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87-787: The Cotabato Trench is an oceanic trench in the Pacific Ocean , off the southwestern coast of Mindanao in the Philippines . Along this trench the oceanic crust of the Sunda Plate beneath the Celebes Sea is being subducted beneath the Philippines Mobile Belt . It forms part of a linked set of trenches along the western side of the Philippines formed over east-dipping subduction zones, including

174-417: 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 entire length of Cascadia at regular intervals of 13–16 months. Tremor occurs deeper on

261-558: A 50 kilometer by 300 kilometer (about 30 mile by 200 mile) area. The movement was the equivalent of a 6.7 magnitude earthquake. The motion did not trigger an earthquake and was only detectable as silent, non-earthquake seismic signatures. 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

348-457: A difference in buoyancy. An increase in retrograde trench migration (slab rollback) (2–4 cm/yr) is a result of flattened slabs at the 660-km discontinuity where the slab does not penetrate into the lower mantle. This is the case for the Japan, Java and Izu–Bonin trenches. These flattened slabs are only temporarily arrested in the transition zone. The subsequent displacement into the lower mantle

435-423: A few millimeters to over ten centimeters per year. Oceanic lithosphere moves into trenches at a global rate of about 3 km (1.2 sq mi) per year. A trench marks the position at which the flexed, subducting slab begins to descend beneath another lithospheric slab. Trenches are generally parallel to and about 200 km (120 mi) from a volcanic arc . Much of the fluid trapped in sediments of

522-426: A high angle of repose. Over half of all convergent margins are erosive margins. Accretionary margins, such as the southern Peru-Chile, Cascadia, and Aleutians, are associated with moderately to heavily sedimented trenches. As the slab subducts, sediments are "bulldozed" onto the edge of the overriding plate, producing an accretionary wedge or accretionary prism . This builds the overriding plate outwards. Because

609-519: 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,

696-564: A magnitude 6.7 earthquake in Seattle would result in 7,700 dead and injured, $ 33 billion in damage, 39,000 buildings severely damaged or destroyed, and 130 simultaneous fires. The Cascade Volcanic Arc is a continental volcanic arc that extends from northern California to the coastal peninsula of Alaska . 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 . The Cascade Volcanic arc

783-543: A megathrust earthquake is proportional to length of the rupture along the fault. 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. Because of the great length of the fault, the Cascadia subduction zone is capable of producing very large earthquakes if rupture occurs along its entire length. Thermal and deformation studies indicate that

870-475: A prominent elongated depression of the sea bottom, was first used by Johnstone in his 1923 textbook An Introduction to Oceanography . During the 1920s and 1930s, Felix Andries Vening Meinesz measured gravity over trenches using a newly developed gravimeter that could measure gravity from aboard a submarine. He proposed the tectogene hypothesis to explain the belts of negative gravity anomalies that were found near island arcs. According to this hypothesis,

957-424: 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

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1044-528: A zone of continental collision. Features analogous to trenches are associated with collision zones . One such feature is the peripheral foreland basin , a sediment-filled foredeep . Examples of peripheral foreland basins include the floodplains of the Ganges River and the Tigris-Euphrates river system . Trenches were not clearly defined until the late 1940s and 1950s. The bathymetry of

1131-615: Is a 960 km (600 mi) fault at a convergent plate boundary , about 100–200 km (70–100 mi) off the Pacific coast, 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 (98 ft). The Oregon Department of Emergency Management estimates shaking would last 5–7 minutes along

1218-476: Is caused by slab pull forces, or the destabilization of the slab from warming and broadening due to thermal diffusion. Slabs that penetrate directly into the lower mantle result in slower slab rollback rates (~1–3 cm/yr) such as the Mariana arc, Tonga arcs. As sediments are subducted at the bottom of trenches, much of their fluid content is expelled and moves back along the subduction décollement to emerge on

1305-422: Is complex, with many thrust ridges. These compete with canyon formation by rivers draining into the trench. Inner trench slopes of erosive margins rarely show thrust ridges. Accretionary prisms grow in two ways. The first is by frontal accretion, in which sediments are scraped off the downgoing plate and emplaced at the front of the accretionary prism. As the accretionary wedge grows, older sediments further from

1392-416: Is determined by the angle of repose of the overriding plate edge. This reflects frequent earthquakes along the trench that prevent oversteepening of the inner slope. As the subducting plate approaches the trench, it bends slightly upwards before beginning its plunge into the depths. As a result, the outer trench slope is bounded by an outer trench high . This is subtle, often only tens of meters high, and

1479-489: Is explained by a change in the density of the subducting plate, such as the arrival of buoyant lithosphere (a continent, arc, ridge, or plateau), a change in the subduction dynamics, or a change in the plate kinematics. The age of the subducting plates does not have any effect on slab rollback. Nearby continental collisions have an effect on slab rollback. Continental collisions induce mantle flow and extrusion of mantle material, which causes stretching and arc-trench rollback. In

1566-422: Is fully exposed on the ocean bottom. The central Chile segment of the trench is moderately sedimented, with sediments onlapping onto pelagic sediments or ocean basement of the subducting slab, but the trench morphology is still clearly discernible. The southern Chile segment of the trench is fully sedimented, to the point where the outer rise and slope are no longer discernible. Other fully sedimented trenches include

1653-600: 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. The major peaks from south to north include: The most active volcanoes in the chain include Mount St. Helens, Mount Baker, Lassen Peak, Mount Shasta, and Mount Hood. Mount St. Helens captured worldwide attention when it erupted catastrophically in 1980 . St. Helens continues to rumble, albeit more quietly, emitting occasional steam plumes and experiencing small earthquakes, both signs of continuing magmatic activity. Most of

1740-824: 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

1827-484: Is recorded as tectonic mélanges and duplex structures. Frequent megathrust earthquakes modify the inner slope of the trench by triggering massive landslides. These leave semicircular landslide scarps with slopes of up to 20 degrees on the headwalls and sidewalls. Subduction of seamounts and aseismic ridges into the trench may increase aseismic creep and reduce the severity of earthquakes. Contrariwise, subduction of large amounts of sediments may allow ruptures along

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1914-412: Is typically located a few tens of kilometers from the trench axis. On the outer slope itself, where the plate begins to bend downwards into the trench, the upper part of the subducting slab is broken by bending faults that give the outer trench slope a horst and graben topography. The formation of these bending faults is suppressed where oceanic ridges or large seamounts are subducting into the trench, but

2001-515: Is what generates slab rollback. When the deep slab section obstructs the down-going motion of the shallow slab section, slab rollback occurs. The subducting slab undergoes backward sinking due to the negative buoyancy forces causing a retrogradation of the trench hinge along the surface. Upwelling of the mantle around the slab can create favorable conditions for the formation of a back-arc basin. Seismic tomography provides evidence for slab rollback. Results demonstrate high temperature anomalies within

2088-565: 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

2175-543: The Manila Trench and the Negros Trench . At its northern end the rate of convergence across this boundary is about 100 mm per year. It is a relatively young structure, forming during the late Miocene to Pliocene . This age is consistent with the estimated age of the sedimentary rocks in the accretionary wedge associated with the trench and the age of adakitic arc rocks on Mindanao thought to date

2262-637: 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

2349-497: 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

2436-444: The shear stresses at the base of the overriding plate. As slab rollback velocities increase, circular mantle flow velocities also increase, accelerating extension rates. Extension rates are altered when the slab interacts with the discontinuities within the mantle at 410 km and 660 km depth. Slabs can either penetrate directly into the lower mantle , or can be retarded due to the phase transition at 660 km depth creating

2523-487: 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 up 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

2610-572: The 1960 descent of the Bathyscaphe Trieste to the bottom of the Challenger Deep. Following Robert S. Dietz ' and Harry Hess ' promulgation of the seafloor spreading hypothesis in the early 1960s and the plate tectonic revolution in the late 1960s, the oceanic trench became an important concept in plate tectonic theory. Oceanic trenches are 50 to 100 kilometers (30 to 60 mi) wide and have an asymmetric V-shape, with

2697-570: 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. A 1996 study published by seismologist Kenji Satake supplemented

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2784-426: The 5.6 magnitude 1993 Scotts Mills earthquake , an oblique-slip quake. Megathrust earthquakes are the most powerful earthquakes known to occur, and can exceed magnitude 9.0, which releases 1,000 times more energy than magnitude 7.0 and 1 million times more energy than a magnitude 5.0. They occur when enough energy (stress) has accumulated in the "locked" zone of the fault to cause a rupture. The magnitude of

2871-470: The Aleutian trench. In addition to sedimentation from rivers draining into a trench, sedimentation also takes place from landslides on the tectonically steepened inner slope, often driven by megathrust earthquakes . The Reloca Slide of the central Chile trench is an example of this process. Convergent margins are classified as erosive or accretionary, and this has a strong influence on the morphology of

2958-499: The Cascadia subduction zone is anticipated to be capable of causing widespread destruction throughout the Pacific Northwest . Prior to the 1980s, scientists thought that the subduction zone did not generate earthquakes like other subduction zones around the world, but research by Brian Atwater and Kenji Satake tied together evidence of a large tsunami on the Washington coast with documentation of an orphan tsunami in Japan (a tsunami without an associated earthquake). The two pieces of

3045-673: 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 ( Crater Lake ) about 7,500 years ago, the Mount Meager massif ( Bridge River Vent ) 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 , Washington; and Portland , Oregon. There are no contemporaneous written records of

3132-570: The Cascadia subduction zone. Sedimentation is largely controlled by whether the trench is near a continental sediment source. The range of sedimentation is well illustrated by the Chilean trench. The north Chile portion of the trench, which lies along the Atacama Desert with its very slow rate of weathering, is sediment-starved, with from 20 to a few hundred meters of sediments on the trench floor. The tectonic morphology of this trench segment

3219-534: The Cayman Trough, which is a pull-apart basin within a transform fault zone, is not an oceanic trench. Trenches, along with volcanic arcs and Wadati–Benioff zones (zones of earthquakes under a volcanic arc) are diagnostic of convergent plate boundaries and their deeper manifestations, subduction zones . Here, two tectonic plates are drifting into each other at a rate of a few millimeters to over 10 centimeters (4 in) per year. At least one of

3306-580: The Cotabato Trench suggests that run-ups of several metres are likely for future earthquakes similar in size to the 1976 event. 6°00′N 123°36′E  /  6.0°N 123.6°E  / 6.0; 123.6 Oceanic trench Oceanic trenches are a feature of the Earth's distinctive plate tectonics . They mark the locations of convergent plate boundaries , along which lithospheric plates move towards each other at rates that vary from

3393-412: The Earth. The trench asymmetry reflects the different physical mechanisms that determine the inner and outer slope angle. The outer slope angle of the trench is determined by the bending radius of the subducting slab, as determined by its elastic thickness. Since oceanic lithosphere thickens with age, the outer slope angle is ultimately determined by the age of the subducting slab. The inner slope angle

3480-601: The Hyndman and Wang diagram (not shown, click on reference link below) the "locked" zone is storing up 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

3567-518: 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 (North American plate). 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

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3654-596: The Makran Trough, where sediments are up to 7.5 kilometers (4.7 mi) thick; the Cascadia subduction zone, which is completed buried by 3 to 4 kilometers (1.9 to 2.5 mi) of sediments; and the northernmost Sumatra subduction zone, which is buried under 6 kilometers (3.7 mi) of sediments. Sediments are sometimes transported along the axis of an oceanic trench. The central Chile trench experiences transport of sediments from source fans along an axial channel. Similar transport of sediments has been documented in

3741-528: The Pacific Northwest region is not well prepared for such a colossal earthquake. The earthquake is expected to be similar to the 2011 Tōhoku earthquake and tsunami , because the rupture is expected to be as long as the 2004 Indian Ocean earthquake and tsunami . The resulting tsunami might reach heights of approximately 30 meters (100 ft). FEMA estimates some 13,000 fatalities from such an event, with another 27,000 injured, which would make it

3828-681: The Pacific Ocean, but are also found in the eastern Indian Ocean , with a few shorter convergent margin segments in other parts of the Indian Ocean, in the Atlantic Ocean, and in the Mediterranean. They are found on the oceanward side of island arcs and Andean-type orogens . Globally, there are over 50 major ocean trenches covering an area of 1.9 million km or about 0.5% of the oceans. Trenches are geomorphologically distinct from troughs . Troughs are elongated depressions of

3915-479: The area of the Southeast Pacific, there have been several rollback events resulting in the formation of numerous back-arc basins. Interactions with the mantle discontinuities play a significant role in slab rollback. Stagnation at the 660-km discontinuity causes retrograde slab motion due to the suction forces acting at the surface. Slab rollback induces mantle return flow, which causes extension from

4002-405: The belts were zones of downwelling of light crustal rock arising from subcrustal convection currents. The tectogene hypothesis was further developed by Griggs in 1939, using an analogue model based on a pair of rotating drums. Harry Hammond Hess substantially revised the theory based on his geological analysis. World War II in the Pacific led to great improvements of bathymetry, particularly in

4089-497: The bending faults cut right across smaller seamounts. Where the subducting slab is only thinly veneered with sediments, the outer slope will often show seafloor spreading ridges oblique to the horst and graben ridges. Trench morphology is strongly modified by the amount of sedimentation in the trench. This varies from practically no sedimentation, as in the Tonga-Kermadec trench, to completely filled with sediments, as with

4176-585: 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, 60 feet (18 m) 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. In the 1960s, underground fractures were uncovered by oil companies in Puget Sound . These were believed to be inactive through

4263-660: 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 at about Vancouver Island in British Columbia . The Explorer, Juan de Fuca, and Gorda plates are some of

4350-434: The continent for about 200 million years, and currently does so at a rate of approximately 40 mm/ yr . At depths shallower than 30 km (19 mi) or so, the Cascadia zone is locked by friction while stress slowly builds up 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 30 km (19 mi)

4437-464: The deadliest natural disaster in American, and North American, history. FEMA further predicts that a million people will be displaced, with yet another 2.5 million requiring food and water. An estimated 1/3 of public safety workers will not respond to the disaster due to a collapse in infrastructure and a desire to ensure the safety of themselves and their loved ones. Other analyses predict that even

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4524-426: The existence of back-arc basins . Forces perpendicular to the slab (the portion of the subducting plate within the mantle) are responsible for steepening of the slab and, ultimately, the movement of the hinge and trench at the surface. These forces arise from the negative buoyancy of the slab with respect to the mantle modified by the geometry of the slab itself. The extension in the overriding plate, in response to

4611-690: The fundamental plate-tectonic structure is still an oceanic trench. Some troughs look similar to oceanic trenches but possess other tectonic structures. One example is the Lesser Antilles Trough, which is the forearc basin of the Lesser Antilles subduction zone . Also not a trench is the New Caledonia trough, which is an extensional sedimentary basin related to the Tonga-Kermadec subduction zone . Additionally,

4698-437: 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. The Cascadia subduction zone is a 1,000 km (620 mi) long dipping fault that stretches from Northern Vancouver Island to Cape Mendocino in northern California. It separates

4785-462: The inner slope as mud volcanoes and cold seeps . Methane clathrates and gas hydrates also accumulate in the inner slope, and there is concern that their breakdown could contribute to global warming . The fluids released at mud volcanoes and cold seeps are rich in methane and hydrogen sulfide , providing chemical energy for chemotrophic microorganisms that form the base of a unique trench biome . Cold seep communities have been identified in

4872-431: The inner slope of the trench. Erosive margins, such as the northern Peru-Chile, Tonga-Kermadec, and Mariana trenches, correspond to sediment-starved trenches. The subducting slab erodes material from the lower part of the overriding slab, reducing its volume. The edge of the slab experiences subsidence and steepening, with normal faulting. The slope is underlain by relative strong igneous and metamorphic rock, which maintains

4959-451: The inner trench slopes of the western Pacific (especially Japan ), South America, Barbados, the Mediterranean, Makran, and the Sunda trench. These are found at depths as great as 6,000 meters (20,000 ft). The genome of the extremophile Deinococcus from Challenger Deep has sequenced for its ecological insights and potential industrial uses. Because trenches are the lowest points in

5046-441: The last 3,500 years, suggesting a return time of about 500 years. Seafloor core evidence indicates that there have been forty-one subduction zone earthquakes on the Cascadia subduction zone in the past 10,000 years, suggesting a general average earthquake recurrence interval of only 243 years. Of these 41, nineteen have produced a "full margin rupture", wherein the entire fault opens up. By comparison, similar subduction zones in

5133-411: The mantle suggesting subducted material is present in the mantle. Ophiolites are viewed as evidence for such mechanisms as high pressure and temperature rocks are rapidly brought to the surface through the processes of slab rollback, which provides space for the exhumation of ophiolites . Slab rollback is not always a continuous process suggesting an episodic nature. The episodic nature of the rollback

5220-524: The ocean floor, there is concern that plastic debris may accumulate in trenches and endanger the fragile trench biomes. Recent measurements, where the salinity and temperature of the water was measured throughout the dive, have uncertainties of about 15 m (49 ft). Older measurements may be off by hundreds of meters. (*) The five deepest trenches in the world Cascadia subduction zone 45°N 124°W  /  45°N 124°W  / 45; -124 The Cascadia subduction zone

5307-593: The ocean was poorly known prior to the Challenger expedition of 1872–1876, which took 492 soundings of the deep ocean. At station #225, the expedition discovered Challenger Deep , now known to be the southern end of the Mariana Trench . The laying of transatlantic telegraph cables on the seafloor between the continents during the late 19th and early 20th centuries provided further motivation for improved bathymetry. The term trench , in its modern sense of

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5394-483: The onset of subduction. The trench is associated with large megathrust earthquakes, including the 1918 Celebes Sea earthquake (M 8.3), the 1976 Moro Gulf earthquake (M 8.0) and the 2002 Mindanao earthquake (M 7.5). The Cotabato Trench is one of the main structure around the Philippines likely to be associated with tsunamigenic earthquakes. The tsunami generated by the 1976 earthquake caused about 4,000 deaths on Mindanao. Modelling of likely further tsunamis along

5481-415: The overriding plate exerts a force against the subducting plate (FTS). The slab pull force (FSP) is caused by the negative buoyancy of the plate driving the plate to greater depths. The resisting force from the surrounding mantle opposes the slab pull forces. Interactions with the 660-km discontinuity cause a deflection due to the buoyancy at the phase transition (F660). The unique interplay of these forces

5568-529: The past 3,000 years or so. The evidence also shows the rupture direction going from north to south in each of these time-correlated events. The 1906 San Francisco earthquake seems to have been a major exception to this correlation, however, as it was not preceded by a major Cascadia quake. The last known great earthquake in the northwest was the 1700 Cascadia earthquake , 324 years ago. Geological evidence indicates that great earthquakes (> magnitude 8.0) may have occurred sporadically at least seven times in

5655-414: 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 up 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

5742-512: The plates is oceanic lithosphere , which plunges under the other plate to be recycled in the Earth's mantle . Trenches are related to, but distinct from, continental collision zones, such as the Himalayas . Unlike in trenches, in continental collision zones continental crust enters a subduction zone. When buoyant continental crust enters a trench, subduction comes to a halt and the area becomes

5829-458: The puzzle were linked, and they then realized that the subduction zone was more hazardous than previously suggested. In 2009, some geologists predicted a 10% to 14% probability that the Cascadia subduction zone will produce an event of magnitude 9.0 or higher in the next 50 years. In 2010, studies suggested that the risk could be as high as 37% for earthquakes of magnitude 8.0 or higher. Geologists and civil engineers have broadly determined that

5916-414: The region 60 kilometers (about 40 miles) downdip (east) of the deformation front (where plate deformation begins) is fully locked (the plates do not move past each other). Further downdip, there is a transition from fully locked to aseismic sliding . In 1999, a group of Continuous Global Positioning System sites registered a brief reversal of motion of approximately 2 centimeters (0.8 inches) over

6003-564: 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 (which is moving in a northwest direction) in other locations such as the San Andreas Fault in central and southern California. Tectonic processes active in

6090-520: 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 . Since no earthquake had been observed to produce it, scholars dubbed it an " orphan tsunami ". Translating the Japanese calendar , Satake found

6177-505: The researchers to come up with an estimated date range for the event; the midpoint was in the year 1701. 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

6264-525: The sea floor with steep sides and flat bottoms, while trenches are characterized by a V-shaped profile. Trenches that are partially infilled are sometimes described as troughs, for example the Makran Trough. Some trenches are completely buried and lack bathymetric expression as in the Cascadia subduction zone , which is completely filled with sediments. Despite their appearance, in these instances

6351-416: The sediments lack strength, their angle of repose is gentler than the rock making up the inner slope of erosive margin trenches. The inner slope is underlain by imbricated thrust sheets of sediments. The inner slope topography is roughened by localized mass wasting . Cascadia has practically no bathymetric expression of the outer rise and trench, due to complete sediment filling, but the inner trench slope

6438-652: The south, just off Cape Mendocino in California, it intersects the San Andreas Fault and the Mendocino Fracture Zone at the Mendocino triple junction . Subduction zones experience various types of earthquakes (or seismicity); 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

6525-444: The steeper slope (8 to 20 degrees) on the inner (overriding) side of the trench and the gentler slope (around 5 degrees) on the outer (subducting) side of the trench. The bottom of the trench marks the boundary between the subducting and overriding plates, known as the basal plate boundary shear or the subduction décollement . The depth of the trench depends on the starting depth of the oceanic lithosphere as it begins its plunge into

6612-477: The subducting oceanic lithosphere is much younger, the depth of the Peru-Chile trench is around 7 to 8 kilometers (4.3 to 5.0 mi). Though narrow, oceanic trenches are remarkably long and continuous, forming the largest linear depressions on earth. An individual trench can be thousands of kilometers long. Most trenches are convex towards the subducting slab, which is attributed to the spherical geometry of

6699-520: 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

6786-412: The subducting slab returns to the surface at the oceanic trench, producing mud volcanoes and cold seeps . These support unique biomes based on chemotrophic microorganisms. There is concern that plastic debris is accumulating in trenches and threatening these communities. There are approximately 50,000 km (31,000 mi) of convergent plate margins worldwide. These are mostly located around

6873-416: The subduction décollement to propagate for great distances to produce megathrust earthquakes. Trenches seem positionally stable over time, but scientists believe that some trenches—particularly those associated with subduction zones where two oceanic plates converge—move backward into the subducting plate. This is called trench rollback or hinge retreat (also hinge rollback ) and is one explanation for

6960-468: 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

7047-399: The subsequent subhorizontal mantle flow from the displacement of the slab, can result in formation of a back-arc basin. Several forces are involved in the process of slab rollback. Two forces acting against each other at the interface of the two subducting plates exert forces against one another. The subducting plate exerts a bending force (FPB) that supplies pressure during subduction, while

7134-455: The trench become increasingly lithified , and faults and other structural features are steepened by rotation towards the trench. The other mechanism for accretionary prism growth is underplating (also known as basal accretion ) of subducted sediments, together with some oceanic crust , along the shallow parts of the subduction decollement. The Franciscan Group of California is interpreted as an ancient accretionary prism in which underplating

7221-513: The trench, the angle at which the slab plunges, and the amount of sedimentation in the trench. Both starting depth and subduction angle are greater for older oceanic lithosphere, which is reflected in the deep trenches of the western Pacific. Here the bottoms of the Marianas and the Tonga–Kermadec trenches are up to 10–11 kilometers (6.2–6.8 mi) below sea level. In the eastern Pacific, where

7308-475: 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

7395-439: The west coast of Vancouver Island, such as Tofino and Ucluelet , are at risk for a sudden, earthquake initiated, 1–2 m subsidence. 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

7482-429: The western Pacific. In light of these new measurements, the linear nature of the deeps became clear. There was a rapid growth of deep sea research efforts, especially the widespread use of echosounders in the 1950s and 1960s. These efforts confirmed the morphological utility of the term "trench." Important trenches were identified, sampled, and mapped via sonar. The early phase of trench exploration reached its peak with

7569-455: The world usually have such earthquakes every 100 to 200 years; the longer interval here may indicate unusually large stress buildup and subsequent unusually large earthquake slip. There is also evidence of accompanying tsunamis with every earthquake. One strong line of evidence for these earthquakes is convergent timings for fossil damage from tsunamis in the Pacific Northwest and historical Japanese records of tsunamis. The next rupture of

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