GEOLOGICAL SOCIETY OF THE PHILIPPINES

 

Seismotectonics of the Magnitude 7.2 Bohol Earthquake of 15 October 2013 from Onshore, Earthquake and Offshore Data: A Key to Discovering Other Buried Active Thrust Faults?

Aurelio, M.A.1, Jeremy M.Rimando1, Kristine Joy L. Taguibao.1, John Dale B. Dianala1 and Al Emil Berador2

1National Institute of Geological Sciences, University of the Philippines
2Mines and Geosciences Bureau, Region 7

Abstract

At 8:12 in the morning of 15 October 2013, a magnitude Mw 7.2 earthquake struck the island of Bohol in south-central Philippines. Infrastructure damage was most intense in places within and in the vicinity of the epicenter between the towns of Sagbayan and Catigbian in the central-west section of the island, as well as in areas prone to liquefaction (coastal and riverside towns of Maribojoc, Loon, Calape, Tubigon, Baclayon, Loboc) landslides (hilly towns of Antequera, San Isidro, Catigbian, Sagbayan), ground subsidence (Cortes, Maribojoc, Loon) and sinkhole formation (Calape, Catigbian, Tagbilaran). A 5 km-long fault ground rupture was expressed in the northwestern town of Inabanga.

Aftershock data, focal mechanism solutions, ground rupture evidence and the trend of damage extent indicate a reverse (thrust) faulting mechanism along a fault plane that generally strikes N60°E and dips about 45° to the SE. Hyprocentral depth was determined to be between 10 and 12 km. Time-lapse analysis of the generation of aftershocks suggests an asymmetrical rupture process showing propagation of the fault shorter to the NE than to the SW. Focal mechanism solutions of several aftershocks with magnitudes >Mw 5.0 suggest strike-slip faulting at the NE end of the fault, while aftershocks on the SW half extend to the offshore area between Cebu and Bohol. Preliminary Coulomb Stress Transfer modeling indicates a post-quake stress buildup to the NW and SE above and below 10 km depth respectively.

The 5 km-long ground rupture at the NE section of the fault represents only about 6% of the expected empirical rupture length of about 90 km. Along-strike projection of the fault trend to the SW requires an offshore trace into the Bohol Strait. Preliminary analysis of offshore seismic reflection profiles between Cebu and Bohol suggests the presence of NE-striking active folds rooted on bifurcating thrust faults dipping SE. Such fold-fault structures replicate the observed onland structure of the Loon-Maribojoc Peninsula manifested as a NE-striking anticline affecting the youngest carbonate sequences of Bohol (Cortes Limestone).

Interestingly, the magnitude Mw 7.2 Bohol Earthquake of 15 October 2013 occurred only within one-and-a-half years after a magnitude Mw 6.9 earthquake devastated nearby Eastern Negros on 6 February 2012. Both earthquakes were generated by previously unmapped faults of the same mechanism (thrust fault) and with close association to folds affecting recent carbonate deposits. The implied genetic relationship between the recent folds and the active thrusts may prove to be a key in discovering other buried active thrust faults elsewhere.


 
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