Remote Sensing and GPR Studies of Faults Bounding the Irosin Caldera, Luzon, Philippines

Abstract

The Irosin caldera, located at the southernmost tip of the Bicol Volcanic Arc, Luzon, Philippines was investigated for regional tectonic structures through remote-sensing and Ground Penetrating Radar (GPR). Four shaded relief images and two slope aspect maps were derived from high resolution Synthetic Aperture Radar (SAR) Digital Surface Model (DSM) and used to carry out lineament analysis of the area. A network of WNW-SE, NNW-SSE and NE-SW-trending structures, particularly the large-scale ones, were consistently delineated in all the generated topographic models. The most prominent lineaments identified are the WNW-SE-trending linear features that border the northern and southern rim of the Irosin caldera. A GPR survey was carried out by common offset method across the southern lineament using a 100 Mhz antenna to locate and verify its continuity. The GPR data provided relatively high-resolution radargrams of the upper 10m of the subsurface. These profiles revealed dense arrays of fractures and faults, with various angles of dip, spatial orientation and depths, which coincide with the projected location of the southern lineament. The results of the survey proved that the southern caldera fault is continuous. The central segment of the southern lineament is most likely overlain by thick volcanic deposits from the recent volcanic eruption of the nearby Bulusan Volcano, making them difficult to identify on the topographic models. Some structures have also been identified outside of the projected location of the fault, and may likely be some of the numerous fractures and ring faults that are commonly expected along the periphery of calderas.