GEOLOGICAL SOCIETY OF THE PHILIPPINES

 

Geologic Impacts of the 15 October 2013 Mw 7.2 North Bohol Fault Earthquake,
Bohol Island, Philippines

Teresito Bacolcol, Mabelline Cahulogan, Baby Jane Punongbayan, Nolan Evangelista, Vilma Hernandez, 
Ericson Bariso, Cathy Pogay, Charmaine Villanueva, Princess Decierdo, Ma. Isabel Abigania, 
Jeffrey Perez, Kathleen Papiona, Lucille Rose Del Monte, Danikko John Rivera, Robjuneliaaa Lim, 
Thessa Quilalang, Milo Tabigue and Leni Torrevillas

Philippine Institute of Volcanology and Seismology
CP Garcia Ave., UP Diliman Campus
Quezon City

Abstract

A M7.2 earthquake struck Bohol Island, Central Philippines, at 8:12:31 am local time on 15 October 2013. Using the data from its National Seismic Network, the Philippine Institute of Volcanology and Seismology (PHIVOLCS) determined the location at 124.070°E and 9.859°N and a focal depth of 12 kilometers. This earthquake generated ground-shaking of Intensities VII (destructive) to VIII (very destructive) in Bohol and the neighboring Cebu and Negros islands and felt intensities within as far.as about 350 km radius from the epicentral region. The PHIVOLCS Quick Response Team (QRT) documented and mapped 6 km of surface rupture that spans from Brgy. New Anonang in Buenavista to Brgy. Napo in Inabanga. The surface rupture would indicate reverse faulting, with strike and dip readings measured at N40-60E and 50SE, respectively. The maximum vertical displacement was measured at 5 meters. Aftershocks distribution and SAR image analysis however showed that the fault would likely to extend towards Loon, on the SW coast. Severe liquefaction, manifested by extreme ground subsidence and fissuring, lateral spreading, buckling and sandboil formation were documented along the SW coastal towns particularly in Loon. Considerable to severe damage to infrastructures and houses brought about by strong ground shaking were also reported and observed in the north- and south-western sector municipalities of Bohol, including Maribojoc, Loon, Tubigon, Calape, Clarin, Inabanga, Buenavista, Danao, Sagbayan, Catigbian, San Isidro, Antequerra, Balilihan and Cortes. Damage to houses and infrastructures were also observed in Tagbilaran and in the southern municipalities of Loboc, Carmen, Lila, Valencia, Albuquerque, Baclayon and Loay. The intense ground shaking also produced sinkholes and minor rock falls to deep-seated landslides in limestone areas and mountains including areas around Maribojoc, Sagbayan, Catigbian and San Isidro. Only minor damage to infrastructures and houses were observed on the towns located in the eastern sector of Bohol. Uplift was also observed in the SW coastlines, with maximum vertical uplift of 1.8 m measured in Brgy. Guiwanon in Loon. As of 15 November 2013, 3567 aftershocks, of which 104 were felt, have been recorded.


 
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