Abstract
According to reconstructed ground motion snapshots of the northern Taiwan area during the Mw 7.0 eastern Taiwan offshore earthquake of 31 March 2002, the composite effects indicated complicated wave propagation behavior in the ground motion of the Taipei basin. A major low frequency pulse arose after the S-wave with the duration of about 20 seconds was observed in northern Taiwan and dominated the radial direction. Observed waveforms of a low frequency pulse show amplification during the seismic wave across the Taipei basin from its eastern edge to western portion. This effect has been considered to be generated by an unusual source radiation, deep Moho reflection or basin bottom surface. In this study, recorded ground motions from a dense seismic network were analyzed using a frequency-wavenumber spectrum analysis for seismic wave propagation properties. We investigated temporal and spatial variations in strong shaking in different frequency bands. Results show that a simple pulse incident seismic wave strongly interacts with inside soft sediments and the surrounding topography of the Taipei basin which in turn extends its shaking duration. Evidence showed that seismic waves have been reflected back from its western boundary of basin with a dominant frequency near one Hz. Findings in this study have been rarely reported and may provide useful information to further constrain a three-dimensional numerical simulation for the basin response and velocity structure, and to predict ground motions of further large earthquakes.
Highlights
Many seismic disasters have been reported to have been induced by strong near source shakes (Archuleta and Hartzell 1981; Shin and Teng 2001)
It has been reported that the strongest ground-motion which was produced by the 331 earthquake occurs in the east margin of the Taipei basin (Wen 2002; Chen 2003; Huang et al 2003)
By inspecting the data of the 331 earthquake, which was recorded by the array distributes over the Taipei basin, it has been affirmed that the simple pulse which propagated into the Taipei basin was observed
Summary
Many seismic disasters have been reported to have been induced by strong near source shakes (Archuleta and Hartzell 1981; Shin and Teng 2001). The Eastern Taiwan Offshore Earthquake of 31 March 2002 (ML 6.8, MW 7.0) which is called the 331 earthquake caused minor damage near its when an earthquake located on the Ilan offshore area, there was minor damage in the Taipei basin instead. This discrepancy may indicate that as the seismic wave propagated through the complex deep structure of north Taiwan especially by following a path from Hualien offshore to Taipei; its focus and magnification may be one of the reasons to induce the strong shaking in the Taipei basin (Yeh et al 1988). Like the Kobe case, the simple pulse recorded in the 331 earthquake can be an origin of induced building damage
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