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96475
From: jordan, [DNS_Address]
Subject: offshore northern California massive crustal squeeze,
|
the U.S. Geological Survey (USGS) suggests today’s earthquake occurred “as a result of strike-slip faulting on a steeply dipping fault,” likely within part of the Gorda Plate, which is part of the Juan de Fuca Plate that subducts beneath Northern California, Oregon and Washington. Today, many aftershocks are extending east of the mainshock. magnitude-6.2 earthquake occurred about 12 miles (20 kilometers) away a year ago today. In the last century, earthquakes above magnitude 7 and more than 40 quakes above magnitude 6 have struck within 155 miles (250 kilometers) this is an area undergoing a massive crustal squeeze, with both shocks the direct products of this slow- motion crash zone. Eureka lies at a ‘tectonic triple junction,’ where three tectonic plates meet. At many other triple junctions, the three fault systems are perpendicular to each other, and so all of the action can be taken up along the major faults. But here, Eli Silver discovered in 1971 that there is a space problem (Silver, 1971). The oceanic plate being extruded at the Gorda Ridge is not spreading parallel to the Mendocino Fracture Zone, the boundary between the Gorda and the Pacific plates, but instead is being pushed toward the Fracture Zone, creating the space problem. This deforms and crushes the Gorda plate as it moves toward the California coast, and it also has deformed the Cascadia megathrust into a sinuous mess. The Cascadia marks where the Gorda plate is shoved under, or ‘subducts’ beneath, the North American plate. Because the three major faults (the San Andreas, Mendocino, and Cascadia) cannot accommodate the deformation, numerous secondary faults form that distribute the deformation onshore and off. |
Responses:
[96488] [96490] [96507] [96494] [96492] [96478] [96477]
96488
From: ryan, [DNS_Address]
Subject: Re: offshore northern California massive crustal squeeze,
URL: https://www.sfgate.com/bayarea/article/california-mendocino-triple-junction-explained-17669321.php
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Why big earthquakes keep hammering California's 'Mendocino Triple Junction' Amy Graff, SFGATE Dec. 21, 2022 Building inspector Kevin Caldwell red-tags a home in Rio Dell, Calif., that lost an awning and deck after an earthquake hit Humboldt County, Tuesday, Dec. 20, 2022. Kent Porter/AP A magnitude 6.4 earthquake that struck off the coast of Northern California's Humboldt County early Tuesday morning occurred in a region that seismologists call the "Mendocino Triple Junction." This point, where three faults and three plates meet, is considered the most seismically active area in California, said Stephen DeLong, a supervisory research geologist with the U.S. Geological Survey. “It’s a very complicated area where a few different plates are going in a few different directions,” DeLong told SFGATE over the phone. In the past 100 years, at least 40 other earthquakes of magnitude 6 or larger, including six earthquakes magnitude 7.0 or larger, have occurred in this area, the USGS said. A magnitude 6.2 quake occurred in this region only a year ago — Dec. 20, 2021. Most notably, on April 25 and 26, 1992, three major earthquakes — magnitudes 7.2, 6.5 and 6.6 — hit in this area, injuring 356 people, damaging or destroying some 1,100 structures, and causing $61 million dollars in damage, according to the Redwood Coast Tsunami Work Group. The area is sometimes informally referred to as the "Triangle of Doom," but the USGS thinks the ominous term is counterproductive. "We find fear to be a negative motivator for preparedness," DeLong wrote in a text. "We want people to use their knowledge of hazards and their experiences in these moderate earthquakes as motivation to prepare for larger quakes. Feeling prepared helps us manage any fear of natural hazards." Tuesday's quake hit at 2:34 a.m., less than 10 miles off the coast, and intensely shook the the towns of Ferndale, Fortuna, Rio Dell and Scotia. Homes were thrown off foundations, roads buckled, gas and water lines broke, and more than 70,000 customers lost power. Two people died, and 12 were injured. Illustration of the Mendocino Triple Junction. Illustration of the Mendocino Triple Junction. Redwood Coast Tsunami Work Group All of this turmoil was triggered by seismic activity in the Mendocino Triple Junction. At this point off the coast of California near Cape Mendocino, three major faults meet: the Cascadia Subduction Zone, the San Andreas Fault and the Mendocino Fracture Zone. Three tectonic plates — the Pacific, North American and Gorda plates — are also butting up against one another. The Gorda Plate is plunging under, or "subducting" beneath, the North American Plate. This movement formed the Cascade Range. The Gorda Plate "is melting under the North American plate," DeLong explained. "The magma moves up [toward the Earth's surface] and erupts out of the volcanoes in Northern California. ... All the volcanos from Lassen Peak and up into British Columbia are because of this subduction zone." Simultaneously, the Pacific Plate is grinding past the North American Plate along the San Andreas Fault and pushing past the Gorda Plate along the Mendocino Fracture Zone. Map of Cape Mendocino, Eureka, Garberville and southwestern part of Hayfork. Seismologists believe the epicenter of the Tuesday quake was on the Gorda Plate, which is the southernmost part of the Juan de Fuca Plate. "That's what we think based on the depth and the focal mechanisms. ... The fact we see it was a left-lateral earthquake," DeLong said. "With the geometry of everything, that makes us think it's in the subducting Gorda Plate." DeLong said it will take USGS researchers a few more days to confirm the epicenter's exact location. As of midday Wednesday, thousands of PG&E customers in the Fortuna area were awaiting the restoration of their power. The town of Rio Dell was among the hardest hit and is still without water and electricity. “It was the most intense earthquake that I’ve felt,” Rio Dell Mayor Debra Garnes told the Los Angeles Times. “It was a long-duration earthquake, so it was not only significant in size at 6.4, it was also long.” |
96490
From: Redhart, [DNS_Address]
Subject: Re: offshore northern California massive crustal squeeze,
|
I was looking at the aftershock maps and topographical maps earlier today, and what struck me is how nearly all the faults run north-south in your area, but this quake..and the one a year ago, is clearly an east-west trending structure. I'm wondering if we have something like a transverse fault..like the Garlock down south. With the san andreas/mendocino fracture cutting off subduction..and seriously slowing down movement of the pacific plate west..it's like there's a split where the pacific north west north american plate has an easier time moving west than it does south of the san andreas. Just a spit ball theory, but if that's true..it can actually add some stress to a cascadia subduction quake at some point as forces now push harder west against that pacific plate pushing east. This was neither the Mendocino/san andreas, or the cascadia subduction fault, nor a intra mountain thrust. These movements show something different..probably powered by the north american plate...where it is not doorstopped by the san andreas movement. have to ponder on it a while. But think Garlock fault(& white wolf/scodie lineament). |
96507
From: jordan, [DNS_Address]
Subject: Re: offshore northern California massive crustal squeeze,
|
Gorda plate, that may be described as accommodating a fold in the plate with a vertical axis (Chaytor et al 2004) |
Responses:
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96494
From: jordan, [DNS_Address]
Subject: Re: offshore northern California massive crustal squeeze,
|
aftershocks suggest left-lateral strike-slip in Gorda plate? onshore events from the 20 Dec 2021 sequence are just to the north of the aftershocks from the 1992 sequence. They are at similar depths as those ’92 quakes and have similar earthquake mechanisms. |
Responses:
None
96492
From: ryan, [DNS_Address]
Subject: Re: offshore northern California massive crustal squeeze,
| maybe a result of the plate "squeezing" that jordan was talking about... |
Responses:
None
96478
From: Cinnamon in Oregon, [DNS_Address]
Subject: Re: offshore northern California massive crustal squeeze,
| Thanks for the details, Jordan. Will get to later |
Responses:
None
96477
From: ryan, [DNS_Address]
Subject: Re: offshore northern California massive crustal squeeze,
|
it seems the aftershocks west of the epicenter are shallower, and those east are deeper...the 6.4 was about 10 miles deep...aftershocks west 4-6 miles deep, those east 12-15 miles deep... thanks for the info... |
Responses:
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