06 FEBRUARY 2012 NEGROS ORIENTAL EARTHQUAKE Geologic Impacts of the 06 February 2012 Negros Oriental Earthquake in Negros Oriental and Cebu REPORT OF INVESTIGATION DEPARTMENT OF SCIENCE AND TECHNOLOGY PHILIPPINE INSTITUTE OF VOLCANOLOGY AND SEISMOLOGY SEPTEMBER 2015 CONTENTS Summary 1 Introduction 3 Regional Impacts of the Earthquake 6 Ground Shaking and Intensity Distribution and Aftershocks Monitoring 6 Damage to Vertical and Horizontal Infrastructures 10 Tsunami 17 Liquefaction 23 Earthquake-Induced Landslides and Rockslides 28 Other Ground Deformations, Coastal Uplift, and Probable Ground Rupture 32 Remaining Hazards 35 Information, Education and Dissemination 36 Campaigns Local Officials and Residents 36 Orientation for the Members of the Media 41 Conclusions and Recommendations 44 Acknowledgement 46 References 47 The 06 February 2012 Negros Oriental Earthquake 1 SUMMARY The 06 February 2012 earthquake that hit Negros with almost the same length as the projected Oriental produced: (1) strong ground shaking, (2) possible fault rupture length of 30-40km. liquefaction, (3) landslides, (4) tsunamis, and (5) ground deformation and coastal uplift. Extensive Information Education Communication Considerable to severely damaged infrastructures (IEC) Campaigns were also conducted in the and houses were observed in the municipalities towns affected to allay the fears of the residents of Ayungon, Tayasan, Jimalalud, La Libertad, and and to explain the aftershock occurrences. An IEC Guihulngan City in the province of Negros seminar was also organized for local media to Oriental. These towns were also the areas that explain the basics of earthquake and its hazards, felt the strongest ground shaking at PHIVOLCS preparedness, and communication protocols Earthquake Intensity Scale (PEIS) VII-VIII followed under the National Disaster Risk (destructive-very destructive). Local tsunami with Reduction and Management Council (NDRRMC). wave heights of less than a meter up to 4-5m impacted Ayungon, Tayasan, Jimalalud, La Results of the Quick Response Team (QRT) Libertad, and Guihulngan City. Unusual waves investigation, the detailed mapping, and studies were also observed as far as municipalities of San that will be conducted will be included in the Jose in the south and Vallehermoso in the north, earthquake-hazard maps that will be produced giving a total length of approximately 100 km for the province of Negros Oriental. These hazard unusual waves related to tsunami in Negros map will greatly aid the province in their disaster Oriental. Unusual sea waves, minor landslides, risk-reduction plans in the future. In addition, the small sink holes and slight damages to following general recommendations apply to all infrastructures and properties were also reported areas in Negros and Cebu Islands that may and documented at Badian, Moalboal, Ronda, experience yet another large magnitude and Dumanjug, Cebu. Five portable seismographs earthquake capable of generating high intensity were installed as temporary seismic networks to shaking in the future: enable accurate aftershocks monitoring. 1. Immediately conduct rapid The 06 February 2012 Negros Oriental building/structure safety evaluation by Earthquake was generated by a NE-trending blind the Building Official’s authorized local thrust fault along the eastern region of Negros civil/structural engineers after a locally Island. This fault plane, if projected, would destructive earthquake so as to promptly possibly surface east of the Negros Island, most inform the occupants of its hazards or likely offshore, explaining the documented safety for use/habitation and to provide coastal uplift ca. ≤1m and extends for appropriate recommendations. Strong approximately 2 km at Guihulngan coast. It is aftershocks may cause subsequent possible that the uplift may extend further south collapse of building weakened by the main shock. 2 PHIVOLCS Quick Response Team 2. Immediately evacuate and demolish 6. Conduct more intensive community-wide severely damaged buildings, facilities and earthquake and tsunami information and houses to prevent further use of these education campaigns and drills, to raise structures and avoid greater risk of local and sustain awareness and proper inhabitants to possible future collapse. response as well as to allay fears and eliminate spread of false information. 3. Relocate communities situated in areas vulnerable to earthquake hazards, such as 7. Strictly implement and conduct quarterly landslides, ground ruptures and tsunami. earthquake drills in schools and government institutions. 4. Activate and strengthen the Provincial Disaster Risk Reduction and Management 8. Strictly implement and monitor the Council (PDRRMC) down to the Barangay National Building Code requirements and DRRM Committee with the capacity its referral codes in the design and building for disaster preparedness, construction of public and private response, and rehabilitation. facilities/structures to minimize severe structural damage or collapse in future 5. Establish short- and long-term community earthquake events. preparedness plans in preparation for future earthquakes. This would include 9. Formally train local masons and building of community-based carpenters on proper construction disaster/emergency response units, practices and use of standard identifying evacuation sites and routes construction materials. and other mitigation strategies. The 06 February 2012 Negros Oriental Earthquake 3 INTRODUCTION Earthquake Information Defense through the local Disaster Risk Reduction and Management Councils (DRRMCs). Based on On 06 February 2012 at 11:49 a.m. Philippine the extent of damages reported, PHIVOLCS Standard Time (PST), a strong earthquake with a deployed a Quick Response Team (QRT) from its body-wave magnitude (Mb) of 6.9 shook the technical staff on field assignment from its Islands of Negros, Cebu, and nearby islands of Canlaon Volcanological Station to do rapid Western Visayas region. The epicenter was damage assessment on the day of the located by the Philippine seismic network at earthquake. Another team was deployed on 07 9.97° N, 123.14° E, which is approximately 5 km February 2012 from its central office in Quezon northwest of Tayasan, Negros Oriental (Figure 1). City to supplement and further assess the The earthquake had a depth of focus of 5-10 km. impacts of the 06 February earthquake. As severe Based on the hypocenter and focal mechanism impact of the earthquake event was realized, solutions by the International Seismic Network additional team members from the main office (ISN), PHIVOLCS and the United States Geological were dispatched. The PHIVOLCS QRT visited the Survey (USGS), the earthquake was determined provinces of Negros Oriental and Cebu from 07 to to be tectonic in origin, and suggests a 28 February 2012 specifically to: dominantly thrust-faulting movement along a NE- SW fault plane (Figure 2). Ensuing aftershocks 1. Conduct and facilitate intensive information recorded by PHIVOLCS permanent seismic drive and allay the fears of affected stations also suggests a NE-SW fault plane and communities in the epicentral region of rupture propagation along the eastern coast of Negros Oriental province; Negros Island. 2. Conduct damage assessment and intensity Damage to or collapse of weak structures due to survey; strong ground shaking was expected in view of its location, magnitude, shallow depth and sense of 3. Further verify, assess, map and document the movement. In particular, the earthquake was impacts of the earthquake, giving particular expected to generate such earthquake-related attention to occurrences of ground rupture, hazards as tsunami, liquefaction and landslides. liquefaction, tsunami and landslides in the towns nearest the epicentral region; and Objectives 4. Install portable seismographs in and around Negros Oriental for aftershocks monitoring. Immediately after the earthquake, damages were assessed and reported by the Office of Civil 4 PHIVOLCS Quick Response Team Figure 1. Map showing the epicentral location of the mainshock of the 06 February 2012 Mb6.9 Negros Oriental Eathquake (yellow star) and the aftershocks (red circles) as of 9 February 2012. The 06 February 2012 Negros Oriental Earthquake 5 Figure 2. Focal mechanism solution for the earthquake of 06 February 2012 11:49AM in Negros Oriental. Data from the stations of PHIVOLCS-NIED SWIFT Broadband Network (RP-Japan Project) and GFZ Network. 6 PHIVOLCS Quick Response Team REGIONAL IMPACTS OF THE EARTHQUAKE Ground-Shaking Intensity strike of the fault plane. As of 16 February 2012, a total of 1,678 aftershocks was recorded, 102 of these Distribution and Aftershocks were felt at intensity V or IV within the epicentral Monitoring region of Ayungon, Tayasan, Jimalalud, La Libertad, and Guihulngan, Negros Oriental. The earthquake was felt most strongly along the eastern coast of Negros Island. Initial intensity reports In the areas of the epicentral region, local residents showed that the strongest ground shaking was felt at observed that the aftershocks were always intensity VII using the PHIVOLCS Earthquake Intensity preceded by rumbling sounds from the ground that Scale (PEIS) in the municipalities of Tayasan, vary in intensity depending on the size of the event. Vallehermoso, and Guihulngan City (Negros Oriental). Vertical shaking is also almost always felt from bigger There were reports of felt intensities as far as 200 aftershocks, preceding a horizontal shaking. kilometers away from the epicentral area. The