2022 Luzon Earthquake

On July 27, 2022, at 8:43:24 a.m. ( PHT), an earthquake struck the island of Luzon in the Philippines. The earthquake had a magnitude of 7.0 , with an epicenter in Abra province. Eleven people were reported dead and 615 were injured. At least 35,798 homes, schools and other buildings were damaged or destroyed, resulting in ₱ (US$) worth of damage.

Earthquake

The tectonics of the northern Philippines and around the island of Luzon are complex. Luzon is bounded to the east and west by subduction zones. In the southern part of Luzon, the subduction zone is located east of the island along the Philippine Trench, where the Philippine Sea Plate subducts westward beneath the Sunda Plate. In northern Luzon, where the July 27 earthquake occurred, the subduction zone location and direction changes, with another trench (Manila Trench) located west of Luzon and the Sunda Plate subducts eastward beneath the Philippine Sea Plate. The complexity of plate tectonics on and around Luzon is evidenced by the diversity of faulting mechanisms in large earthquakes. Magnitude 7 or greater earthquakes in this region since 1970 have exhibited reverse, normal, and strike-slip faulting. These active plate boundaries lead to high seismic activity. Since 1970, 11 other earthquakes of magnitude 6.5 or larger have occurred within 250 km of the July 27, 2022, earthquake. The largest of these earthquakes was a magnitude 7.7 strike-slip earthquake on July 16, 1990, located approximately 215 km south of the July 27 earthquake. The 1990 earthquake killed at least 1,600 people and injured more than 3,000 people. The 1990 earthquake also caused landslides, liquefaction, subsidence, and sand boils in the Baguio–Cabanatuan–Dagupan area.

Characteristics

The earthquake occurred as a result of oblique-reverse faulting. Preliminary analysis indicates that the earthquake occurred on either of a low-angle reverse fault dipping to the east with a small component of left-lateral (strike-slip) motion, or on a steeply dipping reverse fault dipping to the northwest with a small component of right-lateral motion. The earthquake depth, mechanism, and location are consistent with the earthquake having occurred in the Philippine Sea Plate above the Sunda Plate. The Sunda Plate subducts eastward beneath Luzon with the plate boundary located off the western coast of Luzon. According to the USGS, the seismic moment released was 5.4e+19 N-m, corresponding to a moment magnitude of 7.1 (). A finite fault obtained from seismic inversion suggest rupture occurred along a east-dipping thrust fault, and produced a maximum displacement of .

It was reported as 7.3 by the Philippine Institute of Volcanology and Seismology (PHIVOLCS). The report was later revised to an earthquake of 7.0 with the epicenter N 25° W of Tayum, Abra at a depth of .
Despite being a relatively shallow event, no surface ruptures were identified. A focal mechanism analysis indicate rupture on a reverse fault with either a northeast–southwest striking, steep dipping (69°) plane or a north–south, shallow dipping (8°) plane.

PHIVOLCS initially suspected the earthquake may have occurred on the Abra River Fault. Geologists had been aware of the potential for large earthquakes on the fault. The last known earthquake on the fault was in 1868, measuring 4.0–5.0 in magnitude. The Abra River Fault is a northern extension of the Philippine Fault Zone. As it runs through Luzon, it splays into three branches that runs beneath the southern Sierra Madre mountains, the Central Cordillera Mountains before terminating at the northern coast of the island. The three branches strike north-south and are named (west to east) the Vigan-Agao Fault, the Abra River Fault and the Digdig Fault, respectively. The Philippine Fault Zone is associated with Pliocene

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