Cascadia`s Restless Geology: Volcanoes, Earthquakes & Tsunamis

The tectonic forces that shape our region are responsible for both its natural beauty and
its most dangerous hazards. Eastward moving Pacific oceanic plates—from north to
south, the Explorer, Juan de Fuca, and Blanco Plates—are sliding under (subducting
beneath) the North American continental plate. These forces have folded the Cascade
Mountains, scraped off ocean floor deposits to form Washington State’s Olympics and
the Coast Ranges of British Columbia, Oregon and Northern California, and created the
majestic Cascadia volcanoes. But they also produce volcanic eruptions and mud flows,
and earthquakes, tsunami and coastal landslides.
Regional Threats
As Mount St. Helens’ May 1980 eruptions reminded us, Cascade volcanoes are very
much a present danger to those living near their slopes.
Volcanic eruptions have had disastrous consequences for ports and inland navigation.
Ash and pyroclastic flows from Mount St. Helens blocked the deep-draft navigation
channel to the upstream Columbia River ports
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 lowlying 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. The
best estimate of the “recurrence interval” for seven documented large Cascadia
earthquakes and tsunamis is about 500 yearsThere is no way to absolutely predict when
the next event will be, but we are now within the recurrence interval “window.”
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.
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. 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.
Warnings and Evacuation
While no long-term predictions of earthquake occurrences can yet be made, there are
promising short-term warnings being developed once an earthquake begins. 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.
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. 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.
On land, steep slopes are immediately vulnerable to earthquake-generated landslides, but
slides can continue for weeks afterwards, particularly where the ground is saturated.
Some landslides adjacent to water bodies pose another risk: they can produce local
tsunami. More insidious are submarine landslides. 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
Locating Critical Facilities Out of Harm’s Way
Making Coastal Dependent Development More Resilient
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