Posted on behalf of Naomi Lubick

A swarm of tiny earthquakes known as a tremor event (among other monikers) arrived right on schedule beneath Puget Sound this week (see last month’s feature in Nature: The secret chatter of giant faults).

On 8 August, 5 hours of infinitesimally small shaking lit up seismometers placed by the team of researchers from the University of Washington, Seattle, over the past year. That was the first sign that something bigger was afoot. Researchers had been expecting the cluster of deep-earth spasms to arrive, as they are wont to do every 12 to 14 months in the Cascadia region, at the end of this summer. The team missed the opportunity to record last year’s event because it arrived months earlier than expected. “We’re 80-90% sure now” that this is the event they’ve been waiting for, says John Vidale, one of the team leaders.

Team members beat it into the field to finish off the installation on Tuesday, adding the last of the “Texans,” as researcher Steve Malone calls the seismic monitors on his blog of the study.

Their preparations will now come to fruition, the researchers hope, as they set up the last quarter of the instruments in the team’s so-called Array of Arrays yesterday. The network of seismic monitoring devices have been lying in wait to capture the movements along the giant fault that marks the Juan de Fuca subduction zone, where oceanic crust on the Pacific side is diving under the North American continental crust.

Tremor events like these could give seismologists a different view of what happens to subduction faults that lie tens of kilometres below the Earth’s surface and give rise to huge earthquakes.

Vidale says that from his perspective, everything “happened just as planned: it [the tremor event] started just at the edge of the array,” followed by “a little burst of activity” that travelled to the northeast. The shaking “has been accelerating for a couple of days now,” getting stronger, Vidale comments, which increases the team’s confidence that this is the main event.

But the cloud of tiny earthquakes has mainly sat still on the map for the past few days, hovering close to where it began, just 15 km south/southeast of last year’s event.

That puts the start of the tremor slightly deeper, or a little further down on the plane of the subduction fault, than the previous event, says Heidi Houston, one of the lead investigators on the project. Houston has been looking at the past five tremor events, which took place in 2004, 2005, 2007, 2008 and 2009, to see how they might compare with this ongoing one.

With the movement on the subduction zone fault spread out over several weeks, each past event eventually amounted to earthquakes with magnitudes of 6.5-6.7. But they all started in different places and took anywhere from a few days to almost a week to start moving north.

“While they are similar in some respects, they are not replicas,” Houston says. “Maybe what is surprising is how everything” – the tremor event’s on-time arrival, the installation of the Array of Arrays, and even the weather – “is all going pretty much according to plan!”

Image: Heidi Houston and Aaron Wech, University of Washington, Seattle, Array of Arrays experiment.