BOX 6.4 Urban Hazard Characterization

An important objective of earthquake research is to develop the capability to produce detailed maps of earthquake shaking and other seismic hazards for urban areas with high seismic risk, as well as site-specific time histories of ground shaking in these urban areas expected during large earthquakes. These hazard maps and time histories should incorporate local site response and basin effects, rupture directivity and dynamics, and time-dependent probabilities of large earthquakes on all relevant faults. The steps necessary for improving the prediction of ground motions in urban areas include the following:

  • characterization of the recurrence times and time-dependent probabilities of large earthquakes on faults capable of producing strong motions, as described in Section 6.1;

  • instrumentation of urbanized sedimentary basins with dense seismograph arrays that can record both weak and strong ground motions and sensors installed within structures to evaluate building performance during strong shaking, as envisaged under the 1999 ANSS program plan; these arrays should be supplemented with borehole arrays that measure how motions change as waves propagate upward from rock through the sedimentary section;

  • development of detailed three-dimensional models of seismic velocities, density, and attenuation using seismic reflection-refraction, gravity, travel times of seismic waves, surface-wave dispersion, and waveform modeling; detailed measurements of shear-wave velocity and nonlinear properties are required in the upper 300 meters, data on the latter would come from borehole arrays; recordings of small and moderate earthquakes can be used to validate the three-dimensional models;

  • simulations of ground motions for potential large earthquakes using validated three-dimensional models of seismic velocity and attenuation to make detailed maps of various shaking intensity parameters; the most advanced simulations would include dynamic rupture, three-dimensional basin effects, and nonlinear soil response—the latter would be used to assess liquefaction potential (with surficial geology and water table information) and landslide probability (with slope data and material properties); and

  • implementation of the research results through cooperative efforts with engineers, risk evaluators, urban planners, and emergency management planners and education of the general public.

station spacing as short as 1 kilometer, or even less in some areas. These dense arrays should have sufficient dynamic range to record the strong shaking from large events as well as the weaker ground motions from small and moderate earthquakes. Small earthquakes recorded on such networks will furnish the dense data coverage needed for constructing three-dimensional models of sedimentary basins, while the strong-motion data will be essential for validating the ability of numerical models to predict ground shaking in future large earthquakes.



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