Satish K. Pullammanappallil

Blind Shear-Wave Velocity Comparison of ReMi and MASW Results with Boreholes to 200 m in Santa Clara Valley: Implications for Earthquake Ground-Motion Assessment

Multichannel analysis of surface waves (MASW) and refraction micro- tremor (ReMi) are two of the most recently developed surface acquisition techniques for determining shallow shear-wave velocity. We conducted a blind comparison of MASW and ReMi results with four boreholes logged to at least 260 m for shear vel- ocity in Santa Clara Valley, California, to determine how closely these surface meth- ods match the downhole measurements. Average shear-wave velocity estimates to depths of 30, 50, and 100 m demonstrate that the surface methods as implemented in this study can generally match borehole results to within 15% to these depths. At two of the boreholes, the average to 100 m depth was within 3%. Spectral amplifi- cations predicted from the respective borehole velocity profiles similarly compare to within 15% or better from 1 to 10 Hz with both the MASW and ReMi surface-method velocity profiles. Overall, neither surface method was consistently better at matching the borehole velocity profiles or amplifications. Our results suggest MASW and ReMi surface acquisition methods can both be appropriate choices for estimating shear- wave velocity and can be complementary to each other in urban settings for hazards assessment.

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

The December 16, 1954, Dixie Valley earthquake (Ms = 6.8) followed the nearby Fairview Peak earthquake (Ms = 7.2) by 4 min, 20 s. 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 hanging wall 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 postdates a 13 to 15 Ma basalt, imaged in the hanging wall, 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.

Application of simulated annealing inversion on high-frequency fundamental-mode Rayleigh wave dispersion curves

The simulated annealing (SA) inversion technique has been successfully applied for solving various nonlinear geophysical problems. Following previous developments, we modified the SA inversion, yielding 1D shallow S-wave velocity profiles from high frequency fundamental-mode Rayleigh dispersion curves, and validated the inversion with blind tests. Unlike previous applications of SA, this study draws random numbers from a standard Gaussian distribution. The numbers simultaneously perturb both S-wave velocities and the layer thickness of models. The annealing temperature is gradually decreased following a polynomial-time cooling schedule. Phase velocities are calculated using the reflectivity-transmission coefficient method. The reliability of the model resulting from our implementation is evaluated by statistically calculating the expected values of model parameters and their covariance matrices. Blind tests on two field and 12 synthetic Rayleigh dispersion data sets show that our SA implementation works well for S-wave velocity inversion of dispersion curves from high-frequency fundamental-mode Rayleigh waves. Blind estimates of layer S-wave velocities fall within one standard deviation of the velocities of the original synthetic models in 78% of cases.

Recent faulting in western Nevada revealed by multi-scale seismic reflection

Roxanna N. Frary∗†, John N. Louie†, William J. Stephenson‡, Jackson K. Odum‡, Annie Kell†, Amy Eisses†, Graham M. Kent†, Neal W. Driscoll§, Robert Karlin¶, Robert L. Baskin‖, Satish Pullammanappallil∗∗, Lee M. Liberty†† †Nevada Seismological Laboratory, University of Nevada ‡United States Geological Survey, Golden, Colorado §Scripps Institution of Oceanography, University of California, San Diego ¶Department of Geological Sciences and Engineering, University of Nevada ‖United States Geological Survey, West Valley City, Utah ∗∗Optim, Reno, Nevada ††Center for the Geophysical Investigation of the Shallow Subsurface, Boise State University

Surface-wave and refraction tomography at the FACT Site, Sandia National Laboratories, Albuquerque, New Mexico.

We present a technique that allows for the simultaneous acquisition and interpretation of both shear-wave and compressive-wave 3-D velocities. The technique requires no special seismic sources or array geometries, and is suited to studies with small source-receiver offsets. The method also effectively deals with unwanted seismic arrivals by using the statistical properties of the data itself to discriminate against spurious picks. We demonstrate the technique with a field experiment at the Facility for Analysis, Calibration, and Testing at Sandia National Laboratories, Albuquerque, New Mexico. The resulting 3-D shear-velocity and compressive-velocity distributions are consistent with surface geologic mapping. The averaged velocities and V{sub p}/V{sub s} ratio in the upper 30 meters are also consistent with examples found in the scientific literature.

New constraints on fault architecture, slip rates, and strain partitioning beneath Pyramid Lake, Nevada

A seismic compressed high-intensity radar pulse (CHIRP) survey of Pyramid Lake, Nevada, defines fault architecture and distribution within a key sector of the northern Walker Lane belt. More than 500 line-kilometers of high-resolution (decimeter) subsurface imagery, together with dated piston and gravity cores, were used to produce the first comprehensive fault map and attendant slip rates beneath the lake. A reversal of fault polarity is observed beneath Pyramid Lake, where down-to-the-east slip on the dextral Pyramid Lake fault to the south switches to down-to-the-west displacement on the Lake Range fault to the north. Extensional deformation within the northern two thirds of the basin is bounded by the Lake Range fault, which exhibits varying degrees of asymmetric tilting and stratal divergence due to along-strike segmentation. This structural configuration likely results from a combination of changes in slip rate along strike and the splaying of fault segments onshore. The potential splaying of fault segments onshore tends to shift the focus of extension away from the lake. The combination of normal- and oblique-slip faults in the northern basin gives Pyramid Lake its distinctive “fanning open to the north” geometry. The oblique-slip faults in the northwestern region of the lake are short and discontinuous in nature, possibly representing a nascent shear zone. In contrast, the Lake Range fault is long and well defined. Vertical slip rates measured across the Lake Range and other faults provide new estimates on extension across the Pyramid Lake basin. A minimum vertical slip rate of ∼1.0 mm/yr is estimated along the Lake Range fault. When combined with fault length, slip rates yield a potential earthquake magnitude range between M6.4 and M7.0. Little to no offset on the Lake Range fault is observed in the sediment rapidly emplaced at the end of Tioga glaciation (12.5–9.5 ka). In contrast, since 9.5 ka, CHIRP imagery provides evidence for three or four major earthquakes, assuming a characteristic offset of 2.5 m per event. Regionally, our CHIRP investigation helps to reveal how strain is partitioned along the boundary between the northeastern edge of the Walker Lane and the northwest Basin and Range Province proper.

Validating Nevada ShakeZoning Predictions of Las Vegas Basin Response against 1992 Little Skull Mountain Earthquake Records

Over the last two years, the Nevada Seismological Laboratory has devel- oped and refined Nevada ShakeZoning (NSZ) procedures to characterize earthquake hazards in the Intermountain West. Simulating the ML 5.6-5.8 Little Skull Mountain (LSM) earthquake validates the results of the NSZ process and the ground shaking it predicts for Las Vegas Valley (LVV). The NSZ process employs a physics-based finite- difference code from Lawrence Livermore Laboratory to compute wave propagation through complex 3D earth models. Computing limitations restrict the results to low frequencies of shaking. For this LSM regional model the limitation is to frequencies of 0.12 Hz, and below. The Clark County Parcel Map, completed in 2011, is a critical and unique geotechnical data set included in NSZ predictions for LVV. Replacing default geotechnical velocities with the Parcel Map velocities in a sensitivity test produced peak ground velocity amplifications of 5%-11% in places, even at low frequencies of 0.1 Hz. A detailed model of LVV basin-floor depth and regional basin-thickness mod- els derived from gravity surveys by the U.S. Geological Survey are also important components of NSZ velocity-model building. In the NSZ-predicted seismograms at 0.1 Hz, Rayleigh-wave minus P-wave (R − P) differential arrival times and the pulse shapes of Rayleigh waves correlate well with the low-pass filtered LSM recordings. Importantly, peak ground velocities predicted by NSZ matched what was recorded, to be closer than a factor of two. Observed seismograms within LVV show longer du- rations of shaking than the synthetics, appearing as horizontally reverberating, 0.2 Hz longitudinal waves beyond 60 s after Rayleigh-wave arrival. Within the basins, the current velocity models are laterally homogeneous below 300 m depth, leading the 0.1 Hz NSZ synthetics to show insufficient shaking durations of only 30-40 s.

Earthquake Hazard Class Mapping by Parcel in Las Vegas Valley

Clark County, Nevada completed the very first effort in the United States to map earthquake hazard class systematically through an entire urban area. The County and the City of Henderson contracted with the Nevada System of Higher Education to classify about 500 square miles including urban Las Vegas Valley, and exurban areas considered for future development. The Parcel Map includes 10,721 surface-wave array measurements that classify individual parcels on the NEHRP hazard scale. We introduce a C+ Class for sites with Class B average velocities but soft surface soil. The measured Parcel Map shows a clearly definable C+ to C boundary on the west side of the Valley. The C to D boundary is much more complex. Using the parcel map in computing shaking in the Valley for scenario earthquakes is crucial for obtaining realistic predictions of ground motions. Despite affecting only the upper 30 meters, the Vs30 geotechnical shear-velocity from the Parcel Map shows clear effects on 3d shaking predictions computed at frequencies from 0.1 Hz to 1.0 Hz.

Nonlinear optimization for velocities and structures from reflection picks

We present the use of a nonlinear optimization scheme called generalized simulated annealing to invert seismic reflection times for velocities, reflector depths, and lengths. A finite-difference solution of the eikonal equation computes reflection traveltimes through the velocity model and avoids raytracing. We test the optimization scheme on synthetic models and compare it with results from a linearized inversion. The synthetic tests illustrate that, unlike linear inversion schemes, the results obtained by the optimization scheme are independent of the initial model. The annealing method has the ability to produce a suite of models which satisfy the data equally well. We make use of this property to determine the uncertainties associated with the obtained model parameters. Synthetic examples demonstrate that allowing the reflector length to vary, along with its position, helps the optimization process obtain a better solution. This we put to use in imaging the Garlock fault, whose geometry at depth is poorly known. We use reflection times picked from shot gathers recorded along COCORP Mojave Line 5 to invert for the Garlock fault and velocities within the Cantil Basin below Fremont Valley, California. INTRODUCTION Reflection times depend on both the velocity and the depth of the reflectors, making traveltime inversion a nonlinear process. The last decade has seen the development of numerous inversion schemes to obtain velocity and reflector depth from reflection traveltimes. One wa.y is to parametrize both velocity and reflector depth and perform a joint inversion (e.g., Bishop et al., 1985; Stork and Clayton, 1986; Farra and Madariaga., 1988; Williamson, 1990; van Trier, 1990). Most of these met hods involve local linearization of the problem, and require the starting model to be close to the desired final solution. In this paper we examine the use of a nonlinear optimization scheme, namely simulated annealing, to invert reflection traveltimes. We must rapidly compute traveltimes through models by a met hod avoiding ray tracing. It employs a fast finite-difference scheme based on a, solution to the eikonal equation (Vidale, 1988). This accounts for curved rays and all types of primary arrivals, i.e., the fastest arrival, be it a direct arrival or a, diffraction. We test the optimization process on synthetic models and compare its performance with linear inversion schemes that use curved rays. Synthetic examples and optimizations of data demonstrate that the method is not dependent on the choice of the initial model. Another advantage of the simulated-annealing algorithm is that it produces a suite of final models with comparable least-square error. This enables us to choose the model most likely to represent the geology of the region. We also use this property to determine the uncertainties associated with the model parameters obtained. Lastly, we use the scheme to invert reflection times to image the Garlock fault and the velocity within the Cantil Basin below Fremont Valley, California,. The traveline picks are made from the shot records of COCORP Mojave Line 5. Strong lateral velocity variations across the basin make it difficult to image the Garlock fault by standard seismic velocity analysis techniques. This makes it important to use an inversion scheme that will image the velocities with minimum a priori constraints. In addition, the geometry of the Garlock fault at depth is not unequivocally known. The non1inea.r optimization scheme we use can reconstruct velocities, as well as recover estimates of dip or length of the fault. We need not specify the length or dip prior to the inversion. METHODS Simulated annealing avoids local linearization and does not require the calculation of partial derivatives. Ideally? it has the ability to test a series of local minima in search of the global minimum (Kirkpatrick et al., 1983). We make use of a variation of this optimization process called generalized simulated annealing (Bohachevsky et al., 1986). Its basis is a Monte Carlo technique due to Metropolis et al. (1953). The algorithm essentially is a two step process: ( 1) a random guess for the model is made, (2j a decision is made either to accept or reject this new guess. The medium velocity and reflector depth are perturbed simultaneously in order to preserve the nonlinearity of the problem. We perturb the velocity by adding random sized boxes, followed by smoothing with averaging over four adjacent, cells. The boxes can vary between one cell size and the entire model size. To perturb the reflector we add random length lines that are smoothed by averaging adjacent nodes. Again, the added lines can be as small as one grid spacing or as long as the length of the model. The objecive of the inversion is to minimize the least-square error between the observed times and times calculated through the model. New models are accepted when the error is less than that of the previous iteration. Conditional acceptance of models with a larger least-square error helps the inversion escape out of local minima. The generalized annealing schedule is dependent largely on two empirical parameters. They are the initial tern perature and the rate of temperature decrease. These are determined by a process based on the procedure developed by Basu and Frazer (1990). RESULTS We compare the performance of annealing with a linearized inversion scheme. In the linearized inversion. we linearize the nonlinear relationship between traveltime and slowness and the reflector depth. Generally, this linear scheme is cast: in a matrix form and the solution obtained using a standard least-squares technique (Jackson, 1972). We use a singular value decomposition method to do this. We compute the traveltimes by the same method we used in the annealing. To speed up convergence and stabilize the inversion we smooth the velocity perturbations using a Laplacian operator and the reflector depth