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Seismic wavesSeismology from Space

The SESWG is intrigued by the possibility of seismic imaging from space. Spaceborne seismology is a logical extension of spaceborne surface-change detection by SAR, radar, and GPS. The Southern California Integrated GPS Network (SCIGN) has observed near-field strain-wave propagation from the 1999 Hector Mine earthquake. It is likely that a continuously observing spaceborne system could image the occurrence of “silent” or “slow” earthquakes as well as the propagation of ground displacement by surface waves at scales of continents. The required technology, such as 30-m lightweight antennas, warrants early investment, and the specific data requirements for imaging from geosynchronous orbit should be defined.

The Solid Earth Beneath the Oceans

Over 70% of the solid Earth lies beneath the oceans, and many geologic processes of global significance occur on and beneath the ocean basins and margins. Some of these processes have analogues on land, but others are quite distinct. For example, at more than 50,000 km in length, the global mid-ocean ridge system is the dominant contributor to volcanic activity at the Earth's solid surface. Mid-ocean ridges are the locus as well of pervasive hydrothermal systems that host unique biological communities not found at terrestrial volcanoes. Subduction zones, where great faults mark the sites of convergence between two tectonic plates, are found almost exclusively beneath the seafloor. Tsunamis, with their frequently devastating effects at coastal regions, originate by motion at the Earth–ocean interface.

Future satellite missions will extend observations of the solid Earth beneath the oceans to smaller spatial scales and will enhance global understanding of temporal variability. Although some solid-Earth missions will have limited application to submarine processes, NASA should be watchful for future opportunities
to ensure that spaceborne measurements are utilized to their full capacity.

Subsurface Imaging

Spaceborne subsurface imaging was first demonstrated in radar images from the Spaceborne Imaging Radar (SIR-A and SIR-B) missions, from which sub-Saharan drainage channels were identified beneath desert sands. Applications of this capability range from the estimation of ice cap thicknesses to the determination of subsurface properties such as geological structure, lithology, and soil moisture. Radar subsurface imaging combined with hyperspectral imaging could prove extremely valuable for remotely
mapping shallow subsurface characteristics that would make a region more vulnerable to natural hazards. For example, the capability to recognize areas of soft sediment or high moisture content could help delineate regions prone to strong shaking or possible liquefaction during earthquakes.

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