Yi-Ben Tsai

Estimated Seismic Intensity Distributions for Earthquakes in Taiwan from 1900 to 2008

In this study we recreated peak ground accelerations (PGA) and peak ground velocity (PGV) distributions for Taiwan by applying the attenuation relations of Liu and Tsai (2005) to calculate the PGA and PGV values for 1989 Mw ≥5:0 earthquakes in a catalog of earthquakes from 1900 to 2008 with homogenized magnitude (Mw )( Chen and Tsai, 2008). We further combined the PGA and PGV values to obtain corresponding modified Mercalli intensity (MMI) values (Wald, Quitoriano, Heaton, Kanamori, et al., 1999) and their spatial distributions and recurrence intervals. We adopted a logarithmic functional form analogous to the Gutenberg-Richter relation for seismicity to represent the annual frequency of seismic intensity parameters: log10N a blog10PGA, log10N a blog10PGV, and log10N a bI. The regions with high PGA and PGV values are often associated with low b values in these equations. As it is well known that the Mw 7.45 Chi-Chi earthquake of 21 September 1999 had produced high PGA values (in excess of 0:9g) and PGV values (in excess of 300 cm=s), we used these relations to estimate the Poisson probability distributions in Taiwan for MMI ≥ VIII (i.e., PGA ≥ 485g) for recurrence intervals of 30, 50, and 100 years. The results show a wide range of differences in the Poisson probability of MMI ≥ VIII among different areas of Taiwan. For example, for a 50-year interval, this probability at 10 major cities in Taiwan is as follows: Taipei 0.67%, Hsinchu 2.15%, Taichung 5.24%, Chiayi 24.35%, Tainan 1.61%, Kaohsiung 0.04%, Hengchun 4.94%, Ilan 17.67%, Hualien 37.04%, and Taitung 9.82%. These esti- mates should be of interest to city planners, especially for earthquake preparedness planning.

Probability for Simulating Future Earthquakes with Mw ≥6.0 in Taiwan for Seismic Hazard for the Earthquake Catalog from 1900 to 2008

In this study, we use extreme value theory based on Gumbel‐equation derivations to estimate the Gutenberg–Richter a and b parameters for Taiwan. Data are from the augmented, homogenized (in terms of moment magnitude), historic catalog for Taiwan. The island is divided into grids of 0.2° latitude by 0.2° longitude, and Gumbel type 1 statistical analysis is applied. The values of a and b are then used to determine the probability of large earthquakes ( M w≥6.0) occurring at each grid. The results show two relatively high probability paths for large earthquakes, one extending from Hsinchu southward to Taichung, Chiayi, and Tainan in western Taiwan and the other from Ilan southward to Hualian and Taitung in eastern Taiwan, both of which are characterized by low b ‐values. It indicates that future earthquakes can be expected along these paths characterized by low b ‐values. Additionally, maximum peak ground acceleration and maximum peak ground velocity (determined from respective attenuation laws and a gridding regimen of 0.1° latitude by 0.1° longitude for Taiwan) follow similar paths to that of the low b ‐values.

A Seismic Structure Study in the Kaoping Area, Southwestern Taiwan

The difference between S ‐wave and S ‐to‐ P ‐wave conversion ( S P phase) arrival times is enhanced with rectilinear motion detector filtering to describe alluvial‐sediment thickness in the Kaohsiung–Pingtung (Kaoping) plains area. A more complete understanding of the underground structures of the Kaoping area is provided in this paper and explains why the surrounding regions in Taiwan experience more earthquakes than the Kaoping area. Data are based on seismic activity recorded by the portable array for numerical data acquisition (PANDA) for the period from 1995 to 1997. The difference between S ‐wave and S P ‐phase arrival times shows that the sedimentary layer is thicker along the west and southwest coasts. P ‐wave travel‐time residuals, high‐frequency attenuation parameters Kappa, and quality factor Q P , Q S , and coda waves confirm this result. We also determined the orientation of the Chaochou fault using the first motion of P ‐wave arrivals. To the east of the Chaochou fault, stress trends southeast–northwest, while to the west, it trends northeast–southwest. The change in stress trends east and west of Chaochou fault suggests the presence of a highly fluid accretionary wedge in the Kaoping area. The Chaochou fault forms a seismically active tectonic boundary with the uplift of the hanging wall leading to westward tilting of the basement of the Kaoping plains. We demonstrate that these features are the reason there are relatively few earthquakes in the Kaoping area. The presence of a highly fluid accretionary wedge is indicated by a thick alluvial layer in the west and southwest Kaoping coasts; the Peikung High acts as the indenter that may allow seismic energy to escape and reduce the number of earthquakes in the region. Online Material: Figures illustrating calculations of Kappa, Q c , P ‐ and S ‐wave spectra, Q P , and Q S from ground‐motion data.