HIGH-RESOLUTION SEISMIC IMAGING FOR EARTHQUAKE HAZARD ASSESSMENT: APPLICATIONS, EXAMPLES AND FUTURE DIRECTIONS

High-Resolution seismic profiling is currently used in three modes to help assess the earthquake hazard in a region: 1) reconnaissance of extensive areas to look for faults; 2) verification of suspected faults beneath topographic or geophysical features; and 3) characterization of near-surface materials or the geometry and age of faults. Reconnaissance profiling of tectonically active areas can reveal important new features, even if the project is initially designed only to cross a specific fault zone. Where applicable, reconnaissance using marine profiling is preferred because the speed of acquisition is an order of magnitude faster than on land and the costs are correspondingly lower. Verification consists of using seismic reflection profiles to determine whether a geomorphic feature or a geophysical lineament is caused by a fault, whether known faults break young strata near the surface, and to ascertain the exact location of the fault in preparation for sampling with a trench or core. Although verification is simple in concept, faults with small amounts of displacement or predominantly strike-slip motion can be difficult to recognize. Characterization of faults includes determining the style of faulting, the attitude of the fault, the age and amount of motion in the last events, and the recent slip rate. Determining these key fault parameters can be done by directly imaging the fault and displaced bedding, or by analysis of folds and growth strata above deeper faults. Characterization is often done in conjunction with trenching and geologic studies to obtain age dates on key horizons and to better interpret the geologic results. Seismic profiling is also becoming widely used in hazard studies to measure material properties beneath specific sites, including the P and S wave velocity, the depth to bedrock and the reflectivity of the shallow strata. Further developments in seismic profiling will focus on reducing costs, increasing resolution, making more use of S-waves, imaging in 3 dimensions and utilizing seismic techniques in increasingly difficult urban environments.