A strange swarm of earthquakes has been detected off the western coast of the United States. Such swarms are normally triggered by seismic activity, but there’s the rub –or rather the lack of it. This swarm is quite a long way from the local plate boundaries.

More than 600 ’quakes have been detected in the last couple of weeks, with three of 5.0 or higher.

“In the 17 years we’ve been monitoring the ocean through hydrophone recordings, we’ve never seen a swarm of earthquakes in an area such as this. We’re not certain what it means,” says Robert Dziak, a marine geologist at Oregon State University (press release, news coverage from AP, the Eugene Register-Guard, The Oregonian).

Dziak would like to divert a research ship and pick up some water samples from the swarm area. This could help determine whether the earthquakes are being triggered by tectonic or hydrothermal causes.

Previous swarms, says Dziak, were caused by magma getting into the ocean crust and pushing plates apart. This forces a massive increase in the rate of movement of a plate, from a few centimetres a year to a meter in a fortnight, triggering the rash of earthquakes.

So what is causing the swarm where this is no known plate boundary or volcano?

As the Corvallis Gazette Times notes, it is something of a mystery:

Dziak didn’t know what the quakes might mean or what is causing them because the seismic activity is occurring at an unusual place: the middle of the Juan de Fuca Plate, away from any major tectonic boundaries.

The Oregonian adds:

The swarm also is odd because it has not come in the form of a main shock followed by steadily decreasing tremors, known as aftershocks. That’s typically what geologists see when an earthquake occurs on a fault within one of the plates.

It seems somehow appropriate that this mystery has come to light due to some covert Cold War technology. A network of underwater microphones called the Sound Surveillance System (SOSUS) was put together to monitor submarines in the Pacific. After tensions eased between east and west, SOSUS was made available to researchers.

Image: detail from map of the swarm, courtesy of Hatfield Marine Science Center. View full map