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 as simply the southernmost portion of the neighboring Juan de Fuca plate, another Farallon remnant.
The Cascadia subduction zone 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 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 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 tectonic 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. This region can be characterized by transform fault movement, the San Andreas also by transform strike slip movement, and the Cascadia subduction zone by a convergent plate boundary subduction movement. 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. This area is tectonically active historically and today. The Cascadia subduction zone is capable of producing megathrust earthquakes on the order of MW 9.0.
The 2010 Eureka earthquake occurred on January 9 at 4:27:38 pm PST offshore of Humboldt County, California, United States. The magnitude was measured 6.5 on the scale, and its epicenter was located offshore in the Pacific Ocean 33 miles (53 km) west of the nearest major city, Eureka. Additionally, there was a separate earthquake further offshore of Eureka on February 4 with a slightly lower magnitude of 5.9. It was also the most significant earthquake in the Eureka area in terms of magnitude since the 1992 Cape Mendocino earthquakes. It was felt from Santa Cruz County, California in the south, to Eugene, Oregon in the north and to the east as far as Reno, Nevada.
The 1986 Chalfant Valley earthquake struck southern Mono County near Bishop and Chalfant, California at Pacific Daylight Time on July 21. With a moment magnitude of 6.2 and a maximum Mercalli intensity of VI (Strong), the shock injured two people and caused property damage estimated at $2.7 million in the affected areas. There was a significant foreshock and aftershock sequence that included a few moderate events, and was the last in a series of three earthquakes that affected southern California and the northern Owens Valley in July 1986.
The 1992 Cape Mendocino 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 that converge near Cape Mendocino. The total number of aftershocks that followed the events exceeded 2,000.
At 04:56 and 06:19 UTC on October 2, 1969, a pair of earthquakes of magnitude 5.6 and 5.7, respectively, struck the city of Santa Rosa, California, killing one person and damaging buildings. The maximum felt intensity for the two events was VII and VIII (Severe) respectively on the Mercalli intensity scale. The $8.35 million cost of the earthquakes went mostly to replace buildings damaged beyond repair. There were at least 200 aftershocks after the initial pair of earthquakes. At the time they were the largest earthquakes to affect the northern San Francisco Bay Area since the 1906 San Francisco earthquake.
The 1940 El Centro earthquake occurred at 21:35 Pacific Standard Time on May 18 in the Imperial Valley in southeastern Southern California near the international border of the United States and Mexico. It had a moment magnitude of 6.9 and a maximum perceived intensity of X (Extreme) on the Mercalli intensity scale. It was the first major earthquake to be recorded by a strong-motion seismograph located next to a fault rupture. The earthquake was characterized as a typical moderate-sized destructive event with a complex energy release signature. It was the strongest recorded earthquake in the Imperial Valley, causing widespread damage to irrigation systems and killing nine people.
The 1979 Imperial Valley earthquake occurred at 16:16 Pacific Daylight Time on 15 October just south of the Mexico–United States border. It affected Imperial Valley in Southern California and Mexicali Valley in northern Baja California. The earthquake had a relatively shallow hypocenter and caused property damage in the United States estimated at US$30 million. The irrigation systems in the Imperial Valley were badly affected, but no deaths occurred. It was the largest earthquake to occur in the contiguous United States since the 1971 San Fernando earthquake eight years earlier.
The 1991 Sierra Madre earthquake occurred on June 28 at with a moment magnitude of 5.6 and a maximum Mercalli intensity of VII. The thrust earthquake resulted in two deaths, around 100 injuries, and damage estimated at $33.5–40 million. The event occurred beneath the San Gabriel Mountains on the Clamshell–Sawpit Fault, which is a part of the Sierra Madre–Cucamonga Fault System. Instruments captured the event at a number of strong motion stations in Southern California.
The 1979 Coyote Lake earthquake occurred at on August 6 with a moment magnitude of 5.7 and a maximum Mercalli Intensity of VII. The shock occurred on the Calaveras Fault near Coyote Lake in Santa Clara County, California and resulted in a number of injuries, including some that required hospitalization. Most of the $500,000 in damage that was caused was non-structural, but several businesses were closed for repairs. Data from numerous strong motion instruments was used to determine the type, depth, and extent of slip. A non-destructive aftershock sequence that lasted throughout the remainder of the month was of interest to seismologists, especially with regard to fault creep, and following the event local governments evaluated their response to the incident.
The 1957 San Francisco earthquake occurred on March 22 at 11:44:22 local time with a moment magnitude of 5.7 and a maximum Mercalli Intensity of VII. 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.
The 1986 North Palm Springs earthquake occurred on July 8 at with a moment magnitude of 6.0 and a maximum Mercalli Intensity of VII. The shock occurred in a complex setting along the San Andreas Fault Zone where it bisects San Gorgonio Mountain and San Jacinto Peak at the San Gorgonio Pass and was the first in a series of three earthquakes that affected southern California and the northern Owens Valley in July 1986. Numerous strong motion instruments recorded the event, one of which showed relatively high accelerations. Between 29 and 40 people were injured, and financial losses were estimated to be in the range of $4.5–6 million.
The 1892 Vacaville–Winters earthquakes occurred in northern California as a large doublet on April 19 and April 21. Measured on a seismic scale that is based on an isoseismal map or the event's felt area, the 6.4 and 6.2 Mla events were assigned a maximum Mercalli intensity of IX (Violent), and affected the North Bay and Central Valley areas. The total damage was estimated to be between $225,000 and 250,000 and one person was killed. No evidence of fault movement on the surface of the ground was observed as a result of either of the strong shocks. Both occurred in the domain of the San Andreas strike-slip system of faults, but their focal mechanism is uncertain.
The 1932 Eureka earthquake occurred on June 6 at 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.
The 1838 San Andreas earthquake is believed to be a rupture along the northern part of the San Andreas Fault in June 1838. It affected approximately 100 km of the fault, from the San Francisco Peninsula to the Santa Cruz Mountains. It was a strong earthquake, with an estimated moment magnitude of 6.8 to 7.2, making it one of the largest known earthquakes in California. The region was lightly populated at the time, although structural damage was reported in San Francisco, Oakland, and Monterey. It is unknown whether there were fatalities. Based on geological sampling, the fault created approximately 1.5 meters of slip.
The 1898 Mare Island earthquake occurred in Northern California on with a moment magnitude of 5.8–6.4 and a maximum Mercalli intensity of VIII–IX (Severe–Violent). Its area of perceptibility included much of northern and central California and western Nevada. Damage amounted to $350,000 and was most pronounced on Mare Island, a peninsula in northern San Francisco Bay. While relatively strong effects there were attributed to vulnerable buildings, moderate effects elsewhere in the San Francisco Bay Area consisted of damaged or partially collapsed structures, and there were media reports of a small tsunami and mostly mild aftershocks that followed.
The 1979 Saint Elias earthquake affected Alaska at 12:27 AKST on 28 February. The thrust-faulting 7.5 earthquake had an epicenter in the Granite Mountains. Though the maximum recorded Modified Mercalli intensity was VII, damage was minimal and there were no casualties due to the remoteness of the faulting. Damage also extended across the border in parts of Yukon, Canada.
On December 20, 2022, a magnitude 6.4 earthquake struck Ferndale, California in Humboldt County, United States at 10:34:25 UTC, or 2:34 a.m. PST.
