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Part 1 Unearthed earthquakes THROUGH MOST OF THE 20TH CENTURY, North America's Cascadia region was thought incapable of generating earthquakes larger than magnitude 7.5. Any tsunami striking the region's coasts would come from afar, leaving hours for warning and evacuation. Yet by century's end, Cascadia had its own recognized source of earthquakes of magnitude 8 to 9 and of tsunamis that would reach its shores in a few tens of minutes. That recognition began in the early 1980s. Earth scientists were then beginning to debate Cascadia's potential for great earthquakes-shocks of magnitude 8 or higher. Despite hints from oral histories of native peoples, there seemed no way to learn whether great earthquakes had ever struck the region. Fortunately, the earthquakes had written their own history. They wrote it most clearly in the ways that great earthquakes of the 1960s in Chile and Alaska wrote theirs-by dropping coasts a meter or two, by sending sand-laden sea water surging across the freshly lowered land, and by causing shaken land to crack. Those geologic records soon gave Cascadia a recognized history of great earthquakes. In the late 1980s, at bays and river mouths along Cascadia's Pacific coast, researchers found the buried remains of marshes and forests that subsidence had changed into tidal mudflats. They also found that the burial began with sand delivered by tsunami or erupted in response to shaking. In a few places they even found the hearths of native people who had used the land before its submergence and burial. But researchers quickly reached an impasse in this attempt to define, from events recorded geologically, Cascadia's earthquake and tsunami hazards. How great an earthquake should a school or hospital be designed to withstand? How large a tsunami should govern evacuation plans on the coast? There seemed no way to know whether Cascadia's plate-boundary fault can unzip all at once, in a giant earthquake of magnitude 9, or whether it must break piecemeal, in series of lesser shocks. Unearthed earthquakes 7 Earthquake potential Can Cascadia do what other subduction zones have done? CASCADIA'S CONVERGING PLATES pose a triple seismic threat. The subducted Juan de Fuca Plate contains sources of earthquakes as large as magnitude 7. Large earthquakes can also radiate from faults in the overriding North America Plate. And the enormous fault that forms the boundary between the plates can produce great earthquakes, of magnitude 8 or 9. This current picture began taking form in the 1960s, when early ideas about continental drift and seafloor spreading came together as the theory of plate tectonics. The Juan de Fuca Plate was identified as a remnant of a larger tectonic plate that had mostly disappeared beneath North America during 150 million years of subduction. By the early 1980s, geophysicists had shown that the Juan de Fuca Plate continues to subduct at an average rate of 4 meters per century. But there was no consensus on how the plates move past one another. The plate boundary lacked a recognized history of earthquakes, even at the shallow depths where the rocks might be cool and brittle enough to break (pink in block diagram and map, right). An earthquake in 1985 provided disturbing images of what can happen when such a plate boundary fails. On September 19th of that year, a subduction earthquake of magnitude 8 generated seismic waves that devastated Mexico City, 400 km from the earthquake source (facing page, top). More than 300 modem buildings collapsed or were damaged beyond repair, 10,000 lives were lost, and another 300,000 persons were left homeless. Could a great Cascadia earthquake have similar effects at inland cities like Vancouver, Seattle, and Portland? Though few Earth scientists were then taking the idea seriously, some broached the possibility of a Cascadia earthquake of magnitude 9. Cascadia looked like it might have as much source area as the 1964 Alaska earthquake, of magnitude 9.2 (compare the Cascadia and Alaska maps on these two pages). It was even possible to imagine a Cascadia earthquake as large as the 1960 Chile mainshock, the 20th century's largest earthquake at magnitude 9.5. THE THEORY OF PLATE TECTONICS holds that Earth's outer shell consists of moving plates composed of crust and rigid upper mantle (Sullivan, 1991; Oreskes, 2003). Riddihough (1984) used seafloor magnetic anomalies, first mapped by Raff and Mason (1961), to reconstruct the past 7 million years of convergence between the Juan de Fuca and North...

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