INTEGRATED ANALYSIS OF REMOTELY-SENSED IMAGERY IN DESCRIBING THE MORPHOLOGY OF THE EDIFICE AND THE MASS WASTING EVENTS IN MOUNT ARAYAT, PHILIPPINES

摘要

Mount Arayat is an inactive stratovolcano located in the plains of central Luzon, Philippines. It is a morphologically distinct cone that has ceased its constructional processes and is at a stage where erosional processes dominate. Using high-resolution digital terrain models (DTM) and freely available satellite imagery, details of its morphology are enhanced such that features of mass movements are identified. This study emphasizes morphological analysis as a key to understanding the underlying processes that govern the activities in seemingly quiet volcanic landforms. Morphological classification on the basis of digital elevation model value distribution and surface textures from slope and aspect maps defines areal domains of different degrees and controls of erosion. Domains such as the deeply incised valleys of the upper cone, the high slope differences of bottom flanks, and the smooth topography of undisturbed flanks discriminates the zones of weakness of the volcano. Lineaments that offset from the regular radial drainage pattern are dominantly oriented WNW-ESE. We find evidence of debris flows and several landslides and scarps in the north and south flanks of Arayat. A debris flow was detected after the Tropical Storm Ketsana (local name: Ondoy) in 2009 at the southeast flank. Its source and extent was delineated and debris flow simulations using Flo-2D software and the 5m resolution DEM derived from SAR were carried out for other valleys. Results indicate high sediment accumulation mostly from watersheds with headwaters starting at the midflanks and lower than expected accumulation from the deeply incised valleys. Old landslides clearly seen in the lm resolution of LiDAR-derived imagery were observed at the basal portion Mt. Arayat. There are evident expressions of arcuate scarps which follow the WNW-ESE trending lineaments and the rough terrain of the landslide deposits. The ability of LiDAR data to exclude features such as vegetation and infrastructure revealed old landslide morphologies which otherwise would have been overlooked. In Arayat, the lineaments, especially the clear structural controls of the large erosional valleys and the location of old landslides, need to be distinguished and classified into those controlled by regional stress and gravity spreading. This will contribute to the identification of weak zones that have a potential for mass wasting hazards. Furthermore, there is a need to continuously observe all types volcanoes, may they be active or inactive, since their edifices undergo fast changing topography even during periods of quiescence.