Abbott et al., 1998 Fall AGU Poster S11C-03

Geophysical Test of Low-Angle Dip on the Seismogenic Dixie Valley Fault, Nevada

Robert E. Abbott¹, John N. Louie¹, S. John Caskey², Stephen G. Wesnousky³

University of Nevada, Reno Seismological Laboratory and Department of Geological Sciences
San Francisco State University
University of Nevada, Reno Center for Neotectonic Studies and Department of Geological Sciences

Presented at the Fall 1998 American Geophysical Union meeting, San Francisco
(Presentation also available as a 2.4 Mb Acrobat PDF file.)

Abstract

The Dixie Valley fault ruptured in a normal event of Ms 6.8 in August of 1954. Recent geologic study of the fault trace has been used to suggest that much of the trace is characterized by dips as low as 25 degrees. Nodal planes for the event are poorly defined for the event because of coda from the nearby Fairview Peak earthquake (Ms 7.2) that occurred 4 minutes earlier. Our recent collection of high-resolution and medium-resolution seismic reflection profiles across the fault provides data consistent with the low-angle hypothesis.

The high-resolution survey was conducted within 130 m of the rupture with 100 Hz geophones and a sledgehammer source. The medium-resolution line used near-surface explosive sources, 8 Hz geophones, and extended from the rupture to 2.9 km into the basin. Both high- and medium-resolution profiles define the surface as dipping at 25-30 degrees to 1.5 km depth. The high-resolution survey shows fault-surface reflections from within 10 m of the surface to 50 m depth; the medium-resolution survey shows fault-surface reflections from 100 m to 1.5 km depth. Reflections subparallel to the fault-surface may be seen in the granite footwall, from 10 to 200 m depth. Also seen are rollover anticlines and buried rupture-graben structures in the sedimentary hanging wall. A vertically coincident refraction off the fault-surface from a long-offset source is also consistent with the presence of a smooth, shallowly dipping fault from 50 to 700 m depth. Gravity results support a shallow basin model, as required by the shallowly dipping boundary normal fault.

Introduction

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.

Fig. 1: Dixie Valley Rupture Map

Map of Dixie Valley in central Nevada. 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.

Fig. 2: 1998 Traverses

This figure shows the location of our 1998 geophysical field work. High- and medium-resolution seismic reflection profiles were conducted along Cattle Road from the range-front scarp eastward. Gravity transects were conducted across the valley along Settlement and Cattle Roads and along the scarp from Willow Canyon to Brush Canyon.

Field Acquisition and Data Reduction Methods

Fig. 3: Seismic Data Acquisition

This is a view of Dixie Valley near the fault scarp and looking east, along Cattle Road. Receiver locations for the medium-resolution survey are red flags. The medium-resolution profile utilized 8 Hz geophones, explosives at 2 m depth, and extended 3.6 km into the basin. It was composed of 4 stationary setups of 48 receivers with 15.2 meter spacing. The high-resolution profile was conducted within 130 m of the range-front scarp with 100 Hz geophone groups, 2 m spacing, and a sledgehammer source. A linear array of 6 geophones per group was used to reduce ground-roll in the high-resolution survey. In both surveys, off-end and longer-offset shots were recorded to increase fold and gather deep velocity information.

Photo by Karl V. Steinbrugge 1954: Picture of Range-Front Scarp, East Job Canyon

Gravity Data Acquisition

Gravity measurements were made with a LaCoste & Romberg Model G gravity meter. Vertical control is supplied by a Trimble geodetic-quality GPS. Data were reduced to simple Bouguer anomaly with a 2.67 g/cc reduction density and inner-ring terrain corrections to 100 m. Additional data from Schaefer's (1984) gravity study were incorporated to increase station density.

Fig. 5: Processing Flows

Post-Stack Migration	Velocity Analysis and Pre-Stack Migration
Trace Editing and Top-Muting	First Break Picking
Bandpass Filtering 6-8 and 100-120 Hz trapezoidal frequency domain filter	Simulated-Annealing Optimization to determine V(x,z)
f-k Filtering --> Polygonal Filter, 50 trace, 500 ms time window	--> AGC of f-k Filtered Records 500 ms time window
AGC 200 ms time window	Pre-Stack Kirchhoff Migration Includes Lumley-Claerbout operator anti-aliasing
NMO Correction Velocities picked in 200 m/s intervals from CV stacks	Estimation of Noise Data Set by sign flips on random traces
CMP Binning and Stacking 15.2 m bin centers, mid-points assigned to bin fold is variable, no amplitude variation for fold.	Re-Migration of Noise for image of artifacts
Stolt Time Migration 2 km/s migration velocity	Harlan Coherency Enhancement scale image by signal expectation

Velocity Analysis and Acoustical Modeling

Fig. 6: 3-D Velocity Model

This is a velocity model obtained from a simulated-annealing non-linear optimization of first arrival picks (Pullammanappallil and Louie, 1994). A three-dimensional inversion was calculated because of a slig htly crooked-line geometry along the Cattle Road profile. The 3-D model was subsequently "flattened" to 2-D for use in Kirchhoff pre -stack migration as in Chavez-Perez et al. (1998).

Fig. 7: Refraction Experiment

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. The shot geometry and interpretation is given in Figure 8.

Fig. 8: Acoustic Model of Headwave

This is an acoustic model of the vertically propagating headwave shown in Figure 7. The 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), 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).

Results

Fig. 9: 2.0 km/s Stolt-Migrated Stack

This is a post-migrated stack of our medium-resolution Cattle Road profile, following the processing flow given in figure 5. 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.

Fig. 10: Enhanced, Anti-Aliased Pre-Stack Migration

This is a pre-stack migration of the same profile as above, also following the flow given in figure 5. 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.

Fig. 11: Preliminary Gravity Basin Model

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. A 1:1 overlay of the seismic results is consistent with the low-angle hypothesis. 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.

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.

Acknowledgements

This project was funded by the National Science Foundation (EAR-9706255). The W. M. Keck Foundation donated seismic equipment, computers, and modeling software. All geophysical fieldwork was performed by the spring 1998 Geophysical Applications class at the University of Nevada, Reno. 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 to Mike Dennis and Nevada Precision Drilling and Blasting for their work. We also thank Prill Meacham and the Carson City office of the BLM for their assistance and cooperation.