seismic Zone (nmsz) of the central United States and in the central eastern Taiwan region. Preliminary results in both regions reveal that earthquake hypocenters can be efficiently relocated in spite of the very significant lateral structural variations. Tests with data from Taiwan further demonstrate that the resolution of seismic tomography and the relocated seismicity is sensitive to relative distribution of seismic-network stations and background seismicity. Thus, this single-event location program can be applied to relocate all earthquakes in a seismic-network catalog and, more importantly, to allow routine earthquake location for any seismic network using the available 3D velocity models.