PRIMER ON THE 6 FEBRUARY 2012 MAGNITUDE 6.9 EARTHQUAKE AT NEGROS ORIENTAL Department of Science and Technology Philippine Institute of Volcanology and Seismology (PHIVOLCS) 6 February 2012 PDF Print
Friday, 02 March 2012 02:32

What is happening at Negros Oriental, Cebu and nearby islands?
At 11:49 AM on 6 February 2012, a moderate size earthquake of magnitude 6.9 shook Negros, Cebu and nearby islands.  Aftershocks of lesser magnitude followed a few minutes after the main shock. 23 hours later, 896 aftershocks have been recorded by the PHIVOLCS seismic monitoring network.
The main shock and ensuing aftershocks have epicenters on the eastern coast of Negros Oriental, near the municipality of Tayasan.  It is a shallow earthquake, with focal depth of 10 kilometers. Although there are identified active faults in the island, such as the Central Negros Fault, further investigation needs to be done to determine the fault responsible for this earthquake.
Intensity reports showed that municipalities of Tayasan, Vallehermoso, Guihulngan and Dumaguete City (Negros Oriental) felt the strongest ground shaking at PEIS  VII (destructive ground shaking). Furthermore, the surrounding areas of La Carlota City and La Castellana (Negros Occidental), Tanjay and Manjud (Negros Oriental), Argao, Dalaguete, Barili, Cebu City (Cebu) and Clarin (Bohol) felt the ground shaking at PEIS VI (very strong ground shaking).  There were reports of felt intensities as far as 200 kilometers away from the epicentral area.  The strong ground shaking, especially in the epicentral area, caused damage to or collapse of weak structures, liquefaction in low-lying soft grounds, and landslide in steep slopes.  There were also reported sea disturbance in some coastal areas.
Moderate-magnitude  earthquakes have also affected Negros Island in the past!
On 5 May 1925, a magnitude 6.8 earthquake occurred at Southern Negros.  The towns of Bais, Tanjay, Tolong and Siaton felt the strongest intensity of ground shaking at PEIS VI to VII.  There were report of damaged buildings in Bais and Siaton, while in Dumaguete, landslides along ridges and fissures along rivers and mangrove areas.

Why do earthquakes occur at Negros and Cebu Islands?
Negros and Cebu islands are located in a seismically active area in the Philippines.  Instrumental monitoring of earthquakes for the past century has detected many small to large-magnitude earthquakes in Negros Island generated by the Negros-Sulu Trench.  The Negros-Sulu Trench is an earthquake generator located offshore west of the island, roughly parallel to the Philippine archipelago along its length, but veers very close to land at the southern tip of Panay Island.  Another earthquake generator that affects the islands is the Philippine Fault Zone, segments of which pass through the islands of Masbate, Leyte and Eastern Mindanao, and which have also been the locus of small to large-magnitude earthquakes.  Other active faults on the islands are the Central Negros Fault and Cebu Lineaments. Seismicity for these structures are low.

Can these present earthquakes indicate volcanic activity from Kanlaon Volcano?
No.  The origin of the main shock and aftershocks are clearly tectonic. However, there are also known volcanic activities, such as those of Pinatubo Volcano, which some scientists suggested were influenced by an earlier large-magnitude earthquake. PHIVOLCS is closely monitoring the situation.

1PEIS – Philippine Earthquake Intensity Scale
2Small magnitude less 4.9 or less; moderate magnitude 5 to 6.9; large magnitude 7 and above

What can we expect from the current earthquake activity?
The current seismic trend indicates that the magnitude 6.9 earthquake on 6 February 2012 may be the main shock, and the succeeding small magnitude earthquakes are the aftershocks.  The aftershock may continue for weeks to months, but diminishing in number and strength as time passes.  In this case, a stronger earthquake related to this event is no longer expected to occur.  However, there is no absolute way to determine whether another large earthquake would not follow the current activity. The present state of technology in the world is not capable of reliably predicting earthquake occurrences.

What should we do?
The best course of action is preparedness – we can minimize the damaging effects of earthquakes if we prepare ourselves for the event.  Because a large-magnitude earthquake, from the Negros-Sulu Trench, the Philippine Fault Zone, or from any of the known active faults in the Visayas, may affect Negros and neighboring islands as a whole, it is always prudent to prepare for such an eventuality. The conduct of earthquake and tsunami evacuation drill, as well as adherence to the building code are effective means to save lives and properties.
What can we expect in the event of a large-magnitude/high-intensity earthquake?
Strong ground shaking may cause extensive damage to, or even the collapse of houses, buildings, bridges, and other infrastructures.  Collapsed structures usually account for most of the casualties during a strong earthquake.  Falling objects may also cause injuries.

Aside from strong ground shaking, what other seismic hazards are life-threatening?
Landslides, rock falls, and other types of mass movements may occur in mountainous or hilly areas.  Liquefaction may affect low-lying, water-logged areas near the coast or at the banks of rivers.  In general, a strong earthquake occurring offshore may also generate tsunamis – large sea waves – that may sweep coastal areas.  There is practically not enough lead time to warn for a locally-generated tsunami. If a nearby earthquake source generated a tsunami, it may reach the shore in as short a time as 3-5 minutes, and may sweep away nipa huts and boats along with unprepared residents.

What is the role of PHIVOLCS?
PHIVOLCS operates and maintains a network of 64 seismic stations spread across the Philippines. Nine of these – the manned seismic stations of Negros Oriental (Sibulan), Cebu (Lapu-lapu City), Panay Island (San Jose Buenavista, Roxas City, Kalibo), Bohol (Tagbilaran City), as well as unmanned stations of Guimaras Island and Leyte Island (Maasin City and Ormoc City) – are located in or around Negros and Cebu islands.  Additional seismic data came from the Kanlaon Volcano Observatory in La Carlota City, Negros Occidental.  Data from the seismic stations are used to determine the location, magnitude and other characteristics of the earthquakes generated.
Aside from monitoring the occurrences of earthquakes, PHIVOLCS also conducts hazards analyses and assessments, and make these information available to the public.  PHIVOLCS works hand-in-hand with other government agencies in mitigating the damaging effects of earthquakes.