Earthquakes you can't feel.   1 December 2006

A new kind of earthquake has been discovered, which are of such low frequency that they go unfelt on the surface, but according to Japanese researchers, these 'silent quakes' could help to predict the big ones.

"We must know the behaviour of the subducting plate to predict earthquakes, however, we have not understood all processes of subduction yet," said lead researcher Yoshihiro Ito, of the National Research Institute for Earth Science and Disaster Prevention in Tsukuba, Japan.

According a paper published this week online in Science Express, these low frequency earthquakes - that is, quakes that shake the ground back and forth very slowly - are thought to increase stress on the tectonic plates that make up the Earth's crust.

The tectonic plates are in slow but constant motion caused by currents in the Earth's liquid mantle. The plate edges grind against each other, creating frictional stress. The stress between the plates increases until the strain is released by a violent displacement of the Earth's crust. The resulting waves of energy, called seismic waves, are radiated through the ground and cause an earthquake of the type we feel on the surface.

The 'silent quakes' vibrate much more slowly than the quakes we can feel on the surface. The speed of vibration of a quake, known as its period, describes the amount of time it takes the quake to go through one complete vibration - that is, to shake back and forth once. The ground vibrates more quickly as the period of a quake gets shorter.

The newly-discovered quakes have a period of around 20 seconds, compared with already known low frequency earthquakes with a period of around 0.5 seconds. 'Normal' earthquakes that we can feel on the surface have a period of around 0.1 seconds.

The most powerful earthquakes, called 'mega-thrust' quakes, can exceed 9.0 on the Richter Scale and cause enormous damage around the world. A mega-thrust quake was the cause of the tsunami that ravaged much of South East Asia in 2004.

The Kobe mega-thrust earthquake on 17 January 1995, which measured 7.2 on the Richter scale and killed more than 6,000 people, led to the Japanese creating a network of over 800 seismographs along the Nankai subduction zone to help predict earthquakes. These seismographs were used by the research team to conduct their research. The sensitive instruments can detect unfelt seismic signals emanating from deep underground that displace the ground without shaking it.

The high-pressure zone of the fault where the silent quakes form lies next to the locked part of the fault zone where mega-thrust earthquakes occur. Each time a silent quake happens, the researchers believe it places stress on the locked part of the plate interface, so a big mega-thrust earthquake would more likely be triggered during a silent quake event.

"We are considering the monitoring of these activities to have an accurate grasp of the stress condition for the mega-thrust rupture zone," said Ito.