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Earthquakes and
Tsunami in the Cascadia Subduction Zone Challenges Volcanoes As Mount St. Helens’ May 1980 eruptions reminded us, Cascade volcanoes are very much a present danger to those living near their slopes. The town of Orting, Washington sits on a mud and debris flow from the slopes of Mount Rainier _miles to the east; and, an entire arm of Puget Sound—what is now the Green River valley—was filled during a prehistoric eruption of Mount Rainier, know to geologists as America’s most dangerous volcano. Gravity alone can trigger slope failures on Cascadia’s volcanoes and the resulting "lahars" can travel at speeds of up to _ m.p.h., giving little time to evacuate urban populations at risk. Volcanic eruptions have had disastrous consequences for ports and inland navigation. Ash and pyroclastic flows from Mount St. Helens swept down the Toutle and Cowlitz Rivers into the Columbia at Longview, blocking the deep-draft navigation channel and cutting off ocean access to the all upstream Columbia River ports including Portland, Kalama and Longview. Emergency dredging eventually restored navigation. Earthquakes and Tsunami As strain builds and is periodically released along the junction of these moving oceanic and continental plates—the Cascadia Subduction Zone (CSZ)—great earthquakes wrack our coastal region causing the ocean floor to deform and parts of the coastline to subside. These submarine land movements generate tsunami that sweep onshore to inundate low-lying coastal areas with little warning. Except for a magnitude 7.1 event in April 1992 near Cape Mendocino, at the extreme south end of the CSZ, no earthquakes have been recorded along the fault since European settlement occurred in the mid 19th century. But, over the past decade or so, evidence of very large prehistoric earthquakes and tsunami along the CSZ has been uncovered. Based on studies of buried marsh deposits in estuaries in the region, the last major CSZ event is estimated to have occurred about 300 to 280 years. Parish records of a tsunami in Japan, suggest a more precise date and time for the last great Cascadia earthquake—a magnitude 9 event at 9 p.m. local time, January 26, 1700. The best estimate of the "recurrence interval" for seven documented large Cascadia earthquakes and tsunamis is about 500 years, although some of these events occurred less than 300 years apart, while others were at least 700 years apart. There is no way to absolutely predict when the next event will be, but we are now within the recurrence interval "window." The scenario for one of these large CSZ earthquake is sobering: severe ground shaking lasting up to four minutes; liquefaction of saturated, unconsolidated soils such as sand or silt; numerous and possibly massive landslides; land subsidence and permanent flooding in some locations; and a series of large tsunami waves beginning to arrive soon after the event. The potential for loss of life is significant, as is the potential for major damage and disruption of transportation and utility systems, manufacturing and commercial enterprises, and other infrastructure and development. Communities all along the West Coast are vulnerable, but none more than those in northern California, Oregon, and Washington—our Cascadia region. Two other kinds of earthquakes affect parts our region: crustal earthquakes that occur as stress is released in the upper (North American) plate, and intra-plate earthquakes that occur deep in the subducting plate. (The February 28, 2001 Magnitude 6.8 Nisqually event was an intra-plate earthquake.) Crustal earthquakes, since they occur closer to the surface, can cause terrible local damage to structures through severe ground-shaking, but are not felt at such great distances as deep intra-plate events of similar magnitude. Only in the last five years has it been shown that some of our populous metropolitan coastal areas are at significant risk from large, but infrequent crustal earthquakes. Around 930 AD a Magnitude 7+ ground-rupturing earthquake struck along a fault zone in area of Puget Sound that would later become home to the cities of Seattle, Bainbridge Island and Bremerton. To the south of the fault, the land surface—and bed of Puget Sound—rose as much as 20 feet, causing a tsunami that left its tell-tale sand deposits along low-lying shores of Whidbey Island, West Point and the Snohomish estuary, and massive landslides along the region’s steep shoreline bluffs. Evidence of earlier undated earthquakes on the same fault system shows the Seattle Fault to be active and a threat to a growing urban population. Furthermore, computer modeling of the tsunami that occurred 1100 years ago shows Elliott Bay, Harbor Island and the Duwamish River estuary—where the Port of Seattle’s terminals are located—to be at risk from inundation and destructive tsunami-induced currents. Problems Arising from Regional Threats Managing Risk in the Face of Uncertainty Unlike other natural coastal hazards such as floods or hurricanes, earthquakes are not predictable events; they occur with no warning. Consequently, for those who live in a seismically active region like Cascadia, they cannot be avoided—they come with the territory. The most destructive earthquakes—magnitude 7+ shallow crustal and magnitude 8+ Cascadia Subduction Zone events—recur at long, irregular intervals and have not been experienced, firsthand, by anyone alive in this region. The risk they pose, then, becomes an abstract statistical matter, of practical use in designing building codes for structural seismic resistance, but poorly understood by the lay person making choices about where to live and how to protect life and property. The immediate benefits of enjoying a coastal view may seem to outweigh any long-term risks from living in a seismically hazardous geologic setting. And, unlike in the case of, say, weatherizing a house, the pay-back time for investment made in a seismic retrofit to a home is difficult to calculate. Warnings and Evacuation Earthquakes While no long-term predictions of earthquake occurrences can yet be made, there are promising short-term warnings being developed once an earthquake begins. The difference in arrival time between two types of earthquake ground waves can give automated systems a brief interval in which to shut down critical systems (nuclear power plants, subway systems, high speed trains, computer networks, etc.) and even to warn people to take cover before the most damaging ground-shaking occurs. Experimental work at Lawrence Livermore National Laboratory suggests that within a second of receiving the first seismic waves from sensitive instruments, computer models can quite accurately predict the time before peak ground-shaking occurs and the duration of damaging seismic waves. Tsunami While there are no long term warnings for earthquakes, the events are themselves a warning for some co-seismic hazards they can produce: tsunami inundation and, in some cases, landslides. Tsunami waves travel at 400+ m.p.h. in open ocean waters, giving ample time (hours) following a large tsunamigenic earthquake in, say Alaska, to warn of the danger it poses to US West Coast shores. Even local CSZ earthquakes give 10-30 minutes lead time to evacuate populations from an inundation zone to higher ground. Not all large earthquakes generate tsunami, and false alarms are problematic. NOAA’s Deep ocean Assessment and Reporting of Tsunami (DART) tethered buoys are designed to measure actual tsunami waves underway in the ocean and provide a satellite link to broadcast accurate warnings of arrival time, though not inundation depths. Landslides On land, steep slopes are immediately vulnerable to earthquake-generated landslides, but slides can continue for weeks afterwards, particularly where the ground is saturated; hence, these areas are to be avoided after an earthquake until deemed safe again. Some landslides adjacent to water bodies pose another risk: they can produce local tsunami. On steep hillsides where large amounts of displaced material may reach the water, the waves can be particularly destructive. These sub-aerial landslides have caused significant damage and loss of life in Alaska, and property damage in Puget Sound. More insidious are submarine landslides. These are likely to occur in the loose sediments along the over-steepened unstable fronts of estuarine deltas, and may happen even in the absence of ground-shaking. During the Good Friday Alaskan earthquake of 1964, submarine landslides in Seward and Valdez did the greatest damage to the port and harbor areas. Opportunities to Mitigate Regional Threats Evacuation Signage Low-lying coastal areas are being posted with tsunami evacuation signs and route designations to enable residents and visitors to reach higher ground before a tsunami reaches shore. Locating Critical Facilities Out of Harm’s Way It is neither necessary, nor in all cases desirable to locate development outside areas subject to seismic hazards in coastal areas. However, certain kinds of facilities are critical for response to, and recovery following, a damaging coastal earthquake and/or tsunami. Hospitals, Coast Guard, police and fire stations, emergency communications centers need to be functional. Facilities serving vulnerable populations—schools, nursing homes, prisons, etc.—should also be sited out of harm’s way. Oregon’s Senate Bill 379, enacted in 1995, achieves these goals in areas subject to tsunami inundation. (Please click on this link, then scroll down to Section 455.446) Washington State’s growth management act addresses geological hazards and flooding (including tsunami inundation) through its requirement for local counties and cities (in growth counties, at least) to enact critical areas ordinances. (WAC 365-190-080) Making Coastal Dependent Development More Resilient Water-dependent port and industrial facilities, by their nature, cannot be located away from the water’s edge and still function. Docks and cranes, cargo handling yards, commercial and recreational moorage, and marine railways can, however, be made more resilient to earthquake and tsunami hazards: Better engineering design standards for new facilities and retrofitting existing ones; evacuation and assembly plans for the waterfront workforce; containing loose cargo storage sites; and, keeping toxic and hazardous materials secure. A bi-state Sea Grant applied research and outreach project is underway in Washington and Oregon designed to make Pacific Northwest port and harbor communities more resilient to earthquakes and tsunami. Yaquina Bay (Newport and Toledo, Ore.) and Sinclair Inlet (Bremerton and Port Orchard, Wash.) are the two states’ demonstration communities.
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Please send comments or questions to Robert Goodwin at goodrf@u.washington.edu