In 2011, a massive earthquake hit Tōhoku, triggering a tsunami that devastated coastal Japan and surged across the ocean. The triple-plate tectonic junction that spawned the disaster has a mirror image across the Pacific Ocean, a nasty intersection of looming catastrophe in the Pacific Northwest.
Road damage after the 2001 Nisqually earthquake in Washington. Image credit: USGS
Today is the four-year anniversary of the magnitude 9.0 earthquake near the east coast of Honshu in Japan, and the subsequent tsunami that trashed the Fukushima nuclear power plant. In the intervening years, we've learned more about the mechanics of what happened and how, lessons that are applicable closer to home when contemplating the hazard posed by the Cascadia subduction zone in the Pacific Northwest.
Geology of Subduction Zone Earthquakes and Tsunami
Tectonic plates encompass this planet, a flexible, rigid lithosphere combining the crust and uppermost mantle. These plates move at about the rate our fingernails grow, moving a few centimeters per year. Of the three types of tectonic boundary, subduction happens when a denser oceanic plate is pulled or pushed under a less dense oceanic or continental plate. This subduction isn't a smooth process: the plates lock and jerk, creep and snap as energy builds up and is suddenly released in catastrophic earthquakes.
This sudden vertical displacement can trigger tsunami that spread across the ocean, growing into massive waves as they shoal in the shallow water near shore. Tsunami are strange beasts — we can predict how quickly they will travel and where they will go, but not how high the waves will be until it comes on shore, and even then we can be confounded by reverberation and near-shore interference effects.
The 2011 Tōhoku Earthquake and Tsunami
The Tōhoku earthquake took place at 2:46 pm local time on March 11, 2014 on a triple plate junction where three tectonic plates collide. The junction of Philippines, Okhotsk, and Pacific plate are a colossal mess of stress, tension, and earthquakes.
The Pacific Plate is subducting under the Okhotsk microplate, the tectonic plate fragment holding Japan's main island Honshu. Simultaneously, the Philippines plate is diving under the Amur plate holding southern Japan.
Tectonic map of the conjunction of the Amur plate (east) Philippine microplate (south-center), Okhotsk microplate (north-center) and the Pacific plate (west). Purple marks a convergent boundary; blue marks a subduction boundary with teeth pointing down-trench, and red marks a spreading rift. Image modified from Eric Gaba
The Pacific Plate is moving westward at roughly 9 centimeters per year with the Philippines Plate only slightly slower at approximately 7 centimeters per year. The Okhotsk microplate is a slug by comparison, squishing southeast at just under 1.5 centimeters per year.
Seismic history of the Hanshu region wit the slip zone of the Tōhoku mainshock marked with a yellow line. Image credit: USGS
This scraping and shoving has been building up stress for centuries, sparking earthquakes on a regular basis with a major megaquake every few hundred years.
The rupture area was astonishingly large: 200 kilometers wide by 300 to 400 kilometers long. The massive size of this rupture was shocking — our previous models were that segments of a subduction zone acted somewhat independently, not all releasing in a huge zone like this.
Map of the epicenter (star), rupture area (square), and shaking intensity for the earthquake on March 11, 2011. Image credit: USGS
The result was by far the largest earthquake seen in the region for centuries, moving the Honshu coast 4 meters west and sinking it by almost a meter after the plates snapped and released. Out on the sea floor, parts of the Pacific Plate crept almost 80 meters closer to the island.
Tsunami inundation and run-up elevation measurements with earthquake rupture zone marked by blue rectangles. Image credit: The 2011 Tohoku Earthquake Tsunami Joint Survey
Although the strong, prolonged shaking caused substantial structural damage, the intense tsunami compounded the devastation. The waves ran up 39 meters (128 feet) in Miyako and reached up to 10 kilometers (6 miles) inland in Sendai, refracting and amplifying within deep embayments. This broke all hazard projections, although field observations of high-elevation tsunami deposits from 869 proved that it wasn't an unprecedented event.
Tsunami maximum wave height models incorporating tide gauge measurements. Image credit: NOAA
The major earthquake redistributed stress within the region, pushing more locked fault segments over the breaking point until they released as literally hundreds of aftershocks in the following days, and over a thousand within two years. Thankfully, these aftershocks fell off rapidly over time in both magnitude and intensity.
Aftershocks of the 2011 Tōhoku earthquake. Image credit: USGS
The Tōhoku event was neither the largest nor the most deadly in recent history — Chile holds the record for the biggest megaquake, and the Boxing Day Sumatra earthquake and tsunami killed over 230,000 people. Even so, it was undeniably disastrous and the worst felt within the region for over 1,200 years. The combination of earthquake and tsunami killed over 19,000 people and displaced another 130,000, while damaging and destroying at least 370,000 buildings.
Geology of Potential Earthquake and Tsunami In The Pacific Northwest
Tōhoku isn't the only nasty triple junction tectonic mess on the Pacific Rim. North America's Pacific Northwest is home to the Cascadia subduction zone.
From northern California up to northern British Columbia, the Pacific Plate and Juan de Fuca Plate are nudging into the North American Plate.
A spreading ridge separates the Pacific Plate (west) from the Juan de Fuca Plate (center), which subducts under the North American plate (east). Purple marks a convergent boundary; blue marks a subduction boundary with teeth pointing down-trench, red marks a spreading rift, and green marks a transform boundary. Image modified from Eric Gaba
At first glance, the tectonic zones are similar but not identical. But they get more similar close up. From strain measurements, we know the rates are about the same: Vancouver Island is uplifting by about 4 mm/year, and shortening by about 10 mm/year.
We also know the regions have a similar geological history: Cascadia, too, has a history of megaquakes every few centuries, and shallower earthquakes more frequently.
Cross-section of the Cascadia Subduction Zone with notable historic events. Image credit: USGS
Where things get interesting is in pinning down the most recent major event. Instead of over a millennia ago, the Cascadia subduction zone had a M9.0 earthquake on January 26, 1700. We can tag the event down to such a specific date because the tsunami was a virtual mirror of the 2011 Tōhoku tsunami and was added to Japan's historical tsunami records.
Tsunami maximum wave height from a simulation of M9.0 Cascadia earthquake event. Image credit: Kenji Satake/Geological Survey of Japan
The Cascadia subduction zone is still tectonically active. Smaller earthquakes happen relatively frequently, but thanks to the thin population density on most of coastal Vancouver Island, rarely felt unless it happens to hit near one of the few population centers. When they do happen, the impact of these earthquakes can range from the no-big-deal of being unnoticed by anyone but seismologists, to downright destructive if they hit under a city like the 2001 M6.8 earthquake in Nisqually, Washington.
The aftermath of the 2001 Nisqually earthquake proved that even a relatively modest M6.8 earthquake can be horrifically damaging if poorly located. Image credit: FEMA
But the truly scary option is a major M9.0 earthquake like the event in 1700, which hit the Cascadia Zone roughly every 500 years. But a return period is an average, not a firm prediction — of the 13 earthquakes identified in the spotty geological record of the last 6,000 years, they've been as close together as under 200 years and as far apart as 1,000 years between major events. Like Tōhoku, an earthquake of this magnitude wouldn't be catastrophic just because of the shaking, but also because of the tsunami it could generate. Within moments of feeling strong shaking, residents along the coast could find themselves running for high ground as a tsunami inundated their homes.
When the major earthquake inevitably hits the Pacific Northwest one day, I just hope we're better prepared than we are now. With few smaller earthquakes to emphasize the necessity for personal preparation, many residents in the Pacific Northwest rarely contemplate their seismic risk outside the annual ShakeOut drill. While cross-discipline and cross-border disaster preparation is getting better with time, we in North America still lack the political will to fund and roll out an earthquake early warning system.
Everywhere has disasters; the only question is how prepared and comfortable are we with each region's particular risks. Japan is one of the most earthquake-prepared countries on the planet, but still got blindsided by something more intense than they'd prepared for, with more interrelated cascading failures. Let's take the anniversary of their tragedy as an incentive to pause, evaluate our own preparations, and do better.
Pick up a copy of At Risk: Earthquakes and Tsunamis on the West Coast to learn more about the Cascadia Subduction Zone. Learn about the transform boundary south of Cascadia subduction zone with these specials on 2014 earthquakes in Los Angeles and Napa Valley. Parts of this article originally appeared on GeoMika in 2011 as part of a series of explainers on the tsunami. Correction: An earlier draft implied Portland has substantial tsunami risk; it doesn't.