In the past, scientists have not really known what causes peculiar “sounds” (seismic waves) coming from geologic fault lines, which are sometimes (but not always) located between segments of the Earth’s crust that move relative to each other—what are called tectonic plates.

However, they just discovered the cause of these strange sounds to be the slow sliding and grinding of a higher tectonic plate over a lower tectonic plate. For example, these sounds have been heard along such major faults as the San Andreas Fault in California.

Scientists have been studying these soft seismic waves for over five years. With this discovery, they are now calling a series of these “sounds” a silent earthquake, or tremor, Thus, a tremor is a very small magnitude earthquake that can only be heard with very sensitive seismometers, those measuring devices specially made to listen for seismic waves produced by earthquakes and motion within the Earth.

In fact, American geophysicist Gregory C. Beroza, of Stanford University (Stanford, California) says that such a tremor might produce rumbling sounds for hours to weeks. What is important is the possibility that such sounds, heard for hours, days, or even weeks, could be an early warming call for the real thing: a large magnitude (major) earthquake. 

The researchers say that the next time a silent earthquake (tremor) happens it might help scientists forecast an upcoming major earthquake. However, they also admit that much more research and study is needed before they have a good understanding of the physical forces at work underneath the Earth’s surface.