have been scavenged principally from collections made by the resource extraction industry. Programs designed specifically to collect high-resolution subsurface images using the three-dimensional techniques developed in the search for petroleum could advance earthquake science significantly.

Earthquakes occurring beneath the oceans pose significant hazards to the world population, which is becoming increasingly concentrated along continental coastlines. The main risk in the coastal zones of Cascadia, Alaska, Japan, and Indonesia, for example, arises from thrust faults that intersect the surface many kilometers offshore but are fully capable of generating destructive ground motions and tsunamis. To date, most earthquake research has relied on data from arrays of seismometers, geodetic positioning of benchmarks, and geologic mapping confined to land areas. A major objective of earthquake science should therefore be to extend observational systems and data bases into the offshore environments (Box 6.3). For example, bathymetric maps by various new technologies have revealed ancient slumps and the traces of active faults on the seafloor. Systematic bathymetric mapping in active regions could reveal structures with significant seismogenic or tsunamigenic potential.

BOX 6.3 Marine-Based Earthquake Studies

The oceans offer natural earthquake laboratories and research opportunities not available on land. The geologic structure and history of the oceanic lithosphere, which is considerably different and usually much simpler than the continents, provide an excellent starting place for understanding the fundamentals of fault-zone processes. The seismogenic zone for major thrust faults can be sampled directly only by drilling in the offshore region. The international Ocean Drilling Program has already drilled a number of accretionary prisms to provide ground truth for three-dimensional structural models and sample the active décollement. Ship-based acoustic mapping and seismic reflection and refraction surveys can characterize the three-dimensional structure of marine fault systems at a fraction of the cost of terrestrial surveys. The NSF-sponsored Margins Program has placed a high priority on imaging plate boundaries in the offshore region, and it would be extremely advantageous to follow up such studies with earthquake observatories.

Although offshore earthquake observatories are considerably more expensive than land-based stations, there are no longer major technological impediments to installing them as an integral part of plate boundary networks. Such installations address all of the major aspects of the earthquake problem, but they are especially useful in determining how lithospheric deformation is controlled by lithospheric architecture in relatively simple and well-imaged geologic environments, understanding the details of rupture nucleation and propagation in major thrust systems, and characterizing the strain cycle in regions with the highest rates of relative plate motion. Project NEP

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