Vignettes from the Salton Seismic Imaging Project: Student Field Work Experiences


Kathy Davenport (Virginia Tech) and members of the SSIP field crew

Figure 1. SSIP Project map. Red lines are faults; symbols (see index) are seismic sources or seismographs.

Figure 1. SSIP Project map. Red lines are faults; symbols (see index) are seismic sources or seismographs.

In early 2011, the Salton Seismic Imaging Project (SSIP) descended on Southern California. The Salton Trough was part of the Gulf of California focus area for MARGINS, and processes in this setting also address issues of rift initiation and evolution (RIE) important to GeoPRISMS. Over the course of three weeks, we acquired refraction and low-fold reflection seismic data along 7 lines totaling over 750 km, two 3D grids, and an offshore array. About 130 people participated in the data acquisition, including students from 31 different colleges and universities. During this time, 126 shots were fired, totaling 33,329 kg of explosives, and a 3.4-liter GI airgun was fired 2330 times in the Salton Sea. These sources were recorded on land on 2595 single-component seismographs and186 three-component seismographs at 4235 unique sites, as well as 48 three-component ocean bottom seismographs at 78 sites in the Salton Sea. A 42 station broadband deployment was also live during this time. We deployed instruments in sand dunes and snow, on bombing ranges and golf courses, beneath windmills and Joshua trees. We hiked through mesquite, avoided cactus and endangered lizards, and endured the stench of the Salton Sea. It took the best efforts of all the people involved to accomplish this massive data acquisition in the Salton Trough!

On January 23, Steve Skinner and I went to survey station locations along the San Andreas Fault east of Mecca. In this area of the desert few people have passed, so there are very few roads. We drove through washes and desert, looking for the easiest paths possible to reach our tentative waypoints. Jack rabbits and lizards tried to run away from us. When we finally stepped on the fault, with one foot on the Pacific Plate and the other on the North America Plate, looking at Salton Sea and the sunset, at that moment I felt that I was a real geologist.Liang Han, Virginia Tech. January 23, 2011

The Salton Trough is a prime target for investigating rift initiation and evolution and earthquake hazards because it is the northernmost extent of the Gulf of California extensional province. The San Andreas Fault ends in southern California, and strike-slip plate motion is transferred to the Imperial Fault. This step-over created the Salton Trough, a basin extending from Palm Springs to the Gulf of California. Previous studies suggest that North American lithosphere has rifted completely in the central Salton Trough. However, rifting here has been strongly affected by rapid sedimentation from the Colorado River, preventing the onset of seafloor spreading as has occurred in the southern Gulf of California. The 20-25 km thick crust in the central Salton Trough apparently is composed entirely of new crust created by magmatism from below and sedimentation from above. Between the major transform faults, active rifting is manifested by faults observed in modern sediment, abundant seismicity, minor volcanism, very high heat flow, and corresponding geothermal energy production.

Figure 2. Shot gather. The 911 kg shot was at the Imperial Fault. The 1142 seismograms (from Texans, plus vertical components from RT130's) were recorded along Line 2 that extends from the San Diego and Tijuana suburbs across the Peninsular Ranges, Salton Trough and Chocolate Mountains, to the Colorado River.

Figure 2. Shot gather. The 911 kg shot was at the Imperial Fault. The 1142 seismograms (from Texans, plus vertical components from RT130’s) were recorded along Line 2 that extends from the San Diego and Tijuana suburbs across the Peninsular Ranges, Salton Trough and Chocolate Mountains, to the Colorado River.

Based on the paleoseismic record, the southern San Andreas Fault is considered overdue for an earthquake of magnitude >7.5, and other nearby faults have had historic earthquakes with magnitudes >7. Earthquake hazard models and strong ground motion simulations require knowledge of the dip of the faults and the geometry and wavespeed of the adjacent sedimentary basins, but these parameters are currently poorly constrained.

SSIP ultimately will constrain the initiation and evolution of nearly complete continental rifting, including the emplacement of magmatism, effects of sedimentation upon extension and magmatism, and partitioning of strain during continental breakup. To improve earthquake hazard models, we will image the geometry of the San Andreas, Imperial and other faults, the structure of sedimentary basins in the Salton Trough, and the three-dimensional seismic wavespeed of the crust and uppermost mantle.

Constraining all these targets in the Salton Trough requires good instrument coverage in areas that are not always easily accessible. For instance, the deserts of Southern California are home to multiple military training facilities. These include the El Centro Naval Air Facility, whose bombing ranges are the winter training grounds for the Blue Angels, and the Chocolate Mountain Gunnery Range, Marine lands used for live munitions training. The Navy and Marine Corp were very accommodating to our project, providing safety training and time windows where we could safely cross the bombing ranges to deploy and pick up instruments. Of course, we had to work around the daily operations of these facilities, and that was not always easy.

Figure 3. Deploying a Texan seismograph on a wind farm near Palm Springs.

Figure 3. Deploying a Texan seismograph on a wind farm near Palm Springs.

The military assured us they had done sweep along our route so there shouldn’t be any live munitions on the ground. For safety, however, we were warned to avoid anything that appeared to be man-made. It was my role to drive into the desert, drop off the cross-country hikers, then drive around and pick them up on the other side of the bombing range. When I checked in at the operations center I was told that the Blue Angels were flying that day, and they don’t like moving objects on the ground. When I saw them I was to stop driving until they passed by. It seemed like I could drive for no more than a few minutes before the Blue Angels flew overhead and I would have to stop driving. It was pretty awesome to see them flying and executing their performance maneuvers right over our heads! As I stood by the truck awaiting the hikers, a solitary Blue Angel flew by, absolutely directly over my head. In the rush of noise and vibration of the flight, his elevation seemed like it was barely 30 meters. I decided to assume his flight path at that moment was a salute for the good work he thought we were doing.Janet Harvey, Caltech. March 2, 2011. El Centro NAF

Our access to the Chocolate Mountain marine bombing range was scheduled around daily munitions training. This meant we could only be on the range during hours when there was no chance of encountering one of the training groups, making this our earliest deployment – beginning at 3 am! We left the warehouse in El Centro hours before sunrise to give us enough time to get on and off the range before the firing started. Due to the extremely limited access, we could not survey the station locations ahead of time and instruments had to be deployed without precise GPS locations. We scurried around in the dark, planting seismometers as quickly as we could by flashlight, and left the base just as the sun came up. When we returned to retrieve the instruments we only had approximate station coordinates, so we had to scramble around, searching through the brush by flashlight for the buried instruments, with the imposing deadline of live ammunition flying through the air motivating us to find our instruments and get out by our sunrise deadline.Steve Skinner, Caltech. March 2, 2011. Chocolate Mountain Gunnery Range

Much of our work in the Imperial and Coachella Valleys was outside the urban areas and farmlands where the population is concentrated. We worked in the desert, the mountains, and on the Sea. Very often we found ourselves driving in washes or hiking because there were no roads where we needed to be. Bushwhacking, boating, and travelling cross-country led to many adventures for our deployment crews.

During surveying along Hwy 78 towards the Algodones sand dunes we chose a small, sandy side trail that was much safer than the main road. We tested the utility vans we would be using for deployment and learned that carefully driven, empty vans could successfully navigate the sandy road. Unfortunately, on deployment day I was the one driving the van loaded with instruments on this section. As we approached the dunes I saw the access to the side trail, took a deep breath, and began turning the van off the main road. 100 meters later, I learned that through either my lack of utility van experience or the weight of the fully loaded van, our test had failed… we were stuck. When we were pulled free we opted to work from the narrow shoulder on the main road. Later the trail looked more manageable, and much safer than pulling over on the half-shoulder of Hwy 78, so I gave it a second go… and 200 meters later became stuck again. After being pulled out for the second time, we finished our deployment from the main road. I would not try the van on the sandy trail again.Erin Carrick, Virginia Tech. March 1, 2011
Figure 4. Deploying an OBS into the shallow Salton Sea.

Figure 4. Deploying an OBS into the shallow Salton Sea.

The Salton Trough is often a barren and desolate place. Working on the Salton Sea, however, redefines desolate. I never saw another vessel on the water, despite a warning sign at the marina advising in case of emergency to flag down a passing boat, as there are no 911 services or coast guard rescue. We deployed our sound source and streamers off of a ~100’ barge towed behind a dual engine 40’ vessel. The water in the Sea is unbelievably hard on boat engines, precipitating salt quickly and preventing the internal cooling system from working. The Salton Sea also ‘blows out’ very quickly, going from dead calm to ocean size waves in 15 minutes. One nerve-wracking day, the water was as rough as I have ever seen it, one engine was out completely, and the other was screaming with warning sirens, close to overheating too. One may expect that this would be scary for fear of personal injury or lost data or ruined equipment, but the mind changes priorities on the Salton Sea. During the 4-hour ride back to the marina, I was only fearful of how utterly disgusting it would be to be in the water with the millions of dead tilapia. I would surely die from disgust! This particular evening, in true Salton Sea form, the water returned to glass 20 minutes out from the launch, and we enjoyed one of the most beautiful sunsets we had ever seen.Annie Kell, University of Nevada, Reno. March, 2011

The day’s assignment was to deploy two-dozen seismometers and geophones across the southern tip of the San Andreas Fault. We would drive as far as possible, and then pack in the instruments and equipment the rest of the way. Our crew had two extra members on this trip – a reporter and photographer from the Los Angeles Times. We drove into the field area on a path we blazed through the brush a month earlier. On the hike both of the media men were good sports, following us across the dry powdered mud in the heat, asking questions about regional tectonics and the SSIP experiment. After deploying the instruments we began the hike back to the vehicles along an abandoned railroad. All of a sudden we were stopped instantly in our tracks. An overwhelmingly close rattle sounded from just a few yards away and the biggest rattlesnake I have ever seen was coiled right off the tracks. We all backed away slowly. The cameraman, however, jumped into action, switching lenses and approaching the snake head-on until he was no more than a foot from its venomous fangs. Its head bobbed forward and back while he got his shots. This man who had fought in an infantry unit in Vietnam, covered troops in Iraq and Afghanistan, and won a Pulitzer Prize for following undocumented workers from Central America to the USA, had managed to find excitement and danger with a few geoscientists in the Salton Sea, California.Frank Sousa, Caltech. March 13, 2011
Figure 5. Backpacking seismographs across a Naval bombing range. Each person is carrying about 8 Texan seismographs and deployment equipment.

Figure 5. Backpacking seismographs across a Naval bombing range. Each person is carrying about 8 Texan seismographs and deployment equipment.

Onshore SSIP principal investigators are John Hole (Virginia Tech), Joann Stock (Caltech), and Gary Fuis (USGS, Menlo Park), working with Mexican collaborators Antonio Gonzalez-Fernandez (CICESE) and Octavio Lazaro-Mancilla (Univ. Autonoma de Baja California). The onshore work was funded by the NSF MARGINS Program (GeoPRISMS predecessor), the NSF EarthScope Program, and the USGS MultiHazards Program. The marine component, Wet-SSIP, is funded by an NSF Marine Geology and Geophysics Program grant to Neal Driscoll and Alistair Harding (Scripps Inst. Oceanography) and Graham Kent (Univ. Nevada, Reno). Broadband-SSIP is led by Simon Klemperer (Stanford Univ.) with funding from the NSF Geophysics Program. Onshore seismometers were provided by the EarthScope FlexArray and IRIS PASSCAL instrument pools with field support from PASSCAL. The OBSs were supplied by the OBSIP.

“Report from the Field” was designed to inform the community of real-time, exciting GeoPRISMS -related research. Through this report, the authors expose the excitement, trials, and opportunities to conduct fieldwork, as well as the challenges they may have experienced by deploying research activities in unique geological settings. If you would like to contribute to this series and share your experience on the field, please contact the GeoPRISMS Office at info@geoprisms.org. This opportunity is open to anyone engaged in GeoPRISMS research, from senior researchers to undergraduate students.
We hope to hear from you!

Reference information
Vignettes from the Salton Seismic Imaging Project: Student Field Work Experiences, Davenport, K., and members of the SSIP field crew;

GeoPRISMS Newsletter, Issue No. 28, Spring 2012. Retrieved from http://geoprisms.org

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