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