April 9, 2011

El Mayor-Cucapah earthquake anniversary

Posted by Austin Elliott

I have just returned from three weeks of field work at the site of a rather forgotten but significant earthquake that occurred one year ago last Monday, just south of the Mexico-U.S. border in Baja California del norte.

At 3:40pm PDT last Easter Sunday (April 4, 2010), the ground beneath the Sierra El Mayor began to unzip. The seismic energy that was radiated outward continued rupturing roughly northwest-southeast oriented faults, unzipping the crust deep below for nearly 50 kilometers (~30 miles) in both directions, up through the Sierra Cucapah and down through the Colorado River delta into the Sea of Cortez. That left this gash at the surface:

Geologists from UC Davis, University of Kansas, and UCLA admire fresh surface rupture along the Borrego fault in northern Mexico on April 16, 2010.

and it rattled northwestern Mexico and the southwestern U.S. for a few unexpected moments on a beautiful Easter afternoon:


On account of it occurring smack in the middle of a warm, sunny Easter afternoon, it interrupted innumerable egg-hunts, providing a wealth of home video footage from locations far and near. Because of the size of the quake (M7.2), it was felt throughout the desert southwest: Mexicali/Calexico most strongly, San Diego, Tijuana, and Palm Springs of course, then also Los Angeles, Ensenada, Las Vegas, Phoenix, Tuscon, Flagstaff, and so forth. Nevertheless, its location in the deserted mountains west of Mexicali made it a less than significant event to most of the quake-veteran populace of southern California–and no doubt saved countless lives–while leaving a treasure trove of accessible information to earthquake scientists like myself (among many others) to go study.

The earthquake ruptured a series of faults, with land to the east generally moving downward and southeastward, and land to the west generally moving relatively upward and northwestward. This makes it an oblique right-lateral rupture (no matter which side of the fault you stand on, the other side appears to have moved to the right), which is more or less what we expect from this portion of the Pacific-North American tectonic plate boundary. Indeed offset features observed all along the fault tend to demonstrate right-oblique slip, as seen in my field photos below, but detailed mapping conducted by a slew of researchers from the Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), San Diego State University, the USGS, and us at UC Davis (among others) reveal a complex and intriguing distribution of cracks, slip, escarpments, warping, and bending of the ground in a wide swath along the fault zone. Although the magnitude of slip varies substantially with fault geometry and the distribution of cracking, the total displacement across the fault is in the neighborhood of ~3 meters.

Right-oblique displacement of a steep hillside stream channel across a scarp formed by rupture of the Borrego fault. The scarp here is ~1.5 meters high, about the height of a person.

Tire tracks carved before the April 4, 2010 earthquake were offset along with the floor of this wash (we're looking downstream!), and are still visible in this 2011 photo.

Plenty of people are studying the deformation associated with this earthquake, and I’ll post on some of the results as they come out; for now, I’ll describe the goals of my field team when we visit the site of the rupture.

The abrupt disruption of the landscape caused by an earthquake leaves a marker to be recorded over time as wind and water smooth, rework, and erode the land. These markers (the scarps pictured above) are what we use to identify prehistoric earthquakes and understand the longterm behavior of faults. Although we recognize that these scarps form instantaneously in earthquakes and erode gradually over the following millennia, we have not recognized this for long enough to document the rate at which it occurs, and so we have a difficult time calculating the age of an ancient scarp just based on its morphology–its shape in the landscape. Thus our group set out immediately following the April 4 earthquake to begin capturing a time-series of topographic surveys of the fresh escarpment, to quantify the rate of erosion. To make these surveys rapidly and with sufficient resolution to identify the small year-to-year changes, we used a ground-based LiDAR scanner, a Star-Trek-esque device that basically scans its surroundings with a laser and produces a 3D representation as a cloud of data points.

Topographic hillshade image of the LiDAR point cloud collected by myself and fellow grad student Peter Gold, showing the ~1.5m high Borrego fault scarp slicing across a hillside.

The terrestrial LiDAR data we collected (we being Peter Gold and I at UC Davis, under auspices of Dr. Mike Oskin and using the Trimble scanner owned by Dr. Eric Cowgill; and Dr. Michael Taylor and AJ Herrs from the University of Kansas) captured minute features recording the earthquake in the landscape. In the high resolution 3D data we see striations and grooves from sliding along the fault surface, small fissures and cracks away from the main fault rupture, overturned cobbles, uprooted trees, and myriad cross-fault features that were offset by displacement along the fault.

This year Peter and I returned with Mike Taylor and his graduate student Richard Styron to see how these features had changed, and whether the scarp had degraded at all during the winter’s rain storms. I’ll leave you hanging there: next post will be a slide show of nifty field photos from this year’s trip, and some of the neat erosional features we found.

Happy El Mayor-Cucapah quake anniversary!