[11][12], As the largest earthquake in California since the October 1989 event in the Santa Cruz Mountains, the mainshock near Petrolia produced some of the highest ground motions ever recorded (at that time) by the California Strong Motion Instrumentation Program (CSMIP). (https://mendovoice.com/2020/03/modest-earthquake-strikes-off-cape-mendocino/), The location of the 5.2 earthquake that hit at 3:08 p.m. on March 17, 2020. Investigation of more than 1,200 surveys from the North Coast area led to the assignment of an intensity rating of IX (Violent) on the Modified Mercalli scale for the region near Petrolia. The first shock was located six miles north of Petrolia in a sparsely populated part of southwestern Humbolt County. Mendocino triple junction. The speed a tsunami travels is directly related to the depth of the water in which it is traversing. Figure 1: This map shows all quakes with magnitudes of more than 6 near the Mendocino Triple Junction in the last 36 years. Further investigation revealed that a 15 km (9.3 mi) portion of the shoreline between Cape Mendocino and near Punta Gorda had been uplifted by as much as 1 m (3 ft 3 in) near the middle portion, and decreasing amounts near the outer portions of the affected area. Previous Gorda plate events include the July 13 and August 17, 1991, shocks of 6.8 and 7.1 and the M7.3 event on November 10, 1980, west of Arcata. Even the fire station suffered so muc… While the focal mechanism indicated slip on a thrust fault striking N.10°W with a shallow dip of 13° to the east-northeast, the rupture most likely propagated to the west, based on the mainshock location at the southeastern boundary of the aftershock zone. Three major faults, the San Andreas, the … The April 25, 1992, Cape Mendocino, California, earthquake (moment magnitude M=7) is the first well-recorded event to demonstrate the seismogenic potential of the Cascadia subduction zone [Oppenheimer et al., 1993]. 1992 Cape Mendocino Earthquake Videos. And in this case, the strongest waves came ashore in Crescent City three to four hours after the initial surge, but it is possible to be just the opposite where the first waves could be the strongest. On April 25, 1992, at 11:06 A.M. a magnitude 7.1 main shock near Petrolia, California, initiated a series of shocks in the Cape Mendocino area. ... Offshore, about 70 miles W of Cape Mendocino: Nearly 30 earthquake­s were detected in the triple junction region in the past month alone. Also discovered was about 1 m (3 ft 3 in) of coastal uplift near Cape Mendocino and Punta Gorda. WILLITS, 3/18/20 — A decent sized earthquake, measuring a magnitude 5.2 struck off the coast of Cape Mendocino in Humboldt County, at 3:08 p.m. today. The movement of the seawater across the earth's magnetic field produces a large-scale motional electric field. 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. Earthquakes similar to or like 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. Eureka and Arcata (25 miles (40 km) north of the Eel River valley) saw light damage and no injuries, while the unincorporated communities of Weott and Carlotta reported damage of less than $2 million combined. “What about those lights? A post-earthquake lab test of the seismometer and an inspection of the photographically enlarged accelerogram revealed that the limit was hit twice, with a maximum deflection of 31 mm, as the needle bounced off the unit's mass. First large earthquake in historic times to: -be related to the Cascadia subduction zone -produce a local tsunami Q Largest magnitude onshore north coast earthquake in historic times. That's fine, consider a one time contribution to keep local journalism going. No tsunami was detected at Johnston Atoll, 5,050 kilometers (3,140 mi) from Cape Mendocino in the north Pacific Ocean, but bottom pressure recorders registered a maximum amplitude of .4 cm (0.16 in) in 4,000 meters (13,000 ft) of water in the Gulf of Alaska, with 3.75 hours of travel time. [6][7], USGS ShakeMaps showing the mainshock (left) and the second (and slightly stronger) 4:18 PDT aftershock's intensity, National Oceanic and Atmospheric Administration, "Reducing tsunami hazards along U.S. coastlines", "The Cape Mendocino, California, Earthquakes of April 1992: Subduction at the Triple Junction", "Recent large earthquakes near Cape Mendocino and in the Gorda plate: Broadband source time functions, fault orientations, and rupture complexities", Doom and gloom? The quake’s epicenter was about 8.5 miles (14km) west of Petrolia, in the Pacific Ocean. The remaining four were located on structures, including a Highway 101 overpass in Rio Dell, a dam, a one-story supermarket in Fortuna, and a 5-story residential building in Eureka. In Petrolia, the post office, three businesses, and 44 homes were destroyed, and another 68 residences were damaged. It is also the meeting point of three great fault systems: the San Andreas to the south, the Mendocino to the west and the Cascadia to the north. These are earthquakes of magnitude greater than or equal to 6.5, or that caused loss of life or more than $200,000 in damage. Quake ’92 from Ross Rowley. No surface ruptures were found during aerial surveillance, but lateral spreading features were observed on a channel near the mouth of the Eel River. The North Coast has rumbled plenty of times since then. The 1992 Cape Mendocino earthquakes (or 1992 Petrolia earthquakes) occurred along the Lost Coast of Northern California on April 25 and 26. Some were mere seconds while others lasted for hours, but the hydroelectric plant's performance at Ruth Reservoir was deemed acceptable, and power that was not generated locally was unaffected. 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. 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. © Copyright 2021, The Mendocino Voice, Mendocino News Media LLC. An extrapolation of the vertical record led to a maximum acceleration estimate of 2.2g for that site, and the unit was eventually replaced with a higher capacity digital device. The 25 April 1992 magnitude 7.1 Cape Mendocino thrust earthquake demonstrated that the North America—Gorda plate boundary is seismogenic and illustrated hazards that could result from much larger earthquakes forecast for the Cascadia region. The 1992 Cape Mendocino earthquakes (or 1992 Petrolia earthquakes) occurred along the Lost Coast of Northern California on April 25 and 26. [6][8], The initial event caused a number of wood framed homes in Scotia to come off their foundations while the porches of some other homes became detached. That the two strike-slip events followed a thrust event indicated a strong coupling of stresses at the North American and Gorda plate boundaries, and underscored the convoluted nature of the interconnected faults in that area. The northernmost coastal area is one of California's most seismically active regions and, in a 50-year period, the area including the Mendocino Fracture Zone at the southern flank of the Gorda Plate generated about 25 percent of all seismic energy unleashed in the state. The Gorda Plate is undergoing a process of intraplate deformation and experiences large intraplate earthquakes that may be the result of north–south compression of the oceanic crust along the Mendocino Fracture Zone. The shallow tremor (which occurred at a depth of 17km, around 40km from the town of Ferndale) caused minor shaking along the coast but at the time of writing there were no reports of damage or injuries – and, despite the offshore location, … Those three tectonic plates are the North American, Pacific, and Gorda plates. Submerged wetlands and raised marine terraces both illustrate the presence of past events, and radiocarbon dating of rock layers has revealed that three seismic events took place in the last 2,000 years, with the most recent event being the 1700 Cascadia earthquake. A tsunami is NOT expected. The Mendocino Voice | Mendocino County, CA, - Useful news, for all of Mendocino County, CA. Since July 1991 seven earthquakes with Mw > 6 have occurred within a few hundred kilometer distance from the MTJ (Table 1). Most of these were existing landslides that had been re-initiated and the largest of the slides were either slumps or bedding plane failures along the coastal bluffs. The San Andreas Fault continues to lengthen to the northwest and the southeast as the two triple junctions continue their transient motion. 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. The Cape Mendocino earthquakes occurred within the Mendocino triple junction region, where three plates of the earth’s surface, the Pacific, the North American and the Gorda plate meet. The mainshock, located 25 km southeast of Cape Mendocino at 10 km depth, and two deeper M~6.6 aftershocks located 25 km The first packet of energy reached that location in 47 minutes and had a maximum wave height of 35 cm (14 in), and a second, larger packet arrived later with a maximum amplitude of 53 cm (21 in). Evidence of previous events has been found in the form of sequential marine terraces along the coast, with periodic events creating shelves at 300, 1,700, 3,000, and 5,000 years before present. First Humboldt County earthquake to be declared a major disaster by the President. The road between Ferndale and Petrolia was closed for more than a week where about six mostly minor landslides blocked the passage, with the roadway itself sustaining light damage due to sliding or settlement of the road fill in a few instances. A six figure portion of that figure was due to losses at the high school's gymnasium. Another large event (7.3–7.6) occurred on January 31, 1922 (with an aftershock of M7+ the next day) but the sources of these shocks could not be determined with any precision as the first seismographs did not arrive in the area until 1932. The quake’s epicenter was about 8.5 miles (14km) west of Petrolia, in the Pacific Ocean. The epicenter of the 1992 quake is marked by the red star near the lower right corner of the map. Several slump failures between Guthrie Creek and Oil Creek stretched from the bluffs out towards the shore for a distance of 150 meters (490 ft), leaving some of the slide to be eroded by the surf. The North Coast has rumbled plenty of times since then. The 1992 Cape Mendocino Earthquake Back to the Future: Remembering the Cape Mendocino earthquakes of 25 years ago and the preparedness lessons … One of the largest slides occurred along the railroad tracks at the Scotia bluffs where previous slides had taken place. There have been a series of earthquakes in the past month, some small ones in Redwood Valley, as well as a cluster of quakes measuring larger that 5.0 off the coast of Humboldt County near the Mendocino Triple Junction, whether three tectonic plates meet. [10], Damage estimates were as high as $75 million, one third of which was due to bridges and roads, and the remainder of the costs were structure-related. The shock occurred just north of the Mendocino Triple Junction and caused strong ground motion and moderate Cascadia Subduction Zone | Pacific Northwest Seismic Network Using the Point Arena, California, to Hanauma Bay, Hawaii, unpowered HAW-1 cable, we have studied the geopotential across this distance to look for possible tsunami-induced fields that might have been produced following the April 1992 Cape Mendocino earthquake. This is a sparsely populated and undeveloped area in the coastal The total number of aftershocks that followed the events exceeded 2,000. In Ferndale, 29 homes were destroyed and 126 were damaged, along with 51 businesses. Also detailed from this event was that the wave hazard can be of long duration, with wave action lingering for more than eight hours. A large fire was triggered following the first aftershock at a shopping center in Scotia that destroyed four businesses, with the resulting damage at that site alone estimated at $15 million, and was the largest individual financial misfortune of the sequence of earthquakes. Shortly after 11 am on April 25, 1992 the town and with it the greater part of Northern California were shaken by the strongest earthquake … That slide also took about a week to clear. A landslide came within 50 meters (160 ft) of the device and left debris on the road, but a lack of large rocks close to the instrument and no cracking of the rocks near the platform left geologists with no clear explanation for the extraordinarily high vertical component reading of 1.85g. It is a geologic triple junction where three tectonic plates come together. From Point Conception to Cape Mendocino, seafloor faults have been, in the past, mapped in varying ways and without enough detail to assess their earthquake potential. Cape Mendocino Seismic Retrofit. Understanding potential geologic hazards in the Humboldt Bay region, Mendocino Triple Junction Working Group Region PBO Mini Proposal, Petrolia Journal; Town Amid Redwoods Gamely Fights Off Quake, https://en.wikipedia.org/w/index.php?title=1992_Cape_Mendocino_earthquakes&oldid=1017710349, Earthquake clusters, swarms, and sequences, Creative Commons Attribution-ShareAlike License, This page was last edited on 14 April 2021, at 06:48. An earthquake with a magnitude of 5.7 (M5.7) struck off the coast of northern California in the afternoon of 28 January 2015. Mendocino Public Health will issue shelter-in-place order tomorrow, Coronavirus dampens St. Patrick's celebrations in Fort Bragg, Summer 2020: Changes coming to membership and newsletters, Two large quakes rattle Humboldt coast Sunday evening (updated 11:45 p.m.), Swarm of small earthquakes shakes Redwood Valley, Q&A with Cal Fire Mendocino Chief George Gonzalez: “We might have to start trucking in water just for our daily activities at the station level…”, Growing our food future: Mendocino County food summit happening May 22, Small wildfire just south of Ukiah controlled (6:12pm), Thunderstorm brings heavy rain, car wrecks, reports of spot fires to Willits area — small wildfire north of Covelo likely started by lightning — thunderstorms will continue into tonight (update 5:20pm), NCO offers gifts to people vaccinated for COVID in Mendocino Co. — NCO ofrece regalos a las personas vacunadas contra el COVID, Deputies involved in gunfight, no one injured — major search ensues, “Seniors Thriving” event focused on elderly health in Mendocino and Lake counties happening May 20, City of Ukiah now accepting vendor applications for community yardsale, happening June 6, Inmate attempts suicide at Mendocino Jail, Commenting, Corrections, & Letters-to-the-Editor Policy, Watch the Mendocino Co. election forums Oct. 5 & 8. Required fields are marked *. Abstract- The 25 April 1992 Cape Mendocino earthquake generated a tsunami characterized by both coastal trapped edge wave and non-trapped tsunami modes that propagated north and south along the U.S. West Coast. The supermarket, residential building, and dam were 28, 50, and 75 kilometers distant from the epicenter respectively, and recorded peak accelerations of .46g, .34g, and .15g. Simultaneously, the Rivera Triple Junction shifted to the southeast to its current position at 23° N. Once the Pacific Plate and North American Plate connected the boundary became that of a transform fault (San Andreas) due to the northwestward motion of the Pacific Plate relative to the North American Plate. This series of quakes, measuring 7.1, 6.2 and 6.5 respectively, emanated from a complex geological area known as the Mendocino Triple Junction. Both shocks (M6.5 and 6.6) were of intensity VIII (Severe), occurred at a depth of 20 kilometers (12 mi), and exhibited right-lateral motion. Larry Karsteadt, longtime friend and emergency response colleague, commented after last week’s column about the 1992 Cape Mendocino earthquake. The tri-axial analog accelerometer that was in use was physically limited to that value and all three traces had uniformly significant values at three seconds into the recording. We believe in useful news for all of Mendocino. The April 25, 1992, Cape Mendocino earthquake (M w = 7.2) occurred in the immediate vicinity of the MTJ (Figure 1) and The 25 April 1992 magnitude 7.1 Cape Mendocino thrust earthquake demonstrated that the North America-Gorda plate boundary is seismogenic and illustrated hazards that could result from much larger earthquakes forecast for the Cascadia region. With their epicenters offshore, south of Petrolia, this earthquake created a great deal of damage to the small towns of the area, including Ferndale, … 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. Why is the Cape Mendocino earthquake important? [13], The CSMIP Cape Mendocino station was installed in 1978 and was located just 4 kilometers (2.5 mi) from the epicenter on the slope of a ridge in the coast ranges. The initial earthquake was centered 6 miles north of Petrolia in the south­ western part of Humbolt County.