Megathrust Earthquakes

The recent Tōhoku quake that struck Japan on March 11th, is a devastating reminder of the sheer power and velocity of a magnitude 9 earthquake. Such reminders are very rare; in the 20th-century only three earthquakes reached or exceeded magnitude 9.0 worldwide. In the US, there are only two locations where a megathrust earthquake is possible—the Alaska-Aleutian subduction zone and the Cascadia Subduction Zone—the region west of the Cascade Range from southern British Columbia to northern California.

With recent lessons from Japan in mind, I wanted to look at what might happen if an earthquake struck the Cascadia Zone. However, an in-depth look at megathrust quakes has a unique set of challenges because one can’t snap their fingers and inspire nature to shake the ground. As a television journalist I have to be slightly obsessive about capturing enough footage to cover a story because a bunch of talking heads will put the audience to sleep. It’s easy to end up with a bunch of lackluster clips from expert interviewees strung together in a timeline. Fortunately, my early research revealed that the last megathrust earthquake in 1700 left behind some very interesting visuals.

Until recently, Cascadia did not have any written records of local earthquakes larger than magnitude 7.5, nor of transpacific tsunamis. However, in the early 1980’s earth scientists began looking a little closer. Hints from oral histories of native peoples were analyzed in detail, and geologists began uncovering unusual and exciting physical clues, both suggesting that very large earthquakes had struck the Pacific Northwest.

Two of the detectives who pinpointed the timing of the 1700 quake were geologist, Brian Atwater, and tree ring scientist, Dave Yamaguchi. Our camera crew joined this duo for a day in the field on a sunny June morning. Videographer, Tim Griffis, and I followed Atwater a few hundred feet up a stream near Discovery Bay on Washington’s Olympic Peninsula. Atwater scraped away at the riverbank to reveal a light grey layer of sand filled with microscopic siliceous shells of marine diatoms. According to Atwater, the simplest explanation for this unique sand layer is a tsunami from an earthquake in which a tectonic plate, in a seismic shift, abruptly displaced the sea while lowering the adjoining coast.

When a megathrust earthquake strikes, the land can drop by five feet or more. Imagine a tsunami wave powerfully driving towards a shoreline. Sand from the sea is swept up into the current and carried far inland and then deposited, coating the land with a fresh layer of sand.

Once Atwater concluded that the sand layers likely revealed evidence of past tsunamis, he recruited tree ring scientist, Dave Yamaguchi to help him pinpoint when the last giant quake struck. Yamaguchi suggested they look to trees that were drowned by incoming waters from the past giant tsunami. He pointed to ghost forests—groves of weather-beaten tree trunks in tidal marshes and meadows in southern Washington.

When incoming waters from a tsunami rush inland the bases of trees are submerged underwater and the trees are killed. Thousands of rotting stumps can be seen along the banks of tidal streams up and down the west coast. By dating the final ring on one of these trees Yamaguchi determined the past giant quake happened in 1700. According to Yamaguchi, “It’s important for us to know how big the earthquakes in the past have been in this region because the past tells us what can happen in the future.”

Knowing the Cascadia fault is susceptible to giant quakes allows engineers to plan ahead and construct structures to withstand that kind of force.

Although footage from the Tōhoku quake was the primary footage used to illustrate our story about megathrust quakes, my favorite visuals revealed the signs of an earthquake that hit over three hundred years ago. Not only did we witness unique science in action, we shared in the joy of two scientists who clearly love connecting the dots between geologic evidence and history. Plus, it was obvious they enjoyed striding through tidal muck in hip waders giddily digging away the shoreline to reveal an unusual and important discovery.

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