The 1994-5 Colorado Plateau-Great Basin PASSCAL Experiment

by Craig H. Jones, CIRES, Univ. of Colorado, Boulder
on behalf of the Field Team of the CPGB experiment

The expedition's lofty goals were in jeopardy. Equipment had been lost outside of Reno, team members were sick and unable to go into the field, and the team leader had been struck by lightning while setting up equipment. The attempt to form a link between surveys in California and Colorado seemed doomed.

And yet Clarence King's Survey of the Fortieth Parallel went on to become an example of efficiency sufficient to propel the survey's leader into becoming the first head of the U.S. Geological Survey. I reflected on this as I followed in the expedition s footsteps, courting disaster in the name of sc ience. My risks were considerably smaller, but the snow of the mid-November storm flooding over the hood of my truck seem foreboding. I lowered the window and learned the reason: the snow was deeper than the floorboards were high. My truck was now acting as a snowplow on the untravelled dirt track in central Utah. Shifting into the lowest gear I had, I moved forward towards one of our seismometers.

Just 8 days previous I had installed this broadband seismometer under clear skies and without any snow at all. It was one of eleven PASSCAL and one UNR portable broadband seismometers deployed and operating across the Colorado Plateau into the eastern Basin and Range (Figure 1) . The instruments in Nevada had been deployed by Martha Savage and Serdar Ozalaybey in September, 1994; the instruments I and Anne Sheehan were responsible for were only deployed in late October and November owing to difficulties in obtaining permits from the BLM. Much as King's survey filled a gap between the original California State Geological Survey and Hayden's Rocky Mountain Survey, our basic goal was to fill a gap between the Rocky Mountain Front broadband deployment of 1991-2 ( Lerner-Lam and RMF Field Party, 1993) and the permanent broadbands run by the University of Nevada, Reno in western Nevada.

Figure 1. Shaded relief map of the southwestern United States with locations of digital broadband seismic instruments east of California as of 1994.

And, at the moment, that goal seemed a bit of a reach. To get the instruments as early as possible in our grant period, we agreed to a winter deployment with additional months added to allow for equipment problems. With my t ruck now a snowplow moving slowly up the face of the mountains near Meadow, Utah, it seemed an unwise decision. In fact, the winter of 1994-5 proved one of the snowiest in Utah history. Considering the success of perseverance in the case of King, though, I continued to force the truck up the grade towards a seismometer probably hopelessly smothered in snow. At last, several miles from help should the truck become a snowdrift, I stopped and opened the door, pushing aside several inches of snow higher than the bottom of the door ( Figure 2). I gathered my notebook, tools, disk drive, and heavy jacket and began postholing through the snow towards the seismometer site.

Figure 2a. Parking spot in mid-November 1994 near station MDW.

Figure 2b. Installing station CYF. Typical CPGB installation using sewer pipe for vault is apparent. CU-Boulder undergraduate Noah Hughes is examining the recording equipment.

Our seismic deployments built on earlier experiences of many PASSCAL groups. We opted for a simple vault lined with a 13\quot;-diameter PVC sewer pipe; the base, dug as near to bedrock as possible, was a cement paving stone plastered to the soil or rock underneath (Figure 2b). The cover was a water-catcher base from a large pot for houseplants. The CMG-3 ESP inside was cabled to a Reftek -08 DAS with internal GPS clock and two lead-acid car batteries. Two solar panels recharged the system, and a 540 Mb Reftek-housed disk drive recorded the data continuously at 32 bits/sample (compressed) and 25 samples per second. Batteries, sewer pipe, and gray boxes used to protect the DAS and disk were recycled from an earlier experiment in the Sierra Nevada and Death Valley. To avoid long waits in the snow and cold at a station, we had two swap disks and a stripped-down DAS for motel playback at night to a field DAT drive. We were somewhat pessimistic about the amount of time the equipment would be running during the winter, especially storms.

Figure 2c. Station MDW prior to removal of snow, mid-November 1994.

And approaching the thoroughly buried seismometer, my doubts were heightened (Figure 2c). One panel on the ground was totally hidden; only fishing around in the snow allowed me to locate the panel and dig it out. The other, mounted on a wooden frame and fencepost (as were most of our panels) held a nice thick blanket of snow. I dug out the gray box containing the electronics and pulled out the Epsom hand terminal, optimistically hoping form a response from the DAS.

I was shocked to see 11.6 volts from the internal sensor--pretty good for our 1 1/2 year old batteries; furthermore, the disk usage and number of events was consistent with the station having remained up throughout the storm. In point of fact, the station had not lost power at all and would remain up through most of the winter. Overall, our stations recorded data more than 95% of the time, and several stations (including one at 2280 m on the south flank of the Uinta Mountains (Figure 2d)) recorded continuously over the full duration of the experiment. Such success allowed us to start to make a systematic connection between the Rockies and the western Great Basin using receiver functions, surface waves, and shear-wave splitting techniques.

Figure 2d. Anne Sheehan servicing station RCC on the south flank of the Uinta Mountains.

Success had its price: we had about 26 Gb of compressed data to manage. A hallmark of the King Survey had been the rapid processing and distribution of results, a procedure inspired by the recriminations directed at King's mentor and former superior Josiah D. Whitney of the California Survey for the slow publication of results from that work. We wanted to push our data through as soon as possible; our goal was to avoid lengthy post-experiment processing. Fortunately, experience from previous experiments combined with the growing maturity of the PASSCAL data processing software made this quite possible.

We used a Sun workstation with about 8 Gb of free disk space; wit h this setup we could download about 2 months of data from our network at one time--roughly the data from one visit to the field. The station log (aka soh) files were processed for timing glitches and drift; the results from this were used in a modified version of Tom Owens's soh2db TCL script to create a CSS 3.0 schema database. The format of this database and tools developed and maintained by the IRIS JSP center at the University of Colorado permitted us to view the entire two months of data from all stations interactively without manually decompressing the data. In addition, we could search for teleseisms and local events using catalogs converted to the CSS format. Indeed we could (and did) extract full gathers of regional and teleseismic events of interest with a single command; by building this set of event gathers during the experiment, we were able to begin analysis of the data nearly immediately. Total time to create a time-corrected database with event gathers was under a week from the time the field tapes reached the lab. Concurrently, waveforms from US NSN stations in the region were collected from their auto-DMR system and have since been merged into the database.

King's Fortieth Parallel Survey was a reconnaissance to provide uniform, if sketchy, information useful to those looking for opportunity along the transcontinental railroad (then under construction). This was one of the first geological studies of the contrast between the Basin Ranges (as they were then called) and the Plateau country to the east. The recognition of basin-range normal faulting and its impact on the physiography of the region came in part from this work; this style of "fault-block" mountains was one of the few fundamental and original contributions to tectonic theory from the southwestern U.S. The fundamental causes of this deformation and the absence of similar deformation immediately to the east remains a topic of controversy, a topic we have sought to explore.

Our survey was conducted to obtain a uniformly collected, if spatially sketchy, view of the relative roles of crust and mantle in the deformation of the Basin and Range and relative absence of deformation in the Colorado Plateau. Are there fundamental differences between provinces in the crust? Are there any in the mantle? Is the Colorado Plateau a piece of craton lost in the Cordillera? Or is it merely an accident of history that this region has remained nearly undeformed for more than half a billion years? Is its modern elevation due to some difference in the mantle relative to its neighbors, or is it in the crust? We have begun to address these and other questions through study of receiver functions, shear-wave splitting, and surface waves.

Figure 3. Radial receiver functions stacked over tens of earthquakes at each of the CPGB stations. The probable Ps phase from the Moho is in red and a strong intracrustal conversion is in green. Processing and most of the events used are identical across the network.

One example of our work is from a preliminary collection of receiver functions over the region ( Figure 3). These complement sparse refraction constraints on the depth of the Moho in the region and expand upon earlier work in the Rocky Mountains (Sheehan et al., 1995); initial analysis suggests a crust in the Plateau center at the lower end of the refraction estimates (~40 km thick crust) (Jones et al., 1995). This seems to indicate that the plateau owes its elevation to the mantle , similar to (though not as extreme as) the Great Basin to the west. The character of Moho changes from a sharp, single discontinuity in eastern Nevada to a double conversion along the Wasatch Front to a lower amplitude conversion in the Colorado Plateau . The Ps- P travelitime across the crust is somewhat less than might be anticipated.

Other analyses are well underway. Deeper level receiver functions will be constructed in the near future to investigate differences in the upper mantle discontinuities down to the 660 km discontinuity to complement work from the Rocky Mountain and Snake River Plain experiments (Dueker and Sheehan, ms in prep.). Shear-wave splitting measurements from teleseismic SKS and S phases provide information on the strain patterns in the upper mantle; initial results have already been presented (Savage et al., 1995). Surface wave analysis will provide an important constraint on the shear wave structure of the lithosphere in the region with the receiver functions; already an analysis combining these techniques has been done within Nevada using the UNR and portable stations from this deployment (Ozalaybey, 1996, Ozalaybey et al., ms in prep., results map at UNR You can also view the abstract at UNR.).

While our contributions cannot match the observations of the King Survey for primacy (and our perils cannot match theirs for thrills), our work is part of the first detailed look at this orogen using broadband seismic recordings. We hope our efforts provide data equally seminal to our understanding of the diverse tectonics of the region. Armed with this data, we can look in the coming years to better understand the causative processes that generated the geology first systematically described by King and his contemporaries.

Acknowlegements. This project was funded by the National Science Foundation Seismology Program. Although CHJ is responsible for this report and any inaccuracies present, there would be nothing to tell were it not for the efforts in the field of M. K. Savage, A. F. Sheehan, L. Trimble, N. Hughes, and especially Serdar Ozalaybey, who kept the Nevada part of the network running. Discussion of some of the early work is possible because of the efforts of those people and K. Dueker and J. Bartsch .

Additional photos can be found at UNR of the Nevada portion of the deployment

References
Jones, C. H., A. F. Sheehan, L. J. Sonder, M. K. Savage, and S. Ozalaybey, Isolating crustal versus mantle sources of isostatic support of the Colorado Plateau, EOS, 76 [suppl. to no. 46], F619, 1995.

Lerner-Lam, A. L., and RMF Field Party, PASSCAL meets the Rocky Mountain Front, IRIS newsletter, v. 12, no. 1, 1-3,11, 1993.

Ozalaybey, S.., Seismic velocity structure in the western U. S. from shear-wave splitting and receiver functions of teleseismic earthquakes, Ph.D. thesis, University of Nevada, Reno, NV, 1996.

Savage, M. K., A. F. Sheehan, J. E. Bartsch, Seismic anisotropy across the Colorado Plateau and surrounding regions, EOS, 76 [suppl. to no. 46], F604, 1995.

Sheehan, A.F., G. A. Abers, C. H. Jones, and A. L. Lerner-Lam, Crustal thickness variations across the Colorado Rocky Mountains from teleseismic receiver functions,J. Geophys. Res., 100, p. 20,391-20,404, 1995. You can view the abstract of this paper

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