Subsiding deltas, sinking cities, and sea-level rise
Flooding due to groundwater extraction and delta subsidence
The coastal areas of Southeast Asia are some of the most densely populated in the world. The combination of over-exploitation of groundwater, oil and gas extraction, and natural subsidence of deltas has led to rapid subsidence in many areas. Subsidence of coastal land results in an increase in relative sea level, leading to flooding and increased susceptibility to tropical storms and typhoons. The rapid withdrawal of groundwater is also unsustainable for meeting the future water needs of these populations, especially when the pumping results in salt-water intrusion of the aquifers.
Eric and I are working on better understanding the subsidence in various SE Asian cities, using a combination of InSAR and GPS data. For example, we are working in collaboration with scientists from ITB, Indonesia, to study subsidence in Jakarta and Semarang. We plan to install continuous GPS stations in early 2016. And we are processing up-to-date InSAR time series using data from the ALOS, ALOS-2 and TerraSAR-X satellites.
Previous studies have estimated extremely high rates of subsidence for these cities, of 22 cm/yr and 13 cm/yr for Jakarta and Semarang, respectively, over the period 2007-2009 (Chaussard et al., 2013). Leveling data taken during the period of 1982 to 1991 indicate up to 2.4 m of subsidence in northern Jakarta, with rates of up to 25 cm/yr [Abidin et al., 2011]. In addition, in recent years this area has experienced high rates of sea-level rise from oceanographic effects. The societal impacts are clear; large areas of these cities are regularly subject to the dangers posed by “rob” (an Indonesian term for inundation of the land by the sea), with roads impassible and entire neighborhoods flooded.
Regional variations in sea level from tectonic effects
Tectonic changes can have significant effects on crustal deformation, the geoid, and relative sea level (RSL). Indeed, the tectonic impacts on RSL in our region can be greater than those predicted as a result of climate change. In the case of earthquakes, these changes can occur suddenly, as coastlines uplift or subside by up to many meters. The changes can also occur over many decades as a result of interseismic or postseismic processes, or periodically in the form of transient slow-slip events. Although these effects are (mostly) recovered elastically over the course of the earthquake cycle, they are occurring in the context of ever-increasing populations living along affected coastlines, particularly the case in SE Asia. The societal effects of these tectonic-induced sea-level changes are therefore becoming increasingly significant, and important to consider in future projections for sea-level change. Additionally, tide-gauge and gravity measurements made in tectonically active areas cannot be interpreted without consideration and modeling of the tectonic setting.
Along the Sumatra subduction zone, the recent series of great earthquakes, and their ensuing postseismic deformation, have reshaped regional coastlines. Qiu Qiang has been working on modeling these effects using a decade of Sumatra GPS Array (SuGAr) data demonstrating dramatically the ups and downs of land elevation close to the earthquake sources. Vertical coseismic displacements as large as ~2.9 m have been recorded by the SuGAr (an uplift at Nias, during the 2005 Mw 8.6 earthquake), and vertical postseismic rates on the order of tens of mm/yr or greater (e.g., in northern Aceh, one station has been uplifting at a rate of ~34 mm/yr since the 2004 Mw 9.2 earthquake, while in southern Simeulue a station has been subsiding, on average, by ~39 mm/yr since 2005, with higher rates immediately after the earthquake). Further afield, viscoelastic relaxation of the mantle is causing widespread regional changes. For example, postseismic deformation following the 2004 Sumatra-Andaman earthquake has caused subsidence of 20-30 mm/yr along the Thai coastline. This subsidence – and therefore relative sea-level rise - will continue for many years to come.