source parameters of aftershocks and their relationships to the main shocks—important data for studies of rupture propagation, postseismic relaxation, and stress transfer. Recordings of aftershocks have also begun to elucidate the causes of anomalous ground shaking and damage concentration, including basin resonance, basin-edge effects, and Moho reflection (see Section 3.1). Various forms of telemetry are making it possible to monitor state of health and to retrieve ground-motion data in near real time, allowing portable arrays to be integrated with permanent seismic monitoring systems for a wide range of seismic applications.
Investigations of Earth structure have always figured prominently in the study of earthquakes because they frame the interpretation of seismograms in terms of source processes. Indeed, the problems of discovering the space-time structure of faulting and the three-dimensional variations in the Earth’s elastic properties are strongly coupled and must be worked out together, either through joint inversion of the seismograms or iteratively through successive approximations. The primary seismological parameters needed to specify Earth structure are the local speeds of the two basic types of seismic waves, compressional (vp) and shear (vs), their associated attenuation factors, and the mass density (27). The variations in Earth structure that can be resolved are limited by the size and spacing of the seismic array and the distribution of seismic sources used to illuminate the array. Global networks can therefore determine worldwide structure at relatively low spatial resolution (Figure 4.4), whereas regional and local networks give finer details but only within more limited volumes of the Earth (Figure 4.9).
Portable arrays are useful in enhancing the structural resolution at spatial scales below the station spacing of permanent arrays. They can be deployed in two basic modes of observation: (1) to record artificial sources—explosions, mobile ground-shaking devices such as Vibroseis, or marine air guns—by high-frequency sensors (active-source experiments) and (2) to record signals from natural events, either regional or teleseismic earthquakes (passive experiments). PASSCAL experiments use both modes. Shallow structure (in the upper 2000 meters) can be imaged with highly portable, multichannel systems that record waves reflected from subsurface discontinuities, using hammer blows or small charges as sources. For example, in the Los Angeles Region Seismic Experiment (LARSE), researchers used air guns and explosions to construct images of the subsurface structure that may lead to a better understanding of earthquake hazards in southern California (Figure 4.10). These systems have proved very effective in delineating fault planes within sedimentary ba-