Disaster in Slow Motion: Widespread Land Subsidence in and Around Metro Manila, Philippines Quantified By Insar Time-Series Analysis

Extensive land subsidence in and around Metro Manila, largely from overuse of groundwater, is orders of magnitude more rapid than sea-level rise from global warming. It enhances the exposure of its residents to worsening floods and tidal incursions, and greatly exacerbates the storm-surge threat. Additionally, differential movements at pre-existing faults causes considerable damage to overlying properties. The subsidence has been recognized and quantified since the 1990s from the rates at which wells apparently rise as the ground around them sinks, the frequency with which roads have had to be raised, and social surveys of historical flooding and tide heights. To assess the impact of the subsidence and devise appropriate countermeasures requires that the subsiding areas be delineated and their subsidence rates be measured more precisely. Toward those ends, 2003-2010 Envisat and 2007-2011 ALOS PALSAR-1 imagery were processed using the PSInSAR method. During these periods, Manila, CAMANAVA, Rosario, San Pedro, Las Piñas and Dasmariñas subsided by at least 2-4.2 cm/yr, due mainly to over-extraction of groundwater. InSAR data also reveal subsidence in highly urbanized areas that also rely heavily on groundwater along southern segments of the West Marikina Valley Fault, but none on the northern segment. Land subsidence along the coast.