Abstract
Remote sensing techniques have shown their capabilities in detecting ground deformation associated with a variety of natural hazards. However, most studies focus on relatively small areas, providing only spatially limited information, and many rely only on conventional Interferometric Synthetic Aperture Radar (InSAR), which lacks temporal resolution. The goal of this work is to perform systematic InSAR time series surveys at regional scales to improve volcanic and land subsidence hazards characterization in Indonesia and central Mexico. An arc-wide InSAR time series survey along the west-Sunda arc, Indonesia, revealed unambiguous evidence that six dangerous explosive volcanoes inflated during 2007-2009, three of which erupted afterward (Chapter 2). A similar survey along the Trans-Mexican Volcanic Belt (TMVB), Mexico, did not reveal any volcanic inflation, reflecting a lower activity level (Chapter 3). Comparison between the two regions allowed characterization of the cases where InSAR can be used as a forecast tool for volcanic unrest. Closed volcanic systems present deformation cycles, as observed in the west-Sunda arc, eruptions being preceded by edifice inflation, while no significant ground deformation occurs prior to unrest at open volcanic systems (Chapter 3). The TMVB survey also revealed subsidence in the Parícutin lava field (Chapter 5). Detailed study highlighted that processes related to lava emplacement, even decades ago, especially due to cooling result in several centimeters per year of ground deformation. Such processes need to be considered to avoid misinterpretation of deformation fields in terms of changes in the magma plumbing systems at polygenetic volcanoes. Magma reservoir depths were estimated using inverse modeling and regional trends in storage depths were revealed (Chapter 2). A global data compilation showed that these trends are explained by the crustal structure and stress regime of volcanic arcs (Chapter 4). Shallow magma reservoirs preferentially develop in young and thin crusts undergoing extensive or strike slip stresses, while the rates of melt generation in the mantle have no direct effect on crustal magma ascent. These observations have implications for volcanic hazard assessment and confirmed that large-scale surveys are useful to better understand general processes related to magma ascent. Regional InSAR time series surveys were also applied to detect, map, and monitor land subsidence with high spatial and temporal resolutions in these two regions (Chapter 6 and 7). Results obtained show that land subsidence is a widespread problem and immediate actions are necessary to mitigate the risks associated with it. Combining the regional averaged velocity maps with other data sets such as land use, surface geology, and faults maps enabled identification of the parameters that influence the spatial distribution and the temporal evolution of land subsidence. Groundwater extraction appeared as the main cause of the rapid subsidence both in Indonesia and Mexico, confirming that subsidence is associated with a decrease in water access and quality. In Indonesia nine subsiding areas were detected, including six major cities, the fastest subsidence occurring in densely populated coastal areas particularly vulnerable to flooding (Chapter 6). A continuation of the high subsidence rates will put these areas below relative sea level within a few decades. In central Mexico twenty-one subsiding areas including seventeen major cities were identified (Chapter 7). High velocity gradients exist in all the locations and often correlate with preexisting faults, motion on these faults being driven by water extraction. These high gradients present a threat to the integrity of buildings and infrastructures, some of these constructions being included in the UNESCO World Heritage list. The averaged velocity maps also revealed that faults act as barriers to groundwater flow, suggesting that they partition the aquifer systems at depths. Such observations have major implications for aquifer dynamics and need to be considered to achieve a more sustainable use of the groundwater resources at regional scales.