FIGURE 4.5 25 years of CMT solutions (1976-2000) for the region surrounding Africa. The availability of broadband data has made it possible to describe global seismicity in terms not only of location and magnitude, but also of fault mechanism, thereby greatly enhancing our view of active tectonics. SOURCE: Harvard CMT group.

Regional Seismic Networks Owing to their sparse station coverage, global networks do a poor job of detecting and locating events with magnitudes less than about 4.5 (Figure 4.2), and their sampling is too crude for investigating how waves are produced by fault ruptures, especially the near-fault radiation that generates the complex patterns of strong ground motions observed in large earthquakes. To deal with these problems, seismologists have densified station arrays in areas of high (or otherwise interesting) seismicity. Regional networks are collections of seismographic stations distributed over tens to hundreds of kilometers, usually as permanent facilities. The information supplied by regional networks services three overlapping but distinct communities: (1) scientists and engineers