Typhoons trigger earthquakes

Scientists have uncovered evidence that large storms can trigger certain types of earthquake.

Writing in this week's Nature, Taiwan-based researcher ChiChing Liu from Academic Sinica in Taipei together with two scientists from the US, explains how between 2002 and 2007 he and his colleagues used underground strain-sensitive devices to follow how the ground deformed over time.

The researchers found that they could pick up the arrival of typhoons, which occur predominantly in the second half of each year.  These storms are accompanied by very low pressure, which usually causes the ground to swell and this is what the team could see on their subterranean strain-meters. But occasionally they would pick up the reverse - the ground appeared to have shrunk.

They detected eleven events like this, all associated with typhoons. The likelihood of this occurring by chance is less than one in a million, and the only explanation, say the scientists, is that the typhoons are occasionally triggering 'slow earthquakes', which are ground movements that occur over much longer timescales - hours to days - compared with their normal vigorous counterparts.

The storms unleash the slow quakes, say the researchers, by increasing the stress across faults.  This occurs because the arrival of the storm causes atmospheric pressure to drop over land, but to remain unchanged over the sea.  This stretches the fault and if it is primed to move then a quake follows.

Paradoxically, this mechanism might actually help to protect Taiwan, where Philippine Sea plate and the Eurasian plate are running into one another at more than 8cm per year, by helping to periodically 'unload' the fault and preventing the build-up of energy that would otherwise be unleashed subsequently and with potentially deadly effect.