Deep-Focus Seismicity in Southwestern Mindanao: The 2005 Palimbang and 2024 Sangay M7.1 Events
The Philippine archipelago lies within the tectonically active Pacific Ring of Fire, where multiple subduction zones generate frequent earthquakes. Southwestern Mindanao, near Palimbang and Sangay, sits above the Philippine Trench system and the subducting Philippine Sea Plate. This setting produces both shallow crustal events and deep-focus earthquakes that occur within the downgoing slab at depths exceeding 300 km.
On 5 February 2005 at 12:23 local time, a magnitude 7.1 earthquake struck 138 km southwest of Palimbang at a focal depth of 525 km. Such extreme depth indicates rupture within the cold interior of the subducted slab, where brittle failure can still occur despite high confining pressures. Deep events of this type often produce minimal surface shaking relative to their magnitude because seismic energy dissipates over the long travel path through the mantle.
A second magnitude 7.1 earthquake occurred on 11 July 2024, located 106 km west-southwest of Sangay and approximately 90 km from the 2005 hypocenter. Both events share the same magnitude and occurred within the same subduction-related volume, highlighting persistent seismic productivity along this segment of the plate boundary since at least 2000.
The region’s seismic history reflects ongoing convergence between the Philippine Sea Plate and the Philippine Mobile Belt. Intermediate-depth and deep earthquakes cluster along the subducted slab’s trajectory, while shallower activity is concentrated along the trench and overlying crustal faults. The 2005 and 2024 events illustrate that large-magnitude ruptures can occur at widely differing depths within the same tectonic corridor, underscoring the three-dimensional complexity of Philippine subduction.
Continued monitoring by the Philippine Institute of Volcanology and Seismology remains essential for characterizing slab geometry and assessing potential interactions between deep and shallow seismicity in this densely populated portion of Mindanao.