being tested in a real-time mode (see Box 4.2). Seismic monitoring of earthquake activity can enhance the accuracy of the seismicity on which this method is based and provide physical insight into the tectonic processes that contribute to the phenomena. These include changes in earthquake occurrence rates, earthquake clustering, near-simultaneous occurrence of earthquakes over a wide region, and changes in the proportion of moderate- and small-magnitude earthquakes.
Silent earthquakes. During the past decade, newly installed dense Global Positioning System (GPS) monitoring systems in the Pacific Northwest and Japan have revealed the occurrence of “silent earthquakes” on subduction plate interfaces. These silent earthquakes are similar to ordinary earthquakes in that they involve the relative movement of one side of a fault past the other, but they differ from ordinary earthquakes in that this slip occurs over a time interval of days to weeks, not the seconds to minutes of fault movement that occurs in ordinary earthquakes. These silent earthquakes are not completely silent—they cause muffled rumblings that are detected by seismic monitoring instruments. These new observations may constitute the means to observe the evolution of deformation on the plate interface that precede the occurrence of an earthquake. For example, a region of the plate interface that has not slipped recently—but lies adjacent to a region that has—may be a candidate for an impending earthquake. Such observations could be the basis for more focused monitoring of particular regions, providing some prospect for the development of an earthquake prediction capability. Regional networks of seismic recording instruments, such as those planned for the ANSS, provide an important component of the earthquake monitoring system needed for the potential development of such a prediction capability.