Gravity Survey at the Guinsaugon Rockslide-Debris Avalanche Deposit

Masahiko Makino¹, Allan Mandanas² and Sandra G. Catane²
 

¹ Geological Survey of Japan, AIST
² National Institute of Geological Sciences-College of Science
University of Philippines, Diliman, Quezon City

Abstract

On 17 February 2006, a massive landslide occurred at Barangay Guinsaugon, Municipality of St. Bernard, Southern Leyte Province, Philippines. The landslide initiated at the ridge top along the Philippine Fault Zone (PFZ) arc-parallel strike-slip faults. PFZ traverses the entire length of the Philippines, more than 1,200km, from Luzon Island to Mindanao Island. The displacement velocity was estimated to be about 26mm/yr in the Leyte Island from GPS observations. At Southern Leyte, a 7.0 magnitude earthquake occurred in 1948. In order to investigate the large and active tectonic structure near the Guinsaugon landslide, a gravity survey was conducted in April 2006. Locations of 50 gravity stations were determined by the first-static mode of GPS survey with a base station at St. Bernard. The gravity results show 1) a large difference of gravity anomalies, more than 10 mgal, 2) a large gradient of gravity anomalies, more than 10 mgal/km, at Barangay Guinsaugon. The gravity anomalies decrease steeply toward PFZ. On the other hand, gravity anomalies near PFZ remained constant. These results suggest that there is a large subsurface fault with a depth of more than 1 km, similar to subsurface faults found after Kobe earthquake 1995. The surveyed area is underlain by the Central Highland Igneous Complex and Recent alluvium. The gravity basement structure, dipping steeply toward PFZ from Himbungao River, may have enhanced the seismic intensity of the 2.6 magnitude earthquake at the ridge top, where the rockslide-debris avalanche had originated.