Shawn Larsen

Space geodetic measurement of crustal deformation in central and southern California, 1984-1992

A laboratory type of analyzer for quantitatively determining the percent third element content of a hydrocarbon sample. A unique rhodium/americium radioactive source is disclosed.

Model for Basin Effects on Long-Period Response Spectra in Southern California

We propose a model for the effect of sedimentary basin depth on long-period response spectra. The model is based on the analysis of 3-D numerical simulations (finite element and finite difference) of long-period (2–10 s) ground motions for a suite of sixty scenario earthquakes (Mw 6.3 to Mw 7.1) within the Los Angeles basin region. We find depth to the 1.5 km/s S-wave velocity isosurface to be a suitable predictor variable, and also present alternative versions of the model based on depths to the 1.0 and 2.5 km/s isosurfaces. The resulting mean basin-depth effect is period dependent, and both smoother (as a function of period and depth) and higher in amplitude than predictions from local 1-D models. The main requirement for the use of the results in construction of attenuation relationships is determining the extent to which the basin effect, as defined and quantified in this study, is already accounted for implicitly in existing attenuation relationships, through (1) departures of the average “rock” site from our idealized reference model, and (2) correlation of basin depth with other predictor variables (such as Vs30).

Marmousi-2: An Updated Model for the Investigation of AVO in Structurally Complex Areas

We have created an elastic version of the IFF Marmousi model for use in AVO analysis in the presence of complex structure. The model is larger, includes larger offsets, lies in deeper water, includes surface streamer, multicomponent OBC and VSP acquisition, and contains more hydrocarbons than its predecessor. In addition to AVO analysis, we believe these data will be suitable for calibrating emerging technologies including converted wave tomography and vector seismic processing.

Global positioning system measurements of deformations associated with the 1987 Superstition Hills Earthquake: Evidence for conjugate faulting

Large station displacements observed from Imperial Valley global positioning system (GPS) campaigns are attributed to the November 24, 1987, Superstition Hills earthquake sequence. Thirty sites from a 42-station GPS network established in 1986 have been reoccupied during 1988 and/or 1990. Displacements at three sites within 3 km of the surface rupture approach 0.5 m. Eight additional stations within 20 km of the seismic zone are displaced at least 10 cm. This is the first occurrence of a large earthquake (MS 6.6) within a preexisting GPS network. Best-fitting uniform slip models of rectangular dislocations in an elastic half-space indicate 130 ± 8 cm right-lateral displacement along the northwest-trending Superstition Hills fault and 30 ± 10 cm left-lateral displacement along the conjugate northeast-trending Elmore Ranch fault. The geodetic moments are 9.4 × 1025 dyn cm and 2.3 × 1025 dyn cm for the Superstition Hills and Elmore Ranch faults, respectively, consistent with teleseismic source parameters. The data also suggest that postseismic slip along the Superstition Hills fault is concentrated at shallow depths. Distributed slip solutions using singular value decomposition indicate near uniform displacement along the Elmore Ranch fault and concentrated slip to the northwest and southeast along the Superstition Hills fault. A significant component of nonseismic secular displacement is observed across the Imperial Valley, which is attributed to interseismic plate-boundary deformation.

Bayesian Inference and Markov Chain Monte Carlo Sampling to Reconstruct a Contaminant Source on a Continental Scale

Abstract A methodology combining Bayesian inference with Markov chain Monte Carlo (MCMC) sampling is applied to a real accidental radioactive release that occurred on a continental scale at the end of May 1998 near Algeciras, Spain. The source parameters (i.e., source location and strength) are reconstructed from a limited set of measurements of the release. Annealing and adaptive procedures are implemented to ensure a robust and effective parameter-space exploration. The simulation setup is similar to an emergency response scenario, with the simplifying assumptions that the source geometry and release time are known. The Bayesian stochastic algorithm provides likely source locations within 100 km from the true source, after exploring a domain covering an area of approximately 1800 km × 3600 km. The source strength is reconstructed with a distribution of values of the same order of magnitude as the upper end of the range reported by the Spanish Nuclear Security Agency. By running the Bayesian MCMC algorit...

Ground-Motion Modeling of the 1906 San Francisco Earthquake, Part I: Validation Using the 1989 Loma Prieta Earthquake

We compute ground motions for the Beroza (1991) and Wald et al. (1991) source models of the 1989 magnitude 6.9 Loma Prieta earthquake using four different wave-propagation codes and recently developed 3D geologic and seismic velocity models. In preparation for modeling the 1906 San Francisco earthquake, we use this well-recorded earthquake to characterize how well our ground-motion simulations reproduce the observed shaking intensities and amplitude and durations of recorded motions throughout the San Francisco Bay Area. All of the simulations generate ground motions consistent with the large-scale spatial variations in shaking associated with rupture directivity and the geologic structure. We attribute the small variations among the synthetics to the minimum shear-wave speed permitted in the simulations and how they accommodate topography. Our long-period simulations, on average, under predict shaking intensities by about one-half modified Mercalli inten- sity (MMI) units (25%-35% in peak velocity), while our broadband simulations, on average, under predict the shaking intensities by one-fourth MMI units (16% in peak velocity). Discrepancies with observations arise due to errors in the source models and geologic structure. The consistency in the synthetic waveforms across the wave- propagation codes for a given source model suggests the uncertainty in the source parameters tends to exceed the uncertainty in the seismic velocity structure. In agree- ment with earlier studies, we find that a source model with slip more evenly distributed northwest and southeast of the hypocenter would be preferable to both the Beroza and Wald source models. Although the new 3D seismic velocity model improves upon previous velocity models, we identify two areas needing improvement. Nevertheless, we find that the seismic velocity model and the wave-propagation codes are suitable for modeling the 1906 earthquake and scenario events in the San Francisco Bay Area. Online Material: Modified Mercalli intensities and velocity waveforms, and a movie of simulated wave propagation.

Age constraints for the present fault configuration in the Imperial Valley, California: Evidence for northwestward propagation of the Gulf of California Rift System

Releveling and other geophysical data for the Imperial Valley of southern California suggest the northern section of the Imperial-Brawley fault system, which includes the Mesquite Basin and Brawley Seismic Zone, is much younger than the age of the valley itself. A minimum age of 3000 years is calculated for the northern segment of the Imperial fault from correlations between surface topography and geodetically observed seismic/interseismic vertical movements. Calculation of a maximum age of 100,000 years is based upon displacements in the crystalline basement along the Imperial fault, inferred from seismic refraction surveys. This young age supports recent interpretations of heat flow measurements and the evolution of geothermal systems, which also suggest that the current patterns of seismicity and faulting in the Imperial Valley are not long lived. The current fault geometry and basement morphology suggest a northwestward growth of the Imperial fault and a northwestward migration of the Brawley Seismic Zone. If this localized process is representative of more regional tectonic processes along the extent of the Salton Trough, we suggest that this migration is a manifestation of the propagation of the Gulf of California rift system into the North American continent.

3-D Elastic Numerical Modeling of a Complex Salt Structure

Summary Reliably processing, imaging, and interpreting seismic data from areas with complicated structures, such as sub-salt, requires a thorough understanding of elastic as well as acoustic wave propagation. Elastic numerical modeling is an essential tool to develop that understanding. While 2-D elastic modeling is in common use, 3-D elastic modeling has been too computationally intensive to be used routinely. Recent advances in computing hardware, including commodity-based hardware, have substantially reduced computing costs. These advances are making 3-D elastic numerical modeling more feasible. A series of example 3-D elastic calculations were performed using a complicated structure, the SEG/EAGE salt structure. The synthetic traces show that the effects of shear wave propagation can be important for imaging and interpretation of images, and also for AVO and other applications that rely on trace amplitudes. Additional calculations are needed to better identify and understand the complex wave propagation effects produced in complicated structures, such as the SEG/EAGE salt struc

Global positioning system measurements of strain accumulation across the Imperial Valley, California: 1986–1989

Global Positioning System (GPS) data collected in southern California from 1986 to 1989 indicate considerable strain accumulation across the Imperial Valley. Displacements are computed at 29 stations in and near the valley from 1986 to 1988, and at 11 sites from 1988 to 1989. The earlier measurements indicate 5.9 ±1.0 cm/yr right-lateral differential velocity across the valley, although the data are heavily influenced by the 1987 Superstition Hills earthquake sequence. The 1988–1989 GPS displacements are best modeled by 5.0±0.9 cm/yr of valley crossing deformation, but rates calculated from 15 years of terrestrial geodetic measurements (3.7 ± 0.1 cm/yr) fit the data nearly as weh1. The relatively high GPS rates are suspect because some measurements, especially the east-trending displacements, have large errors. There is evidence from GPS and very long baseline interferometry observations that the present strain rate along the southern San Andreas fault is smaller than the long-term geologic estimate, suggesting a lower earthquake potential than is currently assumed. Correspondingly, a higher earthquake potential is suggested for the San Jacinto fault.

Basin Structure Influences on the Propagation of Teleseismic Waves in the Santa Clara Valley, California

We have investigated ground-motion amplification in the Santa Clara Valley (scv) using teleseismic P waves observed during the 1998 deployment of 41 short-period seismometers. The Santa Clara Valley Seismic Experiment (scvse) (Lindh et al. , 1999; Fletcher et al. , 2003) recorded many local and regional earthquakes and seven large ( M w > 6.4) teleseisms. Measured teleseismic P -wave arrival-time delays, relative P -wave amplification, and P -wave energy were used in the analysis. The relative P -wave amplification is found to correlate strongly with the arrival-time delays. In addition, the P -wave energy is found to correlate with the observed teleseismic delays. We also compared observed P -wave arrival-time delays and P -wave amplification with synthetics computed by using 3D finite-difference simulations of the teleseismic wave field to model these parameters using both the University of California, Berkeley (ucb) (Stidham et al. , 1999; Stidham, 1999) and the U.S. Geological Survey (usgs) (Brocher et al. , 1997; Jachens et al. , 1997) 3D velocity models. The results indicate that arrival-time delays on the order of ±0.25 sec correlate strongly with the reported basin depths in the two models. We find that the correlation between the arrival-time delays and basin depth is strongest for the usgs model. However, the ucb velocity model yields wave amplification that better matches the data. The finite-difference simulations indicate that, in general, the observations may be reproduced by either of the 3D velocity models, although refinements to the proposed 3D structure for the scv are needed.