Accretionary Wedge #41: El Mayor-Cucapah Earthquake

The El Mayor-Cucapah earthquake was a M7.2 centered in Baja California, Mexico, that occurred on 4 April 2010. It was the largest earthquake in Baja California since 1892, and the largest to shake southern California since the M7.3 1992 Landers earthquake. It was also the most significant geologic event that I personally have experienced.

The earthquake hit in the middle of the afternoon on Easter Sunday. Seeing as I have not celebrated Easter since I stopped believing in giant bunnies with eggs, and seeing as the first draft my Master’s thesis was due to my committee by the end of April, I was working in the lab that afternoon, all by myself. I had just opened – I swear I’m not making this up – a pdf of an article on ground motion in the greater Los Angeles area when the lab started to shake.

The initial shaking didn’t feel like much. It was side to side, but not a very big motion. My initial thought was that they were surface waves from an upper 4s earthquake on one of the local faults. It reminded me shaking from events that size on the Elsinore and the San Jacinto. But then it got stronger, still side to side, and I went, “Huh.” And then the Rayleigh waves started. These were textbook Rayleigh waves: big rolling motions, but fortunately not very hard or fast. The amount things were moving, no matter how slowly, and the fact that this had been going on for a while, finally triggered my, “Oh crap, this is a big earthquake,” sense, and so I got my ShakeOut on and dropped, covered, and held on. I knew it had to be far away, and I hoped it was in the desert and not, say, under downtown Los Angeles.

I’m a bad judge of time, so I’m not sure exactly how long it lasted, especially since I didn’t notice the P waves. Judging by the amount of music that had elapsed on iTunes, I’d say a good 25 seconds. I stayed under the desk for a bit longer, then got up and immediately started flailing all over Twitter, alternating giddy excitement with as much proper science as I could find. (I tried to go back that far in my Twitter feed to show how ridiculous I was being when the preliminary magnitude was posted. I can only go as far back as December 2010, but I distinctly recall tweeting “six point nine” several times in a row before it got upgraded to 7.2.)

So, that was exciting! And distracting! I didn’t write any more of my thesis that afternoon, that’s for sure. But the most exciting part was yet to come. Namely, about two hours later, I got a text message that said something to the extent of, “Anyone want to go install GPS equipment and look for the fault in Mexico tomorrow?”

Um, yes? Hell yes.

We left Riverside at the crack of dawn the next day, with a Subaru Outback and a Ford Focus full of GPS equipment. We stopped in Calexico, which didn’t have all of its electricity, but didn’t have any obvious damage where we were, then met up with a group from UC San Diego before we all crossed the border. They just waved us across. Immediately, it was clear that Mexicali had fared worse than Calexico. It was by no means total devastation, but most of the power was out, and there were a lot of broken windows and unhappy-looking building facades. But there was also proof that reinforcing your masonry helps: we saw a lot of squat cinder block buildings with no damage, and the ones that did have some chunks that fell off had reinforcing rebar sticking out for all to see.

Reinforce your masonry!

The building on the left was unreinforced, the one on the right had rebar reinforcing the cinder blocks.

As we drove toward the epicenter, the damage became more severe. We did see a few completely collapsed structures, as well as some nasty cracks (that were not offset) in the road. There was also a lot of evidence for liquefaction, despite this being the desert; the area south of Mexicali is irrigated and agricultural. The worst example we saw was a house whose front yard had basically turned into a pond.

Flooding from liquefaction: a hazard even in the desert.

There was no evidence of offset, or even cracking, at the coordinates we had for the epicenter. I’m not sure if this is due to the dip of the fault, or if the coordinates we had that early in the game weren’t quite correct. At any rate, we stopped there – a rocky foothill at the base of a larger mountain – and decided to set up three GPS stations in the vicinity, corresponding with past USGS placemarks, in order to track the postseismic deformation in the area. It is rather difficult, I can tell you now, to set up your tripod in a completely level manner when there are aftershocks rolling past you fairly frequently. We could feel even the small ones, and the idea that these are propagating waves was really driven home: they felt like someone was rippling the ground below us like a carpet, but just one ripple at and then it was gone, as opposed to consistent shaking. Even with no structures around to shake and make noise, there was still a deep audible rumble with each aftershock.

We ate lunch near our GPS stations, small aftershocks rippling past the whole time. I lost track of exactly how many I felt. After lunch, we decided to split up – some people went to go scope out other good sites for GPS, and some went in search of the fault. I was in the latter group. We were all expecting to find surface rupture, since the crust in Baja California is so thin (on account of being so close to a spreading center), and we had been surprised that we didn’t see any such thing at the epicenter. We knew there would be hordes of mappers out there eventually, finding each fault strand and conjugate fracture, but we wanted to see it for ourselves, because how often do you really get to see fresh surface rupture?

We set off looking for the Laguna Salada Fault. At the time, it was thought to be the prime candidate fault for the 2010 earthquake, since it was the one that ruptured in a similarly sized event in 1892, which caused damage in the same area. But the Laguna Salada Fault didn’t want to show us anything. We found plenty of liquefaction features, and lots of spreading cracks in the road, but no long linear offsets. It was perplexing, and given how early we’d all woken up, also quite frustrating. But I, for one, was not having the idea of leaving without finding surface rupture, no matter how exhausted we all were, so I insisted we try going further west on Highway 2.

And that’s where we found this:

Oh. Em. Gee.

Seventeen strands of right-lateral strike-slip rupture slicing the freeway, and slowing cars enough that their drivers could really gawk at what the fault had wrought. Construction crews were already at work patching the road, so we wasted no time. We parked on the shoulder, hopped out with field notebook, measuring tape, and handheld GPS unit, and measured, photographed, described, and took waypoints for all seventeen strands. We measured a total of ~1.2 meters of right-lateral offset across the road.

As it turns out, the reason we hadn’t found the rupture where we thought it would be is because the rupture wasn’t actually on the Laguna Salada Fault, at least not that far north. This was a very complex earthquake, and it ruptured several faults that hadn’t previously been identified or named. As more data was collected, researchers determined that the strike slip faults involved were actually subsidiary branches connecting to a larger normal fault – not actually surprising, given that Baja is in the transition from a transform boundary to the north and a spreading center to the south. After I read that paper, I realized that I had photos showing the normal fault contact as well as photos showing the strike slip ruptures:

Roadcut along Mexico Highway 2. Left: obvious contact between different rock types, with no evidence of rupture. Right: multiple strands of rupture in the hanging wall material.

We were pretty excited to find all of this surface expression. We were also very punchy and exhausted by this point. As a way to cap off the day, we decided to take inspiration from that famous photo of a woman standing next to the 1906 rupture on the San Andreas Fault, but with our rather less proper 2010 twist:

Though who knows, maybe the 1906 shot was considered a wholly undignified pose at the time.

Crossing the border back into the United States took forever, since the northbound crossing structure in Mexicali had sustained damage in the earthquake, so everyone went to the next crossing over. Even the frustration of that (and of getting back to Riverside at 3 AM) was not enough to negate the excitement of this major seismic event, and of being part of the immediate field response to it.

Obligatory offset fence.

All photographs in this post (except for the 1906 one, obviously) are my own. If you’d like to use them for anything, please ask me first.

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3 Responses to Accretionary Wedge #41: El Mayor-Cucapah Earthquake

  1. Kevin says:

    Great post! Really interesting reading. The photographs are fantastic as well.

  2. Austin says:

    Julian, your field photos of the pristine fresh rupture offsetting man-made features are totally spectacular. Way to go! Lucky duck.

  3. Pingback: Accretionary Wedge #41: Most Memorable/Significant Geologic Event That You’ve Directly Experienced « The Accretionary Wedge

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