Gorda Plate

The Gorda Plate, located beneath the Pacific Ocean off the coast of northern California, is one of the northern remnants of the Farallon Plate. It is sometimes referred to (by, for example, publications from the USGS Earthquake Hazards Program)[1] as simply the southernmost portion of the neighboring Juan de Fuca Plate, another Farallon remnant.

Unlike most tectonic plates, the Gorda Plate experiences significant intraplate deformation inside its boundaries. Numerous faults have been mapped in both the sediments and basement of the Gorda Basin, which is in the interior of the plate south of 41.6°N.[2] Stresses from the neighboring North American Plate and Pacific Plate cause frequent earthquakes in the interior of the plate, including the 1980 Eureka earthquake (also known as the Gorda Basin event).[3]

The easterly side is a convergent boundary subducting under the North American Plate in northern California. The southerly side is a transform boundary with the Pacific Plate along the Mendocino Fault. The westerly side is a divergent boundary with the Pacific Plate forming the Gorda Ridge. This ridge provides morphological evidence of differing spreading rates, with the northern portion of the ridge being narrow, and the southern portion being wide. [4]The northerly side is a transform boundary with the Juan de Fuca Plate, the Blanco Fracture Zone.

The subducting Gorda Plate is connected with the volcanoes in northern California, namely, Mount Shasta and Lassen Peak. Lassen Peak last erupted in 1914–1917.[5]

Juan de fuca plate
A map of the Gorda and neighboring Juan de Fuca Plates subducting under the North American plate

See also


  1. ^ "National Earthquake Information Center (NEIC)". usgs.gov. Archived from the original on 25 June 2012. Retrieved 12 December 2014.
  2. ^ Gulick, S., Meltzer, A., and S. Clarke (1998) Seismic structure of the southern Cascadia subduction zone and accretionary prism north of the Mendocino triple junction. Journal of Geophysical Research-Solid Earth, 103(B11).
  3. ^ "Historic Earthquakes". usgs.gov. Retrieved 12 December 2014.
  4. ^ Riddihough, R. P. (1980). Gorda plate motions from magnetic anomaly analysis. Earth and Planetary Science Letters, 51(1), 163-170.
  5. ^ Clynne Clynne, Michael; Christiansen, Daniel; Stauffer, Peter; Hendley, James (21 May 2015). "Eruptions of Lassen Peak, California, 1914 to 1917 — A Centennial Commemoration". usgs.gov. U.S. Department of the Interior.

External links

Coordinates: 41°12′N 126°24′W / 41.2°N 126.4°W

1932 Eureka earthquake

The 1932 Eureka earthquake occurred on June 6 at 00:44:26 local time along the northern coastal area of California in the United States. With a moment magnitude of 6.4 and a maximum Mercalli intensity of VIII (Severe), this earthquake left one person dead from a falling chimney and several injured. The shock was the largest in the area since 1923 and was felt in southern Oregon and northern California.

1957 San Francisco earthquake

The 1957 San Francisco earthquake (also known as the Daly City earthquake of 1957) occurred on March 22 at 11:44:22 local time with a moment magnitude of 5.7 and a maximum Mercalli Intensity of VII (Very strong). It was located just off the San Francisco Peninsula near the San Andreas Fault and was felt in a limited portion of Northern and Central California. There was a non-destructive foreshock and aftershock sequence that lasted for several months. With financial losses of around US$1 million, damage was considered minimal, with one death and forty injuries.

1980 Eureka earthquake

The 1980 Eureka earthquake (also known as the Gorda Basin earthquake) occurred on November 8 at 02:27:34 local time along the northern coastal area of California in the United States. With a moment magnitude of 7.3 and a maximum Mercalli intensity of VII (Very strong), this strike-slip earthquake was the largest to occur in California in 28 years. Although damage was considered light, several loss estimates equaled or exceeded $2 million, and six injuries resulted when two vehicles came down with the partial collapse of a highway overpass on US 101 in Fields Landing. The north coast of California experiences frequent plate boundary earthquakes near the Mendocino Triple Junction and intraplate events also occur within the Gorda Plate.

Due to the regional seismic risk, the nuclear portion of the Humboldt Bay Nuclear Power Plant was shut down in the 1970s. No substantial damage occurred to the two fossil-fuel units that were still operational at the facility. Several types of sensors were installed at the site to capture strong motion data in this seismically-active area, but the majority of records from the event were considered unreliable due to faulty equipment or inadequate maintenance. Only one piece of equipment at the facility provided data by which an estimate of the peak ground acceleration could be made.

1992 Cape Mendocino earthquakes

The 1992 Cape Mendocino earthquakes (or 1992 Petrolia earthquakes) occurred along the Lost Coast of Northern California on April 25 and 26. The three largest events were the M7.2 thrust mainshock that struck near the unincorporated community of Petrolia midday on April 25 and two primary strike-slip aftershocks measuring 6.5 and 6.6 that followed early the next morning. The sequence encompassed both interplate and intraplate activity that was associated with the Mendocino Triple Junction, a complex system of three major faults (including the Cascadia subduction zone, San Andreas Fault, and Mendocino Fracture Zone) that converge near Cape Mendocino. The total number of aftershocks that followed the events exceeded 2,000.

The three shocks damaged and destroyed homes and businesses in Humboldt County and injured up to 356 people, but the single largest loss was due to a post-earthquake fire that consumed a business center in Scotia. Accelerometers that had been in place in the Cape Mendocino area since the late 1970s recorded the event and the readings were moderate to strong, with the exception of the instruments closest to the epicenter, which went off scale a few seconds into the recording. No surface ruptures were present in the epicentral area, but landslides closed roads and railroad tracks for at least a week while cleanup took place. Also discovered was about 1 m (3 ft 3 in) of coastal uplift near Cape Mendocino and Punta Gorda.

As the largest earthquake in California since the 1989 Loma Prieta event several years earlier, the mainshock caused a non-destructive tsunami that quickly reached the coast, and eventually Alaska and Hawaii several hours later. The tsunami was significant not because of its run-up, but because of the speed with which it reached the coast and for how long the waves persisted. Other strong earthquakes have affected the same area, with some that were clearly associated with the (interplate) Mendocino Fracture Zone, and others (like the two shocks on April 26) were intraplate earthquakes that ruptured within the Gorda Plate, but events that are unequivocally associated with the Cascadia subduction zone are very infrequent.

Cape Mendocino

Cape Mendocino, approximately 200 miles north of San Francisco, is located on the Lost Coast entirely within Humboldt County, California, USA. At 124° 24' 34" W longitude it is the westernmost point on the coast of California. The South Cape Mendocino State Marine Reserve and Sugarloaf Island are immediately offshore, although closed to public access due to their protected status. Sugarloaf Island is cited as California's westernmost island.

Cascadia subduction zone

The Cascadia subduction zone (also referred to as the Cascadia fault, or Cascadia) is a convergent plate boundary that stretches from northern Vancouver Island in Canada to Northern California in the United States. 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 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 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.

Explorer Plate

The Explorer Plate is an oceanic tectonic plate beneath the Pacific Ocean off the west coast of Vancouver Island, Canada and is partially subducted under the North American Plate. Along with the Juan De Fuca Plate and Gorda Plate, the Explorer Plate is a remnant of the ancient Farallon Plate which has been subducted under the North American Plate. The Explorer Plate separated from the Juan De Fuca Plate roughly 4 million years ago. In its smoother, southern half, the average depth of the Explorer plate is roughly 2,400 metres (7,900 ft) and rises up in its northern half to a highly variable basin between 1,400 metres (4,600 ft) and 2,200 metres (7,200 ft) in depth.

Explorer Ridge

The Explorer Ridge is a mid-ocean ridge, a divergent tectonic plate boundary located about 241 km (150 mi) west of Vancouver Island, British Columbia, Canada. It lies at the northern extremity of the Pacific spreading axis. To its east is the Explorer Plate, which together with the Juan de Fuca Plate and the Gorda Plate to its south, is what remains of the once-vast Farallon Plate which has been largely subducted under the North American Plate. The Explorer Ridge consists of one major segment, the Southern Explorer Ridge, and several smaller segments. It runs northward from the Sovanco Fracture Zone to the Queen Charlotte Triple Junction, a point where it meets the Queen Charlotte Fault and the northern Cascadia subduction zone.

Fracture zone

A fracture zone is a linear oceanic feature—often hundreds, even thousands of kilometers long—resulting from the action of offset mid-ocean ridge axis segments. They are a consequence of plate tectonics. Lithospheric plates on either side of an active transform fault move in opposite directions; here, strike-slip activity occurs. Fracture zones extend past the transform faults, away from the ridge axis; seismically inactive (because both plate segments are moving in the same direction), they display evidence of past transform fault activity, primarily in the different ages of the crust on opposite sides of the zone.

In actual usage, many transform faults aligned with fracture zones are often loosely referred to as "fracture zones" although technically, they are not.


Gorda may refer to:

Gorda, California, United States, a small town on the Pacific Ocean

Gorda Plate, a tectonic plate located beneath the Pacific Ocean near northern California

Cayo Gorda, a small island of Honduras in Caribbean Sea

Gorda, fat in Spanish and Portuguese

Gorda Ridge

The Gorda Ridge (41°36'19.6"N 127°22'03.1"W), a tectonic spreading center, is located roughly 200 kilometres (120 mi) off the northern coast of California and southern Oregon. Running NE – SW it is roughly 300 kilometres (190 mi) in length. The ridge is broken into three segments; the northern ridge, central ridge, and the southern ridge, which contains the Escanaba Trough.

Juan de Fuca Plate

The Juan de Fuca Plate is a tectonic plate generated from the Juan de Fuca Ridge that is subducting under the northerly portion of the western side of the North American Plate at the Cascadia subduction zone. It is named after the explorer of the same name. One of the smallest of Earth's tectonic plates, the Juan de Fuca Plate is a remnant part of the once-vast Farallon Plate, which is now largely subducted underneath the North American Plate.

Lassen Volcanic National Park

Lassen Volcanic National Park is an American national park in northeastern California. The dominant feature of the park is Lassen Peak, the largest plug dome volcano in the world and the southernmost volcano in the Cascade Range. Lassen Volcanic National Park started as two separate national monuments designated by President Theodore Roosevelt in 1907: Cinder Cone National Monument and Lassen Peak National Monument.The source of heat for the volcanism in the Lassen area is subduction of the Gorda Plate diving below the North American Plate off the Northern California coast. The area surrounding Lassen Peak is still active with boiling mud pots, fumaroles, and hot springs. Lassen Volcanic National Park is one of the few areas in the world where all four types of volcano can be found—plug dome, shield, cinder cone, and stratovolcano.The park is accessible via State Routes 89 and 44. SR 89 passes north-south through the park, beginning at

SR 36 to the south and ending at SR 44 to the north. SR 89 passes immediately adjacent to the base of Lassen Peak.

There are five vehicle entrances to the park: the north and south entrances on SR 89; and unpaved roads entering at Drakesbad and Juniper Lake in the south, and at Butte Lake in the northeast. The park can also be accessed by trails leading in from the Caribou Wilderness to the east, as well as the Pacific Crest Trail, and two smaller trails leading in from Willow Lake and Little Willow Lake to the south.

The Lassen Chalet, a large lodge with concession facilities, was located near the southwest entrance, but was demolished in 2005. A new full-service visitor center in the same location opened to the public in 2008. The Lassen Ski Area was located near the lodge; it ceased operation in 1992 and all infrastructure has been removed.

List of tectonic plates

This is a list of tectonic plates on the Earth's surface. Tectonic plates are pieces of Earth's crust and uppermost mantle, together referred to as the lithosphere. The plates are around 100 km (62 mi) thick and consist of two principal types of material: oceanic crust (also called sima from silicon and magnesium) and continental crust (sial from silicon and aluminium). The composition of the two types of crust differs markedly, with mafic basaltic rocks dominating oceanic crust, while continental crust consists principally of lower-density felsic granitic rocks.

Lost Coast

The Lost Coast is a mostly natural and undeveloped area of the California North Coast in Humboldt and Mendocino Counties, which includes the King Range. It was named the "Lost Coast" after the area experienced depopulation in the 1930s. In addition, the steepness and related geotechnical challenges of the coastal mountains made this stretch of coastline too costly for state highway or county road builders to establish routes through the area, leaving it the most undeveloped and remote portion of the California coast. Without any major highways, communities in the Lost Coast region such as Petrolia, Shelter Cove, and Whitethorn are isolated from the rest of California.

The region lies roughly between Rockport and Ferndale. At the south end, State Route 1, which runs very close along the coast for most of its length, suddenly turns inland at Rockport before merging with U.S. Route 101 at Leggett. At the north end, State Route 211 begins its journey at Ferndale, heading towards Highway 101 in Fernbridge. Section 511 of the California Streets and Highways Code still says that "Route 211 is from Route 1 near Rockport to Route 101 near Fernbridge", but it is unlikely that the portion south of Ferndale will be built. Most of the region's coastline is now part of either Sinkyone Wilderness State Park or King Range National Conservation Area. In 1912, biologist Dr. Bill Johnson explored and documented the geology and wildlife of the region now known as King Range National Conservation Area. Dr. Johnson spent most of his time at Randall Creek, which he nicknamed "Cumacie" due to the overbearing summer wind storms.

Mendocino Fracture Zone

The Mendocino Fracture Zone is a fracture zone and transform boundary over 4000 km (2500 miles) long, starting off the coast of Cape Mendocino in far northern California. It runs westward from a triple junction with the San Andreas Fault and the Cascadia subduction zone to the southern end of the Gorda Ridge. It continues on west of its junction with the Gorda Ridge, as an inactive remnant section which extends for several hundred miles.

Technically, a fracture zone is not a transform fault, but in the case of the Mendocino, the term has been loosely applied to the active fault segment east of the Gorda Ridge as well as to the true fracture zone segment west of it. Many seismologists refer to the active segment as the Mendocino Fault or Mendocino fault zone. The fault section demarcates the boundary between the northwestward-moving Pacific Plate and the eastward-moving Gorda Plate. The Gorda Plate is subducting beneath the North American Plate just offshore of Cape Mendocino. Where the Mendocino Fault intersects the undersea trench of the subduction zone, it also meets the San Andreas Fault. This seismically active intersection is called a triple junction, and specifically the Mendocino Triple Junction.

In Tsunami studies, energy focusing around the fracture zone has been noted, leading to increased wave heights in the area around Crescent City, California. The fracture zone is referred to as the Mendocino Escarpment in these studies, descriptively rather than named from its geological origin.

Mendocino Triple Junction

The Mendocino Triple Junction (MTJ) is the point where the Gorda plate, the North American plate, and the Pacific plate meet, in the Pacific Ocean near Cape Mendocino in northern California. This triple junction is the location of a change in the broad plate motions which dominate the west coast of North America, linking convergence of the northern Cascadia subduction zone and translation of the southern San Andreas Fault system. The Gorda plate is subducting, towards N50ºE, under the North American plate at 2.5 – 3 cm/yr, and is simultaneously converging obliquely against the Pacific plate at a rate of 5 cm/yr in the direction N115ºE. The accommodation of this plate configuration results in a transform boundary along the Mendocino Fracture Zone, and a divergent boundary at the Gorda Ridge.Due to the relative plate motions, the triple junction has been migrating northwards for the past 25–30 million years, and assuming rigid plates, the geometry requires that a void, called slab window, develop southeast of the MTJ. At this point, removal of the subducting Gorda lithosphere from beneath North America causes asthenospheric upwelling. This instigates different tectonic processes, which include surficial uplift, crustal deformation, intense seismic activity, high heat flow, and even the extrusion of volcanic rocks. This activity is centred on the current triple junction position, but evidence for its migration is found in the geology all along the California coast, starting as far south as Los Angeles.

Pacific Plate

The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At 103 million square kilometres (40,000,000 sq mi), it is the largest tectonic plate.The Pacific Plate contains an interior hot spot forming the Hawaiian Islands.Hillis and Müller are reported to consider the Bird's Head Plate to be moving in unison with the Pacific Plate. Bird considers them to be unconnected.

Triple junction

A triple junction is the point where the boundaries of three tectonic plates meet. At the triple junction each of the three boundaries will be one of 3 types - a ridge (R), trench (T) or transform fault (F) - and triple junctions can be described according to the types of plate margin that meet at them (e.g. Transform-Transform-Trench, Ridge-Ridge-Ridge, or abbreviated F-F-T, R-R-R). Of the many possible types of triple junction only a few are stable through time ('stable' in this context means that the geometrical configuration of the triple junction will not change through geologic time). The meeting of 4 or more plates is also theoretically possible but junctions will only exist instantaneously.



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