Instrumental improvements and densification of the regional seismic networks, as proposed in the ANSS initiative, will contribute substantially to these objectives. As currently planned, the ANSS will comprise about 6000 seismic stations in urban areas with significant seismic risk (Box 6.1). About half of these instruments will be free-field and half will be installed in buildings and other structures. This network will improve seismic hazard maps and will also enable engineers to correlate ground motions with building performance. The deployment of seismic instrumentation that records both strong and weak motions will unite the efforts of strong-motion and network seismologists whose often separate studies of site response, scattering, attenuation, and high-frequency radiation would benefit from enhanced collaboration.

A major new source of structural data will come from the USArray component of the EarthScope project. The “Big Foot” array will provide nearly uniform structural control nationwide on lateral scales of tens of kilometers (Figure 6.6), while the high-density “flexible” array will allow the imaging of specific features at higher resolution using active-source techniques. These portable arrays can also be used to record aftershocks from larger earthquakes. In addition to providing information about rupture processes and driving stresses, such studies will yield valuable information on fault waveguides for constraining crack density, continuity of fault planes, and evolution of fault strength through the seismic cycle. Portable arrays recording background seismicity will also shed light on the effect of basins and basin edges on ground motion. Probing the detailed structure of sedimentary basins in high-seismic-risk areas will require extensive use of reflection seismology techniques using Vibroseis trucks or other high-energy sources and, at a few selected locations, measurements from deep boreholes.

In regions of high seismicity, crustal and uppermost mantle structure can be resolved using waveform data from local and regional earthquakes recorded on permanent and temporary stations. High-priority targets include the investigation of fault-related offsets of major structural features, such as the Moho discontinuity, which shed light on the rheology of the lower crust and connect observed surface motions with underlying mantle flow. A better knowledge of Moho topography would also improve the prediction of strong motions from SmS reflections at ranges of 50 to 150 kilometers. Similar data are needed on intracrustal reflectors.

As richer seismic data sets are collected, a primary challenge will be to set up a computational framework for the systematic refinement of three-dimensional wave-speed and attenuation models and the use of these models in the calculation of synthetic seismograms. This framework should be based on a unified structural representation that includes not only seismic propagation parameters but also geologic, gravimetric, and



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