Oblique Blind Faulting Underneath the Luzon Volcanic Arc During the 2022 M_w 7.0 Abra Earthquake, the Philippines

Abstract

On 27 July 2022 (UTC), the M w 7.0 Abra earthquake struck the northwestern region of Luzon, the Philippines. This earthquake is the largest crustal seismic event in northern Luzon since the 1990 M w 7.7 earthquake (Silcock & Beavan, 2001;Yoshida & Abe, 1992). The earthquake occurred ∼20 km east of Vigan city, beneath the western flank of the Cordillera Central, with a hypocenter depth of 15 km (PHIVOLCS, 2022) (Figure 1). The earthquake is characterized by an oblique faulting and the focal mechanism of the mainshock exhibits two nodal planes dipping ∼70° to the north-west and ∼30° to the east, respectively (Bonita et al., 2015;Ekström et al., 2012;Punongbayan et al., 2015; USGS, 2022a) (Table S1 in Supporting Information S1). Strong ground shaking generated by the mainshock was perceived throughout Luzon island, including Manila, one of the most populous metropolises in the world, ∼300 km away from the epicenter (PHIVOLCS, 2022). The emergency field investigation after the mainshock reported massive damages of local infrastructures, induced landslides in mountainous areas, and liquefaction in western Luzon (Perez et al., 2023). The distribution of aftershocks shows a north-south trend, roughly parallel to the strike of geological units in this region (Figure 1). However, no coseismic surface ruptures were found, leaving the seismogenic fault of the mainshock concealed. According to the focal mechanism (Figure 1), this event is likely associated with an oblique blind fault.