The source of mysterious rumblings that occur in fault zones has been discovered 鈥� they are caused by the very slow slide of one tectonic plate underneath another. The gentle seismic rumblings, called tremor, can only be detected by highly sensitive instruments, but the researchers suggest they might help forecast major earthquakes.
Tremor was first observed about five years ago in Japan and has since been detected on several major faults including the San Andreas fault in California, US. 鈥淚t shakes the Earth for hours, days or even weeks at a time,鈥� says Gregory Beroza of Stanford University, US.
But it was not previously known what causes tremor. Some researchers attributed it to fluids moving deep underneath the Earth鈥檚 surface. But Beroza and graduate student David Shelly have shown tremor is in fact a swarm of small-magnitude earthquakes. They did this by examining seismic records of tremor from the Shikoku region of Japan and comparing them to examples of single low-magnitude earthquakes.
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鈥淒avid found that the wiggles that tremor makes on seismographs matches the wiggles of the low-frequency earthquakes,鈥� Beroza explains.
Silent earthquakes
The fact that tremor consists of a series of low-frequency earthquakes, of magnitude 1 or 2, indicates it is caused by one plate slowly slipping under another in a series of 鈥渟ilent earthquakes鈥�.
鈥淭his is the same mechanism by which regular earthquakes are generated but with a twist. The slip in deep tremor happens more slowly than in ordinary earthquakes,鈥� says Beroza.
The researchers believe their discovery is important because research suggests giant earthquakes are preceded by such silent earthquakes. Their work also allowed them to pinpoint the slip causing the tremor with unprecedented precision.
鈥淚n Japan, the deep section of the fault where slow earthquakes form is particularly significant, because it lies next to the shallower locked portion of the fault, where big quakes periodically strike,鈥� says Beroza.
鈥淪o each time a slow earthquake happens, it adds stress to the locked section and increases the likelihood of a magnitude 8 mega-thrust,鈥� he adds. 鈥淭herefore, knowing when a slow earthquake has occurred could be useful in seismic hazard forecasting.鈥�
Roger Musson, at the British Geological Survey, says the research is convincing in showing how tremor can be linked swarms of small earthquakes. But he cautions that more research is needed to understand the significance of tremor fully.
鈥淭he researchers consider that periods of increased [tremor] may indicate times of heightened danger of large earthquakes 鈥� but one could also argue to the contrary that tremor is actually partially unloading the fault zone,鈥� reducing the danger, says Musson. 鈥淭he significance of slow-slip events in terms of earthquake forecasting is still highly unclear.鈥�
Journal reference: Nature (vol 446 p 305)
