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GEOLOGIC IMPACTS OF THE 15 OCTOBER 2013 Mw7.2 NORTH BOHOL FAULT EARTHQUAKE, BOHOL ISLAND, PHILIPPINES PDF Print
Monday, 28 October 2013 06:21

QRT Report of Investigation
Conducted on 16-25 October 2013
(as of 28 October 2013)

 

SUMMARY

 

At 8:12:31 am local time, on 15 October 2013 a Mw7.2 earthquake occurred in Bohol island. The epicenter was plotted at 6 kilometers S24W (9.86 deg N, 124.07 deg E) of Sagbayan municipality at a focal depth of 12 kilometers. The earthquake produced 1) a surface rupture, 2) strong ground shaking, 3) liquefaction, and 4) landslide. Several sinkholes also appeared after the earthquake. Considerable to severe damage to infrastructures and houses were reported and observed in the north- and south-western municipalities of Bohol, including Maribojoc, Loon, Tubigon, Calape, Clarin Inabanga, Buenavista, Danao, Sagbayan, Catigbian, San Isidro, Antequerra, Balilihan and Cortes. These towns were also the areas that felt the strongest ground shaking at PEIS VII-VIII (destructive to very destructive ground shaking). Damage to houses and infrastructures were also reported in Tagbilaran and in the southern munipalities of Loboc, Carmen, Lila, Valencia, Albuquerque, Baclayon and Loay.

 

1. Surface Rupture

The Mw7.2 Bohol Earthquake was generated on the north-western sector of Bohol island by a NE-SW trending reverse fault (hereto named as North Bohol Fault or NBF), which had no previous surface manifestation. The PHIVOLCS Quick Response Team (QRT) measured surface ruptures ranging from 0.10 meter to as much as 5 meters in vertical displacements exposed in Brgy. Anonang in Inabanga Municipality. As of this writing, the mapped surface of the NBF is 6 kilometers long, from Brgy. New Anonang in Buenavista to Brgy. Napo in Inabanga. The NBF generally trends N40E and dips at 50SE. The PHIVOLCS QRT continues to map the probable extension of NBF northeast of Inabanga towards Getafe and southwest towards Loon and Maribojoc.

Surface rupture

The longest, continuous individual trace mapped by PHIVOLCS QRT is approximately 2 kilometers in Brgy. Anonang. In this barangay, surface rupture trends N40E. The surface rupture manifested as prominent fault scarps, which range from 2-5 meters of vertical displacements. Other geomorphic manifestations observed in Brgy. Anonang include 2.5 meter vertical displacement of Cawasan Creek in Sitio Calubian, producing a small waterfall at the point where the fault transects the creek. Other typical features associated with reverse faulting, such as scallops, bulges and warps, were also observed in the deformation zone, which extended as wide as 30 meters in some places.

Further south-westward of Brgy. Anonang in Sitio Tangub, Brgy. Liloan Norte, PHIVOLCS QRT measured a shift in the trend of the fault trace to N55-60E and a decrease in the vertical displacement to 0.15 meter. PHIVOLCS QRT observed the presence of a spring along the trace of the surface rupture. Residents of this sitio confirmed that the spring appeared only after the earthquake.

In the next barangay of Napo, still in Inabanga and north of Inabanga River, the surface rupture trends N40-60E, with vertical displacements ranging from 0.10 to 0.40 meter.

Towards the NE from Brgy. Anonang in Inabanga to Brgy. New Anonang in Buenavista, the surface rupture trending N40E is manifested by displaced rice paddies. Vertical displacements range from 0.3 to 0.5 meter.

The fault probably terminates in Sitio Haligi, Brgy. New Anonang in Buenavista, where an originally flat-lying ricefield became gently sloping. The difference in elevation is measured at 0.10 meter. This feature is different from the previously observed abrupt break along the surface rupture and is common along the termini of active faults.

Although the PHIVOLCS QRT has not yet finished mapping the SW extension of NBF, it is likely that the fault extends towards north of Maribojoc. A sea-ward shift of the high tide mark after the earthquake was observed by the residents in Brgy. Punta Cruz, Maribojoc. This shift in high tide mark may imply that Maribojoc is on the upthrown block of the reverse NBF. The PHIVOLCS QRT measured the shift to be about 50 meters.

Part 2: Ground shaking

Part 3: Liquefaction

Part 4: Earthquake-induced landslides

tcb/mtc/mlp/asd

 
PRIMER ON THE 15 October 2013 Magnitude 7.2 BOHOL EARTHQUAKE Philippine Institute of Volcanology and Seismology – Department of Science and Technology Wednesday, 16 October 2013 PDF Print
Wednesday, 16 October 2013 13:16

What happened at Bohol?

At 8:12 AM on 15 October 2013, Tuesday, a destructive earthquake of magnitude 7.2 shook the island of Bohol and nearby provinces. Smaller-magnitude earthquakes followed afterwards, and as of 1:00 pm, 16 October 2013, 885 earthquakes have been recorded by the PHIVOLCS seismic monitoring network. At least 15 events were reportedly felt in the epicentral area. The main shock and succeeding aftershocks were located in the vicinity of Bohol. These recorded events were shallow, with a depth of at most 32 kilometers. Based on spatial distribution of succeeding events and characteristics of the earthquake, the event is tectonic in origin.

Based on preliminary intensity reports, the strongest ground shaking at PEIS VII was felt at Tagbilaran City and several cities in the province of Cebu. Neighboring island provinces of Cebu, Negros Occidental, Negros Oriental, Camiguin, Panay, Leyte, and several areas in northeastern Mindanao felt the earthquake at varying intensities of PEIS I-VI.

Moderate-magnitude (M5 to 6.9) earthquakes have also affected Bohol Island in the past!

On 08 February 1990, a magnitude 6.8 earthquake occurred at Bohol generated by an offshore reverse fault east of the island. Sixteen municipalities felt the strongest intensity of ground shaking at PEIS VIII. There were reports of severe property damages, numerous casualties, hundreds injured, and several thousands homeless. The towns of Jagna, Duero, Guindulman, Garcia Hernandez, and Valencia experienced tsunami inundation.

Why do earthquakes occur in Bohol?

Bohol Island is one of the seismically active areas in the country. Instrumental monitoring of earthquakes for the past century has detected many small to moderate-magnitude earthquakes in Bohol Island. There is at least one known earthquake generator on the island, the East Bohol Fault. In addition, there are other local faults which can be sources of small to large magnitude earthquakes. Earthquakes can also occur offshore or undersea because of local offshore faults near the island or trenches in the vicinity of the region.

Can these present earthquakes indicate volcanic activity?

No. There are no volcanoes in Bohol Island.

What can we expect from the current earthquake activity?

The current seismic trend indicates that the magnitude 7.2 earthquake on 15 October 2013 is the main shock, and the succeeding small magnitude earthquakes are the aftershocks. Aftershocks are expected, some of which will be felt. These may continue for weeks to months, but diminishing in number and strength as time passes. In this case, a higher magnitude earthquake related to this event is no longer expected to occur.

What can we expect after a large-magnitude/high-intensity earthquake like this?

People are reminded to be cautious of structures visibly weakened or with signs of damage by the 15 October 2013 earthquake, as these may be further damaged by succeeding earthquakes. 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.

What should we do?

Report from the field mentioned several building, houses and other infrastructures that sustained minor to major damages. In case of houses with visible damage, it is best to contact the Municipal Engineering Office for advice. Engineers from the municipal government and other authorities should inspect buildings and other infrastructures to determine their integrity and recommend appropriate action to concerned affected groups or individuals. Check for tension cracks on the ground that may have resulted by the strong ground shaking. These areas can initiate landslides during intense rainfall.

The best course of action is preparedness. In case of another felt earthquake, it is recommended that people protect themselves by doing the “duck, cover and hold”. In homes and offices, heavy furniture should be strapped to the walls and appliances be secured to prevent them from toppling and causing injuries to persons.

Aside from strong ground shaking, what other seismic hazards are life-threatening?

Landslides, rock falls, and other types of mass movement may occur in mountainous or hilly areas. Liquefaction, manifested by sandboils or lateral spreading may affect low-lying, water-saturated, sandy areas near the coast or at the banks of rivers. In general, since the destructive earthquake occurred inland, no tsunami was generated.

What is the role of PHIVOLCS?

PHIVOLCS operates and maintains a network of 69 seismic stations spread across the Philippines. Twenty five of these – the manned seismic stations of Tagbilaran City, Palo, Leyte, Lapu-Lapu City, Camiguin, San Jose De Buenavista, Roxas City, Capiz, Kalibo, Aklan, Sibulan, Negros Oriental, Surigao City, Cagayan de Oro City and Dipolog City as well as unmanned stations of Maasin, So. Leyte, Ormoc City, Butuan City, Borongan, Eastern Samar, Guimaras and Ipil are located in or around Bohol. 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 the information available to the public.

PHIVOLCS works hand-in-hand with other government agencies in mitigating the damaging effects of earthquakes. Please visit our website at http://www.phivolcs.dost.gov.ph for earthquake information, volcano updates, and education materials on earthquakes and volcanoes. Earthquake observations may also be reported to PHIVOLCS at telephone numbers (02) 929-9254 and (02) 426-1468 to 79, local 124 and 125.


 
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