Newly discovered fault line may have been source of M 7.2 Bohol earthquake


The Philippine Institute of Volcanology and Seismology (PHIVOLCS) stated that a newly discovered fault may have been the source of the M 7.2 earthquake that rocked Central Visayas, Philippines on October 15, 2013. State seismologists said the earthquake's was the equivalent of 32 Hiroshima atom bombs. By October 16, 2013, 885 earthquakes have been recorded by the PHIVOLCS seismic monitoring network.

At least 15 events were reportedly felt in the epicentral area. The main shock and succeeding aftershocks were located in the vicinity of Bohol. These recorded events were shallow, with a depth of at most 32 kilometers. Based on spatial distribution of succeeding events and characteristics of the earthquake, the event is tectonic in origin.

October 15, 2013 M7.2 earthquake map (Credit: PHIVOLCS/SOEPD)

The current seismic trend indicates that the magnitude 7.2 earthquake on October 15,2013 is the main shock, and the succeeding small magnitude earthquakes are the aftershocks. Aftershocks are expected, some of which will be felt. These may continue for weeks to months, but diminishing in number and strength as time passes. In this case, a higher magnitude earthquake related to this event is no longer expected to occur.

Regional active fault systems and trenches (Credit: PHIVOLCS)

After closer examination of data, seismologists found that quake's real epicenter is located between the municipality of Catigbian and Sagbayan in Bohol and not in the town of Carmen as the agency earlier stated. New re-examined data suggests that the source of the earthquake was probably a different fault system that transects Bohol island. Bohol Island is one of the seismically active areas in the country. Instrumental monitoring of earthquakes for the past century has detected many small to moderate-magnitude earthquakes in Bohol Island.

There is at least one known earthquake generator on the island, the East Bohol Fault. In addition, there are other local faults which can be sources of small to large magnitude earthquakes. Earthquakes can also occur offshore or undersea because of local offshore faults near the island or trenches in the vicinity of the region.

Preliminary Finite Fault  results for the October 15, 2013 M 7.2 5 km E of Balilihan, Philippines earthquake. Surface projection of the slip distribution superimposed on GEBCO bathymetry. Red lines indicate major plate boundaries. Gray circles, if present, are aftershock locations, sized by magnitude.(Credit: USGS/Bird/NEIC/GEBCO)

The Philippine Islands straddle a region of complex tectonics at the intersection of three major tectonic plates (the Philippine Sea, Sunda and Eurasia plates). As such, the islands are familiar with large and damaging earthquakes. The province's limestone structure made identifying active faults difficult to identify, despite the occurrence of an earthquake near the new fault line in 1996.

On February 8, 1990, a magnitude 6.8 earthquake occurred at same area generated by an offshore reverse fault east of the Bohol island. The Philippines' comprehensive geohazard maps from 2007 already suggested the possibility that a strong earthquake might occur in the province. 

Seismotectonics of the Philippine Sea (Credit: USGS)

Meanwhile, the National Disaster Risk Reduction and Management Council (NDRRMC) raised the death toll to 161 as of October 17, 2013. The infrastructural damage have reached PHP 563 million (around 12 million USD). You can also check up for updates with quake analysis at Earthquake-Report, with excellent videos, images and maps.

Feature image: USGS shake intensity map (Credit: USGS)

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