Researchers reflect on an 'ominous quiet' in Pacific Northwest seismic behavior

If you've noticed a quiet in the region's seismic activity, you're not alone. According to a new paper by researchers at the Pacific Northwest Seismic Network, researchers have also picked up on a strange lack of significant earthquakes in the region -- and they're not sure why.

The last big quake to hit the area was the Nisqually quake, with a magnitude of 6.8, back in 2001. Washington hasn't had a major earthquake in a while, and that seems unusual to researchers.

Though there have been rumbles, tremors, and even swarms of quakes, there have only been nine earthquakes at magnitude 4 or grater since 2009 in the Pacific Northwest, outside of those at Mount St. Helens or on the Cascadia Subduction Zone.

That's significantly less than the decade before that, which had 14 -- which is already less in turn than between 1989 and 1999, when there were 24.

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"The largest event in the past decade was also smaller than the largest event in any previous decade," wrote Stephen Malone, professor at the University of Washington's Earth and Space Sciences department. "It seems inescapable that strong seismicity has taken a break."

What the paper can't conclude is exactly what the cause is. Malone, who is also a former director of the PNSN, noted that it is "beyond the purview of this opinion piece" to say confidently what the possible causes for the calm is.

He does note a number of possibilities though, including differing kinds of seismograms and ranges of magnitude between PNSN and other agencies that monitor this sort of thing, or a general lack of historical data to see a larger pattern forming. There's also a slight correlation in leadership at the PNSN, which could affect how data is read and reported.

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"Although the recent quiescence in the PNW described here may be just random chance, it is possible that it fits into some model of seismicity variability on a regional scale," Malone wrote. "Alternatively, with a new director (Harold Tobin) having just taken over leadership of the PNSN, we have the opportunity to test the hypothesis that regime change can have a profound effect on seismicity."

In an interview with Northwest Public Broadcasting, Tobin told reporters that scientists are coming around to the idea that there may be a non-random cycle for earthquakes to follow.

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"The essential question is, does that mean we're in a quiet period now and what was before that was normal?" Tobin told NWPB last week. "Or have we returned to normal after a decade or so of an unusually large numbers of earthquakes?"

Either way, the report isn't meant to be read as a prelude to greater seismic activity in order to make up for a quiet period; as the Seismological Research letters wrote ahead of the opinion paper, "this observation is little more than a curiosity and not an indication of likely lower or higher earthquake rates in the future."

The best thing that residents can do (aside from make sure their structures are earthquake-proof) is just be prepared. After all, we may still not know how to predict earthquakes, but there are ways to prepare.

You can read the full opinion piece here.