Deep Tremor Shakes the Earth

Brett Israel

Date: 14 July 2010 Time: 07:07 AM ET

The Earth doesn’t shake only after violent volcanoes or earthquakes. A recently discovered phenomenon known as "deep tremor" also rumbles the globe.

Technically speaking, the process is called non-volcanic tremor. It was once thought to be caused by magma flowing beneath volcanoes. Today scientists know that this process is triggered deep underground, well in advance of earthquakes, in the area where two tectonic plates scrape past each other.

Non-volcanic tremor isn't as obvious as a volcano, and the phenomenon was previously overlooked as background noise in seismic data. Its main characteristics are a long but soft shaking — a steady grumble deep within the Earth.

With increasingly sensitive seismic technology, deep tremor events have now been detected in subduction zones about 19 miles (30 kilometers) underground. In these zones, one of the Earth's rocky plates dives under another one and sinks into the underlying mantle (the scorching molten rock that underlies the Earth's crust). It's this shifting and sliding of the plates that produces non-volcanic tremor. There is still debate, however, on the precise location of tremor relative to where the plates rub together and exactly how it is generated.

Subduction zones are also places where large earthquakes rupture, so understanding how tremor occurs could allow scientists to develop a useful earthquake-warning tool, suggests a study detailed in the July 15 edition of the journal Nature. Observations of tremor could potentially predict quakes by providing valuable information on plate motion and stress accumulation along faults.

Non-volcanic tremor is not likely to be determined by how long two plates rub together, but by the hardness of the plates where they touch, researchers found.

The new study analyzed deep tremor in the Nankai subduction zone in Japan. Other subduction zones such as Alaska, Cascadia in the Pacific Northwest, and Costa Rica have had documented cases of non-volcanic tremor.

This article was provided by OurAmazingPlanet, a sister site to LiveScience.