centers and the virtual strong-motion database (in fact, a meta-database) recently set up by the Consortium of Organizations for Strong-Motion Observation Systems. However, as in broadband regional seismology, the U.S. effort falls short of the Japanese, who have created a database system called Kyoshin Net (K-Net), managed by the National Research Institute for Earth Science and Disaster Prevention, to archive and distribute data from the dense array (25-kilometer spacing) of 1000 digital strong-motion stations deployed throughout Japan (24). In Taiwan, a strong-motion network of 614 stations provided unprecedented strong ground-motion data during the 1999 Chi-Chi earthquake.

The 1999 Izmit, Turkey, and Chi-Chi, Taiwan, earthquakes (M 7.4 and 7.6, respectively) have substantially increased the number of strong-motion records for large earthquakes, allowing detailed mapping of the ruptures in time and space (Figure 4.8). Yet, despite more than 70 years of strong-motion seismology, the data coverage remains poor. There are few strong-motion recordings for subduction-zone earthquakes with magnitude greater than 8 and none for magnitude greater than 9. Intraslab earthquakes of M 7 and larger are also poorly sampled, yet they pose a substantial hazard to major cities around the world, as evidenced in the 2001 El Salvador earthquake (M 7.6) and the 1949 (M 7.1), 1965 (M 6.5), and 2001 (M 6.8) events beneath the Seattle-Tacoma metropolitan area. Likewise, there are no close-in recordings (closer than 50 kilometers) of intraplate earthquakes in the central and eastern United States for magnitudes greater than about 5.2 and few worldwide for interplate earthquakes with magnitudes greater than about 7.3. The improved national monitoring structure planned in the framework of the ANSS is clearly needed to remedy this situation (see Chapter 6).

Portable-Array Studies

Portable arrays of seismometers augment the data from permanent monitoring networks by increasing the recording of seismicity in reconnaissance studies and during periods of anomalous activity, including aftershock sequences and swarms. They are also used to image the architecture of fault systems and other aspects of crustal structure, such as sedimentary basins, that affect the amplitude and duration of strong motions. Until recently, this mode of operation was limited to short-period seismometers with low dynamic range, but large pools of broadband instruments are now efficiently organized within the IRIS Program of Array Seismic Studies of the Continental Lithosphere (PASSCAL) (25) and the USGS (26). Subsets are available for deployment after a major earthquake in a coordinated effort called the Rapid Array Mobilization Program (RAMP). These deployments have been used to determine the

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