By Kris Elvis N. Alferez
The conduct of the 2019 Ground Subsidence Hazard Mapping target area commenced on March 12, 2019 in the Municipality of Buug, Zamboanga Sibugay. This annual program is part of the continuing Geohazards Assessment and Mapping Program of MGB that has now expanded to include subsidence hazard due to sinkhole collapse and karst subsidence. Sinkholes are present in areas characterized by karst topography and underlain by limestone or calcareous lithologies.
The work program generally aims to assess and locate the presence of ground subsidence, sinkhole collapse and caves using geomorphological information and employ a geophysical survey instrument, Ground Penetrating Radar (GPR) to ascertain and quantify subsurface cavities. The program will also determine the subsidence susceptibility of the study area as a result of sinkhole and cover collapse particularly in selected local government infrastructure sites such as school building grounds other priority areas identified by the Local Government Units (LGUs) which are commonly used as evacuation sites during natural disaster; and after carrying out the survey, conduct awareness campaign on sinkholes and other karst features, and issuance of threat advisories to LGUs to the hazard subsidence due to sinkhole collapse.
The methods to be employed during the field campaign includes inventory of pre-delineated sinkholes interpreted from the 2013 NAMRIA Interferometric Synthetic Aperture Radar (IfSAR) satellite image, ground penetrating radar (GPR) surveys of selected areas in the municipality, gathering of anecdotal accounts, Information, Education and Communication (IEC) Campaign, and issuance of “Threat Advisories” on the concern barangays in the Municipality of Buug where sinkhole and ground subsidence can develop.
Buug is a municipality in the eastern section of Zamboanga Sibugay located 7°43’ 52.9” N and 123°03’ 35.8” E in western Mindanao and covers an area of about 134.06 km2. It is bounded by the Municipality of Kumalarang on the east, Municipality of Diplahan on the west and Bayog bounds on the north. The Dumanquillas Bay bounds the coastal barangays in the south. Based on the 2015 population census, it has a total of 36,634 inhabitants distributed in the 27 barangays.
The municipality is approximately 208 km northeast of Zamboanga City and can be accessed by any mode of land transport via national highways Asian Highway 26 (AH26). Road network within the peninsula links Buug to other urbanized cities in the region such as Dipolog City, Pagadian and Zamboanga City.
The 27 barangay centers can be easily accessed via a series of mixed paved, unpaved, sheeted and concreted access tracks from the national highway. Access to far flung barangays can be hampered by seasonal rain and irregular public transport serving these areas.
Geological and Geomorphological Assessment
During the first course of the activity, the team conducted an in situ outcrop/subcrop rock identification and sampling, paleontologic and physical analysis which are vital information needed for the finalization of technical report. Delineation of lithologic boundaries/geologic contacts and structures were also conducted within the municipality to delineate areas with high concentration of carbonate rocks which are prone to Karst landform formation. As for initial result, the lithological boundaries and structures from the existing Regional Geologic Map of Zamboanga Peninsula are expected to be modified based on ground validation/field observations conducted.