The geopolitical realities of remotely sensed data became plain in the aftermath of the 11th September attack by terrorists on the United States. The US National Imagery and Mapping Agency (formerly the Defense Mapping Agency of the Department of Defense) placed a moratorium on release of digital elevation data derived from NASA’s February 2000 Shuttle Radar Topography Mission, “in the interests of national security”. The SRTM, which used radar interferometry from dual antennae on a 60 metre long boom, was intended to satisfy the huge demand from Earth scientists for digital elevation models of the continents for a large range of applications, ranging from accurate hydrological mapping to sophisticated mathematical analyses of landforms. An accurate, high-resolution DEM is central to rapid topographic mapping of those many parts of the world where published scales do not exceed 1:250 000. NIMA also maintains the classified DTED Level-1C global elevation data set, derived from a variety of sources, including clandestine aerial and satellite photography, and which has a resolution as precise as 30 metres. SRTM data are reported to be more revealing. At the heart of cruise-missile guidance and the real-time imaging radar used for navigation in low-flying, all-weather military aircraft lies DTED Level-1C data. Such facilities are not known to be in the possession of, or under development by any agencies other than the military of a small number of developed countries, for obvious economic reasons. Oddly, elevation data as revealing as DTED Level-1C for the whole of the USA and its territories are still available freely from the US Geological Survey. Anyone “targeting” installations, either for military or more innocent purposes, need look no further than the growing number of commercial image providers who sell satellite images with spatial resolutions as good as 1 metre. Indeed, some such companies currently promote their wares through images of Manhattan Island in the aftermath of 9th September, and there is a thriving business in selling aerial photographs of real estate with resolutions up to the 10 centimetre level.
Browsing through the archive of data from the Terra satellite, particularly those from the ASTER instrument (visit http://edcimswww.cr.usgs.gov/pub/imswelcome/ ), reveals a disproportionate focus on Afghanistan compared with much of the rest of the world. The majority of Afghan images were captured before 11th September, and the area is hardly a priority for scientific research. ASTER produces stereographic images with a 15 metre resolution, suitable for producing high-quality digital elevation models that rival those of SRTM. It would not be surprising to discover that US and British Special Forces engaged in Afghanistan not only carried large-scale topographic maps derived from ASTER images, but also commercial Ikonos 1-metre images, that are capable of pinpointing vehicles and concentrations of people. Nor is it surprising that relief agencies, intent on delivering humanitarian supplies to emergencies of many different kinds in nearly unknown terrain, rarely if ever have such sophisticated navigational aids.
Interferometric radar and faults of the Mojave Desert
Though it requires considerable computing power and specialized software, the use of “before” and “after” radar data to detect small-scale subsidence or shifts in the horizontal plane, is a potentially powerful tool in neotectonics (see Radar analysis of Turkish earthquake, August 2001 Earth Pages). Motion detection by such radar interferometry becomes even more useful as historic radar images accumulate. The workhorse for radar interferometry is the European Space Agency ERS series of satellites, which produce synthetic aperture radar images about 150 km wide along the same track, orbit after orbit. The system has operated since 1992, so there are rich possibilities for multitemporal use of the distance-measuring capacity inherent in radar imagery. Means of assessing the regional build-up of strain in seismically active areas are important in earthquake prediction, and such synopses help understand the tectonics at work there.
In terms of seismicity and tectonics there is no better studied area than that extending from the Pacific coast of southern California across the San Andreas Fault and the Mojave Desert. Radar interferometry provided by 25 pairs of ERS images from 1992 to 2000 produces a spectacular picture of the gradual development of ductile strain underlying this risky area (Peltzer, G. et al. 2001. Transient strain accumulation and fault interaction in the Eastern California shear zone. Geology, v. 29, p. 975-978). Unsurprisingly, shear strain along the San Andreas fault system shows up well. The Garlock Fault that marks the NW flank of the Mojave is apparently resting after 10 thousand years of motion that averaged 7 mm per year. The authors focus on displacements associated with the diminutive, by Californian standards, Blackwater and Little Lake fault systems, which trend SE-NW to link the epicentres of the 1872 Owens Valley earthquake and that at Landers in 1992. Within 10 km of these aligned faults are clear signs of a step in strain rate, that suggests that the lineament lies above a major, active ductile shear zone; perhaps the birth of a new fault system. Should this system fail in a brittle fashion it is likely to result in an event with a magnitude greater than 7 on the Richter scale, and a surface break more than 100 km long. Peltzer et al. have achieved a test of concept for interferometric radar’s use in seismic risk assessment, that can be deployed anywhere, given the computing resources. Their work transcends after-the-event studies that do little to assist the victims of earthquakes.