The Trembling Earth

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Join my SSA special session: When and Why do Earthquake Ruptures Stop?

The clock is ticking on abstract submission for the April 17-19 annual meeting of the Seismological Society of America. Julian Lozos (of Seismogenic, and of course of the PhD program at UC Riverside) and I are convening one of the special sessions, entitled “When and Why Do Earthquake Ruptures Stop? Evaluating Competing Mechanisms of Rupture Termination.”

I highly encourage any of you who think you have answers to that question to submit an abstract for a poster or talk in our session.

The detailed request is below, but I’ll emphasize here that this question is near and dear to my heart as essentially the broad topic of my PhD dissertation research. I can describe that in a future post, but if you want to hear the deets, come to our session!

I should also emphasize that the deadline is seriously rapidly approaching: Thursday, January 10 at 5pm Pacific (UTC -8)!   Eek!

SSA 2013 Special Session:

When and Why do Earthquake Ruptures Stop? Evaluating Competing Mechanisms of Rupture Termination

Summary:

Cessation of coseismic fault rupture has been suggested to result from a variety of mechanisms, ranging from fault-specific static properties to transient, rupture-history-driven dynamic effects. Field and modeling evidence alike implicate static or quasi-static properties such as fault geometry, frictional asperities or regions of creep, and time-dependent poro-elasticity as strong controls on rupture endpoints. However, static, dynamic, and quasi-dynamic numerical models, as well as mounting instrumental and field evidence demonstrate that, as stress evolves over multiple seismic cycles, transient effects may periodically overcome established static barriers, allowing rupture to continue. While much work has been done to investigate the effects of individual mechanisms on rupture cessation, the next step is to merge disparate studies of competing mechanisms in order to understand their relative roles within a given fault system. We invite presentations that summarize findings from numerical models, laboratory tests, observational analyses, and field and paleoseismic investigations that address various mechanisms that inhibit earthquake ruptures. We encourage comparison of these effects with one another, as well as discussion of how to evaluate which properties may dominate rupture through a given fault system, and of how to determine which effects are persistent over multiple earthquake cycles.

Conveners:

Austin Elliott (University of California Davis, ajelliott@ucdavis.edu)

Julian Lozos (University of California Riverside, jlozo001@student.ucr.edu)

See you in Salt Lake City!

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Ten years ago Denali shook

Saturday [November 3, 2012]  marked the ten-year anniversary of the largest quake to hit the U.S. since 1964, and the 1906 SF quake before that. The M7.9 Denali earthquake tore a ~250 mile gash through Alaskan glaciers and pine forests along the Denali Fault, which runs beside the eponymous mountain also known as Mt. McKinley, North America’s highest peak.

A detailed topographic survey of the rupture reveals fresh scarps along the sharp linear trace of the fault where it has been offsetting river valleys for millennia, one earthquake at a time. The fault runs from upper left to lower right across this image, and the large deflections of each canyon represent the cumulative result of dozens of 2002-sized earthquakes. This data set is available to view in Google Earth from http://www.OpenTopography.org

Much like the Haida Gwaii earthquake last week, the Denali quake shook mainly sparsely populated areas, so it was much more probable that anyone feeling it would be on the outer fringes of the shaking from the massive temblor. Indeed footage from a home in Anchorage, 160 miles from the epicenter, shows slow rocking–that telltale sign that some place moderately far from you is really getting hammered. Despite the low frequency of the shaking in this video, its strength is clear, and it lasts a very long time–another sign that you’re on the fringes of a huge earthquake.

One of the many legacies of this earthquake was that it put the trans-Alaska oil pipeline to the test. The pipeline crosses the Denali fault nearly perpendicularly, and was constructed with the knowledge and anticipation of offset along the fault. For ~1,000 feet on either side of the fault, the pipeline’s supports rest in tracks that allow it to shift laterally and bend as the ground beneath it carries the tracks in opposite directions. The structure was designed anticipating an earthquake of magnitude 8.0 with 20 feet of coseismic lateral displacement. At nearly the anticipated size, the Denali quake was a resoundingly successful test of earthquake engineering, which spared us an enormous environmental disaster.

A comparison of the 2002 Denali and 1857 San Andreas fault ruptures. The modern recordings from Alaska help guide our expectations about a contemporary repeat of the southern California quake.

Both its size and its geometry suggest that this rupture may be an excellent modern analogue to the earthquake that ripped along the San Andreas Fault in 1857, before southern California was heavily populated. It thus serves as an excellent source of modern data (seismic recordings, satellite imaging, GPS velocities) to help understand what a repeat of the San Andreas rupture will be like. For example, seismic records from the Denali quake are used in structural engineering tests of seismic safety design, simulating the type of shaking that may be expected around Los Angeles in the next SAF quake.

The USGS hosts a great set of photos from the spectacular Denali fault rupture, including offset glaciers, newly formed waterfalls across the scarp, huge landslides, and my personal favorite: this unfortunate tree that was growing directly atop the fault trace and got sheared in half.

A tree, taking advantage of the groundwater source along the Denali fault, suffers the consequences of its opportunism. Photo credit: Peter Haeussler, USGS.

El Mayor-Cucapah earthquake anniversary

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!

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