Cascadia subduction zone
The Oregon Department of Emergency Management estimates shaking would last 5–7 minutes along the coast, with strength and intensity decreasing further from the epicenter.
The zone varies in width and lies offshore beginning near Cape Mendocino, Northern California, passing through Oregon and Washington, and terminating at about Vancouver Island in British Columbia.
[3] Major cities affected by a disturbance in this subduction zone include Vancouver and Victoria, British Columbia; Seattle, Washington; and Portland, Oregon.
[4] During low tide one day in March 1986, paleogeologist Brian Atwater dug along Neah Bay with a nejiri gama, a small hand hoe.
[7] The pair happened upon a section of "ghost forest", so-called due to the dead, gray stumps left standing after a sudden inundation of salt water had killed them hundreds of years ago.
[5] Originally thought to have died slowly due to a gradual rise in sea level,[2] closer inspection yielded a different story: the land plummeted up to two meters during an earthquake.
[5] In 1995, an international team led by Alan Nelson of the USGS further corroborated these findings with 85 new samples from the rest of the Pacific Northwest.
All along British Columbia, Washington State, and Oregon, the coast had fallen due to a violent earthquake and been covered by sand from the subsequent tsunami.
[7] A further ghost forest was identified by Gordon Jacoby, a dendrochronologist from Columbia University, 60 feet (18 m) underwater in Lake Washington.
[6] In the 1980s, geophysicists Tom Heaton and Hiroo Kanamori of Caltech compared the generally quiet Cascadia to more active subduction zones elsewhere in the Ring of Fire.
They found similarities to faults in Chile, Alaska, and Japan's Nankai Trough, locations known for megathrust earthquakes, a conclusion that was met with skepticism from other geophysicists at the time.
[4] Japanese annals, which have recorded natural disasters since approximately 600 CE,[2] had reports of a sixteen-foot tsunami that struck the coast of Honshu Island during the Genroku era.
[2] Translating the Japanese calendar, Satake found the incident had taken place around midnight of 27–28 January 1700, ten hours after the earthquake occurred.
As the edge of the plate sinks and becomes hotter and more molten, the subducting rock eventually loses the ability to store mechanical stress; earthquakes may result.
Unlike other subduction zones on Earth, Cascadia currently experiences low levels of seismicity and has not generated a megathrust earthquake since January 26, 1700.
Tremor, a type of slow fault slip, occurs along almost the entire length of Cascadia[11] at regular intervals of 13–16 months.
[16] Intraslab earthquakes, frequently associated with stresses within the subducting plate in convergent margins, occur most frequently in northern Cascadia along the west coast of Vancouver Island and in Puget Sound, and in southern Cascadia within the subducting Gorda plate, near the Mendocino triple junction offshore of northern California.
The Cascadia subduction zone, which forms the boundary between the Juan de Fuca and North American plates, is a very long sloping fault that stretches from mid-Vancouver Island to Northern California.
[22] In 2004, a study conducted by the Geological Society of America analyzed the potential for land subsidence along the Cascadia subduction zone.
It postulated that several towns and cities on the west coast of Vancouver Island, such as Tofino and Ucluelet, are at risk for a sudden, earthquake initiated, 1–2 m subsidence.
[23] Studies of past earthquake traces on both the northern San Andreas Fault and the southern Cascadia subduction zone indicate a correlation in time which may be evidence that quakes on the Cascadia subduction zone may have triggered most of the major quakes on the northern San Andreas during at least the past 3,000 years or so.
[35] The arc consists of a series of Quaternary age stratovolcanoes that grew on top of pre-existing geologic materials that ranged from Miocene volcanics to glacial ice.
[3] The Cascade Volcanic arc is located approximately 100 km inland from the coast, and forms a north-to-south chain of peaks that average over 3,000 m (10,000 ft) in elevation.
