2014 Volume 123 Issue 1 Pages 133-142
A gravity survey was carried out in and around Bulusan volcano in February 1996. The gravity stations totaled to 225, which were restricted to roads. The value of density used for both terrain and Bouguer corrections was 2,300 kg/m3 (2.3 g/cm3), the value commonly used in volcanic terrains and the surface layer density is considered to be geologically low. A semi-circular feature with steep gravity gradient was recognized in the Bouguer anomalies from the east through the south and west of Bulusan volcano. This semi-circular feature corresponds clearly to the southern rim of the Irosin caldera. However, the northern caldera rim is not clear from gravity anomalies. In contrast, the result of three-dimensional analysis of residual gravity anomalies indicates that, the gravity basement has a circular structure, with its diameter significantly smaller than that of topographic depression. It is important to note that this circular depression is similar to funnel-shaped (or inverted cone) caldera rather than to a piston-cylinder type of caldera. The mass deficiency of the Irosin caldera was estimated to be 1.1 × 1010 tons by applying the Gaussian theorem gravity anomalies.