AMF Lagmay (a,b), RN Eco (a,b), R Ybanez (a,b)
(a) Nationwide Operational Assessment of Hazards, Department of Science and Technology, Philippines
(b) National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City
Early morning at 8:12 AM of 15 October 2013, a Magnitude 7.2 earthquake struck Bohol Island in the Central Philippines region of Visayas (Figure 1). The earthquake’s focus was at 12 kilometers depth with its epicenter located at 9.86 latitude and 124.07 longitude, 6 kilometers southwest of Sagbayan, Bohol (Phivolcs, 2013). Initial and preliminary reports by the USGS sent a few minutes after the earthquake, had the temblor pegged at Magnitude 7.2 occurring at 8:12 AM (local time) with the epicenter 2 km South of Nueva Vida Sur, Bohol, Philippines. The USGS record on the earthquake was later revised to Magnitude 7.1 with focal depth of 20 kilometers. Based on preliminary reports on the earthquake intensity, ground shaking was highest at Intensity VII (Philippine Earthquake Intensity Scale or PEIS), felt at Tagbilaran, Bohol. In other places, ground shaking effects registered intensity VI in Hinigaran, Negros Occidental, Intensity V in Iloilo City and La Carlota, Intensity IV in Masbate City, Roxas City, San Jose, Culasi, Antique, Guihulngan, Negros Oriental and Intensity III in Davao City (Phivolcs, 2013). Later reports coming from near the epicenter of the quake show images of centuries-old churches flattened to the ground and collapsed buildings giving rise to intensities measuring VIII or higher (Figure 2). To date (26 October 2013), the National Disaster Coordinating Council (NDRRMC) has reported 213 fatalities, 742 injured and 8 missing. A total of 56,933 houses were damaged, 44,430 of which were totally destroyed, with 35,622 houses partially affected. Out of the 41 bridges affected by the earthquake, 3 are not passable while 2 out of the 18 roads damaged are impassable (NDRRMC, 2013a). The total estimated cost of the damage to infrastructure is 1,644,230,000 Philippine Pesos (US\$38.21 million). Landslide occurrences were reported in 25 villages in Bohol and 13 villages in Cebu (NDRRMC, 2013a) with reports of sinkholes and landslides damming rivers raising fears of possible floods.
As of 6:00 PM, October 26, 2013, 2779 aftershocks have been recorded by the Philippine Institute of Volcanology and Seismology, 75 of which were felt (NDRRMC, 2013a). The earthquake epicenters plot in a northeast-southwest trend spanning approximately 100 kilometres from mainly inland to offshore areas southwest of Bohol Island (Figure 1a). Focal mechanism solutions generated by the Global Centroid Moment Tensor (CMT) project, which depict the type of slip movement of the earthquake fault, show a reverse fault (dominant vertical motion) with a slight strike-slip (lateral fault-slip motion) component for the main shock (Figure 3). Two significantly large aftershocks recorded at 4:36 PM (local time) on the day of the main shock had reverse and strike-slip focal mechanisms, respectively. The Ms 5.9 (Mw 5.7) was a reverse fault-related aftershock while the Ms 5.5 (Mw 5.6) earthquake was related to strike-slip fault movement based on their focal mechanism solutions (Figure 3).
The only mapped active fault in Bohol Island is the East Bohol Fault (Phivolcs, 2000). It was originally believed to be the source of the M7.2 earthquake but it would appear based on the locus of earthquake epicenters and initial field reports that an unmapped fault or faults about 20-25 kilometres north of the East Bohol Fault were responsible for the M7.2 earthquake and most of the aftershocks (Figure 1a). Lineament analysis of digital topography show northeast-southwest-trending structures cutting across the northern portion of Bohol Island, passing through several municipalities of Bohol in the northern portion of the island. In the village of Anonang, municipality of Inabanga, where there were several aftershocks recorded, displacement of originally gently sloping to flat ground formed a northeast- to southwest-trending wall as much as 3 meters high and extends more than five kilometers long (Figure 4). The length measurement of the raised wall due to reverse faulting is currently being determined in the field and through lineament mapping using high-resolution imagery.
In the worldwide database of earthquakes, a 3 meter displacement of a fault approximately corresponds to a magnitude 7.2 earthquake (Figure 5; Wells and Coppersmith, 1994). Based on this, the fault seen very well exposed in Barangay Anonang, Inabanga is most likely to have been responsible for the magnitude 7.2 earthquake in Bohol. This earthquake triggered more than 2500 aftershocks. But the trend of the epicenters of these earthquakes which span more than 100 kilometers and reach nearly the southern part of Cebu island, do not follow exactly the orientation of the fault trace found in Inabanga municipality (see Figure 1). The reverse fault in Inabanga also does not account for the strike-slip focal mechanism solution of an earthquake recorded in the afternoon of the devastating Bohol event (see figure 3). There are other lineament structures found in the northern part of Bohol and they could correspond to a fault system defined by the large number of earthquakes triggered by the main shock. These too must be mapped out in detail when possible.
The nomenclature of newly discovered faults or geological formations for that matter, are normally based on where they are best exposed and described. Because Inabanga municipality is where the reverse fault was first seen and archetypal for the fault that generated the fatal M7.2 earthquake of Bohol, we propose to name the fault, the Inabanga Fault. This also would prevent complications in the future should there be any other active faults that will be discovered and mapped in the large area affected by the temblor, north of Bohol.
3. Past earthquakes
The East Bohol Fault was responsible for generating the M6.8 Bohol earthquake on 8 February 1990, which generated tsunamis as high as 2 meters and significant inundation in the southeastern coast of the island. The epicenter of the 1990 Bohol earthquake was located 17 kilometers east of Tagbilaran City with intensities reaching VIII on the Rossi-Forrel scale in the town of Jagna, Duero and Guindulman in Bohol (Umbal et al., 1990). The temblor exacted six fatalities with more than 200 injured. Approximately 46,000 people were displaced with at least 7,000 rendered homeless. Damage to property for the 1990 Bohol earthquake was estimated at 154 million pesos. Another Magnitude 5.6 earthquake struck on 27 May 1996 and was centered in the municipality of Clarin, Bohol. Damage brought by the 1996 earthquake was confined to poorly built structures and/or old wooden, masonry, limestone walls of houses and buildings, generally due to ground shaking. There were no reports and observations attributed to other earthquake hazards such as liquefaction, ground subsidence, landslide and any other geologic ground disturbances (Phivolcs, 1996a).
4. Tectonic Framework of the Philippines
The Philippine archipelago is a mature island arc that is at present being accreted to the eastern margin of the Eurasian Plate (Ben-Avraham, 1978;Hamilton, 1979; Holloway, 1982; Karig, 1983; McCabe et al., 1982; Pubellier et al., 1996; Rangin et al., 1985). It is composed of a complex mixture of terranes (Hamilton, 1979; Geary et al., 1988; Encarnacion, 2004) formed through plate interaction of the Philippine Sea Plate, Eurasian Plate and the Indo-Australian Plate (Figure 6). The entire archipelago is characterized by a system of subduction zones, collision zones, and oblique slip faults. The actively deforming region of the Philippines is a zone known as the Philippine Mobile Belt or PMB (Figure 6; Gervasio,1967), flanked on both sides by subduction zones. West-dipping subduction zones are the East Luzon Trough and the Philippine Trench. Generally east-dipping subduction zones are the Negros Trench, Sulu Trench, and Cotabato Trench. In between these west- and east-dipping subduction zones is the left-lateral Philippine Fault (Allen, 1962; Barrier et al., 1991; Aurelio et al., 1991), which straddles the entire length of the PMB. There are many active faults in the archipelago but the closest active faults in the Bohol region, are the Cebu lineaments, Central Negros Fault, West Panay Fault and the Central Mindanao fault. The tectonic structures in the Philippines accommodate the stress imparted by the ongoing northwest movement of the Philippine Sea Plate towards Eurasia (Fitch, 1972; Karig, 1975; Ranken et al., 1984; Huchon, 1986). Southwest of the PMB is the Palawan-Mindoro block, an aseismic region of the Philippines of continental affinity (Hamilton, 1979; Holloway, 1982). A summary of the major tectonic structures bordering the Philippines is presented in Table 1 with their corresponding ages of formation (Table 1).
The trenches in the east and west of the archipelago are major sites of seismicity and where marginal basins surrounding the Philippines are consumed. The South China Sea (Pautot et al., 1986), Sulu Sea and the Celebes Sea Basins (Rangin and Silver, 1991) subduct along the east-dipping Manila (Cardwell et al., 1980; Hamburger et al., 1983), Negros (Rangin et al., 1988) and Cotabato trenches (Schluter et al., 2001), respectively. The Sulu Trench is the locus where the Sulu Sea is consumed (Hinz and Block, 1991) while the Philippine trench is the site where the Philippine Sea Plate subducts (Karig, 1975).
The Bohol earthquake is a devastating event that emanates from one of the many faults that straddle the Philippine islands. There are many active faults that have been mapped and are potential sites for devastating earthquakes. Many active faults listed by Phivolcs are near urban centers, populated by millions of people. Metro Cebu, which is the second most populous Metropolitan area in the Philippines after Metro Manila, came out relatively unscathed with only 13 deaths compared to the 199 fatalities in Bohol Island where the earthquake emanated. A year and a half earlier, on 6 February 2012, a shallow focus Mb 6.9 earthquake generated by an unmapped thrust fault and referred to as a hidden fault, struck Negros Island immediately west of Cebu. That earthquake caused considerable damage killing 51 people with 62 missing and presumed dead (NDRRMC, 2013b). The most recent fatal temblor is the 2013 Bohol earthquake, which released underground, tremendous amount of energy equivalent to more than 30 Hiroshima atomic bombs. That amount of energy, which can trigger an earthquake, can only be built up again on the same fault system over a long period of time, perhaps a hundred years or more. In the meantime, the reverse fault found and best seen in Inabangan, Bohol, in most probability, will be quiet and will not pose imminent danger from earthquake hazards except for landslides and further destabilization of already weakened infrastructure triggered by aftershocks, strong winds and heavy rainfall. These recent events are a wake up call for all residents of the Philippines to brace for possible earthquakes that may strike the country elsewhere, anytime.
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