The October 15 2013 Bohol Island Philippines earth quake and its damaging effects An investigative report Lessandro Estelito GARCIANO1 and Patrick TACLIBON2 1Associate Professor Department of Civil Engineering De La Salle University 2401 Taft Avenue Malate Manila 1004 Philippines E mail lessandro garciano dlsu edu ph 2Graduate Student Department of Civil Engineering De La Salle University 2401 Taft Avenue Malate Manila 1004 Philippines E mail patrick taclibon dlsu edu ph Key Facts Hazard Type Earthquake Date of the disaster October 15 2013 Location of the survey Bohol Island Philippines Date of the field survey October 26 and 27 November 4 and 5 2013 Survey tools digital cameras and measuring tapes Key findings 1 The epicenter of the earthquake was approximately 6 km S24W of Sagbayan Bohol with a focal depth of 12 km The 7 2 magnitude earthquake was of tectonic in origin that was generated from a reverse thrust movement of a previously uncharted faul line This new fault line is now named as the North Bohol Fault Line 2 Damage from this earthquake was significant and widespread that included roads and bridges schools and hospitals government and private buildings houses seaports and airports 3 Landslides liquefaction sinkholes and lateral ground fissures where observed in many 4 The strong lateral movement also damaged the nation s cultural heritage structures such as centuries old churches and watchtowers places Key Words earthquake liquefaction cultural heritage structures damage to infrastructures 1 INTRODUCTION Bohol is an island located south of Manila Phil ippines On October 15 2013 at approximately 08 12 PST an earthquake with a magnitude of 7 2 vigorously shook this island and its neighboring islands e g Cebu Negros and Surigao see Fig 1 According to the Philippine Institute of Volcanology and Seismology PHILVOLCS the epicenter 9 86oN 124 07oE of the earthquake was approxi mately 6 km S24oW of Sagbayan Bohol with a focal depth of about 12 km1 The earthquake was caused by a reverse thrust movement of a previously unidentified fault see white dashed line in Fig 2 and geologists are at present mapping the entire fault trace The solid blue line in this figure is the trace of the East Bohol Fig 1 Bohol Island Philippines 1 fault line This earthquake registered a PEIS Phil ippine Earthquake Instensity Scale of VIII that is characteristic of a very destructive earthquake with the presence of liquefaction lateral spreading and considerably damaged buildings and bridges Fig 2 Epicenter and aftershocks source PHILVOLCS According to a report by the National Disaster Risk Reduction and Management Council NDRRMC the number of fatalities reached 222 as of November 3 2013 Furthermore the number of persons reported to have sustained substantial inju ries is more than three times this number In fact out of the approximately 1 2 million residents of Bohol island 74 907 familes representing more than 30 of the population were displaced by this earthquake This report presents the findings gathered by the authors during a four day survey augmented by re ports from different sources The paper is divided into five chapters starting with the introduction with Chapter 2 briefly ex plaining the geologic and tectonic setting of Bohol Island Chapter 3 expounds on the damage caused by the earthquake and Chapter 4 summarizes the findings 2 GEOLOGICAL AND TECTONIC SETTING 1 Geological and tectonic setting Bohol Island is the 10th largest island in the Phil ippine archipelago and is located in the Central Vi sayas Region This island has an approximate land area of 4 117 26 km2 and houses a population of around 260 persons per square kilometer in 47 mu nicipalities2 The island of Bohol and most of the Philippines is contained in the Philippine Mobile Belt PMB which is a zone of deformation and active seismicity that accommodates by large the stresses caused by the northwestward movement of the Philippine Sea Plate This zone is bounded in the west by east dipping subduction zones in the Manila Sulu Negros and Cotobato trenches and in the east by west dipping subduction zones mainly following the outline of the Philippine trench In addition within the PMB lies the left lateral Philippine fault that is roughly 1400 km in length and spans almost the whole of the country Lastly southwest of the PMB located is the aseismic Palawan Mindoro block3 see Fig 3 Fig 3 Overview of the Philippine fault system 2 Fault setting The earthquake in Bohol was tectonic in origin and exhibited a reverse thrust movement as observed during the field observations The earthquake was generated by a new fault that now bears the name North Bohol fault1 According to an interview with the PHIVOLCS through a local news media the government agency found it hard to recognize the formation of the new fault despite possible signs that emerged in the earthquake back in 1996 due to the prevailing lime stone structure of the island that easily wears off the feign structures features that might have suggested the materialization of the new fault4 3 Main shock and aftershocks As of November 3 2013 more than 3198 after shocks have been recorded by PHILVOLCS with 94 of these felt by the general population2 2 Fig 5 tilting of two residential houses due to column failure 2 Damage to bridges A total of 41 bridges were also reported damaged The two spans of Abatan bridge which connects the Municipalities of Cortes and Tagbilaran Fig 6 fell to the river The abutment tilted towards the em bankment causing the spans to fall off Damaged to reinforced concrete piles were also observed In Catigbian a bailey bridge with wooden deck swayed to one side while the other end fell into the river Fig 7 3 DAMAGE TO STRUCTURES This recent earthquake was one of the strongest in Philippine history The damage was widespread it affected houses bridges schools roads highways public and private building flood control structures seaports airports and hospitals According to NDRRMC site report as many as 222 persons lost their lives 976 were injured and 8 persons are still unaccounted What was heart breaking also was the the damage to our country s heritage structures such as Spanish era churches and watchtowers Landslides sink holes ground fissures and liquefaction were also reported in many areas The following sub chapters are the damage ob served during the site inspection and assessment 1 Damage to houses Damage to houses was enormous and affected dwellings made of