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The 2018 February 6 M w 6.3 Hualien earthquake caused severe localized damage in Hualien City, located 20 km away from the epicentre. The damage was due to strong (>70 cm s−1) and sharp (duration ∼2.5 s) velocity pulses. The observed peak ground-motion velocity in Hualien City symmetrically decays with distance from the nearby Milun fault. Waveforms observed on the opposite sides of the fault show reversed polarity on the vertical and N–S components while the E–W component is almost identical. None of the published finite-fault slip models can explain the spatially highly localized large velocity pulses. In this study, we show that an M w 5.9 strike-slip subevent on the Milun fault at 2.5 km depth, rupturing from north to south at ∼0.9 V s speed, combined with site effects caused by surficial layers with low S-wave speed, can explain the velocity pulses observed at the dense strong-motion network stations. This subevent contributes only 25 per cent of the total moment of the 2018 Hualien earthquake, suggesting that a small local slip patch near a metropolis can dominate the local hazard. Our result strongly suggests that seismic hazard assessments should consider large ground-motion variabilities caused by directivity and site effects, as observed in the 2018 Hualien earthquake. [ABSTRACT FROM AUTHOR]

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 (s) 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-s anomalies in areas with known active faults in the southwestern and easternmost parts of Taiwan. In contrast, high-VP, high-VS and low-s 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-s anomaly at depths of 23-40 km, which may reflect ascending fluids from the upper (or uppermost) mantle. The low-V and high-s 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 foldand- 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. [ABSTRACT FROM AUTHOR]