Cascading ruptures on near-orthogonal strike-slip faults controlled by simple shear: Insights from the 2019 Cotabato earthquake quartet, Philippines

Near-orthogonal ruptures within the elastic crust cannot be explained by the classic Mohr-Coulomb theory. Instead, simple shear is a promising hypothesis to explain near-orthogonal ruptures, and we test this hypothesis on the 2019 Cotabato earthquake sequence. In 2019, an earthquake quartet struck the Cotabato province on Mindanao island, Philippines: M_w6.4 on October 15 (EQ1), M_w6.6 on October 29 (EQ2), M_w6.5 on October 31 (EQ3), and M_w6.8 on December 15 (EQ4). This was the first documented earthquake quartet involving four similar-size moderate strike-slip events in such a short period. The sequence ruptured the Sindangan-Cotabato-Daguma Lineament, which was formed during the collision of the Sundaland-Eurasia Plate and the Philippine Mobile Plate in the Late Miocene and has not hosted any large earthquake in the past century. We initially estimated the fault orientation by surface wave relocation of M>4.7 events, and then retrieved the fault slip distributions of the major earthquakes using Interferometric Synthetic Aperture Radar (InSAR) images. The geodetic inversion reveals that EQ1, EQ2, and EQ4 ruptured the NW-trending M’Lang and Makilala-Maulungoon faults, while EQ3 ruptured the NE-trending Makilala fault. Near orthogonal (88°-93°) nature between the NW-trending faults (EQ1/EQ4) and the NE-trending fault (EQ3) can be explained by the rotation of the conjugate faults due to simple shear since ~7 Myr. We find that the stepover widths between the near-parallel faults associated with EQ1, EQ2, and EQ4 may have limited the dynamic triggering of EQ2 and EQ4. Coulomb stress transfer models suggest that the coseismic slip of EQ1, EQ2, and EQs 1+2 could have triggered EQ2, EQ3, and EQ4, respectively. Fault orientation rotation modelling reveals the fault starting the near-orthogonal cascading sequence is the one accommodating the majority of the rotation, possibly because of the instability associated with rotations. Our study suggests that the shallow segments of the M’Lang and Makilala-Maulungoon faults did not fail, and that these remain a potential seismic hazard.