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1: Introduction

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    We are not used to the idea of earthquakes near my home in the Pacific Northwest. Earthquakes are a threat to California, Japan, and Alaska, but surely not to Seattle, Spokane, Portland, and Vancouver. That was certainly my own view in 1977, when I moved to Corvallis, Oregon, even though I had been studying earthquakes for many years—in California, of course. My neighbor said, “Earthquakes? Bob, you gotta be kidding!”

    On the other hand, the Pacific Northwest is flanked by a huge offshore active fault more than seven hundred miles long at the base of the continental slope: the Cascadia Subduction Zone. Subduction zones are where masses of crust collide, and a block of oceanic crust is forced down deep into the Earth’s interior. Subduction zones around the Earth produce most of the world’s great earthquakes. Unlike most of the other subduction zones, the Cascadia Subduction Zone has not suffered an earthquake since local written records have been kept. Modern seismographs show very little microearthquake activity on this subduction zone. I assumed, as did most of my scientific colleagues, that subduction in the Pacific Northwest is nonviolent, and that the oceanic crust somehow eases beneath the major cities of the Northwest without building up strain that would be released by earthquakes.

    But in 1983, I heard a presentation by John Adams, a young New Zealand geologist transplanted to the Geological Survey of Canada. Adams stated that there might be an earthquake hazard in the Pacific Northwest. He had learned that a little-known federal agency, the National Geodetic Survey, routinely re-levels U.S. highway survey markers, and he decided to compare old level lines with more recent ones. Changes in the relative elevation of survey monuments and benchmarks along Pacific Northwest highways could provide evidence of the slow buildup of tectonic strain, ultimately leading to an earthquake.

    If there were no warping of the Earth’s crust, re-leveling highway markers would be a pretty boring job. Each survey would be exactly like the previous one. But the re-leveling done by the National Geodetic Survey in the Pacific Northwest was not the same between surveys. It showed an ominous change. The highways crossing the Coast Range are being tilted slowly toward the Willamette Valley in Oregon and Puget Sound in Washington. Could this mean an increase of strain in the Earth’s crust, like a diving board being bent, and possibly a future rupture and earthquake?

    As a student of natural disasters, I worry about needlessly alarming the public. What would be the reaction of people in major cities like Seattle, Tacoma, and Portland to such bad news? “Cool it, John,” I said.

    Good scientist that he is, Adams ignored my advice and published his results anyway. What was the result? Nothing! For the average person, the idea was too far-fetched. The media did not pick up on the story, and Adams’ research paper was read-only by other scientists. I breathed a sigh of relief, but I also began to worry that my early assumption of a slippery subduction zone might be wrong. So I waited for scientific confirmation from other sources.

    Evidence was not long in coming. In 1984, Tom Heaton and Hiroo Kanamori, two seismologists from the California Institute of Technology (Caltech), published a comparison of the Cascadia Subduction Zone with others around the world. They knew that Cascadia was unusually quiet but, otherwise, the geologic setting was the same as that of other subduction zones that had experienced catastrophic earthquakes, like those off the coasts of Chile and Alaska. The oceanic crust in the Cascadia Subduction Zone is relatively young, which means that it has cooled from the molten state only a few million years ago (a short time for a geologist). Because it is hotter than other oceanic crust, it is also lighter and more buoyant, meaning that it is not likely to slide smoothly beneath the continent. (The comparison I use is that of trying to stuff an air mattress beneath a floating raft.) Other subduction zones similar to Cascadia have been visited in this century by earthquakes of magnitudes greater than 8. Could it be that the reason for the lack of seismic activity here is that this subduction zone is completely locked? Maybe the time during which records have been kept, less than two hundred years, is too short for us to conclude that the Pacific Northwest is not earthquake country.

    At the same time, Jim Savage and his colleagues at the U.S. Geological Survey (USGS) were re-surveying geodetic benchmarks and finding evidence of horizontal contraction of the crust of western Washington, which could be explained as a response to the eastward driving of the oceanic plate beneath the continent, further evidence that the Cascadia Subduction Zone is locked but building up strain.

    Two years after Heaton and Kanamori published their model of a locked subduction zone, Brian Atwater of the USGS in Seattle was paddling his kayak up the Niawiakum Estuary of Willapa Bay, in southwestern Washington. The purpose of his trip was to examine the soft sediments along the banks of the estuary, which he was able to observe only at very low tide. This young sediment, only a few hundred years old, might contain evidence to support or refute the ideas that were being advanced about earthquakes.

    There, Atwater made an astonishing observation. Just beneath the marsh grass is gray clay containing microscopic marine fossils, evidence that it had once been deposited beneath the surface of the sea. Below the gray clay is a soil and peat layer from an older marsh, together with dead cedar stumps from an ancient forest. These stumps had been covered by the marine gray clay, in which the present marsh grass had grown. Why are the fossil forest and the fossil marsh overlain by clay with marine fossils? Atwater concluded that the old marsh flat and the coastal western redcedar forest had suddenly dropped down and been covered by Willapa Bay. Not gradually, but instantly! What could have caused this?

    Atwater talked about his discovery to George Plafker, also of the USGS. Plafker told him that the same thing had happened after great subduction-zone earthquakes in southern Chile in 1960 and in the Gulf of Alaska in 1964. Coastal areas had subsided and had been inundated permanently by the sea, drowning forests and marshes. Atwater made the comparison and thought the unthinkable. The marshes and coastal forests of the Pacific Northwest had been downdropped during a great earthquake!

    The evidence for earthquakes that I had been looking for was falling into place, and the news wasn’t good. At this point, Don Hull, the State Geologist of Oregon, and I decided to hold a scientific workshop the evening before the Oregon Academy of Sciences meeting in Monmouth in February 1987, to address the question: Is there a major earthquake hazard in Oregon or not? We invited John Adams, Tom Heaton, and Brian Atwater, as well as other scientists, including skeptics who had previously advocated the idea that no earthquake hazard exists on the Cascadia Subduction Zone.

    Everybody agreed to come, and the atmosphere was electric. The Oregonian newspaper got wind of the meeting, and their science writer, Linda Monroe, wanted to cover it. I was nervous about having the press there because I wanted the scientists to be completely candid, not worrying about a front-page doomsday quote in a major newspaper. But Monroe asked me to trust her, and I did. Her coverage was responsible, and her presence did not detract from the give-and-take of the meeting.

    As it turned out, Linda Monroe had a scoop. There was no argument, no controversy! Most of the scientists at the meeting were so impressed with the results presented by Adams, Heaton, and Atwater that the no-earthquake opposition retreated to the sidelines. The meeting marked a paradigm change, a fundamental change in our thinking about earthquakes in the Northwest. Attendees at the Oregon Academy meeting and readers of the Oregonian got the word the next day. Oregon, as well as the rest of the Pacific Northwest, is indeed Earthquake Country! None of us felt as safe after that day as we thought we had been the day before.

    The third edition, updated nearly three decades later, tells the earthquake story of the Pacific Northwest. (This includes the west coast of Canada, and perhaps from a Canadian perspective, it should be the Pacific Southwest.) The book presents the evidence for earthquakes, the location of major faults, the danger from tsunamis, the importance of ground conditions, and what we as individuals and as taxpayers and voters can do to make our homes and our communities safer from earthquakes. There are lessons from the Northwest experience to be learned elsewhere in the United States, Canada, and other parts of the world where the earthquake threat is greater than that perceived by the general public.

    In 2013, encouraged by legislators representing coastal communities, state agencies convened a different set of meetings to address the question: what is the cost of doing nothing, or of taking only modest steps? The meetings included engineers, emergency managers, scientists, planners, political leaders, and members of the general public. These meetings led to appraisals of our area’s resilience against the next subduction-zone earthquake. The results were published by the Cascadia Regional Earthquake Workgroup (CREW, 2013) and by the geological surveys of British Columbia, Oregon, and Washington.

    This analysis showed that the costs of doing little or nothing would be catastrophic, with tens of thousands of deaths and many billions of dollars in damages. Areas struck by the next Cascadia earthquake would be devastated to the extent that some parts, particularly coastal regions, could take as long as a generation to recover. This would be a catastrophe unmatched in the history of the United States or Canada.

    A major purpose of this book is to alert enough people so that this doomsday scenario doesn’t happen. You, the reader, must be part of the solution.

    We cannot prevent earthquakes, but we can learn to live with them and to survive them. When the inevitable earthquake strikes, we can be ready.

    But today, we are not.

    Suggestions For Further Reading

    Cascadia Regional Earthquake Workgroup, 2013, Cascadia Subduction Zone earthquakes: A magnitude 9.0 earthquake scenario and update, 2013, 23 p. Also available as Washington Division of Geology and Earth Resources Information Circular 116, Oregon Department of Geology and Mineral Industries Open-File Report O-13-22, and British     Columba Geological Survey Information Circular 2013-3.

    This page titled 1: Introduction is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Robert S. Yeats (Open Oregon State) via source content that was edited to the style and standards of the LibreTexts platform.