Alternate Author Name(s)

Dr. Chih-Ping J. Lu, MA '73, PhD '76

Document Type

Dissertation

Date of Award

1975

Keywords

Geophysics, Taiwan Earth (Planet), Crust Earth (Planet), Mantle, Seismology, Taiwan, Tectonics

Degree Name

Doctor of Philosophy (PhD)

Department

Geological Sciences

First Advisor

Francis T. Wu

Second Advisor

Leon Thomsen

Third Advisor

W.D. MacDonald

Abstract

The crustal structures of Taiwan and the upper mantle structures in the vicinity of Taiwan have been determined for the first time through body wave studies. Travel times of local crustal events recorded both at the portable and the Telemetered Seismograph Network of Taiwan (TSNT) determine the fine structures of the local crust. A two-dimensional gravity model, constrained by the seismic data, determines the location of the Moho. The results indicate that the crust of Taiwan is 23 km thick on the west side, thickening eastward to 35 km in the central part of the island. It then decreases abruptly down to about 16 km in eastern Taiwan.

Velocity structures for P-waves in the apparent mantle are derived from direct measurements of apparent velocities (dΔ/dT) and studies on later arrivals with the TSNT stations. Short-period P-waves from earthquakes between 3° and 30° are used. A velocity-depth model for the upper 750 km of the mantle is derived. The essential figures of this model are: 1) a low-velocity zone followed by a high velocity gradient on its bottom near 125 km, 2) a first order discontinuity near 440 km, and 3) a second order discontinuity near 600 km. The model is similar to the one near Japan derived by Kanamori (1967), but significantly different from those for the western United States (Archambeau et al., 1969, among others).

Travel-time delays and the Pn distribution in the island indicate that the upper mantle beneath Taiwan is inhomogeneous in the E-W direction.

The crustal models derived above enable us to locate microearthquakes of Taiwan more precisely. The dense and accurate epicenters of the microearthquakes delineated a number of major active fault zones in central and eastern parts of Taiwan. Judging from both the seismicity and focal mechanism solutions, these fault zones are essentially thrusts of crustal origin. They are likely to be the intraplate structures.

A compilation of both large and micro- earthquake data in the Taiwan region as well as the crustal and upper mantle structures across the island suggest that a strip of the Philippine Sea plate is drifting northward along the Longitudinal fault zone of eastern Taiwan and a north-south trending strike-slip fault segment (right-lateral) near longitude 122.5° E. A local northerly downthrust is inferred at the junction of northern Taiwan and southwestern Ryukyu Islands.

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