using strainmeters and tiltmeters, which are more sensitive to short-term transients than network-based geodesy (Figure 4.19). However, no precursory strain signals have been identified reliably (75), presumably because the nucleation zones at depth are too small to cause measurable strains at the Earth’s surface.
The role of subseismic slip events in setting the stage for future earthquakes is unknown. Such events have been observed within several plate boundary fault zones (see above), but so far no convincing relationship to seismic fault slip has been demonstrated. The deformations due to subseismic events decay much more rapidly with distance from the source than seismic waves, which makes them hard to observe. Moreover, available measurement techniques leave important parts of the space-time spectrum poorly covered (Figure 4.19). Establishing the relationship between subseismic strain and future earthquakes is a clear target for research in tectonic geodesy.
Earthquake geology was pioneered by the postearthquake investigations of Charles Darwin, G.K. Gilbert, and B. Koto in the nineteenth century. It has since evolved into three subdisciplines: neotectonics, paleoseismology, and fault-zone geology (76). The methodology and research issues comprised by the first two are the subject of this section; the third is incorporated into a following section on fault and rock mechanics.
Geologists have steadily improved their acuity in reading subtle features of the geologic record, setting the stage for the process-oriented investigations that now lead the geologic study of active fault systems. For example, earthquake geologists have collaborated with paleoclimatologists to improve understanding of the youngest part of the sedimentary record—the Holocene, comprising rocks up to about 10,000 years old—which contains substantial information about prehistoric earthquakes. At the same time, technological developments have contributed new tools for geologic exploration in both space and time.
Remote Sensing Landforms are the most readily accessible expression of active tectonics because they can be viewed remotely, for example, by space-based tonal images from System Probatoire pour l’Observation de la Terre (SPOT) or Landsat satellites (Figure 4.20). The details of land-form topography are particularly useful in measuring the rates of fault slip and associated deformations. For many years, stereopairs of aerial