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The northeastern convergence of the Philippine Sea plate toward the Eurasian plate causes the major western Philippine Sea plate boundary to subduct toward the northwest or west directions. However, this phenomenon is not clearly observed along the plate boundary between Luzon and Taiwan. Careful examination of deep seismicity in the southern Taiwan area from the earthquake catalog reported by the Central Weather Bureau shows two seismic zones dipping toward the opposing directions. The first dips toward the east from the surface down to 150 km in depth, while the second dips westward at depths between 150 and 200 km. These two seismic zones are confirmed further by seismogram observation and modeling results generated by two deep faults in the southern Taiwan area. The eastward seismic zone clearly results from the Eurasia plate subduction along the Manila trench, while a small section of the westward seismic zone might likely be a residual slab from the ancient subducted Philippine Sea plate. Based on the subduction speed obtained from GPS observations and the subducted Eurasian plate geometry, we can further estimate the eastward Eurasian plate subduction started at least 3.35 million years ago. This result is roughly consistent with the volcanic ages (3 - 4 Ma) observed in the arc between Luzon and Taiwan. [ABSTRACT FROM AUTHOR]

In this study, ground strains over a soft sediment-filled plain were estimated for nineteen moderate earthquakes in northeastern Taiwan. To this end, abundant accelerograms recorded by two independent seismic arrays, LLSST and SMART1, were analyzed. These two dense seismic arrays are both highly overlapping in terms of space and operation duration. Spatial analyses herein for the LLSST array suggest that good estimates of ground strains can be obtained using only three 3-component seismometers recording at stations configured in a simple triangle with maximum separation of about 100 m. In the case of the Hualien earthquake (ML = 6.5) of 20 May 1986, a peak shear strain of 210 μ occurs with direct S-waves whilst strain of 186 μ occurs with surface waves dominated by a period of about 2.5 sec. However, the peak vertical gradient accompanied by shear waves is in the range 1500 ~ 2000 μ, which is larger than that of horizontal differential motion by a factor of about 10. For further applications in mapping ground deformation fields at a given region, the single-station method is examined by comparing its estimates with those of a displacement gradient algorithm. A moving cross-correlation method is proposed to obtain phase velocities for a sequence of waves, particularly when ground motion contains obvious surface-wave energy. Consequently, estimates of peak horizontal strain at the LLSST site are in the range 1 ~ 150 μ for the nineteen moderate earthquakes and the logarithm of these appears to be proportional to earthquake magnitudes. Finally, a simple test was implemented to verify the feasibility of this method when used with large station spacing (2 ~ 3 km), simulated by the SMART1 array. In general, when taking strain estimated from LLSST data as a basis, the results from SMART1 reveal that differences would not be over 50%. [ABSTRACT FROM AUTHOR]