A dense temporary seismic deployment across the Southern San Andreas Fault

View of the Coachella Valley Preserve. The vegetation contrast line marks the Mission Creek fault.

Researchers from several SCEC institutions supported a NSF funded UCSD/USC joint project to understand more about this relatively quiet segment.                      

Article contributors: Pieter-Ewald Share, Chris Milliner, Mark Benthien, Jason Ballmann, and Yehuda Ben-Zion

Volunteers gather near Thousand Palms Canyon to listen to a demonstration of how to install and set up the seismic instruments they are about to deploy.Team members listen to a demonstration of how to install the seismic instruments.

In early March, a temporary array of more than 300 IRIS-PASSCAL and University of Utah seismometers across the southern San Andreas Fault (SoSAF) was installed by two dozen faculty, staff, postdocs and students from the University of Southern California (USC), UC San Diego (UCSD), University of Utah, San Diego State University (SDSU), UC Riverside (UCR), JPL, and the USGS. The deployment area, southwest of Joshua Tree National Park in the southern California desert, lies along the northern edge of the popular Coachella Valley, 100 miles east of Los Angeles. The day-long work supported a joint UCSD/USC project funded by NSF, with one main goal to determine the properties and geometry of the San Andreas Fault zone in that area. If we can better understand the subsurface structure of the SoSAF, we can more accurately analyze geodetic data in terms of fault processes, and model effects like dynamic rupture and shaking intensity associated with the next “Big One”.

Everyone assembled at Thousand Palms Canyon in the Coachella Valley Preserve, a large natural area near Palm Springs. After an orientation by SCEC Director Yehuda Ben-Zion and some quick lessons on proper installation procedures, the group was divided into seven teams of three or four people, with each team being responsible for roughly 50 seismometers. Every installation required a precisely located foot-deep hole dug in the sometimes unyielding desert floor. The instrument had to be firmly seated in the hole, leveled, oriented with respect to geographic north, set up with a handheld computer, and finally, gently buried with its top no more than four inches beneath the surface to allow it to access precise timing data from GPS satellites.

Map of a section of the northern Coachella Valley, showing strands of the San Andreas Fault and locations of deployed sensors.The array of sensors crossed several strands of the SoSAF within Thousand Palms Canyon. As one team member activates the seismometer, another notes the location and number of the instrument.As one team member activates the seismometer in its shallow pit, another notes its location.
Screenshot of tweet by Chris Milliner at the start of the day, showing initial training.Tweet by Chris Milliner (JPL) of picture from initial training before deployment of sensors begins.

Little San Bernardino Mountains north of the field area, at sunset.At the end of the day-long deployment, the teams were treated to views of the setting sun painting the Little San Bernardino Mountains.

The seismometers were installed in a pattern consisting of one 4-km-long line across, and perpendicular to, the two local strands of the SoSAF in the area-- the Banning fault and the Mission Creek fault. Centered at each intersection of these strands with the main line were two dense 2D grids, ringed by several scattered locations farther afield. Another more distant set of instruments was installed at the base of the nearby Little San Bernardino Mountains. This configuration was developed to allow high-resolution imaging of the structure and seismic properties of the San Andreas fault zone at depth and over a broad region, which in turn could be used for improved modeling of data and answering questions about the mechanics and evolution of the San Andreas fault zone.

The Coachella Valley Preserve is a popular hiking destination, and many of those out on the trails stopped to inquire about the installation that was occurring. Most were interested in knowing exactly where the local trace of the San Andreas Fault was located, and whether they could see it. Volunteer teams were more than happy to point out the notable vegetation lineament -- a line defined by a change in the density of plant growth on either side -- across the Mission Creek strand (see top image). They also helped hikers locate fault-shaped landforms, and even assisted with identification of some of the many desert wildflowers that were in bloom.

With hard work and a little extra effort, the teams managed to complete the entire installation in a single day, wrapping up as darkness fell across the landscape. In a way, this deployment could be seen as a microcosm of SCEC: science through collaboration, aimed at understanding our earthquake risks, communicated to the public. This time-tested formula for success remains a guiding principle of our center, which will hopefully continue in the years to come. Much appreciation goes to the participants for their involvement in this satisfying day of field work!    



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