Preface to the Focus Section on the 6 February 2018 Mw 6.4 Hualien, Taiwan, Earthquake

Abstract

An intense earthquake swarm with several tens ofMw 5+ class events began shaking offshore eastern Taiwan on 4 February 2018. Two days after the swarm began, an ML 6.26 (Mw 6.4) earthquake struck eastern Taiwan on 6 February 2018 at 11:50 p.m. (local time). This earthquake originated offshore approximately 15 km north of the city of Hualien at a depth of about 10 km and ruptured toward the southwest, with a surface rupture inland within the city of Hualien along the Milun fault. This surface rupture resulted in severe damage to several tall buildings and caused 17 deaths, as well as 289 injuries. The centroid moment tensor (CMT) focal mechanism from the Taiwan Central Weather Bureau (CWB) and the generalized Cut-and-Paste (gCAP) from the Broadband Array in Taiwan for Seismology demonstrated that this Mw 6.4 event had mainly a strike-slip focal mechanism with a minor thrust component dipping to the west. It also had a large compensated linear vector dipole (CLVD) component, consistent with the U.S. Geological Survey CMT solution and the real-time moment tensor solution (Lee et al., 2013). The field investigation report of the Central Geological Survey (CGS, 2018) suggested a possible involvement of two identified surface ruptures, one along the Milun fault and another along the Linding fault, despite the origination of the mainshock offshore northeast of Hualien. However, the observed surface rupture along the Milun fault is a strike-slip fault dipping to the east. The discrepancy between the focal mechanism of the mainshock and the observed surface rupture inland along the Milun fault has focused attention on this event and also raised doubt about the rupture mechanism of the 2018 Hualien earthquake. Historical events in this region have been studied and characterized by the features of previous earthquake swarms (Shyu, Chen, et al., 2016). Significant historical damaging earthquakes occurred on 22 October 1951, when three Mw 7+ class earthquakes happened within 12 hrs. Several surface rupture patterns of the 6 February 2018 event were found to be similar to those of the 22 October 1951 events along the Milun fault. The Taiwan Earthquake Model (TEM) project published a National Seismic Hazard Map in December 2015 (Wang et al., 2016) and showed that the Hualien region has about an 80% probability of experiencing an earthquake with the CWB intensity V (> 80 Gal) or larger in next 50 yrs. The seismogenic structures (Shyu, Chuang, et al., 2016) that were adopted for the seismic hazard map revealed a short recurrence interval of the Milun fault of 70 yrs and thus the highest seismic potential, with a probability of an Mw 6.4 earthquake of about 42% in the next 50 yrs. Unfortunately, this high-seismic-hazard potential did not receive enough attention at the time, resulting in significant damages and casualties during the 2018 sequence. Seismic recordings from the CWB strong-motion network and the P-alert system (Wu et al., 2013), an earthquake early warning (EEW) system operated by the Taiwan Earthquake Research Center, both showed that this earthquake struck inland eastern Taiwan with high intensity, yielding a peak ground acceleration (PGA) of more than 400 Gal and a pulse-like velocity motion of about 100 cm=s. The low-cost high-performance dense P-alert network has proven to be extremely helpful in understanding the physical generation of this strong motion, especially in relation to building damage. Exactly 2 yrs before the 6 February 2018 Hualien earthquake, the 6 February 2016 Mw 6.4 Meinong earthquake occurred in southwestern Taiwan. This earthquake also generated large pulse-like velocity motions that were responsible for much damage (Kanamori et al., 2017) and that also were recorded by the dense P-alert network (Wu et al., 2016). Lin et al. (2018) used the unfiltered dense P-alert records from the Meinong earthquake to identify the location and the slip patch on the fault plane responsible for the generation of the destructive pulse-like velocity motion. Jan et al. (2018) showed that the near-real-time intensity map for the Meinong earthquake derived from the P-alert system is helpful to identify the rupture direction and thus the identification of a mainshock rupture plane. In the focus section on the 6 February 2018 Mw 6.4 Hualien, Taiwan, earthquake, we present 11 articles that investigate various aspects of this recent earthquake sequence. Some articles examine data from the CWB strong-motion network and the P-alert system, and other studies perform joint inversions of seismic and geodetic data to explain the rupture characteristics of this earthquake. Some articles also include discussions of the ground deformation based on geodetic observation, site and geotechnical investigations, examination of the aftershocks, and exploration of the possible velocity changes after the earthquake. Wu et al. (2018) documented the reliable performance of the P-alert network and its ability to provide on-site EEWand a map of expected ground shaking within 2 min of the mainshock origin time that is in good agreement with the patterns of observed damage in the area. Lee, Lin, et al. (2018) mapped the spatial slip distribution of the Hualien earthquake using three fault planes, namely an offshore western dipping fault plane and two shallow faults associated with the identified