Study of the earthquake frequency-magnitude distribution and its relation with crustal deformation and stress state in the Taiwan orogeny

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.