Surface Rupture and Fault Characteristics Associated With the 2020 Magnitude (MW) 6.6 Masbate Earthquake, Masbate Island, Philippines

Abstract

On 18 August 2020, Masbate Island was struck by a magnitude (M_W) 6.6 earthquake. This seismic event represents the second occurrence of a strong earthquake (M > 6) in 17 years, emphasizing the necessity for further investigation into the characteristics of this event. In this study, we employ Interferometric Synthetic Aperture Radar, seismicity analysis, and field investigations to comprehensively characterize the coseismic and postseismic slip associated with the event. Our findings reveal a 50-km-long fault rupture along the Masbate segment of the Philippine Fault, with ∼23 km surface rupture mapped onshore, despite the occurrence of interseismic creep. The slip distribution demonstrates decreasing displacements northwestward toward the creeping section, with a maximum left-lateral displacement of 0.97 m near the epicenter. Toward the southeast offshore, the rupture terminates at a left stepover of a fault. While the surface rupture appears relatively straight and narrowly concentrated, the secondary ruptures and mapped offshore faults reveal a more complex transtensional fault structure in the southeastern part of Masbate Island. This fault complexity represents an asperity that facilitates high-stress accumulation and rupture initiation. Postseismic slip persists for several months along the onshore creeping segment. Based on comprehensive measurements of both cumulative and coseismic slip along the Masbate fault segment, we calculate a slip rate ranging between 2.8 and 3.8 cm/year and a recurrence interval of 16–41 years for earthquakes similar to the 2020 earthquake. Our study highlights how heterogeneity in fault properties, including geometry and coupling state, influences the distribution of slip and magnitude of earthquakes. The 2020 Masbate earthquake provides valuable insights into the rupture dynamics and fault behavior of the Philippine Fault in the Masbate region.