Bohol Fault System, or also known as (BFS), is a

reverse fault In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

system in

Bohol Bohol (), officially the Province of Bohol ( ceb, Lalawigan sa Bohol; tl, Lalawigan ng Bohol), is an island province of the Philippines located in the Central Visayas Regions of the Philippines, region, consisting of the island itself and 75 m ...

province, Philippines. This fault system contains 3 segments: the newly found North Bohol Fault following the

2013 Bohol earthquake The 2013 Bohol earthquake occurred on October 15 at in Bohol, an island province located in Central Visayas, Philippines. The magnitude of the earthquake was recorded at 7.2, with epicenter of Sagbayan, and its depth of focus was . It affe ...

, the South Offshore Fault, and the East Bohol Fault. The North Bohol fault is located in

Inabanga Inabanga, officially the Municipality of Inabanga ( ceb, Munisipyo sa Inabanga; tgl, Bayan ng Inabanga), is a 3rd class municipality in the province of Bohol, Philippines. According to the 2020 census, it has a population of 48,534 people. The ...

and near

Clarin Clarin or Clarín may refer to: Geography *Clarin, Bohol, a municipality in the province of Bohol, Philippines * Clarin, Misamis Occidental, a municipality in the province of Misamis Occidental, Philippines *River Clarin, a river in Ireland Media ...

, the South Offshore fault affects the southern towns, while the East Bohol fault starts at the SW part of Bohol from Loay and goes east towards Pilar.

Bohol fault segments

North Bohol fault

The North Bohol fault or Inabanga fault is a reverse fault located at Anonang, Inabanga which was found on 15 October 2013 during the Bohol earthquake. According to the

Philippine Institute of Volcanology and Seismology The Philippine Institute of Volcanology and Seismology (PHIVOLCS, ; tl, Surian ng Pilipinas sa Bulkanolohiya at Sismolohiya) is a Philippine national institution dedicated to provide information on the activities of volcanoes, earthquakes, and ...

, a new fault occurs only once in a century. The North Bohol fault, shaped as a hanging wall and also known as the "Great Wall of Bohol", became one of the tourist attractions in Bohol province.

South Offshore fault

The South Offshore Fault is adjacent to the town of

Anda Anda or ANDA may refer to: Places China *Anda, Heilongjiang, a city in Heilongjiang, China *Anda railway station, a railway station in Anda, China Iran *Anda, Iran, a village in Fars Province, Iran Norway *Anda, Norway, an island in Øksnes mun ...

. It affects the southern coastal towns from Anda in the east to

Maribojoc Maribojoc, officially the Municipality of Maribojoc ( ceb, Munisipalidad sa Maribojoc; tgl, Bayan ng Maribojoc), is a 4th class municipality of the Philippines, municipality in the Philippine Province, province of Bohol, Philippines. According ...

in the west. The epicenter of the destructive magnitude 6.8 earthquake on February 8, 1990 was near Anda as shown in the map to the right.

East Bohol fault

The East Bohol fault is a dominantly strike slip fault located at the southern portion of Bohol province, opposite to the North Bohol fault.

Bohol earthquake

The 2013 Bohol earthquake was a deadly event which happened on 15 October, striking the provinces of

Cebu Cebu (; ceb, Sugbo), officially the Province of Cebu ( ceb, Lalawigan sa Sugbo; tl, Lalawigan ng Cebu; hil, Kapuroan sang Sugbo), is a province of the Philippines located in the Central Visayas region, and consists of a main island and 167 ...

Negros Oriental Negros Oriental ( ceb, Sidlakang Negros; tl, Silangang Negros), officially the Province of Negros Oriental, is a province in the Philippines located in the Central Visayas region. Its capital is the city of Dumaguete. It occupies the southeaste ...

Masbate Masbate, officially the Province of Masbate ( Masbateño: ''Probinsya san Masbate''; tl, Lalawigan ng Masbate), is an island province in the Philippines located near the midsection of the nation's archipelago. Its provincial capital is Masbate C ...

and

Leyte Leyte ( ) is an island in the Visayas group of islands in the Philippines. It is eighth-largest and sixth-most populous island in the Philippines, with a total population of 2,626,970 as of 2020 census. Since the accessibility of land has be ...

. The 7.2 earthquake produced a ~50‐km‐long, ~12‐km‐wide northeast trending zone of uplift with an ~8‐km‐long discontinuous ground rupture indicating predominantly reverse‐slip movement on a southeast dipping fault. Displacement along the northeastern segment of the NBF in Inabanga is significantly larger in the northeast than in the southwest. In the northeast, the displacement was mostly 2 m and reached a maximum of 5 m, while in the southwest, the displacement was mostly less than 1 m. The maximum and average displacements, which were measured along the nearly continuous approximately 2‐km‐long ground rupture in Sitio Cumayot, Barangay Anonang, Inabanga, were around 5 and 2 m, respectively. Documentation of the nearly continuous northern terminus of the earthquake ground rupture revealed its association to preexisting scarps of the previously unmapped, Quaternary‐active North Bohol Fault. Trenching across the rupture at four sites not only reveals the geometry and kinematics of the fault but also shows at least one or two pre‐2013 surface rupturing events. Onshore geologic mapping and offshore seismic reflection profiles demonstrate the presence of an island‐wide, northeast‐southwest trending fold‐and‐thrust belt through which deformation related to the regional shortening across the Visayan Sea Basin in the central Philippines is likely distributed.

References