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Geophysical confirmation of low-angle normal slip on the historically active Dixie Valley fault, Nevada

Robert E. Abbott

Seismological Laboratory and Department of Geological Sciences, University of Nevada, Reno

1664 North Virginia Street, Reno, NV 89557; Voice: 775-784-4251, FAX: (775) 784-1833,

John N. Louie

Seismological Laboratory and Department of Geological Sciences, University of Nevada, Reno

1664 North Virginia Street, Reno, NV 89557; Voice: 775-784-4219, FAX (775) 784-1833,

S. John Caskey

Department of Geosciences, San Francisco State University

1600 Holloway Avenue, San Francisco, CA 94132, Voice: (415) 405-0353, FAX: (415) 338-7705,

Satish Pullammanappallil

Optim L.L.C., University of Nevada, Reno Seismological Laboratory

1664 North Virginia Street, Reno, NV 89557, Voice: (775) 784-6613 FAX: (775) 784-1833,

Geophysical confirmation of low-angle normal slip on the historically active Dixie Valley fault, Nevada

Abstract. The 16 December 1954 Dixie Valley earthquake (MS=6.8) followed the nearby Fairview Peak earthquake (MS=7.2) by four minutes, twenty seconds. Waveforms from the Fairview Peak event contaminate those from the Dixie Valley event, making accurate fault-plane solutions impossible. A recent geologic study of surface rupture characteristics in southern Dixie Valley suggests that the Dixie Valley fault is low-angle (<30) along a significant portion of the 1954 rupture. To extend these observations into the subsurface we conducted a seismic reflection and gravity experiment. Our results show that a portion of the Dixie Valley ruptures occurred along a fault dipping 25° to 30°. As such, the Dixie Valley event may represent the first large, low-angle normal earthquake on land recorded historically. Our high-resolution seismic reflection profile images the rupture plane from 5 to 50 m depth. Medium-resolution reflections, as well as refraction velocities, show a smoothly dipping fault plane from 50 to 500 m depth. Stratigraphic truncations and rollovers in the hangingwall show a slightly listric fault to 2 km depth. Gravity profiles conservatively constrain maximum basin depth and define overall geometry. Extension along the low-angle section may have occurred in two phases during the Cenozoic. Current fault motion post-dates a 13 to 15 Ma basalt, imaged in the hangingwall, and inherits from a fault formed during an earlier extensional pulse, concentrated at 24.2 to 24.4 Ma. The earlier extension suggests extraordinary slip rates as high as 18 mm/yr, resulting in the formation of the low-angle fault break. Sections of the Dixie Valley fault where there is no evidence for current low-angle slip correlate well with areas where no pre-15 Ma slip has been documented.

Introduction

Despite growing geological and geophysical evidence arguing for the existence of low-angle normal faults that have accommodated large amounts of extension, the paradox of the near-complete absence of low-angle normal-mechanism earthquakes in the seismic record remains. Slip on low-angle normal faults is not predicted in Andersonian theory [Anderson, 1942] and studies of earthquake focal mechanisms, both regional [Doser and Smith, 1989] and global [Jackson, 1987], show an extreme scarcity of large (M>5.5) normal fault mechanisms with dip less than 30°. However, several researchers [e.g. Abers et al., 1997; Hatzfeld et al., 2000; Johnson and Loy, 1992] have presented compelling evidence that substantial extension has occurred along low-angle normal faults in the brittle upper crust.

Theories to resolve the seismicity paradox fall into two categories: those that do not require brittle slip at low angles (e.g. “rolling hinge” models [Wernicke and Axen, 1988] and flexural rotation [Buck, 1988]), and those that argue either for long earthquake recurrence intervals [Wernicke, 1995], or a current rarity of active low-angle faults [Burchfieletal., 1992].

Compelling evidence that brittle slip is possible on at least one low-angle normal fault would have important ramifications for both fault mechanics theory and seismic hazard calculations. Here, we present the results of a seismic reflection and gravity experiment to test whether part of the 16 December 1954 Dixie Valley Earthquake (MS=6.8) produced slip on a low-angle normal fault.

Regional and Tectonic Setting

Dixie Valley, Nevada, lies in the north-central portion of the Basin and Range province (Figure 1). The Basin and Range is a region that has experienced a large amount of intraplate extension in the Cenozoic. Much of the extension is accomplished by high-angle (50°-70°) normal faulting, with several large seismic events recorded historically (e.g. 1915 Pleasant Valley, 1954 Fairview Peak-Dixie Valley, 1983 Borah Peak). The faulting has created predominately north-south-trending mountain ranges and sedimentary basins. Dixie Valley is one such basin; bounded by the Stillwater Range on the west and the Clan Alpine Range on the east (Figure 1). The Dixie Valley fault, site of the 1954 Dixie Valley earthquake, is the east-dipping range-bounding normal fault along the eastern front of the Stillwater Range.

The 1954 fault trace lies along the southern portion of the Stillwater Range and separates Mesozoic and Tertiary footwall rocks from late Tertiary and Quaternary basin fill. Miocene and Oligocene volcanic rocks and granitic plutons related to the Stillwater caldera complex [John, 1995] and Mesozoic metasedimentary rocks represent the “geophysical basement”. The basin fill at the surface consists of alluvial fan and lacustrine deposits [Wilden and Speed, 1974].

The 1954 Dixie Valley Earthquake

The 16 December 1954 Dixie Valley earthquake was the last of a series of large earthquakes that took place within a period of 6 months in central Nevada. The preceding events were the Rainbow Mountain sequence (MS=6.6 and 6.4 on 6 July 1954, MS=6.8 on 24 Aug. 1954) and the Fairview Peak earthquake (MS=7.2 on 16 Dec. 1954). The Fairview Peak event preceded the MS=6.8 Dixie Valley earthquake by four minutes and twenty seconds. Focal mechanisms for the Fairview Peak and Rainbow Mountain events indicate NNW-striking normal-oblique faults with dips ranging from 60 to 78°.

Fault plane solutions for the Dixie Valley event are poorly constrained because the arrivals are obscured by waveforms from the Fairview Peak event. Doser [1986] used waveform modeling to determine fault geometry with a strike of N10°W and a dip of 60°E; however, due to contamination of the Dixie Valley waveforms, “large changes in strike and dip did not significantly change the waveform shape” [Doser, 1986]. Similarly, Okaya and Thompson’s [1985] solution of N11°W, 62°E cannot be relied upon. They noted that: “Of the four focal parameters (depth, dip, strike, and slip), only changes in depth are significant; changes in fault plane strike, dip, or slip have negligible effect” [Okaya and Thompson, 1985]. The Dixie Valley fault plane solution (N8°E, 49°E) of Hodgkinson et al. [1996] using leveling and triangulation benchmarks suffers from a paucity of data (very few pre-rupture survey benchmarks) in the rupture region, such that the triangulation network is unable to document slip along most of the rupture.

Geologic Evidence for Low-Angle Dip on the Dixie Valley Fault

Caskey et al. [1996] conducted the most recent and detailed study of the surface faulting characteristics of the Fairview Peak and Dixie Valley earthquakes. They noted substantial geologic evidence for low-angle dip for the Dixie Valley fault along an approximately 20-km-long portion of the rupture zone. Geologic evidence for a low dip angle lies between The Bend in the north, to just north of Coyote Canyon in the south (Figure 1). Geological evidence for low-angle dip at the surface in Caskey et al. [1996] include: 1) three point fault plane reconstructions, 2) shallow subsurface modeling of the rupture-trace graben, 3) fault-parallel fracture sets in the footwall, and 4) geometry of the Stillwater rangefront.

Previous Geophysical Work in Dixie Valley

Dixie Valley has been the subject of numerous geophysical investigations. The studies primarily focused on the northern part of the valley, around 40 km north of our study area. Okaya and Thompson[1985] combine seismic reflection and gravity data to model northern Dixie Valley as a half-graben with one major normal fault dipping 50°E to the northwest (the Dixie Valley fault), and three smaller, west-dipping normal faults to the southeast. This model is inconsistent with Smith’s[1967] aeromagnetic study north of The Bend (Figure 1). Smith maps pre-Tertiary basement under Dixie Valley as a graben within a graben.

Schaefer[1983] collected widespread gravity data throughout Dixie Valley. A portion of Schaefer’s [1983] Bouguer anomaly map is reproduced in Figure 2. As can be seen in the figure, two 4-5 mGal local gravity lows are near the latitudes of Coyote Canyon and Wood Canyon, consistent with a more moderately dipping rangefront fault (or uplifted bedrock) between these two latitudes. It should be noted that the shape of these anomalies is very poorly constrained and the gravity lows may have other explanations unrelated to a change in rangefront fault dip. An east-west linear transect of 24 gravity points near the latitude of Little Box Canyon (Figure 3) is consistent with a low-angle Dixie Valley fault, assuming reasonable bedrock-alluvium density contrast.

Meister[1967] and Herring[1967] conducted seismic refraction experiments near the latitude of IXL Canyon (Figure 3). Meister [1967] interpreted southern Dixie Valley as a composite graben, based on several short refraction lines parallel to the rangefront, and an east-west cross-valley profile. The east dipping Dixie Valley fault was interpreted to be a combination of high-angle normal faults and flat terraces, resulting in a “staircase-like” fault geometry. These data allow for an alternate interpretation, however. In our evaluation of Meister’s[1967] data, a single low-angle dip normal fault can replace the previous stair-step structure. Herring’s[1967] experiment assumed, a priori, high-angle dip and the assumed geometry was used to test a “sideswipe-refraction” technique.

Methods

Field Data Acquisition

Figure 3 shows the location of our geophysical transects. The Cattle Road profile consists of two seismic reflection profiles and a gravity profile, while the Scarp profile consists solely of gravity coverage. The trend of the Cattle Road profiles is within 10 of the gradient of the gravity field near the rangefront. If the gravity gradient reflects the direction of true dip of the Dixie Valley fault, than apparent dips measured from the geophysical profiles will be underestimated by no more than 0.5.

The medium-resolution Cattle Road profile extended 3.6 km into the basin and utilized 8 Hz geophones. It was composed of 4 stationary setups of 48 receivers with 15.2 meter spacing. The 132 source points were 2-7 kg of high explosive buried 2 m below the surface. Off-end and longer offset shots were recorded to increase fold and improve deep velocity information. Maximum source-receiver offset for the medium-resolution line was 2.8 km, and maximum fold was 24.

The high-resolution Cattle Road profile overlapped the medium-resolution profile close to the 1954 rupture. It was conducted within 150 m of the rangefront scarp using 100 Hz geophone groups with 2 m spacing. 67 sledgehammer source points were rolled through the array. Six inline geophones per group were used to reduce noise from ground roll. Maximum source-receiver offset for the high-resolution line was 124 m. Maximum receiver fold was 32.

Gravity data along Cattle Road were acquired with a LaCoste and Romberg Model G gravity meter. Gravity coverage started at the scarp and extended eastward 12.5 km into the basin at a 250 m average station spacing. Vertical control was supplied by a geodetic-quality GPS.

Gravity data were also collected parallel to the rangefront scarp (Scarp Gravity, Figure 3). The Scarp profile was located along a line where depth-to-bedrock was assumed to be approximately constant, and therefore any gravity variations would be largely due to density variations within the geophysical basement. In this way, errors in depth-to-bedrock calculations from our 2.5-D forward model along the Cattle Road profile can be estimated.

Seismic Data Processing

Conventional processing and post-stack migration-Conventional seismic data processing techniques were used to remove noisy traces, mute direct waves and attenuate other unwanted arrivals. The medium-resolution line was bandpass filtered (6-8 Hz, 100-120 Hz trapezoidal filter), and then filtered with a polygonal, 48 trace, 500 ms f-k domain filter. The f-k filter was designed to eliminate 400 m/s Rayleigh waves. After AGC, constant velocity stacks at 200 m/s intervals were used to pick stacking velocities. Common depth points were binned at 15.2 m intervals, with no amplitude variation with fold. The subsequent CDP stack was then Stolt-migrated at a constant 2000 m/s velocity.

The high-resolution seismic line was bandpass filtered (8-12 Hz, 200-202 Hz) and f-k domain filtered (48 trace, 125 ms). Stacking velocities were chosen using the same method as with the medium-resolution line. Common depth points were binned at 2 m intervals and the resulting CDP stack was Stolt-migrated using the rms stacking velocities.

Velocity Analysis- Due to complex geometry, strong lateral velocity variation, and steep dips (for seismic imaging) in the subsurface at Dixie Valley, we chose to compute pre-stack depth migrations. For input into our pre-stack migrations, we obtained a detailed velocity image of the subsurface by performing a nonlinear optimization on first arrivals picked off raw shot-gathers. The optimization technique employs a generalized simulated annealing algorithm [Pullammanappallil and Louie, 1994] to invert first arrivals for subsurface velocity structure. We used a commercial package, SeisOpt @2D™ (Copyright Optim LLC, 1998-1999), that implements this method.

The simulated annealing algorithm is a Monte-Carlo based estimation process that has the property of being independent of the starting model and has the ability to find the global minimum (i.e. solution) for a highly nonlinear problem. These characteristics make the algorithm a very effective tool for velocity estimation. Travel time inversion is a highly nonlinear problem because any perturbation in the velocities alters the path of the ray propagation, changing the travel times recorded at the surface geophones. This non-linearity makes linear methods dependent on the starting model; that is, the accuracy of the final velocity model is dependent on a good initial guess. The method employed by SeisOpt @2D™ “tests” several thousand models before arriving at the optimized velocity model. The only inputs required by the algorithm are the first arrival picks and survey geometry (source and receiver coordinates). In addition to the final velocity model, SeisOpt @2D™ outputs a ray coverage or “hit count” plot that shows what parts of the model were sampled by the seismic array. The algorithm outputs only the velocities in the subsurface that have been sampled by the rays.

A total of 6117 first-arrival picks from 134 shot gathers were used for the optimization. SeisOpt @2D™ can handle only two-dimensional array geometry. Hence, in order to overcome a bend in the medium-resolution seismic line, we project the source locations to a straight line while maintaining the true offsets of the source-receiver pairs. As a result of this projection, the optimized velocities might show some lateral smearing in the vicinity of the bend in the profile. The resulting velocity model was used in a pre-stack migration algorithm to image the seismic reflectors directly.

Pre-stack migration- In order to use the optimized velocities to perform a pre-stack Kirchhoff migration, we first extended the velocities down to a depth of 2.0 km. Like Pullammanappallil and Louie[1994] and Chavez-Perez et al.[1998] we extended the optimized velocity models for migration by finding the maximum constrained velocity value in each column of the velocity model, and substituted that value into the column of the model everywhere below the depth of the maximum velocity. We then performed a severe lateral smoothing below the depth of first arrival constraint.

The resulting velocity model was used in a pre-stack migration algorithm to image the seismic reflectors directly. The pre-stack Kirchhoff-summation algorithm was originally used to image the San Andreas fault zone by Louie et al.[1988], and modified by Louie and Qin[1991] to account for reflection ray paths that may bend significantly through strong lateral velocity variations. In Dixie Valley we did not attempt to image near-vertical structure, as previous work did, but we needed to account for strong ray bending through the velocity contrasts at the edge of the basin, and through any lateral variability in the Tertiary basalts within the basin section. So we added to the algorithm a dip-dependent obliquity factor.

Another addition is the migration-operator antialiasing criterion of Lumley et al. [1994], which leads to high-cut filtering of the seismic traces. Completely preventing the spatial aliasing of the migration operator leads to discontinuous coverage of depth points for our medium-resolution survey, and detracts from the lateral continuity of reflections. Thus, for the aliasing calculation, we used a receiver spacing that is half of the actual 15.2 m spacing. This apparent spacing yields a mild operator antialiasing effect that only removes the worst-aliased frequencies of the most steeply-dipping structure, while retaining the lateral continuity of the unaliased near-horizontal structure.