See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/311681886 Seismotectonics of the 6 February 2012 Mw 6.7 Negros Earthquake, central Philippines Article · December 2016 DOI: 10.1016/j.jseaes.2016.12.018 CITATIONS READS 2 1,017 7 authors, including: Mario A. Aurelio John Dale Dianala University of the Philippines University of Oxford 39 PUBLICATIONS 758 CITATIONS 5 PUBLICATIONS 2 CITATIONS SEE PROFILE SEE PROFILE All content following this page was uploaded by Mario A. Aurelio on 24 March 2018. The user has requested enhancement of the downloaded file. Journal of Asian Earth Sciences 142 (2017) 93–108 Contents lists available at ScienceDirect Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes Full length Article Seismotectonics of the 6 February 2012 Mw 6.7 Negros Earthquake, central Philippines ⇑ M.A. Aurelio a, , J.D.B. Dianala a, K.J.L. Taguibao a,1, L.R. Pastoriza b,2, K. Reyes b, R. Sarande c, A. Lucero Jr. d a National Institute of Geological Sciences, Velasquez St. corner C.P. Garcia Avenue, National Science Complex, University of the Philippines, Diliman, Quezon City 1101, Philippines b Energy Development Corporation, Julia Vargas Ave. Cor. Meralco Ave., Ortigas Center, Pasig City, Philippines c Philex Mining Corporation (Negros Exploration Group), Brixton St., Pasig City, Philippines d Mines and Geosciences Bureau, Regional Office No. 7, Mandaue City, Philippines article info abstract Article history: At 03:49 UTC on the 6th of February 2012, Negros Island in the Visayan region of central Philippines was Received 27 February 2016 struck by a magnitude Mw 6.7 earthquake causing deaths of over 50 people and tremendous infrastruc- Received in revised form 10 December 2016 ture damage leaving hundreds of families homeless. The epicenter was located in the vicinity of the east- Accepted 10 December 2016 ern coastal towns of La Libertad and Tayasan of the Province of Negros Oriental. Earthquake-induced Available online 16 December 2016 surface deformation was mostly in the form of landslides, liquefaction, ground settlement, subsidence and lateral spread. There were no clear indications of a fault surface rupture. The earthquake was trig- Keywords: gered by a fault that has not been previously recognized. Earthquake data, including epicentral and Mw 6.7 February 2012 Negros Earthquake hypocentral distributions of main shock and aftershocks, and focal mechanism solutions of the main Negros Oriental Thrust Fold-thrust systems shock and major aftershocks, indicate a northeast striking, northwest dipping nodal plane with a reverse Basin inversion fault mechanism. Offshore seismic profiles in the Tañon Strait between the islands of Negros and Cebu Low strain rate show a northwest dipping reverse fault consistent in location, geometry and mechanism with the nodal Seismic hazard assessment plane calculated from earthquake data. The earthquake generator is here proposed to be named the Negros Oriental Thrust (NOT). Geologic transects established from structural traverses across the earth- quake region reveal an east-verging fold-thrust system. In the latitude of Guihulngan, this fold-thrust sys- tem is represented by the Razor Back Anticline – Negros Oriental Thrust pair, and by the Pamplona Anticline – Yupisan Thrust pair in the latitude of Dumaguete to the south. Together, these active fold- thrust systems are causing active deformation of the western section of the Visayan Sea Basin under a compressional tectonic regime. This finding contradicts previous tectonic models that interpret the Tañon Strait as a graben, bounded on both sides by normal faults supposedly operating under an exten- sional regime. The Negros Earthquake and the active fold-thrust systems that were discovered in the course of the structural analysis provide strong arguments for basin inversion processes now affecting the Visayan Sea Basin, albeit under very slow strain rates derived from previous GPS campaigns. The occurrence of the earthquake in an area where no active faults have been previously recognized and char- acterized by slow present-day strain rates underscores the necessity of paying more attention to and exerting more effort in the evaluation of earthquake hazards of regions that are seemingly seismically quiet, especially when they underlie highly urbanized areas. Ó 2016 Published by Elsevier Ltd. 1. Introduction 2012, causing extensive damage to infrastructure and killing at least 50 people living in towns settled on the eastern coast of the The island of Negros in the central Philippines was struck by a island. Initial calculations by several earthquake monitoring agen- magnitude Mw 6.7 earthquake towards midday of 6 February cies including the Philippine Institute of Volcanology and Seismol- ogy (PHIVOLCS), the National Earthquake Information Center of the ⇑ Corresponding author. United States Geological Survey (NEIC-USGS) and the National E-mail address: [email protected] (M.A. Aurelio). Research Institute for Earth Science and Disaster Resilience (NIED) 1 Department of Earth Science and Technology, Akita University, 1-1 Tegata of Japan immediately after the earthquake located the epicenter a Gakuen-machi, Akita City 010-8502, Japan. few kilometres east of the coast of central Negros, while the depth 2 Department of Earth Sciences, Sciences Labs, University of Durham, Durham DH1 of focus was estimated at around 10 km. Although the initial focal 3LE, United Kingdom. http://dx.doi.org/10.1016/j.jseaes.2016.12.018 1367-9120/Ó 2016 Published by Elsevier Ltd. 94 M.A. Aurelio et al. / Journal of Asian Earth Sciences 142 (2017) 93–108 mechanism solution (FMS) indicated reverse faulting, its shallow Cenozoic sedimentary packages of the Visayan Sea Basin (Bureau hypocentral depth and the large epicentral distance from the axis of Energy Development, 1986; Porth et al., 1989), that also under- of the Negros Trench given the known subduction angle lies the neighboring islands of Cebu and Bohol. The rest of the (Cardwell et al., 1980; Aurelio, 2000a), did not argue for a island is composed of an active volcanic arc superimposed over subduction-related earthquake that would have been generated older volcanic arcs mostly dominated by metalliferous late Oligo- by this trench, an east dipping subduction zone located about cene to Pliocene intrusions and their equivalent extrusives 100 km west of the island. Quick Response Teams (QRTs), led by (Aurelio and Peña, 2010)(Fig. 2). This active volcanic arc, repre- government institutions that included the Philippine Institute of sented by 3 andesitic strato volcanoes from north to south by Mt. Volcanology and Seismology (PHIVOLCS), the Lands Geological Sur- Mandalgan, Mt. Kanlaon and Mt. Cuernos de Negros, is associated vey Division of the Mines and Geosciences Bureau (MGB-LGSD) and to the east-dipping Negros Trench which is the closest known the National Institute of Geological Sciences of the University of the active subduction zone located about 100 km to the west. Prior Philippines (UP-NIGS) deployed immediately to the ground mainly to the earthquake, there were no known active faults traversing to assess damage, but also to conduct immediate co-seismic ground the island, based on official active fault maps (PHIVOLCS, 2000). deformation, mostly noted the absence of a clear fault surface rup- Otherwise, faults have been inferred to exist in certain locations ture. Prior to this earthquake, existing maps in the Philippines did on the island but these have yet to be confirmed by further studies. not indicate the presence of any active fault that would correspond However, field observations during this work and geologic maps of to a seismic generator. This paper presents results of a study that the island (Aurelio and Peña, 2010) indicate the presence of Plio- includes onshore structural field observations, earthquake data anal- cene – Quatenary sedimentary formations and carbonate terraces ysis and offshore seismic profile interpretation, in an attempt to presently located at relatively elevated positions (see Section 7), characterize the earthquake fault, which has otherwise generated suggesting uplift in the recent geologic past. This observation con- debates amongst earthquake researchers, as well as to better under- forms to those seen to affect sedimentary packages in the offshore stand the seismotectonic setting of Negros Island in the light of the region (Tañon Strait) of the Visayan Sea Basin (Rangin et al., 1989). recent seismic event. This work provides valuable insights to earth- Motion vectors derived from GPS measurements (Aurelio, 2000b; quake hazard assessment, especially of regions where active faults Rangin et al., 1999; Kreemer et al., 2000), when fixed relative to are unknown or have not been mapped. Palawan Island in western Philippines, indicate that Negros island, like the neighboring islands of Panay, Cebu and Bohol is moving at a slower northwestward rate than blocks located further to the 2. Regional tectonic setting east. This low translation rate is generally interpreted as the result of the effect of impingement of the North Palawan Block (NPB), a Earthquake-prone Philippines is surrounded by oppositely- buoyant microcontinental lithosphere that rifted from mainland verging subduction systems, and traversed by active transcurrent Asia (Holloway, 1982; Hall, 2002), against the western edge of faults all throughout the length of the archipelago (Cardwell the PMB in central Philippines. Consequently, GPS-derived strain et al., 1980; Aurelio, 2000a; Aurelio et al., 2013a)(Fig. 1). To the rates are likewise relatively low in the vicinity of continental west, 4 subduction systems dip east or southeast, namely from impingement (Rangin et al., 1999; Kreemer et al., 2000). the north; (1) Manila Trench where the South China Sea (SCS) oceanic lithosphere is consumed underneath the Luzon Arc; (2, 3) the Negros and Sulu Trenches where the Sulu Sea (SS) oceanic 4. Earthquake data lithosphere subducts underneath the Negros-Zamboanga-Sulu- Jolo Arcs; and (4) the Cotabato Trench where the Celebes Sea 4.1. Epicentral location and depth of focus (CS) oceanic lithosphere underplates the Sarangani-Sangihe Arcs.