The Northwest's nearly annual earthquake is underway

By JAKE ELLISON, SEATTLEPI.COM STAFF

Updated 6:00 pm, Tuesday, January 12, 2016

There is a major earthquake happening beneath northwest Washington right now, and seismologists are uncertain how dangerous this roughly annual "slow earthquake" or "slow slip" is -- or even if it is dangerous at all.

Every 12 to 15 months, the equivalent of the 6.8 magnitude 2001 Nisqually earthquake kicks off a weeks-long journey from southern Vancouver Island to south of Tacoma along the famously dangerous Cascadia Subduction Zone ... you know, where there will one day be another 9-plus megathrust earthquake that devastates the region and leads to a major tsunami.

Among the differences is that you can't feel these slow quakes, the scientists explain, because they're sloooowwwwww:

Graphic by the Pacific Northwest Seismic Network showing how long a slow earthquake plays out.

The researchers sit back and watch it run its course without knowing if it will or can lead to a bigger, faster and more devastating quake, or if it's just something the Earth does and signals no impending dangers.

Snapshot of the Pacific Northwest Seismic Network's Interactive Tremor Map with a date range of Dec. 21 to Jan. 11.

The current slow slip may, writes a researcher at the Pacific Northwest Seismic Network at the University of Washington, have caused the magnitude 4.8 earthquake northeast of Victoria, British Columbia that occurred on Dec. 29.

Here's what the group wrote on its blog:

While the Victoria earthquake occurred on the southeastern edge of where tremor, and hence the slow earthquake was occurring, the Victoria earthquake was deeper than the tremor. The Victoria earthquake occurred within the subducting Juan de Fuca plate while the slow earthquake is occurring on the boundary between the Juan de Fuca and North American plates. Despite the difference in location between the Victoria earthquake and the slow earthquake, the two events could still be related. Changes in the distribution of built up energy due to the slow earthquake could have triggered the Victoria earthquake.

Relation to a megathrust? The PNSN writes:

The relationship between slow slip and megathrust earthquakes is unresolved and very much an area of ongoing research. However, a large body of work from Cascadia as well as other subduction zones worldwide (e.g. Japan, Mexico, Alaska, Costa Rica, New Zealand) suggests that these events are an integral part of the greater earthquake cycle. That doesn't mean they have no effect. We honestly don't know and need more than 10 years of data to answer that. But it does mean they are neither anomalous nor acutely ominous. The activity is a good reminder that we are sitting on an active fault and need to be prepared, but it's important to keep a broad spatial and temporal perspective.

The current episode isn't distinctly more anomalous or sinister than the previous one. Or the 8 before that. Or the dozens we've observed in northern California. Or the likely hundreds that have occurred all along the subduction zone since that last great earthquake. It isn't necessarily good.

It isn't necessarily bad. It just happens.

So don't panic. Or, go ahead and panic just enough to get prepared. Here are some photos of the 1964 quake in Alaska that hit 9.2 magnitude ... just to get your earthquake fear meter jumping:

Image 1 of 29

Alaska Earthquake March 27, 1964. Subsidence of the graben at the head of the Government Hill landslide in Anchorage tore apart an elementary school and converted the schoolyard into a jumble of fissures, ... more

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