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 Seismology
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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