About the Wauna Earthquake — And That Other One 1,100 Years Ago

Perhaps the most surprising part of last month’s magnitude 4.2 earthquake — speaking for my mind now, not my poor heart, which rocked me harder than the quake — was finding out where it had occurred. It originated right here on the Key Peninsula, just north of Lake Holiday and Doyle Pond northwest of the SR 302 and KP Highway intersection.

How did we end up with an earthquake to call our own? The answer involves a fault zone only discovered in the last quarter-century, and it ties the Key Peninsula to deep, potentially catastrophic movement across Cascadia.

First, some basics. An earthquake is a release of tension in the Earth’s crust. Along a zone of rupture, which may stretch for many miles in large earthquakes, huge blocks of the crust suddenly slip past each other. From this epicenter, shock waves travel outward in concentric rings, fading as they get farther away.

Our earthquake’s hypocenter was 17 miles underground (the epicenter is the point on the surface), a figure calculated by triangulating the shock waves’ arrival times at dozens of seismographic stations in a regional network maintained by the University of Washington and the United States Geological Survey.

Around here, some folks had things rattling in their workshops for 15 seconds. Others felt two quick jolts and nothing more. I could swear I heard it before it hit, a low whooshing rumble like a truck in the driveway, followed by shaking that lasted a few seconds at most.

One friend said she got excited; she loves the deep power of things like thunder and fighter jets. Another was reading in his recliner and felt nothing.

The scale for earthquake magnitude is logarithmic, meaning that each successive number represents a tenfold increase in earthquake magnitude. The 2001 Nisqually earthquake, a magnitude 6.8 that I rode out in an early computer lab full of folding tables and metal chairs, was about 400 times larger in magnitude than this 4.2 quake. The magnitude 7.1 earthquake I experienced in Anchorage in 2018, when I ran outside and had to wait long seconds for the shaking to stop, and was shocked when most Alaskans shrugged and went about their day, was about twice as large as the Nisqually quake. And the magnitude 8.8 earthquake I experienced in Chile in 2010 — well, that was a whole different beast, 50 times bigger than the Alaska quake.

That’s a story for another time, but maybe now you can give my heart some grace for pounding when this little shake hit. My daughter just grinned. When I asked her how her first earthquake felt, she said, “Like the house was moving over bumps — very fastly.”

The USGS has a form on its website where you can report your experience, including your emotional response. It’s more than an exercise in the humanities. Local terrain and geology have a big impact on how an earthquake’s shaking is expressed. Solid bedrock, for example, shakes far less than mud or infill. Shake intensity maps, created using seismograph data as well as the firsthand reports of those who felt the earthquake, show that the amount of shaking and patterns of damage are hardly as simple as concentric rings.

This one may have been small — it says a lot that the theme was excitement rather than fear, even close to the epicenter — but the fault responsible, known as the Tacoma Fault Zone, is capable of much larger quakes.

The relatively small offshore Juan de Fuca Plate, which stretches the length of the Cascade Range, is being shoved under the North American Plate. Its disintegration in Earth’s mantle fuels the Cascade volcanoes.

Such a tectonic plate margin may seem simple in outline, but at a regional scale, islands, basins, mountain ranges, and bits of ancient island arcs are all caught up in continental movement, resulting in complex regional networks of fractures, uplifts, strike-slips, and subsidence.

Several east-west fault zones cross Puget Sound, with blocks between them rising or folding back to accommodate an inexorable north-south squeeze.

The Tacoma Fault Zone was found as part of a Puget Sound-wide effort in the early 2000s to use advanced subsurface mapping to better understand where large earthquakes might occur. The Tacoma Fault Zone runs from Allyn across the northern part of the Key Peninsula, roughly under the Purdy spit, across the Gig Harbor Peninsula and southern Vashon Island, and possibly as far as Federal Way.

After the mapping data revealed it as the southern boundary of the Seattle Uplift, a “pop-up structure” just to our north, researchers set out to find on-the-ground evidence of past quakes. They discovered deformed sediment in shoreline bluffs. More tellingly, they found places in Allyn and Burley, north of the fault, where coastal marshes had been lifted above the high tide line, suddenly altering their plant communities. South of the fault, in the intertidal zone of Wollochet Bay, they found a still-rooted Douglas-fir stump, evidence that the elevation there had suddenly fallen.

The evidence was consistent among sites. The fault had ruptured right to the surface — land on the north side pushed up; land on the south side pushed down — with an offset up to 12 feet. By dating freshwater peat layers, buried plants, and the stump’s outermost rings, they deduced that the earthquake occurred about 1,100 years ago.

Subsequent work has uncovered more evidence for that quake, now estimated to have been a magnitude 7.1 that was a full rupture of the fault plane, 35 miles in length and extending from the surface to 9 miles in depth.

That was around 900 CE. Now hundreds of thousands of people and buildings are within striking distance of the Tacoma Fault Zone. Geologists say similar earthquakes are almost certain to recur, though the timeline is unknown. Both the USGS and U.S. military have modeled the modern effects of such a quake. Possible impacts include a tsunami in the Port of Tacoma, liquefaction throughout the Puyallup floodplain, landslides, thousands of injuries, and thousands of damaged buildings, with many collapsing.

Heck, if it happens again and again, our peninsula might just end up underwater.