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Anomalous Seismic Attenuation along the Plate Collision Boundary in Southeastern Taiwan: Observations from a Linear Seismic Array

Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan cplee{at}earth.sinica.edu.tw

Naoshi Hirata

Earthquake Research Institute, University of Tokyo, Tokyo, Japan

Bor-Shouh Huang and Win-Gee Huang

Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan

Yi-Ben Tsai

Geosciences Department, Pacific Gas & Electronic Company, California

Lateral variations of seismic attenuation are investigated using data from a linear seismic array deployed across southern Taiwan. The attenuation parameter t^* is obtained by fitting the amplitude spectra of P and S waves with a theoretical spectrum using an ² model. The observed t^* data are then plotted against the travel time, back azimuth, and focal depth, respectively, to explore the spatial variations of t^* for P and S waves. Significant lateral variations of seismic attenuation are found between the eastern mountainous and western plain areas of Taiwan. Large attenuation contrast with depth in the east indicates that an anomalously high-attenuation zone is located in a shallower area north of the array in southeastern Taiwan. A 2D raytracing method is applied to show that the probable depth of the anomalous zone is at about 15–20 km. This anomalous attenuation zone coincides with an area marked by low-P and low-S velocities as well as a total absence of seismicity. The area is also marked by other prominent manifestations of active collision between the Eurasian and Philippine Sea plates, for example, high mountain ranges, rapid uplift, and high heat flows.