Strong ground motion and pulse-like velocity observations in the near-fault region of the 2018 M _w 6.4 Hualien, Taiwan, earthquake

The maximum observed peak ground acceleration (PGA) and peak ground velocity (PGV) at various stations during the 2018 Hualien, Taiwan earthquake were 594 Gal and 146 cm=s, respectively. Pulse-like velocities were observed at all stations within a distance of 4 km from the Milun fault. The horizontal spectral accelerations of the pulse-like records indicated two obvious amplifications at periods of roughly 1 and 2 s. Natural frequencies of 0.8–1.5 Hz were observed in the region near the Milun fault using microtremor measurements. The spectral acceleration peak at periods of roughly 2 s is mostly seen in the east–west direction, indicating a typical fault-normal seismic radiation from the fault rupture. Consequently, we contend that the amplifications of spectral acceleration at approximately 1 and 2 s were caused by site amplification and the rupture front, respectively. The site amplification at approximately 1 s may have been one reason for the collapse of medium-rise buildings during this earthquake. Evident soil nonlinearity resulted in smaller horizontal than vertical PGA at many stations in the near-fault region.