was installed in Japan in 1988 by the National Research Institute for Earth Science and Disaster Prevention (1), and the first image of earthquake faulting using interferometric synthetic aperture radar (InSAR) was constructed in 1992. Paleoseismologists produced a preliminary 1000-year history of major ruptures on the San Andreas fault in 1995 and discovered a prehistoric moment magnitude (M) 9 earthquake in the Cascadia subduction zone in 1996. The first three-dimensional simulations of dynamic fault ruptures using laboratory-derived, rate- and state-dependent friction equations were run in 1996.

The unprecedented flow of new information opened by these advances is stimulating research on many fronts, from fault-system dynamics and earthquake forecasting to wavefield modeling and the prediction of strong ground motions. This chapter summarizes the state of the art in the main observational disciplines; it focuses on new technologies for observing the active Earth, and it highlights through a few examples the richness of the data sets now becoming available for basic and applied research.

Seismology lies at the core of earthquake science because its main concern is the measurement and physical description of ground shaking. The central problem of seismology is the prediction of ground motions from knowledge of seismic-wave generation by faulting (the earthquake source) and the elastic medium through which the waves propagate (Earth structure). In order to do this calculation (forward problem), information must be extracted from seismograms to solve two coupled inverse problems: imaging the earthquake source, as represented by its space-time history of faulting, and imaging Earth structure, as represented by three-dimensional models of seismic-wave speeds and attenuation parameters. Because seismic signals can be recorded over such a broad range of frequencies—up to seven decades (2)—seismic signals can be used to observe earthquake processes on time scales from milliseconds to almost an hour, and they provide information about elastic structure at dimensions ranging from centimeters to the size of the Earth itself.

Seismic waves span a wide range of amplitude, as well as frequency. The ground motions in the vicinity of a large earthquake can have velocities greater than 1 meter per second and accelerations exceeding the pull of gravity (1g = 9.8 m/s²). The lower limit of seismic detection is typically eight orders of magnitude smaller, set by the level of the ambient ground