REMOTE SENSING/Active Sensors 419 rocks This raises the exciting prospect of being able to relate the current day surface deformation to neotectonic processes and geomorphological features of the past few hundred to thousands of years Volcanoes Earthquakes and volcanoes can share a similar cycle of surface straining associated with the repeated charging of their systems by external forcing (plate motion and magma buoyancy) (see Volcanoes) In the volcanic case, dilational strain during charging of the magma reservoir is released suddenly, perhaps with shallower planar rupture deformation (dyking and surface fissuring) InSAR is having an impact on our knowledge of the volcano cycle as it has for earthquakes In the volcano case, we also have the possibility of using the volume of erupted magma during an eruption to constrain the magma loss event in the reservoir However, the complementarity of InSAR and seismicity in defining the source mechanism of earthquakes is not available in volcanoes Because of this and the inherently more complex three-dimensional shape of pressured magma reservoirs, progress in refining their location and dynamics has been slower Many volcanoes are susceptible to gravitational instability and in particular to spreading under their own load if there is a suitable weak basal layer This has been measured by InSAR A complex mixture of spreading forces, local fault accommodation of these, regional tectonic movements, and pressurizing magma reservoirs mean that the INSAR-measured deformation at many volcanoes may reflect a much more complex system than hitherto realized (Figure 4) Subsidence There are a number of natural and anthropogenic or human-induced mechanisms that can produce shallow and hence local subsidence of the Earth’s surface: for example, soluble rock dissolution, landslides, mining, groundwater/oil extraction, and tunnelling InSAR has been shown to be valuable in monitoring all of these Because of their small spatial extent, it is easier to calibrate the absolute motion Local subsidence events are of considerable concern in urban environments and a variant of the InSAR technique has been developed to take particular advantage of this Buildings and roofs of a variety of shapes and orientations serendipitously provide a source of very strong radar scatterers In the ‘permanent scatterer’ technique of InSAR, the phase change information from these scatterers alone is used in a time series of images (typically at least 20–30) By making some assumptions about the nature of the atmospheric effects and with good knowledge of the topography, Figure A differential radar interferogram of the Etna volcano in Sicily created using two ERS SAR images from 1995 and 2000 Each colour cycle from blue to red represents one fringe of relative motion of 28 mm The parts of the scene where the phase data are too noisy are replaced by a shaded relief image of the topography The displacement pattern of the ground sur face shows apparent LOS uplift of >10 cm of the summit region of the volcano relative to the flanks Also indicated by the F F0 symbols is evidence of relative movement along two faults on the southern flanks The image is about 40 km wide The ERS data were supplied by the European Space Agency these sources of error can be removed and a line of sight (LOS) motion history of the set of scatterers can be measured to accuracies of a few millimetres See Also Engineering Geology: Natural and Anthropogenic Geohazards Europe: Mediterranean Tectonics Remote Sensing: Passive Sensors Sedimentary Environments: Deserts Solar System: Venus Tectonics: Earthquakes Volcanoes Further Reading AGU (1992) Magellan at Venus reprinted from Journal of Geophysical Research 97 (E8, E10), Washington, DC: American Geophysical Union Amelung F, Jonsson S, Zebker H, and Segall P (2000) Wide spread uplift and ‘trapdoor’ faulting on Galapagos volca noes observed with radar interferometry Nature 407: 993 996