Image courtesy of the Southern California Earthquake Center

I have to admit, I’ve only been perpherially following the earthquake swarm currently going on in Southern California, near the end of one of the strands of the San Andreas fault out below the Salton Sea (see the bottom of the map above). The “Bombay Beach” swarm (as I’ll call it … beats me what most people are calling it) is being closely monitored by the USGS. However, from what I had heard from a friend of mine with a more structure/tectonics background, looked like tectonic quakes with “beachball focal mechanism solutions” that were dominantly strike-slip following the regional trends. Heck, there is even some sense that we can “see” the motion caused by the swarm, and this stretch of the fault hasn’t had a major earthquake since 1680.

However, Eruptions reader Thomas Donlon sent me a link to the Arizona Geology blog today that suggest that the earthquakes might be magma related – I stress might. A 3D rendering of the earthquakes from the swarm – all between 0.1 and 4.8 on the Richter Scale – are reminiscent of what you might expect from magma movement and cooling.

Now, I am no expert in the geology of the area and yes, a strike-slip system is not your typical home for volcanism, but it is not unprecedented. Volcanism can occur in “pull apart” areas along a strike-slip fault where the fault, similar to the situation with the San Andreas near the Salton Sea. Also, evidence of magmatism have been seen before in that area of the fault system. The Salton Sea area is no stranger to volcanism as well during the Quaternary along with the Salton Sea geothermal field. So, it wouldn’t be out of the question that these swarms might be magma moving/intruding under the Salton Sea. Now, if (and I stress if) an eruption were to take place, I’m not entirely sure what form it might take: rhyolite domes like the Salton Buttes, cinder cones like the San Francisco Volcanic Field in Arizona, beats me. However, it is an interesting prospect to think that volcanism might occur in southern California.

I’m not saying this swarm means an eruption is likely to happen, or even possible based on the earthquake data collected so far, but I thought I would mention that it is possible. Feel free to add your two cents and correct my speculation. Interesting stuff, no doubt.


  1. #1 Tim Stone
    March 31, 2009

    The visuals of dome-building activity (for example) directly beneath the Salton Sea stagger the imagination! Imagine the phreatics (is that a word?). Boiling salt water, huge plumes, the dome rises from the middle of the lake with steamings and hissings and all form of visual drama, the sea completely disappears leaving salt formations everywhere. I think you should sell the movie rights!

    Fortunately for Southern California, the prevailing winds would tend to carry the plumes inland…

    Great post, Dr. Klemetti! I’m all of a sudden much more interested in the e-swarm down there!

  2. #2 Callan Bentley
    March 31, 2009

    Isn’t that what’s going on with Mount Edgecumbe, near Sitka, Alaska? (Pull-apart on the Queen Charlotte fault system?)

  3. #3 Lee Allison
    March 31, 2009

    Just southeast of the Bombay Beach swarm are a number of mud pots and carbon dioxide vents ( These phenomena are not surprising in a active pull-apart region, where the crust is thinned.

  4. #4 EKoh
    March 31, 2009

    Paraphrasing Arnie Pye: “I can’t see through rock Kent!”

    I know that they are reporting the focal mechanisms as strike-slip, but it would be interesting if they were localized and transtensional. Does anyone have a link to a site with the focal mechanisms?

  5. #5 gg
    March 31, 2009

    Last fall researchers found an underwater volcano off the coast of Washington:

    Canada has them, too, even in the Arctic:

    Thanks for this post. From my amateur point of view, this is intriguing. I believe in the interconnectedness of earth forces.

  6. #6 eklemetti
    March 31, 2009

    Here is some real information on the Bombay Beach earthquake swarm from the Southern California Seismic Network. {Hat tip to Ed for pointing this page out.

  7. #7 George
    March 31, 2009

    The Southern end of the Salton Sea has a buried spreading center (buried by several km of Colorado River sediment from times over geological history when the river has emptied into the Salton basin). This is the same spreading center that formed Baja California and the Gulf of California. Basically, the way I understand it is that the Farallon plate subducted, North America has overridden the spreading center between the Farallon (what is left of it is now called the Cocos plate in that area) and Pacific plates. There is more information here.

    I read one geologist several years ago predict that the Gulf of California could one day extend all the way into Death Valley.

  8. #8 Thomas Donlon
    March 31, 2009

    The intensity of the earthquake storm has subsided. Yet, any pressure that was added to the San Andreas fault remains. So when this fault may rupture – I don’t know.

    But, I appreciate the fascinating perspective and learning about the remote possibility of an a volcano acting up in this area.

    Also, I am very, very slowly learning about what those symbolic balls mean that signify the earthquake type and wave direction. I think I will need more clarity than the Wikipedia article provided as to how to read the ball symbols.

  9. #9 eklemetti
    March 31, 2009

    I tried to find a good resource for the “beach ball” diagrams and how to interpret on the web, but had little luck. Does anybody know of a site that has a good primer for their usage and interpretation (or does anyone want to make one?)

  10. #10 Kim
    April 1, 2009

    I started writing a primer on focal mechanisms a while back, but then I found the USGS page explaining them.

    The beachballs look like what I would expect for small earthquakes on the San Andreas system, but according to the Southern California Seismic Network, the seismologists think the other fault solution (NE-striking) is the correct one.

  11. #11 Thomas Donlon
    April 1, 2009

    If anyone wants to explain the “beach ball” – what I mainly would like to know is what in the “beach ball” indicates which way the ground on each side of the fault moved. “ancillary plane” and other technical speak have confused me when I read the wikipedia article.

  12. #12 Kipper
    January 10, 2010

    I don’t know squat about geology but in watching the movements along the San Andreas over the last few months I was able to call the 6.5 off of Ferndale the day before it happened.
    I’m not sure about the magma end of it, but we aren’t out of the woods as of yet and the swarm is still building.
    Something bigger is in the air.

  13. #13 Watashi
    January 22, 2010

    I feel like something is in the air (or under the ground)too. Over the last few months there were multiple quakes magnitude 6.0+ in the Gulf of California, followed by earthquake swarms in Mexicali, then the Salton Sea area, all of them right next to Obsidian Buttes. Then a week ago 2 4.0+ ones in Beaumont. What all of these have in common is they were all right on the boundary of the North American and Pacific tectonic plates, as was the 6.5 off the coast of Northern California. There were a total of 1000 quakes a day in California this week. I have monitored the site for years and 300-500 has been the average.

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