Lubrication helps explain slow, silent earthquakes

VANCOUVER — The “slow” quakes emanate from the deep like clockwork every 14 months on Canada’s west coast.

They typically release the energy of a Magnitude 7 earthquake, but the powerful tectonic events are almost imperceptible because they occur slowly over two weeks, instead of in sudden jolts that last just seconds.

“They would represent a pretty big earthquake if they happened like a regular quake, but because they are so slow you are never able to feel anything,” says Pascal Audet, a seismologist at the University of Ottawa, and co-author of a report released Wednesday on the subtle, silent quakes.

The “slow” or “slow-slip” quakes are an intriguing but poorly understood feature of subduction zones like the one along the west coast where a giant oceanic plate dives under the continent.

While more famous for massive and often deadly earthquakes, many of the world’s subduction zones also frequently experience the slow subtle variety, says Audet.

There is a slow quake every 14 months under Vancouver Island, he says, while southern Japan has one every six months.

“The frequency varies from place to place,” says Audet, who has been poring over seismic data from quake zones with Roland Burgmann at the University of California Berkeley.

They report in the journal Na- ture that the slow quakes appear to be the result of geological forces that “lubricate” deep sections of the subduction zones.

As oceanic plates are shoved beneath the continents, pressure and temperatures rise, releasing fluids stored in the descending slabs of rock, Audet said.

The fluids, rich in silica, that are released allow the giant plates to slowly slip past each other — a process that typically takes 10 days to two weeks — before the movement stops.