Mysterious slow earthquakes, which happen over the course of anywhere from hours to months, could help prevent faster, larger earthquakes by relieving stress within the Earth, researchers suggest. 

Regular earthquakes are caused by rapid movements along cracks in the Earth's crust, called faults. Slow earthquakes, on the other hand, take much longer to rupture.

A variety of slow quakes seem to exist: Slow slip events can last for days to weeks, shifting the Earth as much as an ordinary earthquake of magnitude 6 to 7 would in mere moments. Non-volcanic tremors caused by the shifting and sliding of tectonic plates can weakly rock the Earth for hours to weeks, and seem to be swarms of hundreds to thousands of tiny seismic pulses of magnitude 2 or smaller. Very low-frequency earthquakes last for tens of seconds and shake at magnitudes of 3.5 to 4.

The connections between all these kinds of slow earthquakes have been murky. To learn more, a group of scientists investigated the Nankai trough, an active fault zone near southwestern Japan that is rocked by giant earthquakes every century or so — most recently in 1946, when a magnitude-8.2 event killed an estimated 1,300 people. The researchers focused on all three types of slow quakes, which occurred in the western margins of the zone in 2003 and 2010.

The team found that a long-term slow slip event seemed to have caused shallow earthquakes of very low frequency near the Nankai trough as well as nonvolcanic tremor at depths of 18 to 25 miles (30 to 40 kilometers).

The investigators suggest that slow earthquakes might draw away some of the energy that builds up before major earthquakes, essentially helping to defuse them. They added that looking for activity linked to slow quakes could therefore shed light on whether a major quake is coming.

"This is very important because the next large earthquake is anticipated to occur in several tens of years and the determination of the slow earthquake activity might help to assess the earthquake damage estimation," researcher Hitoshi Hirose, a seismologist at the National Research Institute for Earth Science and Disaster Prevention in Tsukuba, Japan, told OurAmazingPlanet.

Hirose noted the sources of the major earthquakes anticipated from the Nankai trough and the shallow slow earthquakes the researchers detected there are both offshore. One future avenue of research to learn more about both would be the development of an ocean bottom observation network, "which is now in part being developed by Japan's Agency for Marine-Earth Science and Technology," Hirose said.

Hirose and his colleagues detailed their research in the Dec. 10 issue of the journal Science.

This article was reprinted with permission from OurAmazingPlanet.

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