The spatial and temporal features of earthquakes in the highly-active region of northern Mindoro, Philippines, exhibit unusual properties, including possible interaction of several tectonic elements that gives rise to a very intense, occasionally-destructive and temporally-clustered seismicity. The site is host to the most intense seismicity in the country with a large number of moderate and large magnitude earthquakes being regularly recorded. The latest destructive earthquake caused rupture on a formerly-unmapped fault along Aglubang River and led to tsunami waves and heavy casualty on the coastal communities of northern Mindoro.

Seismicity maps show the dense earthquakes in northern Mindoro to be possibly caused by the interaction of several structures. In cross sections, the seismicity in northern Mindoro appears to terminate at only 200 km depth, although the geometry of a Benioff Zone is clouded by an unusual vertical column that lead up to near Taal Volcano on the surface. Manila Trench curves eastward into Verde Island Passage where the trench is pinched out and apparently displaced southward into the thrust zones of middle and southern Mindoro. A north-east trending fault cuts the northwest corner of the island near Abra de Ilog, along a structure that appears to be intermittently active, A large fault had been mapped on the western edge of the Calapan-Naujan flatlands, although the rupture caused by the 1994 earthquake indicates that the threat of tectonic movement is likely shifting from this large structure to other younger structures. To the east, the Sibuyan Sea Fault approaches northern Mindoro, but appears to either veer north into the southern part of the Luzon’s Sierra Madre, or north and west into Taal Volcano, or even possibly due west where it may blend into the Lubang Fault.

The recent earthquakes suggest sustained interaction of all the structures in northern Mindoro, with all of the seismogenic features being intermittently active at all times. This may indicate that the whole region is responding as one volume to tectonic stresses, with strain in each structure leading to stress relief in the others. Likewise, the earthquakes occur as clusters in time, separated by periods of reduced activity. Each temporal cluster of earthquakes starts with moderate magnitude events whose hypocenters occur at various depths. The range of magnitude and position then spreads, culminating in a period of intense seismicity over a large part of the region which is then capped by a larger-than-average event. A period of lower-than-average seismicity then ensues until the next sequence starts. Such a pattern was followed by the Verde Island earthquakes of 1942 and 1994, and the Abra de Ilog earthquake of 1972. If only the Verde Island events are considered, then a recurrence interval of 52 years between large events is inferred. However, if the deformation in this region is indeed volumetric rather than planar, and that all the faults and tectonic structures are interacting with one another, the Abra de Ilog event of 1972 then becomes a likely part of the sequence. In this case, the recurrence interval between large events for the whole region is from 22 to 30 years.