The sound of silent earthquakes.

The sound of silent earthquakes

During an ordinary earthquake, things happen quickly. A breaking fault can move at speeds of over a kilometer a second, generating high-frequency seismic waves that travel through the Earth. In fact, seismologists use high-frequency waves with a period of one second to detect earthquakes. Sometimes, though, the Earth rings with very low-frequency vibrations of periods up to an hour long--so-called "silent quakes" because they usually avoid detection. Gregory C. Beroza and Thomas H. Jordan of the Massachusetts Institute of Technology in Cambridge have designed a program to detect and study these low-frequency waves.

Like a bell that rings only in a fundamental tone and certain harmonics, the Earth vibrates with discrete frequencies. By searching through seismic data for any low-frequency signals that match the Earth's known modes, Beroza and Jordan have identified several silent quakes. The origin of these waves is unclear, and certain silent quakes might not be earthquakes at all. However, Jordan suggests some low-frequency vibrations represent accelerated creep along a fault. The researchers hope that studying these silent quakes will help decipher how fractures grow -- an important key to understanding and predicting earthquakes.