Shallow Dip on the 1954 Dixie Valley Fault

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A seismogenic normal fault with shallow dip, Dixie Valley

Co-authored by Robert Abbott and Steve Wesnousky

Estimates of extension in the Basin and Range range are commonly above 100 percent (Hamilton, 1978; Wernicke et. al, 1988; Proffett, 1977). Examination of earthquake mechanisms in the western United States reveals the complete absence of large events occurring on normal faults with dips less than 38 degrees (Doser and Smith, 1989). A global study by Jackson (1987) shows a similar limit (approximately 35 degrees) on normal fault dip and reveals that faults are essentially planar and dip steeply down to the brittle-ductile transition. As well, frictional constraints have been used to argue that it is more favorable to create new, steeply-dipping faults than accumulate slip on low-angle normal faults.

Given planar faults and minimum fault dip of 38 degrees, simple geometric relations (Jackson and McKenzie, 1983) can be used to show that the maximum extension possible from a single fault system with rotation of dip is 40%. Beyond 40%, extension must be taken up by a new set of high-angle faults cutting the old system, or by aseismic slip on faults which have rotated to a low angle.

In contrast, several researchers have compiled observations to argue for the existence of Quaternary seismic slip on low-angle normal faults (Axen et al., 1998; Abers, 1983; Burchfiel et al., 1987, Johnson and Loy, 1992). Also, it has been shown that it is energetically more feasib le to accommodate large amounts extension on normal faults of low dip (Forsyth, 1992).

Here, we present the results of a seismic reflection and gravity experiment to test whether or not part of the 16 December 1954 Dixie Valley Earthquake (Ms=6.8) produced slip on a low-angle normal fault.

The Dixie Valley event is the only candidate for a historical normal faulting earthquake that could validate the occasional occurrence of earthquakes on listric normal faults. The fact it was obscured from seismological analysis by a preceeding and possibly triggering event may be related to the conditions needed for such shallow normal faulting.

The extent of the 1954 Dixie Valley surface rupture is denoted by the white arrow. The large error estimate in the epicenter location is a result of contamination of the waveforms by the nearby Fairview Peak (Ms=7.2) earthquake, four minutes in advance. There is a correspondingly large error in fault dip determination.

There is no doubt about the timing and location of the surface rupture of the Dixie Valley earthquake. Ground rupture was observed by local prospectors within minutes, and seismologists visited within hours. The Dixie Valley fault ruptures did not occur during the Fairview Peak event, but during the Dixie Valley earthquake itself four hours later.

Detailed mapping of the rupture graben and its geometry over topographic features led John Caskey to the conclusion that it must have a dip of 30 degrees or less. The graben is huge, 15 m wide, and quite uniform along 50 km of the rupture. The dip of the slip face is about 50 degrees. This suggests via the volume of the graben that the main fault plane shallows in dip to 25-40 degrees, depending on the depth of the bend.

In March 1998 I and a class of 10 undergraduate and graduate students undertook an NSF-funded geophysical study of the Dixie Valley fault. High- and medium-resolution seismic reflection profiles were conducted along Cattle Road from the range-front scarp eastward. The medium-resolution survey extended 3.6 km east of the scarp. Gravity transects were conducted right across the valley along Settlement and Cattle Roads and along the scarp from Willow Canyon to Brush Canyon.

Thanks are due to the W. M. Keck Foundation, which donated seismic equipment, computers, and modeling software. Class participants were Ana Cadena, Travis Rabe, Matt Herrick, Mandy Johnson, Andrew Rael, Tom Blechen, and Evan Hobson. Additional field assistance was rendered by Christine Mann, Jim Ollerton, and John Oswald. Simulated-annealing of first-arrival picks was performed by Sathish Pullammanappallil of OPTIM LLC. Thanks also to Mike Dennis and Nevada Precision Drilling and Blasting for their work. Prill Meacham and the Carson City office of the BLM provided essential assistance and cooperation.

This raw, long-offset shot gather from our medium-resolution profile shows a headwave propagating vertically from the fault surface (highlighted). The coincident first-arrivals across the entire receiver array limit the range of the dip of the fault.

This acoustic model assumes a basement-alluvium velocity contrast of 2 to 1, along a 30-degree dipping fault plane. The array of receivers is shown near the fault scarp. Energy from the blast, at flag number 101 (2.1 km east), reflects and refracts off the fault interface. This geometry reproduces the vertically propagating headwave seen on the shot record. Similar refractions along the high-resolution seismic line (not shown) demonstrate that the fault plane continues along this dip to the surface. The synthetics were produced by a finite-difference solution to the scalar wave equation described by Helmberger and Vidale (1988).

The acoustic model matches the data very well, showing the smooth simultaneous footwall headwave with lower amplitude than the direct basin arrival. The smoothness of the headwave proves that the Dixie Valley rupture cannot be a series of steeply-dipping stairsteps into the basin. For at least 700 m east of the rupture, and 350 m depth, this basin-bounding normal fault must dip at about 30 degrees.

Above is a post-stack migrated section of our medium-resolution Cattle Road profile. The fault plane reflector, dipping eastward at 25-30 degrees, can be traced to its surface outcrop. Reflections sub-parallel to the fault can be seen in the footwall, suggesting foliation in the granite. Highly reflective Tertiary basalt layers in the hanging wall begin to obscure the fault reflections at about 500 ms. However, the basalt layers can be traced to the extension of the fault at depth, where they are seen to terminate, after forming small roll-over anticlines. The roll-over anticlines support a listric fault geometry.

The lower plot is a pre-stack migration of the same profile as above. Such a migration yields a structural section with accurate depths, as we first employed the simulated-annealing velocity optimization of Optim L.L.C. and S. Pullammanappallil to accurately estimate lateral velocity variations from the survey's first-arrival data. The resulting velocity model, though not shown here, also demonstrates the shallow dip of the basin boundary. The migration used the Lumley-Claerbout operator anti-aliasing criterion, yielding a longer-wavelength view of subsurface reflectivity. With it, we can see below the reflective basalts in the hanging wall, and image the shallowly-dipping fault to 1.5 km depth.

By overlaying the pre- and post-stack sections, we can make a line-drawing interpretation that shows all the prominent reflective structures. The lines follow strong and continuous reflectors that are not the upward-arcing migration artifacts. This interpretation emphasiszes the development of the Dixie Valley basin by growth faulting along a shallow-dipping margin.

We used our gravity survey to independently cross-check our seismic interpretation of a shallow-dipping normal fault. This is a modeled basin geometry of the gravity data obtained along the Cattle Road Profile. The scatter in the observed points is due to insufficient elevation control as the roving-mode GPS data has yet to be fully rectified. Even so, the shape of the anomaly is not affected by the scatter and it is compatible with a low-angle geometry. The gradational density in the basin fill follows a regional scheme used by Blakely et al. (1996) in their gravity inversions. Modeling was done with the GM-SYS package by Northwest Geophysical Associates.

A 1:1 overlay of the seismic results is consistent with the low-angle hypothesis. Note that by using basin-fill densities that increase with depth, we are being conservative with respect to the shallow-dip interpretation. The increasing densities with depth push the basin bottom down deeper than any constant-density modeling would.

Within 150 meters of the 1954 scarp we also performed two high-resolution geophysical surveys. A high-res seismic reflection line used 48 channels of 100-Hz geophones in 6-phone groups 2 m long, with a group spacing of 2 m. A 6 kg sledge against a 20x20x3 cm steel plate, also at 2 m intervals, provided the source. The simple migrated brute stack above shows relative depth resolution as good as two meters. The 30-degree dipping fault plane is clear, as are the sub-parallel foliations in the granite below, and sub-horizontal alluvial layers in the headwall.

This line-drawing interpretation of the high-res reflection section emphasizes the initial development of the rollover geometry at 40 m depth, with growth faulting leading to strata dipping towards the fault. In addition the line drawing points out the hints of buried grabens from previous earthquake sequences, at 10, 25, and 35 meters depth.

Above the seismic interpretation is an analysis of our series of time-domain electromagnetic (TEM) soundings across the 1954 scarp, using a square transmitter loop 40 m on a side. As the survey worked to the left (west) the footwall rose to within the ~50 m depth range that the equipment can detect. The footwall is clearly more conductive than the headwall. As the headwall consists of dry alluvium, the very low resistivities atop the 1954 graben may result from groundwater entrained in the 30-degree dipping fracture sets within the granite. Such porosity within the granite could explain both the observed headwall reflections, and a spring on the fault about 300 m south of the Willow Canyon survey line, Willow Spring.

Conclusions

Our results indicate that slip along a section of the 16 December 1954 Dixie Valley earthquake rupture took place along a fault plane of unusually low dip (25-30 degrees). In this regard, it is the first large historical earthquake for which slip on a low-angle normal fault has been documented.

Selected References

Abers, G. A., 1991, Possible seismogenic shallow-dipping normal faults in the Woodlark-D'Entrecasteaux extensional province, Papua New Guinea: Geology, 19, 1205-1208.
Axen, G. J., J. M. Fletcher, E. Cowgill, M. Murphy, P. Kapp, I. MacMillan, E. Ramos- Velazquez, and J. Aranda-Gomez, 1998, Range-front fault scarps of the Sierra El Mayor, Baja California: Formed above and active low-angle normal fault?: in submission Geology.
Blakely, R. J., R. C. Jachens, J. P. Calzia, and V. E. Langenheim, 1996, Cenozoic basins of the Death Valley extended terrain as reflected in regional-scale gravity anomalies: in submission GSA Spec.. Pub..
Burchfiel, B. C., K. V. Hodges, and L. H. Royden, 1987, Geology of Panamint Valley- Saline Valley pull-apart system, California: Palinspatic evidence for low-angle geometry of a Neogene range-bounding fault: J. Geophys. Res., 92, 10,422- 10,426.
Caskey, S. J., S. G. Wesnousky, P. Zhang, and D. B. Slemmons, 1996, Surface faulting of the 1954 Fairview Peak (Ms 7.2) and Dixie Valley (Ms 6.8) earthquakes, Central Nevada: Bull. Seis. Soc. Am., 86, 761-787.
Chavez-Perez S., J. N. Louie, and S. K. Pullammanappallil, 1998, Seismic depth imaging of normal faulting in the southern Death Valley basin: Geophysics, 63, 223-230 97-1409.
Doser, D. I., and R. B. Smith, 1989, An assessment of source parameters of earthquakes in the cordillera of the western United States, Bull. Seis. Soc. Am, 79, 1383-1409.
Forsyth, D. W., 1992, Finite extension and low-angle normal faulting: Geology, 20, 27- 30.
Hamilton, W., 1978, Mesozoic tectonics of the western United States, in Howell, D. G., et al., Eds, Mesozoic paleogeography of the western United States: Soc. Ec. Paleontologists and Mineralogists, Pacific Coast Paleogeography Symp., 33-70.
Helmberger, D. V., and J. E. Vidale, 1988, Modeling strong motions produced by earthquakes with two-dimensional numerical codes: Bull. Seis. Soc. Am., 78, 109-121.
Jackson, J., and D. McKenzie, 1983, The geometrical evolution of normal fault systems: J. Struct. Geol., 5, No. 5, 471-482.
Jackson, J. A., 1987, Active normal faulting and extension: in Coward, M. P. et al. Eds, Continental Extensional Tectonics, Geol. Soc. Am. Spec. Pub., 28, 3-17.
Johnson, R. A., and K. L. Loy, 1992, Seismic reflection evidence for seismogenic low- angle faulting in southeastern Arizona: Geology, 20, 597-600.
Lumley, D. C., D. Claerbout, 1994, Anti-aliased Kirchhoff 3-D migration: Expanded Abstracts, Soc. Explor. Geoph., Ann. Internat. Mtg., Los Angeles
Proffett, J. M., Jr., 1977, Cenozoic geology of the Yerington district, Nevada, and implications for the nature and origin of Basin and Range faulting: Geol. Soc. Am. Bull., 88, 247-266.
Pullammanappallil, S. K., and J. N. Louie, 1994, A generalized simulated-annealing optimization for inversion of first-arrival times: Bull. Seismo. Soc. Amer., 84, 1397-1409.
Schaefer, D. H., J .M. Thomas, B. G. Duffrin, 1983, Gravity survey of Dixie Valley, west-central Nevada: USGS OFR 82-111, 17 p.
Wernicke, B., 1995, Low-angle normal faults and extension: A review: J. Geophys. Res., 100, No. B10, 20,159-20,174.
Wernicke, B., G. L. Axen, and J. K. Snow, 1988, Basin and Range extensional tectonics at the latitude of Las Vegas, Nevada: Geol. Soc. Am. Bull., 100, 1738- 1757.

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Presented by invitation to the Geophysics Section of Science Wellington, New Zealand, on September 17 1998. Updated with new processing in December 1998.