On 2016 February 6 the South Taiwan earthquake (Mw 6.4) occurred in the Meinong District of Kaohsiung, southern Taiwan, at a depth of 17 km. It caused 117 fatalities and widespread damage to infrastructures, especially in the Tainan city. To clarify the generating mechanism of this damaging earthquake, we determined high-resolution 3-D images of P- and S-wave velocity (VP, VS) and Poisson's ratio (σ) in the epicentral area. We used 105 712 P- and 61 250 S-wave arrival times of 8279 local earthquakes (1.5 ≤ M ≤ 6.4) recorded at 41 seismic stations in South Taiwan during 2000-2011. In the upper crust (depth ≤ 10 km), the most remarkable feature is low-VP, low-VS and high-σ anomalies in areas with known active faults in the southwestern and easternmost parts of Taiwan. In contrast, high-VP, high-VS and low-σ anomalies become dominant in the lower crust. The hypocentre of the 2016 South Taiwan earthquake is located in a boundary zone where seismic velocity and Poisson's ratio change drastically in both the horizontal and vertical directions. Furthermore, the hypocentre is underlain by a vertically elongated high-σ anomaly at depths of 23-40 km, which may reflect ascending fluids from the upper (or uppermost) mantle. The low-V and high-σ anomalies in the upper crust coincide with areas of low heat flow, negative Bouguer gravity anomaly, and low magnetotelluric resistivity, which may reflect crustal fluids contained in the young fold-and-thrust belt. These results suggest that the 2016 South Taiwan earthquake was triggered by ascending fluids from dehydration of the subducting Eurasian slab, invading into active faults with a high loading rate.
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