The NSF funding was motivated by Caskey's detailed mapping (in his UNR Ph.D. thesis, and other references described in the proposal) of an unusually wide, 15 m graben developed against the fault during the 1954 earthquake. (A photo of a cabin in the graben is on the cover of the Seismological Lab's brochure.) Simple volume analysis suggests such a graben can only develop on a fault that is listric at shallow depth (<100 m). Since the shallow fault dip is 50-60 degrees, Caskey's analysis suggests a deeper dip of 20-40 degrees. He finds consistent evidence of such shallow dip for more than 15 km along the fault trace.

If Caskey's shallow-dip hypothesis is true, then the Dixie Valley fault represents the last hope for a truely seismogenic, shallowly dipping fault. Since the 1954 Dixie Valley earthquake occurred only 4 minutes after the Fairview Peak earthquake, seismological evidence of the dip of the earthquake's mechanism is lost in the coda and aftershocks of the preceding event. The potential for a seismogenic, shallow-dipping normal fault has considerable implications outside Dixie Valley. While in many areas geologists have proposed normal-faulting activity on large shallow-dipping structures, seismologists have yet to find any evidence of a shallow-dipping normal-faulting earthquake. This leaves open the question of whether large extensional decollements slip aseismically, or can generate earthquakes and thus considerably raise the seismic hazard assessed for certain populated regions.

Aside from the 1954 ground-rupture studies, geophysical efforts in Dixie Valley have been concentrated near the geothermal production area, almost 30 miles to the north of our field site. The fault there did not rupture in the 1954 earthquake. Last year, Sathish Pullammanappallil (UNR Ph.D. '94) and Bill Honjas (UNR M.S. '93) of the Center for Economic Migration and Tomography (CEMAT), a UNR Seismo Lab/William Lettis and Assoc. consortium, conducted a DOE-funded re-analysis of industry seismic reflection lines in the Oxbow field. They found evidence for steeply dipping normal faults defining blocks stepping down into the basin; a model proposed years ago from sketchy gravity and refraction data. A paper describing their results will be available on the class shelf in LME 320. To be shallowly dipping in southern Dixie Valley and steep in northern Dixie Valley, the fault would have to change character substantially along strike. This would not be unusual for a long normal fault, and it strikes northwesterly in the Oxbow geothermal area while striking northerly in our area.

We will make our seismic refraction, reflection, gravity, and magnetic measurements along a dirt road running 2 km mostly east from the Dixie Valley fault scarp at Willow Canyon, to Dixie Valley Road (Nevada Highway 121). From that intersection Cattle Rd. runs 8 km further east to cross the valley. Gravity and magnetic measurements will continue east as far as possible. This transect happens to be at 39 deg 38 min 15 sec north latitude, and is about 25 miles north of US Highway 50 (yellow on the field area map in GIF or PDF). A few kilometers north at the Hwy 121 intersection with Settlement Rd., an additional gravity and magnetic transect may cross the valley.

The water table is often at the surface but may be below the bedrock interface near the scarp. The terrain is flat to gently sloping, occasionally incised by 5 m deep gullies. Vegetation is low sagebrush. Dirt tracks criss-cross the area. The valley is almost entirely public land, with few restrictions as long as we stay downhill of the fault scarp. Geologic and topographic maps, and some previous publications, are available to the class.

Our task is to profile the Dixie Valley fault using a variety of geophysical methods. We will try to record a 100-200 m long high-resolution sledgehammer seismic reflection line across the scarp at Willow Canyon, using MSM's new 48-channel seismic recorder and 100 Hz geophone strings, to attempt to image colluvial wedges, graben structure, and to trace fault reflections to the surface. We can use source and receiver spacings between 1 and 5 meters. Using the 8 Hz geophones or 100 Hz strings and the 48 channel, 750 m cable, we will conduct a medium-resolution survey from the scarp to the highway. A contractor will drill and blast 2 m deep shot holes with 1-5 lb dynamite charges. We will also analyze these data with refraction techniques.

Gravity measurements can probably be made of 100-200 stations spaced between 50 and 300 m, for a total profile length of 5 to 30 km. Each of these stations must be surveyed in to 1 foot elevation accuracy with the School's new geodetic GPS system. Magnetic measurements may be made at twice as many stations over twice the distance, or half the spacing. In the area of the hammer seismic line near the scarp, electromagnetic soundings may be made at 6-10 locations with 10 to 100 m apertures, and we can record shallow ground conductivities at 10-50 m intervals in grids or over lengths up to 10 km.

• Magnetic
• Gravity
• Electromagnetic
• Seismic reflection (incl. Rob Abbott)

Guidelines for proposing the survey plans are below. Certain questions need to be answered soon, and preparations begun early. These parts of the plans should be prepared by Mar. 9, and will be reviewed and discussed by the class that week. Each team should see J. Louie as soon as possible to begin preparations. The remainder of each plan must be finished by Mar. 11, so we can act on the plans during the days before we go in the field.

Finished plans should also include complete and detailed checklists of every item that will go to the field, data sheets and/or software disks, instrument operation instructions, maps showing proposed survey locations, and schedules for work by each team. Each team should turn in one set of plans on March 9, which I will evaluate and use to affect your final report grades.

• **What instruments are needed for the survey?
• **What instruments are available to us?
• **Who oversees the instruments? Are they available?
• **Are the instruments in good order? Have they been recently tested in the lab and on the ground? Need they be calibrated?
• **What supplies are needed to operate the instrument and record data? Batteries? Special paper? How are batteries to be charged, or sensitive materials stored?
• **If the instruments need repair or supplies need to be obtained, can this be done before departure?
• What provision can be made for instrument failure in the field? Would any tools be useful? Spare parts?
• **How and when are the other participants to be trained in the use of the instruments? What manuals are available? Can brief instructions for field use be written? Data entry forms prepared?
• **What items and procedures need to be put on a checklist that can be completed during mobilization?
• **How can the instruments be shipped to the field area? Are they especially sensitive? Do special arrangements need to be made to borrow them from the Bureau of Mines, or other Universities?
• **Are there materials or supplies that can or must be obtained at the field area? Where and when will this be done?

• What location accuracy is needed for survey stations? Do they need to be surveyed in? Could pace-and-compass locations be adequate?
• What site or geologic factors will contribute to successful bedrock profiling with this technique? Where could these be present in the field area?
• What site or geologic factors will contribute to successful shallow profiling with this technique? Where could these be present in the field area?
• In what parts of the field area would this technique help to constrain the interpretations from another technique?
• In what parts of the field area would the use of this technique be difficult? Are there access or surveying problems?
• How will the instruments be moved to different sites in the field area? When will vehicles be required? Will some stations have to be reached on foot?
• How many people and how much time are needed for each station, or each experiment?
• How much area or how many kilometers of profile can we do while we are in the field?
• How can a schedule be set up so everyone uses each of the instruments in the field?
• Where will the survey stations be? What profiles or areas will they cover? In what order will they be measured?
• How will the data be labeled and stored when it is collected to avoid loss or confusion later?
• What data quality-control procedures can be used? Can data be immediately reduced or plotted in the field to check for accuracy?
• Can initial results be used to guide the other techniques? Or to adjust survey plans on the fly?

• What procedures will be used to reduce and interpret the data? Would any results of the other surveys be needed? Would this survey's results be useful to another's?
• How and when will the data be disseminated to the other students? How will needed accessory information, such as station locations and instrument settings, be provided?

• Are there any items or concerns that need to be added to this list?

References

Compton, 1962, Manual of Field Geology, chapters 2, 3, 4, 11.

Dobrin and Savit, 1988, Introduction to Geophysical Prospecting, on the shelf in LME 320, pages 3-8 and as noted below:

• **What instrument would be easiest to use while providing enough accuracy? Alidade and plane table? Laser transit?
• **What maps, airphotos, and remote-sensing images are available? In what forms will copies be needed before, during, and after field work?
• What horizontal and vertical control has already been established in the field area? Are control points accessible? How will they be tied in?

• **What special precautions need to be taken to assure the stability of the gravimeter? Are extra batteries needed?
• Are any absolute gravity control stations available near the field area? When should they be measured?
• What gravity measurements have been made previously in the field area? How will our survey improve upon that work?
• Where should control stations be established, and how often should they be measured?
• Are any data reduction or modeling packages available? Can they work in the field?
• What sources of local rock density measurements are available?
• What accuracy is needed to make a useful interpretation of basement topography? What procedures will enhance accuracy?

• **Are the instruments working and ready for the field? What can be rented or borrowed?
• **What power sources are required?
• What is the maximum electrode spread, loop area, or time gate? What is needed to sound to our target depths?
• What are the expected depths of penetration for each of the available instruments?
• Are special procedures needed to properly ground electrodes in materials such as dry gravels? What tools and supplies will be needed?
• What will be the effect of the water table?
• What interpretation or analysis packages are available? Can analysis be done in the field? How will data be downloaded?

• **What instruments are available and working?
• **Are recording base stations available? Gradient instruments?
• What magnetic surveys have previously been performed in the field area? How will our survey improve on them?
• Where should drift control stations be located, and how often should they be measured?
• What accuracy is needed to detect the target basement topography? What if the bedrock changes from volcanic to metasedimentary?
• What will be the effect of volcanic materials within the sediments?
• What interpretation or analysis packages are available? Can analysis be done in the field? How will data be downloaded?

• **What recorders, cables, and geophones are available? Need any be rented or borrowed?
• What seismic surveys have been performed in the region? What were their results?
• What seismic velocity measurements are available for our area? If none, what are the likely velocities, and their contrasts at the basement interface?
• What line lengths are needed to locate basement refractions for different target depths? What are the possible dips?
• Which geophones should be used? Can an S-wave experiment be conducted?
• Will hammer blows provide enough seismic energy? Should another source be considered? At which sites will hammer surveys have the most chance of succeeding?
• How will the blaster tie into the Bison triggering system?
• How will proper operation of the roll-along switch be assured?
• What arrivals will likely be observed? How will they be interpreted?
• What fan shots (off-line) will be possible and helpful in interpretation?
• What areas are most conducive to getting good high-resolution data? What condition of the water table is helpful?
• **Where are the buried colluvial wedges of older earthquakes most accessible to reflection surveying?
• How small a hammer source can be used? What tests will need to be done in the field on different sources?
• What interpretation or analysis packages are available? Can analysis be done in the field? How will data be downloaded?