photographs, in combination with topographic and geologic maps, have been primary sources of data for earthquake geologists (Figure 4.21). Digital topographic data from more precise remote-sensing platforms have been poised for some time to substantially improve the measurement and interpretation of tectonic landforms (77), but progress has been frustratingly slow. In a few wealthy countries, such as the United States or Taiwan, digital elevation models (DEMs) are available at 30- to 40-meter postings, which is fine enough to be useful for neotectonic and postseismic studies (Figure 4.22); however, the resolution across most of the world is considerably poorer (1-kilometer postings are common). NASA’s Shuttle Radar Topography Mission (SRTM) collected the first global, high-resolution topographic data set, sampling 80 percent of the land surface at 30-meter resolution (78), but national security interests have thus far prevented the release of these data.
Landforms associated with one or a few earthquakes are often so small that study requires resolution of just a few centimeters. Land-based laser-ranging “total stations” have replaced the plane table and alidade as the geologist’s means of producing detailed maps. A new technology that holds great promise for the rapid mapping of the ground surface at very high resolution is light detection and ranging (LIDAR), the laser-based equivalent of radar. LIDAR systems can be mounted on light aircraft equipped with inertial and GPS guidance systems to obtain vertical resolution at the decimeter level (79). An example of data from the 1999 Hector Mine earthquake is shown in Figure 4.23.
Methods based on the electromagnetic spectrum cannot be used to map active tectonic structures on the seafloor, where most major plate boundaries are found. Surface ships with multibeam sonar systems can map bathymetry in swaths several times as wide as the ocean depth, yielding DEMs with a resolution comparable to those currently available for much of the land surface (80). This mapping capability has thus far been focused on the ridge-transform systems of the mid-ocean spreading centers. Less detailed work has been done in oceanic trench environments and on the active continental margins (Figure 4.24). Side-scan sonar systems towed in midwater use the amplitude of acoustic reflection to image small-scale features not visible by swath mapping, such as the lineations due to faulting. Sleds of instruments towed on cables within tens of meters of the seafloor can collect bathymetric data at decimeter levels, although their deployment costs are very high and they are therefore used to survey only small regions of high interest. In shallow water, swept-frequency (“chirp”) sonar systems can penetrate shallow sediments to return detailed images of sedimentary layering and its disruption by earthquake faulting.