Yesterday's Magnitude 7.2 Mexicali Earthquake

john's picture

About 40 years ago a Japanese seismologist by the name of Kiyoo Mogi proposed the idea that before a large earthquake occurs, there are a series of earthquakes of increasing magnitude that occur as a "run-up" to the main event.  These events surround the eventual fault that breaks in the main earthquake in a kind of "donut" pattern, with the main earthquake then occurring in the "hole" of the "donut".  Since that time, seismologists call this type of precursory pattern a "Mogi Donut". 

The concern with the Mexicali earthquake is the section of the San Andreas fault that last broke about 500 years ago, the piece that runs from Bombay Beach near the Salton Sea, up to San Gorgonio and Cajon Pass.  The 1992 Landers earthquake broke several faults in the Mojave Desert at the northern end of this part of the San Andreas.  The Mexicali earthquake has now ruptured faults in the region at the southern end of this section of the San Andreas.  The question we have is whether these two events, the June 28,1992 M7.3 Landers earthquake, and the April 4, 2010 M7.2 2010 Mexicali earthquake, represent a donut pattern forming.

Another possibility is that yesterday's event seems to be on the Laguna Salada fault, the southern extension of the Elsinore fault that runs northeast of San Diego.  This would seem to directly threaten the region between San Diego and Los Angeles.

We are tracking these events with forecast technology on   Under the "Tools" tab, you can look at the hazard viewer to see all of the events that occurred near and on top of a high risk area that we had previously identified (1-year California forecast).


cindygc's picture

Easter Sunday in Gilbert AZ,  I was at my brothers home laying on his couch watching TV.  Very strangely, his large TV started swaying back and forth, then it felt like someone was pushing the couch back and forth as well.  I yelled if anyone in the house could feel anything, and my brother excited yelled back (he was in another room) that at first he thought his stomach was getting upset.  Then his wife upon overhearing our conversation showed us the chandalier in the dining room that was still swaying.  I immediately looked on the internet wondering if anyone else had felt it.  It was too early.  We didn't hear anything about it until that evening.  I guess I should have come to this web site?  I saw John interviewed on TV today and knew I had to bookmark this website!  A very timely one!  I grew up in Northern AZ and don't recall ever feeling anything like this.  My Grandmother grew up by Tombstone.  When she was little (1880-ish ?), there was a bad earthquake.  The school house was adobe and she got trapped inside.  It could have killed her.  It definately traumatized her.  She avoided adobe buildings for the rest of her life!  What is the possilble damage that Arizona could sustain?

john's picture

Cindy --

Perhaps it was the May 3, 1887 Bavispe earthquake in Sonora, Mexico.  According to the Tombstone Times as well as the June 1, 1887 Tombstone Epitaph, this earthquake had an estimated magnitude of 7.2.  It destroyed the town of Tombstone, and did extensive damage in surrounding areas.  Another earthquake near Phoenix occurred at 5:48 am on April 18, 1906.  According to a paper in the Bulletin of the Seismological Society of America, volume 86, pages 921-924, 1996, by Steeples and Steeples, it was very likely an aftershock of the 1906 San Francisco earthquake.  For the 1887 earthquake, you can use one of the tools on the Open Hazards web site to estimate the amount of ground shaking.  In the Hazard Viewer, there is a button called "Query Location".  If you click on this, you'll see a green house appear, and a red earthquake symbol.  If you place the house at Tombstone, and the red earthquake about 80 km south, you will see ground shaking in the little box on the left side of the screen listed in %g, where g is the acceleration of gravity.  For a magnitude 7.2 earthquake near Bavispe, the ground shaking was about 8% g on soft ground, or Modified Mercalli Intensity V to VI.  This is enough to destroy most unreinforced masonry houses.

-- John

screen23's picture

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Talmadge's picture

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