Lori Dengler | Tickling from the tri-junction area this week – Times-Standard

Wednesday night just before 11pm, my phone buzzed weakly. I put it on silent at night and figure if something calls for my attention I'll hear a few beeps. I ignored this and was able to fall asleep soon after. Good thing I did because I would hear another noise 22 minutes later and two more before midnight.

Get alerts when earthquakes of magnitude 3 or greater occur on the North Coast. The next morning there were four on my phone and a text from geologist and good friend Thomas Dunklin who lives in Petrolia. It was short and to the point, “Dang. Going wild tonight. It's all so close.” Really close – four earthquakes within two miles of Thomas' house and all of them are considered superficial.

For Thomas, who lived directly above the outposts, the shaking was already severe. But the vibrations from each lasted only a few seconds, and Thomas knew they were not large earthquakes. The final USGS rating for the sequence was 3.7 at 10:56 p.m., 4.0 at 11:18, 4.4 at 11:58, 3.9 at 11:59, and six smaller earthquakes of magnitude 1-2. Larger quakes were felt in the Cape Mendocino River Valley, Eel River Valley and a few from Mendocino to McKinleyville. No damage was reported, and the sequence ended early Thursday morning.

These earthquakes attracted not only Thomas' attention, but also the attention of others. I was bombarded with questions. What do they mean? Are they aftershocks from the December 2022 earthquake? Could it be a harbinger of something bigger? Do they radiate “strength” and help reduce the odds of a much larger earthquake? Why didn't I get ShakeAlert on the phone before I felt the vibration?

Placing these earthquakes in a geological context answers some of these questions. The first one is easy. They are not aftershocks from the M6.4 Ferndale earthquake that occurred on December 20, 2022. The epicenter of the earthquake was about 18 miles offshore of False Cape. Distance is not the most important factor, it was in a different tectonic system. Depth, that third dimension, is important on the North Coast where the surface does not reflect what is going on below.

The Cascadia subduction zone forms a complex, multi-layered structure. The Gorda Plate is a giant slab of Earth's material that is slowly being pulled eastward. It has created two different geological worlds – the thin wedge of material above where we live (the North American Plate), and the deeper Gorda Plate below. The huge Cascadia Fault separates these two worlds. In McKinleyville where I live, this front is located about 8 miles below our house. Eastward it deepens, reaching 12 miles at Willow Creek and about 20 miles below Weaverville.

Both the North American Plate and the subducted Gorda Plate contain faults that reflect their different histories and stress regimes. The fabric of North America is reflected in the north-northwest trend of rivers, mountain ridges, and mapped fault zones. The deeper Gorda plate faults beneath the Cascadia front trend toward the east. The December 2022 Ferndale earthquake struck one of Gorda's buried faults, rupturing a 10-mile stretch from the shore of False Cape to south of Ferndale.

The earthquakes that occurred last Wednesday night were relatively shallow at depths of about six miles below the surface and in the realm of the North American plate. The 2022 Ferndale earthquake was nearly twice as deep, in the Gorda plate below the subduction zone interface. The 2022 earthquake produced a series of powerful aftershocks, but all of these aftershocks were also in the Gorda plate.

Whenever an earthquake occurs, there is always a very small possibility that it serves as a harbinger of something bigger. The USGS calculates this probability when an earthquake of magnitude 5 or greater occurs in the United States. Smaller earthquakes can be foreshocks, but it is very rare for them to precede a magnitude 6 or larger earthquake.

I delved into the history of earthquakes at Cape Mendocino to search for possible precedents for the tremors. This is the Mendocino Trijunction, a very complex region affected not only by subduction zone stratigraphy but also by pressures imposed by the buttressing of the Pacific Plate to the south. It is not a point, but a widespread area of ​​high seismic activity extending onshore and offshore.

Considering only the relatively shallow plate earthquakes in North America, the most notable event is the M7.2 Cape Mendocino earthquake of April 25, 1992, the largest land earthquake in the history of the North Coast instrument. It was preceded by several foreshocks, a magnitude 5.3 a month ago, a M6.0 Honeydew earthquake eight months ago, and possibly a magnitude 5.4 in January 1990. This two-year period of shallow magnitude 5 to 6 earthquakes was unusual. There were no obvious first aftershocks in the hours or days immediately preceding 7.2.

Last Wednesday's quakes occurred between the Honeydew and Cape Mendocino quakes, and were caused by the same pressures that caused those larger tremors. There were a number of earthquakes in the M4 range and several sequences not dissimilar to Wednesday. None of them led to major earthquakes. I might raise my eyebrows if a shallow M5 happens.

When it comes to “seismic elimination” and reducing the threat of a major earthquake, the answer is an emphatic “no.” These small earthquakes were not at the subduction zone interface where stress builds up and will eventually produce the next great subduction zone earthquake. And even if they were, they would have released a paltry amount of that strain. It would take approximately five M5 earthquakes every day for 500 years to equal the pressure release of a single M9 earthquake.

No one who felt any of Wednesday's quakes received advance notice of the shaking. This is not because the system did not work. All earthquakes were below the M5 threshold for ShakeAlert and Wireless Emergency Alert (WEA) messages. Calculations were made, the initial magnitude estimate was 4.6 and notifications were sent to ShakeAlert team members 5 seconds after the earthquake. There is no reason to alert the public to such modest events that are unlikely to result in injuries, and when the only people who feel a severe tremor are too close to the epicenter to get alerted before the tremor arrives.

Magnitude 4 earthquakes are relatively common in the North Coast region. A magnitude 4.0 was reported early Saturday morning off Petrolia Beach, bringing the total number of 2024 magnitude 4 and larger earthquakes in California's northern coast and adjacent offshore area to 21. Fortunately, most of these quakes are occurring offshore And too far from population centers to be felt. Or make the news

Wednesday night was probably just a titillation of the triple-cross and is unlikely to mean something bigger is coming soon. Only time will tell. But there are many fault systems in our area that are quite capable of producing larger earthquakes at any time, and there is a very good chance that no earthquakes will precede them.

Note: Learn more about the dynamic Mendocino triple junction region on Thomas Dunklin's virtual field trip at https://rctwg.humboldt.edu/capemendo92.

Lori Dengler is professor emeritus of geology at Cal Poly Humboldt and an expert in tsunami and earthquake hazards. Questions or comments about this column, or want a free copy of the “Living on Shaky Ground” preparedness magazine? Leave a message at 707-826-6019 or email [email protected].

Originally published: October 26, 2024 at 11:32 am

What Are The Main Benefits Of Comparing Car Insurance Quotes Online

LOS ANGELES, CA / ACCESSWIRE / June 24, 2020, / Compare-autoinsurance.Org has launched a new blog post that presents the main benefits of comparing multiple car insurance quotes. For more info and free online quotes, please visit https://compare-autoinsurance.Org/the-advantages-of-comparing-prices-with-car-insurance-quotes-online/ The modern society has numerous technological advantages. One important advantage is the speed at which information is sent and received. With the help of the internet, the shopping habits of many persons have drastically changed. The car insurance industry hasn't remained untouched by these changes. On the internet, drivers can compare insurance prices and find out which sellers have the best offers. View photos The advantages of comparing online car insurance quotes are the following: Online quotes can be obtained from anywhere and at any time. Unlike physical insurance agencies, websites don't have a specific schedule and they are available at any time. Drivers that have busy working schedules, can compare quotes from anywhere and at any time, even at midnight. Multiple choices. Almost all insurance providers, no matter if they are well-known brands or just local insurers, have an online presence. Online quotes will allow policyholders the chance to discover multiple insurance companies and check their prices. Drivers are no longer required to get quotes from just a few known insurance companies. Also, local and regional insurers can provide lower insurance rates for the same services. Accurate insurance estimates. Online quotes can only be accurate if the customers provide accurate and real info about their car models and driving history. Lying about past driving incidents can make the price estimates to be lower, but when dealing with an insurance company lying to them is useless. Usually, insurance companies will do research about a potential customer before granting him coverage. Online quotes can be sorted easily. Although drivers are recommended to not choose a policy just based on its price, drivers can easily sort quotes by insurance price. Using brokerage websites will allow drivers to get quotes from multiple insurers, thus making the comparison faster and easier. For additional info, money-saving tips, and free car insurance quotes, visit https://compare-autoinsurance.Org/ Compare-autoinsurance.Org is an online provider of life, home, health, and auto insurance quotes. This website is unique because it does not simply stick to one kind of insurance provider, but brings the clients the best deals from many different online insurance carriers. In this way, clients have access to offers from multiple carriers all in one place: this website. On this site, customers have access to quotes for insurance plans from various agencies, such as local or nationwide agencies, brand names insurance companies, etc. "Online quotes can easily help drivers obtain better car insurance deals. All they have to do is to complete an online form with accurate and real info, then compare prices", said Russell Rabichev, Marketing Director of Internet Marketing Company. CONTACT: Company Name: Internet Marketing CompanyPerson for contact Name: Gurgu CPhone Number: (818) 359-3898Email: [email protected]: https://compare-autoinsurance.Org/ SOURCE: Compare-autoinsurance.Org View source version on accesswire.Com:https://www.Accesswire.Com/595055/What-Are-The-Main-Benefits-Of-Comparing-Car-Insurance-Quotes-Online View photos