Posted on 12/10/2024 10:03:32 PM PST by Robert A Cook PE
Now, only 5 days after a Mag 7.0 earthquake shook the Cape Mendocino Fracture Zone offshore of the CA coast, aftershocks continue hourly. Over 420 earthquakes in this 110 km x 20 km region are already listed. More happen each hour.
Why do hundreds of small earthquakes offshore of northern California matter? Nobody lives there, no businesses nor voters nor industry is destroyed. No FEMA trailers nor FEMA money is needed of the earthquake is underwater.
The Cape Mendocino Fracture Zone is the southern border of three tectonic plates being pushed east underneath the US continental plate. The Gorda Plate is furthest south - pushing under northern California and south Oregon, then the larger Juan de Fuca plate pushes underneath Oregon and Washington State, then the Explorer Plate is furthest north - moving east underneath Seattle and Vancouver BC. These plates are divided by three east-west fracture zones: the Cape Mendocino Fracture Zone - hitting California at the end of the San Andres Fault north of San Francisco, the Blanco Fracture Zone is next, then the Sovanco Fracture Zone divides the Juan de Fuca and Explorer Plates.
When the Juan de Fuca ocean floor tectonic plate moves, the plate moves at these fracture zones on its borders, pushing the tectonic plates it collides with up out of its way. The horizontal movement is bad enough - creating Mag 8 to Mag 8.5 earthquakes on each border as the plates slide horizontally to each other. But it is the vertical movement as the Juan de Fuca plates slides under the continental plate that creates immense tsunami waves like the ones in Indonesia and Japan that kill tens of thousands coastal residences in minutes.
The frequency of 1000 km long earthquakes of Mag 9 and greater off of the BC-Washington-Oregon-CA coast has been increasing the past 4000 years. (Between each Mag 9 earthquake is a "smaller" Mag 8.0 or Mag 8.7 earthquake moving "only" 220 to 440 km of coastline homes.) Since 2200 BC, now some 19 such Mag 8 to Mag 9 earthquakes have struck these coastlines; on average less than 200 years between earthquakes and tsunamis.
Why should a US reader care?
Because a Japanese and British Columbia reader should care as well: The last large earthquake off of the Washington-Oregon-CA coastline occurred Jan 27 1700 - a date known because villages in Japan were destroyed by a tsunami later that day. Because every coastal tribe living near sea level in coastal BC-WA-OR-CA in January 1700 was killed, 104 years before Lewis and Clark reported only empty beaches and wilderness next to rivers flowing with clear water and plenty of fish.
We are overdue for that coastal earthquake.
I live in WA state near the water in the Puget Sound region so this has been on my mind of late.
Does anyone graph with time the intensity of earthquake aftershocks occurring in that fracture zone? Normally, aftershocks diminish with intensity with time, after a large quake. But if intensities start ramping back up, esp. if the large quake was still a full point or more below past “monster” events that have occurred in a seismic / fracture zone, real trouble may be brewing.
420 Quakes? California? Duuude - wait til the munchies kick in..
No.
But, yes, you are correct. 420 aftershock is not usual after a Mag 7 earthquake.
But. On a tectonic plate fracture zone boundary that is:
Overdue for a Mag 8.5 earthquake. (Now 324 years since a large quake, at a space that sees 19 Mag 8.5 quakes in the past 3600 years)
That has been seismically quiet (less than 5 quakes per week) for the past 22 years.
That saw a Mag 6.0 earthquake on its northern boundary October 30?
Yes, the region becomes “interesting.”
Sure, we all make fun of Uranus, but seriously? Nothing about de Fuca? Really?
:^) Thanks Robert A Cook PE.
Interesting little tidbit, per YouTuber TheEarthMaster, the NSGS hasn’t been logging all the earthquakes that have been occurring in the area...
If you want read an article that will keep you up at night about tsunamis in the Pacific North West, try this. It will definitely open your eyes to the risk.
Honest Question,
How do these guys even know there were 8.5 quakes centuries before we were here and able to even measure them?
For background, first see the graph here showing the calculated dates and magnitudes of 10,000 Years of Cascadia Fault (Large) Earthquakes.
Most important to us today, the past 5000 years (3000 BC to today’s 2024 AD) shows an increasing frequency of very intense Mag 9+ earthquakes, but a very slight decrease in their intensity (from Mag 9.08 and 9.09 down to “only” Mag 9.0, 8.7, 8.87, 8.9.)
The dates and amplitude (Magnitude) of these past earthquakes are estimated from the amount of debris and the thickness of mud and type of debris and area of common debris swept “downhill” from the Pacific slopes into layers on the Pacific Ocean floor. Like volcanic layers of lava and rock put down land, or tree rings bored out of a long-lived tree on land, each ocean floor borehole can be matched by thickness, date, and type to other boreholes up and down the coast to remarkable accuracy.
Yes, there are uncertainties in date, in relative magnitude, and in range (length and width) of each underwater earthquake slide. That is to be expected.
The January 27 1700 Magnitude earthquake is unique though because it can be timed to the hour because of the tax records of villages destroyed in Japan by the tsunami that was created by that earthquake. Only one or two local Indian tribes tell of that earthquake and tsunami though: The rest of the potential witnesses were killed that night by the 10 meter (40 foot) waves crushing inbound in the dark.
https://projects.oregonlive.com/maps/earthquakes/timeline
Far out, man!
Thx for your links
Lots of discussion about the frequency of subduction quakes in, not only that fault, but all such faults worldwide.
The facts are that they have occured at different intervals and not like clockwork.
The volcanism is the main focus of our discussions. Since the material that feeds the volcanic systems in that string along the mountain ranges, the discussion is, what will a serious event do in regs’ to the volcanos? That matetial is directly connected under the subduction.
Cant be predicted. Except, there has been a slight increase in activity.
The thing is, a large earthquake, although disastrous, will pale in comparison to a volcanic eruption say...around Seattle.
Thanks so much! Makes a lot of sense!
There were tsunami deposits left on our west coast and elsewhere, as well as the historical account left from Japan of giant tsunami. The deposits themselves leave organice matter that can be carbon dated, and disturbance to the land itself that geologists can date by various means.
The link in comment #9 explains in exact detail how the scientists determined this. It also explains how the people who make little effort to understand these problems or make any kind of plans for self survival will win the Darwin awards of NONSURVIVAL. Ignorance is the cause of much death. Avoid ignorance if you want to be a life survivor.
If you would like more information about what’s happening in Oregon, please FReepmail me. Please send me your name by FReepmail if you want to be on this list.
GOD is sick and tired of that state
An outstanding article! Concise, clear, good explanations - with iving examples throughout.
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