Before the 20th century there were few seismometers. So Earth’s dynamic geophysical processes were poorly understood. Today the potential for understanding those processes is enormous: The number of seismic instruments is continually increasing, their data are easily stored and shared, and computing power grows exponentially. As a result, seismologists are rapidly discovering new kinds of seismic signals in the frequency range 0.001-100 Hz, as well as relatively large nonseismic displacements, monitored by the global positioning system, occurring over days or weeks.
Such new signals manifest geophysical events very different from classical earthquakes. A classical earthquake occurs when one crustal block slips past another at a fault surface (see the box on page 33). The stored elastic energy released in such an earthquake, broadcast over a wide frequency range, produces a seismic signal that stands out against background noise (see figure 1(a)). By contrast, a glacial earthquake due to the...
