Over on the W.O.O.D. page a discussion of the recent 6.x and now 7.x quakes in the mostly empty area north of the Mojave Desert has been going on.
I made a very lengthy comment there that I’ve decided to elevate to a posting. Both to provide a place to discuss what’s happening in California now, and so that I can put up a visual aid (you can embiggen greatly):
This map is from: https://www.volcanoesandearthquakes.com/map/California
You can see how the California Volcanoes make a kind of a line down from near Lake Tahoe and toward the Gulf Of California. Not on this map is that Mexican volcanoes continue on down the Gulf of California and are more active than those in California (or have been in the last few hundred years).
That is a Rift Zone where (just like in Iceland) an ocean ridge spreading zone comes ashore. That is why the Salton Sea (where the first green triangle volcano at the bottom is located) is below sea level. It is also why Death Valley is below sea level. There’s several thousand feet of rubble in the bottom of Death Valley or it would be even deeper; but as it rips open, dirt and rocks from the mountains around it erodes in trying to fill it. Were it not for the compression forces shoving up the mountains of California (as the Pacific plate slams into the North American plate) this area would be as volcanic as Iceland.
The erosion of the Grand Canyon filled in a silt dam of sorts that keeps the Gulf of California from extending to the Salton Sea (or further) inland.
Notice how the recent quakes follow that path of the volcanoes.
At the “bend” in the east border of California is a blue blob. That’s Lake Tahoe. Not shown on the map is the “extinct” volcano of the Sutter Buttes. Take a line from Lake Tahoe out toward the ocean to that green volcano triangle (Clear Lake – a volcanic caldera just north of The Geysers at Geyserville…) and about 1/2 way from Sacramento to Chico basically on that line is the Sutter Buttes. Just like Mount Shasta and Mount Lassen (in that cluster of volcanoes in the north end of California) it popped up closer to the midline of the State.
By looking at this map and noticing that the activity is all back in that volcanic field area, and not on the more coastal San Andreas system, you can see why I’m not thinking it is much of a ‘feature’ that this big quake didn’t hit Los Angeles. It could well indicate a renewal of rifting and volcanic activity. Do note that this is Highly Unlikely and even if it were happening, it is on Geologic Time Scale and “now” can mean we see results in 5000 years…
Here’s my comment from the other thread:
@Per The Quake:
I’d actually thought of saying “A 6.x? It might be a pre-shock…” but didn’t. Oh Well. One set of prescience points down the tube ;-)
ANY time there’s an out of the blue 5-6.x quake, it can be a pre-shock to the main event. Make sure your “quake kit” is outside and your water storage is full. Keep things that way for a week or so. (Long time readers here will remember me saying things like “Checking my quake kit and preparedness stuff” after some 6.x quake somewhere on the West Coast. That’s why.
WHERE this quake is located worries me. That’s a spreading zone / rift zone. Usually we get a sliding motion along the San Andreas and related faults. Essentially the Pacific plates moving north / North America plate moving south, relative to each other. There’s been a long duration of quiet on those plates at that boundary. Now this.
This is not a “sliding sideways” zone, it is a ripping apart zone. IF the plate motions have changed direction, that could be a Really Big Deal. For one thing, there’s a set of mountains near where I grew up, the Sutter Buttes
that WERE a 6000 foot tall volcano that just popped up through the broad flat central valley floor. Nowhere near all “the usual” volcanic features and mountains. For decades I wondered “Why there?”. Eventually (after a few geology classes) found out that California moves sort of in blocks. The spreading zone from the Gulf of California up through the (below sea level) Salton Sea / Death Valley / Owens Valley etc. eventually at about Mono Lake / Lake Tahoe hits one of those block edges and runs back to the coast (where it picks up again and eventually merges into that Cascadia Fault Zone that will eventually destroy Seattle and Portland…)
Those block edges are presently held closed by the sliding pressure of the plates in opposite directions. This is also why volcanoes down near L.A. (out into the Mojave) have not popped up. (But DID in the past tens of thousands of years ago).
IF (and it is a very big IF) the pressure N/S on those blocks has slackened and we are in more of an unzip rifting motion now, we can get (as we have had before in geologic history…) volcanoes anywhere along that line up from the Salton Sea to Lake Tahoe, along the block edges through the Sutter Buttes area (and The Geysers / Geyserville over by the coast in trendy Napa Wine Country…)
It has been about 6 million years since the Buttes formed, and they have eroded down to about 2000 feet now, so it isn’t like this is a common feature. You MUST keep geologic time in mind thinking about this stuff. It might never happen again, ever. OTOH, were it starting Right Now, we might not notice for a few thousand more years. (Or it could start smoking tomorrow…)
IIRC, the last volcano in the area east of Los Angeles (on the road to Phoenix) was about 9,000 years ago. (Just yesterday by geologic scales). Further north, Mount Lassen popped off about 1914 and Mt. Shasta grew a big cinder cone next to it (that you can see from the freeway as it goes right past it) at the same 9,000 years ago time:
The point being that IF (again, a very big IF) we transitioned from compression N/S on those E/W blocks that make up California into some kind of torque or worse, rifting along that rift line: This State can pop and has popped volcanoes from Los Angeles up the length and even in the Central Valley and including places like Lassen and Shasta.
These are NOT extinct volcanoes. (Heck, not even dormant, really. 9000 years is, in geologic time scales “happening now”…)
So getting 7 sized quakes in the rifting valleys area is a big deal. IMHO.
While I HOPE it is just settling of some strike / slip block and “nothing unusual”, if it starts to be a pattern, well…. then you start a smokers and volcanoes watch.
FWIW I’d expect Mexican volcanoes in the Gulf of California area to show more activity as a precursor event. Their’s are generally more active and recent, and I’d expect them to get more active and “do something” first in a renewal of rifting.