Study of the earthquake frequency-magnitude distribution and its relation with crustal deformation and stress state in the Taiwan orogeny
Date Issued
2016
Date
2016
Author(s)
Chen, Yen-Ling
Abstract
We analyze 343,581 earthquakes and 2,640 focal mechanisms from reprocessed CWB event catalog of Taiwan to map spatial variations of their frequency-magnitude distribution, i.e., b-value, and explore its possible dependence on faulting type and stress pattern within the crust in and around Taiwan. The b values estimated from three basic faulting types of the seismicity in the entire investigation region show significant differences in a 95% confidence interval, with the lowest of 0.82±0.02 for thrust, the intermediate of 0.89±0.03 for strike-slip, and the highest of 1.03±0.09 for normal events, which are consistent with those observed for worldwide and regional seismicity. We investigate the correlation of the earthquake frequency-magnitude distribution with the style of faulting and stress in Taiwan. The b-values estimated for three types of focal mechanisms show significant differences with the lowest for thrust, intermediate for strike-slip, and highest value for normal events, consistent with those found in global and other regional seismicity. Lateral distribution of the b-values shows a good correlation with the predominant faulting mechanism, crustal deformation and stress patterns. The two N-S striking thrust zones in western and eastern Taiwan under the larger E-W shortening and differential stress yield the lower b-values than those in the in-between mountain ranges subject to the smaller extensional stress and dominated by strike-slip and normal faults. The termination of the monotonically decreasing b-value with depth at ~15-20 km corroborates its inverse relationship with stress and the existence of the brittle-plastic transition in the weak middle crust beneath the Taiwan orogen. In summary, general dependence of the b-value with the style of faulting and stress is first verified in the Taiwan orogen. Regional b-value variations correlate well with the crustal deformation and stress regimes manifested from the local tectonics. The depth-varying b-value reveals the brittle-plastic transition at ~15-20 km and a weak middle crust under Taiwan.
Subjects
Taiwan orogeny
frequency-magnitude distribution
crustal deformation
focal mechanism
Type
thesis
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