Abstract
The 1994 Mw 7.1 Mindoro Earthquake and the 2017 Mw 5.9 Batangas Earthquake Sequence both occurred in offshore southern Batangas and devastated southern Luzon and Mindoro. These earthquakes exhibited NW-striking right-lateral slip in an area presumably defined by a WNW-striking left-lateral fault, therefore implying the existence of previously unmapped offshore faults. High resolution multibeam bathymetry grid and subbottom profiles revealed a conjugate strike-slip fault system under an approximately EW-directed extension. NW-striking right-lateral faults (F1 Faults: Central Mindoro Fault, Aglubang River Fault, and Batangas Bay Fault System) bound the western part of the study area. On the other hand, a series of almost parallel NE-trending left-lateral and normal faults (F2 Faults: Macolod Corridor, North Verde Fault System, Central Verde Fault System, South Verde Fault, and Northeast Mindoro Fault System) approach the F1 faults from the northeast. The distribution of the 1994 and 2017 earthquakes suggests that the possible rupture areas for these events are the Aglubang River Fault and the southwest Batangas Bay Fault System, respectively. These two traces appear to be connected and a restraining bend is suggested to have acted as a rupture barrier between the two events. Coulomb stress transfer modeling showed that the 1994 earthquake promoted the failure of the 2017 earthquake. Furthermore, results from the stress transfer models showed stress increase on the F1 faults (Batangas Bay Fault System and Central Mindoro Fault) and the northern F2 faults (North Verde Fault System and Central Verde Fault System). The newly recognized faults redefine the knowledge of the neotectonic structure of the area but are still consistent with the ongoing east-west extension in southern Luzon and the overall extension in northern Central Philippines. These faults pose seismic hazards, and more studies are needed to determine their seismogenic potential.
Highlights
On November 11, 1994, 19:15 GMT, an Mw 7.1 earthquake struck south off Verde Island
The patterns define conjugate fault systems forming under approximately EW-directed extension
The northwestern distribution of the 2017 aftershocks was limited by the Calumpan Peninsula
Summary
An onshore ~35 km-long rupture with right-lateral slip was observed along the Aglubang River Fault in northeast Mindoro. It was accompanied by a tsunami with a maximum vertical runup of 8.5 m (PHIVOLCS Quick Response Team, 1994). The focal mechanism solution of the mainshock exhibits an NNWtrending right-lateral fault which agrees with the observed onshore rupture. Almost 23 years later, on April 8, 2017 at 07:09 GMT, an Mw 5.9 earthquake event occurred in Batangas Bay, just around 30 km to the northwest of the 1994 mainshock. Focal mechanism solution correlated to the northwest trend of the relocated events reveals a northwesttrending right-lateral fault (Chen et al, 2020). The only known offshore fault is the WNW-trending Verde Passage Fault which moves in a left-lateral motion
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