Anthony Lomax @ALomax@qoto.org

#EarthquakeReport #EarthquakeToot for M 6.4 #Earthquake in Mendocino triple junction (Triangle of Doom) region

aftershocks suggest left-lateral strike-slip in Gorda plate

felt broadly, about 92%g in Ferndale

read more from last year's report
https://earthjay.com/?p=10125

https://earthquake.usgs.gov/earthquakes/eventpage/nc73821036/executive

hi res map here

https://earthjay.com/earthquakes/20221220_norcal/20221220_norcal_interpretation.jpg

You can see many aftershocks from today's magnitude 6.4 #earthquake in Northern #California
You see the lines are almost flat before the mainshock and then there is #aftershock after aftershock following it. Largest 4.6 but many between 2.5 and 4. This sequence will likely continue for some time.
https://earthquake.usgs.gov/earthquakes/eventpage/nc73821036/executive

M 6.1, Near Coast of Northern California
2022-12-20 10:34:26 40.62 -124.08 10 km A
http://geofon.gfz-potsdam.de/eqinfo/event.php?from=rss&id=gfz2022yvtn

Time ago, the now scary bird site, was useful for science, knowledge building, hazard communication, etc...

We wrote a paper on that: Rapid collaborative knowledge building via Twitter after significant geohazard events.

https://gc.copernicus.org/articles/3/129/2020/

Today's M5.1-5.3 #earthquake near #Midland (red) is located at a few 100 km from the M5.4 event (blue) that happened in Western #Texas one month ago.
(Map generated at https://earthquake.usgs.gov/earthquakes/eventpage/us6000j9vq/executive )

Mylonite

Down in the hot middle to lower crust where temperatures are higher, rocks behave differently. At geologic timescales they can flow, shear, and deform plastically without fracturing under shear stresses that produce strain, and generally do not produce earthquakes. This ductile environment is where the deep fault shear zones are. These zones are generally wider than faults and can accommodate displacements from centimeters to tens of kilometers. Nearby rocks may show little, or no strain at all.

Movement of the shear zones form foliated, deformed, lineated rocks called mylonite, and lab experiments show the fine-grained textures are formed by recrystallization, grain-size reduction, and metamorphic reactions. These textures can provide clues as to the displacement, or the relative movement of the rocks on either side of the shear zones, called shear-sense indicators. In the example shown below, the sense of movement is “dextral” or to the right (to the left is called “sinistral”). Dextral movement has caused “drag” lineations to form on either side of the rock. If you turn the rock upside down in your imagination, the sense of movement is still dextral facing the shear zone.

The shear-sense indicators in rocks are important tectonic clues for structural geologists to determine how mountains formed and plates collided.

More information from EGU Blog authors Samuele Papeschi, Sandra McLaren and Hannah Davies are at the link: https://blogs.egu.eu/divisions/ts/2021/01/25/features-from-the-field-shear-zones-and-mylonites/

More on mylonites here: https://en.m.wikipedia.org/wiki/Mylonite

Photo of metagranite mylonite and interpretation and annotation is by Samuele Papeschi.

#mylonite #ShearZones #ShearSenseIndicators #rocks #geology #ScienceMastodon

Just posted this 👇🏼 on the scary bird site.
Will see what happens.

Le plateau de #Caussols, situé au-dessus de #Grasse, c’est un spot de #biodiversité (vipère d’Orsini, fritillaires,…) mais aussi une richesse #géologique : vaste poljé et multiple formations karstique. #nature #naturephotography #geology #cotedazur #AlpesMartimes

Another year reviewed by the ISC: the plots show the 2020 ISC #locations and the number of collected (blue), reviewed (green) and relocated (red) events per month. For year 2020 the ISC collected data for 622,960 events, reviewed 55,751 and relocated 41,554 of them, using & reviewing nearly 7 million seismic phases!

Mercator size vs. true country size
by Neil Kaye

M 6.5, Tonga Islands
2022-12-04 19:24:13 -15.46 -173.10 10 km A
http://geofon.gfz-potsdam.de/eqinfo/event.php?from=rss&id=gfz2022xtey