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Typhoons can trigger imperceptible, slow earthquakes, researchers say.

Scientists report in the journal Nature that, in a seismically active zone in Taiwan, pressure changes caused by typhoons “unclamp” the fault.

This gentle release causes an earthquake that dissipates its energy over several hours rather than a few potentially devastating seconds.

The researchers believe this could explain why there are relatively few large earthquakes in this region.

Alan Linde from the Carnegie Institution for Science in the US and colleagues monitored movement of two colliding tectonic plates in eastern Taiwan.

They used three borehole “strainmeters” – highly sensitive instruments deep below the ground.

“These detect otherwise imperceptible movements and distortions of rock,” explained co-author Selwyn Sacks, also from the Carnegie Institution.

Gentle relief
The instruments picked up 20 “slow earthquakes”, each lasting from several hours to more than a day. Of these, 11 co-incided exactly with typhoons.

The authors described the possibility that this coincident timing was by chance as “vanishingly small”.

“It's rare that you see something so definitive, especially when it's something new,” Dr Linde told BBC News.

Their findings could provide clues about why there are relatively few large earthquakes in this region.

Here, the colliding plates move so rapidly that they build mountains at a rate of almost 4mm per year. Dr Linde said that in geological terms that was almost like “growing mushrooms”.

“It's surprising that this area of the globe has had no great earthquakes and relatively few large earthquakes,” Dr Linde commented.

“By comparison, the Nankai Trough in southwestern Japan has a plate convergence rate of about 4cm per year, and this causes a magnitude 8 earthquake every 100 to 150 years.

“The activity in southern Taiwan comes from the convergence of the same two plates, and there the Philippine Sea Plate pushes against the Eurasian Plate at twice that rate.

“This fault experiences more or less constant strain and stress build-up.”

He described how the fault “dipped steeply” westward from near the east coast so that it is under the land area. So the landward side is under constant strain to move upward.

When a typhoon passes over the land, the air pressure on the land is lowered. That slight change in force “unclamps” the fault and allows it to move.

“But this change is quite small,” said Dr Linde. “So for the typhoon to be a trigger, the fault must be precariously close to failure.”

The frequent, slow earthquakes this causes are “totally imperceptible” from the ground. And Dr Linde thinks it is sensible to assume that they may reduce the frequency of larger, more damaging earthquakes.

But this is extremely hard to show because, as he puts it, “how do you prove something that doesn't happen?”