TIDE GAUGE AND SATELLITE ALTIMETRY DATA FOR POSSIBLE VERTICAL LAND MOTION DETECTION IN SOUTH EAST BOHOL TRENCH AND FAULT

Coupled with the occurrence of regional/local sea level rise on urbanized coastal cities is the possibility of land subsidence that contaminates the measurement by the tide gauge (TG) sensors. Another technology that could possibly check the in-situ data from tide gauge is satellite altimetry. The sea surface height (SSH) measured from satellite altimeter is compared with the observed tide gauge sea level (TGSL) to detect vertical land motion (VLM). This study used satellite altimeter retracked products near the TG Stations in Tagbilaran, Bohol; Dumaguete, Negros Oriental; and Mambajao, Camiguin located in the vicinity of the South East Bohol Trench and Fault (SEBTF).Based on the results, the TG site in Tagbilaran is undergoing land subsidence. The rate of VLM is around 5 mm/year from 2009 to 2017. The same trend was manifested in the GNSS observed data in the PHIVOLCS monitoring station in Tagbilaran and the geodetic levelling done in the area. After the October 15, 2013 earthquake in Bohol, downward trends of around 27 mm/year and 17 mm/year were observed from GNSS measurements and SSH-TGSL difference respectively. These different rates may be due to the distance between the two sensors. The comparison between SSH and TGSL in Dumaguete showed small difference with a VLM rate of 1.8 mm/year. The difference in SSH-TGSL in Mambajao is quite large with a downward rate of 9.4 mm/year. This result needs to be further investigated for TG or TGBM instability or monitored for a possibility of land uplift.