ABSTRACT
The coastal regions of Central Visayas show geomorphologic indications suggestive of significant compression related to the Palawan Microcontinental Block (PCB)-Philippine Mobile Belt (PMB) collision. In northeastern Negros Oriental, compressional regime brought about by such collision is expressed by the recent uplift of Quaternary coastal sediments and coral reefs, a co-seismic, reverse faulting event causing the Mw 6.9, February 6, 2012 earthquake. Deformation is also manifested by the tilted orientation of the Cenozoic clastic and carbonate rocks exposed along the coast or road cuts in northeastern Negros Oriental. Regionally, this may be related to the deformation of the Visayan Sea Basin resulting from the aforesaid collision. Similarly, in Bohol, the most recent uplift of carbonates is attributed to the 15 October Mw7.2 North Bohol Fault (NBF) earthquake, also resulting from reverse fault movement. The seismic event caused uplift of the coral reefs by as much as 2.5 meters (or more) in several coastal sections of western Bohol. Smaller islands consisting of uplifted carbonate platform between Bohol and Cebu are aligned southwesterly, consistent with the PMB-PCB collision.
The uplift of carbonate platform resulted in the formation of cliffy karstic coast most prominent in western Bohol and islands west of the province. This translates to lower vulnerability to coastal hazard, particularly coastal inundation. However, while a compressional regime would relate to a seaward retreat of the shoreline, a landward retreat is being witnessed in many areas in Negros Oriental and Bohol mainland following the most recent earthquakes. Isostatic rebound of the land following the seismic event, alongside the general rise in relative sea-level based on the 5-year data, has slowly caused a landward shift of the shoreline in recent years. In Negros Oriental, the restricted area of the Tañon Strait between Negros Oriental and Cebu island could also be an influential factor. The resultant influences of the Visayan Sea in the north and the Bohol Sea in the south could create a complex tidal activity in which high tide can be out of sync for a number of days with those areas just outside of this strait. The passage of Typhoon Odette in 2021 demonstrates how wave impacts and storm surges within the Tañon Strait could cause dramatic changes to the coastal morphology and significant erosion of beach sediments as demonstrated in northern Negros Oriental. On a local scale, anthropogenic activities have enhanced coastal erosion in both Negros Oriental and Bohol. These activities include (1) the extraction of beach sands; (2) the extraction of coral reefs (also serving as beach sediment sources) which enhances wave impacts alongshore; (3) the destruction of mangroves which are vital for trapping sediments and; (4) the erection of structures (port, coastal defenses, perimeter fences; community growth along the foreshore area) that interferes with the sediment transport alongshore. Clearly, coastal hazards studies in an actively deforming region requires a combined output of vertical geologic movement, relative sea level change and anthropogenic activities.