ERRATIC CHARACTER OF THE BOHOL FAULT

A Discourse
By ASTERIO N. COQUILLA
SB Member, Antequera, Bohol

Much has been said of the 7.2 magnitude earthquake that struck Bohol on October 15, 2013 but so far, no clear explanation or much of an analysis has been brought forth as regards the origin, nature and extent of the tectonic activity that resulted to such an erratic earth movement that occurred beneath the central part of Bohol. It seems the Phivolcs itself was puzzled as to the type of earthquake that happened that it could not give a satisfactory description and analysis of the phenomenon. Accordingly, the said earthquake was not of the normally occurring type of earth disturbance that was of tectonic origin. It was a unique type that must have happened only in Bohol. Phivolcs could not even give an accurate analysis because they were puzzled on why such a phenomenon happened which defied accepted standards. Of course they were silent on such a matter to which they could hardly give an explanation.
The experts were surprised why there were many fault lines practically radiating from the Carmen-Sagbayan area going outwards in different directions towards the periphery of the western half of Bohol island. This is not of the usual type where a single fault line would move and only the areas near it or within its range would be affected. Here, it was different, there were highly visible separate lines of destruction coupled with huge, sometimes deep linear ground cracks radiating from the central part of the island towards the coastal towns of the south-western portion. There was a long fault line identified by men from Phivolcs which radiated towards Inabanga-Clarin area. Another line radiated towards San Isidro town and towards Tubigon. Also identified was a line of destruction going towards Loon-Calape area, another fault line was prominently noticed going from Sagbayan towards Catigbi-an, then Balilihan and then Antequera going to Maribojoc via the Abatan bridge area, making its exit to the sea and possibly towards the island of Cebu. There was also another line which traversed through Cortes town and still another through a part of Tagbilaran City. There was also a fault line which had its exit in Loay and Albur area after traversing through Batuan, Bilar and Loboc.
Accordingly, the epicenter of the earthquake was some 8 kms. deep below the Carmen-Sagbayan area and the destruction radiated in different directions towards the periphery of the south western part of the island. This is the Bohol Fault, a unique type of fault which is now the subject of intense study by experts in the field. The Bohol Fault which has its center of origin below the Carmen-Sagbayan area is seemingly like a cobweb which, if its center is disturbed, energy radiates outwards in all directions towards the periphery. It is also similar to what happens when a stone is thrown towards the glass window of a car. From the point of impact, the damage radiates in all directions towards the periphery. However, in the 2013 earthquake, only the western part of Bohol was badly affected while in the 1994 Bohol earthquake, the other half of the island, the eastern part, was the portion severely affected. That’s how unique the Bohol Fault is, as far as this point of view is concerned. Any other analysis is most welcome.

ASTERIO N. COQUILLA

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