clustering and to provide the high-resolution structural information necessary for investigating the architecture of fault systems and predicting ground motions. Probing the structure of sedimentary basins will require the extensive use of artificial-source reflection and refraction seismology, as well as seismographic data from deep boreholes to calibrate the effects of near-surface layering. Plausible 10-year objectives are to determine the structure of high-risk urban areas well enough to model the surface motions from deterministic seismic sources at all frequencies up to at least 1 hertz and to formulate useful, consistent, stochastic representations of surface motions up to at least 10 hertz.
Geodetic instrumentation should be deployed for observing crustal deformations within active fault systems with enough spatial and temporal resolution to measure all significant motions, including aseismic events and the transients before, during, and after large earthquakes. This endeavor will require combining pointwise measurements using strainmeters and GPS with continuous deformation images from a dedicated U.S. InSAR satellite mission. Laser and radar altimetry are needed to produce the precise digital elevation models for investigating surface faulting and the deformations caused by buried faults.
The determination of fault slip rates and rupture histories over many earthquake cycles will require the combination of geologic field study and high-precision age dating. At present, the long-term slip rates of most major faults in North America are either unknown or, at best, constrained by geologic measurements at only one or two sites. Slip-rate data are especially lacking in contractional provinces, where many questions still remain about how strain is partitioned among the major faults and between seismogenic faults and aseismic folding. New techniques of tectonic geomorphology can address these issues. To investigate the important problem of earthquake clustering, paleoseismologists must date slip episodes at particular points on a fault well enough to establish event sequences. The objective should be catalogs of large earthquakes spanning thousands of years on all major faults where such paleoseismic investigations are feasible.
Better information on microscale processes is needed to formulate realistic macroscopic representations of the strength variations and the dynamic response of fault materials. Advances in laboratory-based techniques will be required to elucidate the dynamic phase of fault response, in which rapid large slips may cause large temperature excursions and weakening by pressurization of pore fluids and/or melt generation. Field examination of exhumed faults should explore the mechanical importance of shear localization structures and the evidence for fluids and/or local melting. Borehole data are needed to extrapolate laboratory results on laboratory-scale samples to natural faults, elucidate the generation of