Cascadia subduction zone
The Cascadia subduction zone is a long convergent plate boundary, about off the Pacific coast of North America, that stretches from northern Vancouver Island in Canada to Northern California in the United States. It is capable of producing 9.0+ magnitude earthquakes and tsunamis that could reach 30 m high. 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. It is a very long, sloping subduction zone where the Explorer, Juan de Fuca, and Gorda plates move to the east and slide below the much larger, mostly continental North American plate. The zone varies in width and lies offshore beginning near Cape Mendocino, Northern California, passing through Oregon and Washington, and terminating in Canada at about Vancouver Island in British Columbia.
The Explorer, Juan de Fuca, and Gorda plates are some of the remnants of the vast ancient Farallon plate which is now mostly subducted under the North American plate. The North American plate itself is moving slowly in a generally southwest direction, sliding over the smaller plates as well as the huge oceanic Pacific plate in other locations such as the San Andreas Fault in central and southern California.
Tectonic processes active in the Cascadia subduction zone region include accretion, subduction, deep earthquakes, and active volcanism of the Cascades. This volcanism has included such notable eruptions as Mount Mazama about 7,500 years ago, the Mount Meager massif about 2,350 years ago, and Mount St. Helens in 1980. Major cities affected by a disturbance in this subduction zone include Vancouver and Victoria, British Columbia; Seattle and Tacoma, Washington; and Portland, Oregon.
History
Tradition
There are no contemporaneous written records of the 1700 Cascadia earthquake. Orally transmitted legends from the Olympic Peninsula area tell of an epic battle between a thunderbird and a whale. In 2005, seismologist Ruth Ludwin set out to collect and analyze anecdotes from various First Nations groups. Reports from the Huu-ay-aht, Makah, Hoh, Quileute, Yurok, and Duwamish peoples referred to earthquakes and saltwater floods. This collection of data allowed the researchers to come up with an estimated date range for the event; the midpoint was in the year 1701.Ghost forests
During low tide one day in March 1986, paleogeologist Brian Atwater dug along Neah Bay with a nejiri gama, a small hand hoe. Under a top layer of sand, he uncovered a distinct plant—arrowgrass—that had grown in a layer of marsh soil. This finding was evidence that the ground had suddenly sunk under sea level, causing saltwater to kill the vegetation. The event had happened so quickly that the top layer of sand sealed away the air, thus preserving centuries-old plants.In 1987, Atwater mounted another expedition paddling up the Copalis River with Dr. David Yamaguchi, who was then studying the eruptions of Mount St. Helens. 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. Originally thought to have died slowly due to a gradual rise in sea level, closer inspection yielded a different story: the land plummeted up to two meters during an earthquake. Having initially tested spruce using tree-ring dating, they found that the stumps were too rotted to count all the outer rings. However, upon having examined those of the western red cedar and comparing them to the living specimens meters away from the banks, they were able to approximate their year of death. There were rings until the year 1699, indicating that the incident had occurred shortly thereafter. Root samples confirmed their conclusion, narrowing the time frame to the winter of 1699 to 1700.
As with the arrowgrass site, the banks of the Copalis River are lined with a layer of marsh followed by a layer of sand. Jody Bourgeois and her team went on to demonstrate that the sand cover had originated with a tsunami surge rather than a storm surge.
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.
A further ghost forest was identified by Gordon Jacoby, a dendrochronologist from Columbia University, underwater in Lake Washington. Unlike the other trees, these suffered from a landslide rather than a dip in the fault during a separate event around 900 CE.
Activity
In the 1960s, underground fractures were uncovered by oil companies in Puget Sound. These were believed to be inactive through the 1990s.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.
Orphan tsunami
A 1996 study published by seismologist Kenji Satake supplemented the research by Atwater et al. with tsunami evidence across the Pacific. Japanese annals, which have recorded natural disasters since approximately 600 CE, had reports of a sixteen-foot tsunami that struck the coast of Honshu Island during the Genroku era. Since no earthquake had been observed to produce it, scholars dubbed it an "orphan tsunami". Translating the Japanese calendar, Satake found the incident had taken place around midnight of 27–28 January 1700, ten hours after the earthquake occurred. The original magnitude 9.0 earthquake in the Pacific Northwest had thus occurred around 9 pm Pacific Standard Time on 26 January 1700.Geophysics
The Cascadia subduction zone is a long dipping fault that stretches from Northern Vancouver Island to Cape Mendocino in northern California. It separates the Juan de Fuca and North America plates. New Juan de Fuca plate is created offshore along the Juan de Fuca Ridge.The Juan de Fuca plate moves toward, and eventually is pushed under the continent. The zone separates the Juan de Fuca plate, Explorer plate, Gorda plate, and North American plate. Here, the oceanic crust of the Pacific Ocean has been sinking beneath the continent for about 200 million years, and currently does so at a rate of approximately 40 mm/yr.
At depths shallower than or so, the Cascadia zone is locked by friction while stress slowly builds as the subduction forces act, until the fault's frictional strength is exceeded and the rocks slip past each other along the fault in a megathrust earthquake. Below the plate interface exhibits episodic tremor and slip.
The width of the Cascadia subduction zone varies along its length, depending on the angle of the subducted oceanic plate, which heats as it is pushed deeper beneath the continent. 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. On the Hyndman and Wang diagram the "locked" zone is storing energy for an earthquake, and the "transition" zone, although somewhat plastic, could probably rupture.
The Cascadia subduction zone runs from triple junctions at its north and south ends. To the north, just below Haida Gwaii, it intersects the Queen Charlotte Fault and the Explorer Ridge. To the south, just off Cape Mendocino in California, it intersects the San Andreas Fault and the Mendocino fracture zone at the Mendocino triple junction.
Recent seismicity
Subduction zones experience various types of earthquakes ; including slow earthquakes, megathrust earthquakes, interplate earthquakes, and intraplate earthquakes. Unlike other subduction zones on Earth, Cascadia currently experiences low levels of seismicity and has not generated a megathrust earthquake since January 26, 1700. Despite low levels of seismicity compared to other subduction zones, Cascadia hosts various types of earthquakes that are recorded by seismic and geodetic instruments, such as seismometers and GNSS receivers.Tremor, a type of slow fault slip, occurs along almost the length of Cascadia at regular intervals of 13–16 months. Tremor occurs deeper on the subduction interface than the locked area where megathrust earthquakes occur. The depth of tremor along the subduction interface in Cascadia ranges from 28 km to 45 km, and the motion is so slow that it is not felt at the surface by people or animals, but it can be measured geodetically. The highest density of tremor activity in Cascadia occurs from northern Washington into southern Vancouver Island, and in northern California. Tremor in Cascadia is monitored by the Pacific Northwest Seismic Network's semi-automatic tremor detection system.
The majority of interplate earthquakes, or earthquakes that occur near the boundaries of tectonic plates, near the Cascadia subduction zone occur in the forearc of the overriding North American plate in Washington, west of the Cascade Volcanic Arc and east of where tremor occurs. These earthquakes are sometimes referred to as crustal earthquakes, and they are capable of causing significant damage due to their relatively shallow depths. A damaging magnitude 7 interplate earthquake occurred on the Seattle Fault around 900–930 CE that generated 3 meters of uplift and a 4–5 meter tsunami. A substantial number of forearc interplate earthquakes also occur in northern California. Much less interplate seismicity occurs in Oregon compared to Washington and northern California, although Oregon hosts more volcanic activity than its neighboring states.
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 1949 Olympia earthquake was a damaging magnitude 6.7 intraslab earthquake that occurred at 52 km depth and caused eight deaths. Another notable intraslab earthquake in the Puget Sound region was the magnitude 6.8 2001 Nisqually earthquake. Intraslab earthquakes in Cascadia occur in areas where the subducting plate has high curvature. Much of the seismicity that occurs off the coast of northern California is due to intraplate deformation within the Gorda plate. Similar to the distribution of interplate earthquakes in Cascadia, intraslab earthquakes are infrequent in Oregon, with its strongest earthquake since statehood being the 5.6 magnitude 1993 Scotts Mills earthquake, an oblique-slip quake.