Subsidence Susceptibility Mapping in Tropical Island Karst: A comparison of  approaches used in the Municipality of General Luna in Siargao Island,  Philippines

Hugo  Marie Krystel D.; Agot Ross Dominic D.; Manzano Liza Socorro J.; Esmeralda Aquila Kristian B.; Abracia Aaron Miguel C.; Madrigal Madonna Feliz B.; Ondona April C.; Dela Torre Angelo Ma. Gabriel P.; Isip Marcius Elaeo G.; Rivera John Michael D.; Umali Julius Vincent P.; Belesario Nelgie Ann C.

Authors

Hugo, Marie Krystel D. Department of Environment and Natural Resources - Mines and Geosciences Bureau (DENR – MGB), Quezon City, Philippines; University of the Philippines -Diliman, Quezon City, Philippines
Agot, Ross Dominic D. Department of Environment and Natural Resources - Mines and Geosciences Bureau (DENR – MGB), Quezon City, Philippines; University of the Philippines -Diliman, Quezon City, Philippines
Manzano, Liza Socorro J. Department of Environment and Natural Resources –Mines and Geosciences Bureau (DENR –MGB), Quezon City, Philippines
Esmeralda, Aquila Kristian B. Department of Environment and Natural Resources –Mines and Geosciences Bureau (DENR –MGB), Quezon City, Philippines
Abracia, Aaron Miguel C. Department of Environment and Natural Resources –Mines and Geosciences Bureau (DENR –MGB), Quezon City, Philippines
Madrigal, Madonna Feliz B. Department of Environment and Natural Resources - Mines and Geosciences Bureau (DENR – MGB), Quezon City, Philippines; University of the Philippines -Diliman, Quezon City, Philippines
Ondona, April C. Department of Environment and Natural Resources –Mines and Geosciences Bureau (DENR –MGB), Quezon City, Philippines
Dela Torre, Angelo Ma. Gabriel P. Department of Environment and Natural Resources –Mines and Geosciences Bureau (DENR –MGB), Quezon City, Philippines
Isip, Marcius Elaeo G. Department of Environment and Natural Resources –Mines and Geosciences Bureau (DENR –MGB), Quezon City, Philippines
Rivera, John Michael D. Department of Environment and Natural Resources –Mines and Geosciences Bureau (DENR –MGB), Quezon City, Philippines
Umali, Julius Vincent P. Department of Environment and Natural Resources –Mines and Geosciences Bureau (DENR –MGB), Quezon City, Philippines
Belesario, Nelgie Ann C. Department of Environment and Natural Resources –Mines and Geosciences Bureau (DENR –MGB), Quezon City, Philippines

Keywords:

Geohazards, Karst, Subsidence, Susceptibility Mapping, Sustainable Tourism

Abstract

Karst subsidence hazard susceptibility mapping is an integral part of the National Geohazards Assessment and Mapping Program (NGAMP) initiated by the Mines and Geosciences Bureau (MGB) in 2013. It aims to comprehensively assess geohazards in karst regions in the Philippines. The mapping process involves three primary procedures: remote sensing interpretation of IfSAR[1]derived Sink Depth Maps using ArcGIS, geological and geomorphological assessment, and geophysical survey using Ground Penetrating Radar (GPR). This method considers sinkhole distribution, other karst features, ground movement evidence, and GPR survey results to identify subsidence-prone areas. The approach generates Karst Subsidence Susceptibility Maps indicating highly vulnerable regions, considering the unpredictable nature of sinkhole collapse.

To enhance the methodology and susceptibility classifications, the PhilKARST Program of MGB introduced the statistical Weights of Evidence (WoE) analysis and employed in General Luna, a prominent tourist destination in Siargao Island, Philippines. This approach ensures consistency and reproducibility by examining the correlation between hazard inventories and fifteen (15) conditional factors.

The resultant weights are determined to generate a subsidence susceptibility index map with Low, Moderate, High, and Very High ratings. Evaluation of true- and false-positive rates using available training data indicated an initial success rate of 82.2% for subsidence susceptibility modeling, signifying reliable results. The enhanced susceptibility map for General Luna in Surigao del Norte, displays lower susceptibility classifications in areas with fewer or no sinkhole occurrences. This differs from the output map of the previous method, which exclusively highlighted highly susceptible areas based on sinkhole distribution, other karstic features, and GPR surveys.

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