We investigate a new algorithm for computing regularized solutions of the two-dimensional magnetotelluric inverse problem. The algorithm employs a nonlinear conjugate gradients (NLCG) scheme to minimize an objective function that penalizes data residuals and second spatial derivatives of resistivity. We compare this algorithm theoretically and numerically to two previous algorithms for constructing such 'minimum-structure' models: the Gauss-Newton method, which solves a sequence of linearized inverse problems and has been the standard approach to nonlinear inversion in geophysics, and an algorithm due to Mackie and Madden, which solves a sequence of linearized inverse problems incompletely using a (linear) conjugate gradients technique. Numerical experiments involving synthetic and field data indicate that the two algorithms based on conjugate gradients (NLCG and Mackie-Madden) are more efficient than the GaussNewton algorithm in terms of both computer memory requirements and CPU time needed to find accurate solutions to problems of realistic size. This owes largely to the fact that the conjugate gradients-based algorithms avoid two computationally intensive 14-1 Rodi and Mackie tasks that are performed at each step of a Gauss-Newton iteration: calculation of the full Jacobian matrix of the forward modeling operator, and complete solution of a linear system on the model space. The numerical tests also show that the Mackie-Madden algorithm reduces the objective function more quickly than our new NLCG algorithm in the early stages of minimization, but NLCG is more effective in the later computations. To help understand these results, we describe the Mackie-Madden and new NLCG algorithms in detail and couch each as a special case of a more general conjugate gradients scheme for nonlinear inversion.

We use Global Positioning System (GPS) estimates of horizontal site velocity to constrain slip rates on faults comprising the Pacific-North America plate boundary in southern California and northern Mexico. We enlist a simple elastic block model to parameterize the distribution and sum of deformation within and across the plate boundary. We estimate a Pacific-North America relative plate motion rate of 49 _+ 3 mm/yr (one standard deviation), consistent with NUVEL-1A estimates. We are able to resolve robust slip rate estimates for the southernmost San Andreas, San Jacinto, and Elsinore faults (26 _+ 2, 9 +_ 2, and 6 _+ 2 mm/yr, respectively) and for the Imperial and Cerro Prieto faults (35 _+ 2 and 42 _+ 1 mm/yr, respectively), accounting for about 86% of the total plate motion. The remaining 14% appears to be accommodated to the west of these fault systems, probably via slip along the San Clemente fault and/or the San Miguel, Vallecitos, Rose Canyon, and Newport-Inglewood fault systems. These results are highly consistent with paleoseismic estimates for slip rates implying that off-fault strain accumulation within the deforming zone of the plate boundary is largely elastic. We estimate that the seismically quiescent, southernmost San Andreas fault has incurred about 8.2 m of slip deficit over the last few hundred years, presumably to be recovered during a future large earthquake. Introduction Sometime around 30 Ma, the ancestral East Pacific Rise collided with the trench that then separated the North America and Farallon plates [Atwater, 1970] bringing the Pacific and North America plates into contact. Although the details of the early evolution of this boundary are hazy, by about 20 Ma the existence of a right-lateral proto-San Andreas transform near the present-day continental borderland is probable [e.g., Sedlock and Hamilton, 1991]. Eastward migration of the proto-San Andreas transform to its present location and the associated rifting of the Gulf of California appear to be as recent as 5 Ma. The NUVEL-1A global plate motion model [DeMets et al., 1990, 1994] predicts Pacific-North America relative motion across southern California of 46 _+ 1 mm/yr. This estimate represents plate motions averaged over the last 3 Myr. Recently, however, DeMets [1995] compared seafloor spreading rates in the Gulf of California (used in the calculation of the NUVEL-1A rate) with the NUVEL-iA closure fitting rate (CFR) for Pacific-North America relative motion (estimated in the absence of these spreading rate data) and found that the NUVEL-1A model may underestimate the true rate by as much as 8-9%. That is, the true rate of Pacific-North America relative motion may be closer to 50 mm/yr. It is the ongoing motion of these plates that drives the contemporary accumulation of elastic strain within the plate boundary and is therefore ultimately responsible for earthquakes throughout the region. •Now at the Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts. While the San Andreas fault accommodates a significant frac...

This paper develops a finite element method which gives accurate numerical approximations to magnetotelluric data over two-dimensional conductivity structures. The method employs a simple finite element technique to find the field component parallel to the strike of the structure and a new numerical differentiation scheme to find the field component perpendicular to strike. Examples show that the new numerical differentiation scheme is a significant improvement over the standard finite element procedure when meshes of poor quality are used. Algorithms for incorporating the differentiation scheme into the finite element matrix equation and for computing partial derivatives of magnetotelluric data with respect to mesh parameters are derived in order to simplify the computation needed to do the inverse problem.

Several laboratory and scaled model investigations suggest that organic contaminants affect the surface electrical properties of exposed soils/rocks and therefore produce measurable induced polarization (IP) signatures. However, there is little field evidence of an IP methodology for contaminant mapping. A 2D time-domain IP method is developed for mapping the FS-12 contaminant plume at the Massachusetts Military Reservation (MMR) located in Cape Cod, Massachusetts. The FS-12 plume consists of approximately [Formula: see text] of fuel that erupted from a broken underground pipeline in the early 1970s. Benzene and ethylene dibromide (EDB) are the primary contaminants at FS-12, with concentrations exceeding the allowed maximum concentration levels (MCL), while other constituents of the plume did not exceed their MCL. Therefore, the contaminants of interest are benzene and EDB, partly because of their health risk and partly because they present the highest concentrations (2400 and [Formula: see text], respectively) among the plume constituents and are therefore more likely to be related to the polarization source. IP data were acquired along a survey line that partially transects the plume extending over contaminated and uncontaminated zones and were inverted to give 2D resistivity and chargeability plots to [Formula: see text] depth and a horizontal extent of [Formula: see text]. By separately inverting IP data derived from time windows located at short and long decay times, a time-domain gross (spectral) chargeability difference is produced. Both the chargeability and gross spectral chargeability difference show good agreement with the known location of the plume from monitoring wells, with the IP chargeability section suggesting contaminant distribution detail that cannot otherwise be inferred from the sparse borehole distribution.

The paper presents an application of the two-equation k-e model to the problem of three-dimensional free jets issuing from rectangular orifices. The turbulence model has been modified so that plane and round jets may be predicted with the same empirical input. The continuity, momentum, and turbulence equations are solved using the fmite difference procedure of Patankar and Spalding for three-dimensional parabolic flows and results are presented for aspect ratios of 1,5,10, and 20. The decay of axial velocity is well predicted. The behavior of the half-widths, however, is not well predicted when no lateral velocities are specified at the orifice; the measured crossover of jet major and minor axes is not obtained. The possible existence of a lateral velocity field at the orifice cross section is examined and its ability to produce the observed jet inversion is demonstrated. ProfIle shapes in the orifice short-axis direction are in good agreement with measurements, and, when inlet lateral velocities are specified, the long-axis profIles are also predicted fairly well. The measured "saddle-shape" of the profIles in this direction is, however, not obtained; this will require further changes to the turbulence model.

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Coseismic surface deformation associated with the Mw 6.1, April 23, 1992, Joshua Tree earthquake is well represented by estimates of geodetic monument displacements at 20 locations independently derived from Global Positioning System and trilateration measurements. The rms signal to noise ratio for these inferred displacements is 1.8 with near-fault displacement estimates exceeding 40 mm. In order to determine the long-wavelength distribution of slip over the plane of rupture, a Tikhonov regularization operator is applied to these estimates which minimizes stress variability subject to purely right-lateral slip and zero surface slip constraints. The resulting slip distribution yields a geodetic moment estimate of 1.7x10 is N m with corresponding maximum slip around 0.8 m and compares well with independent and complementary information including seismic moment and source time function estimates and main shock and aftershock locations. From empirical Green's function analyses, a rupture duration of 5 s is obtained which implies a rupture radius of 6-8 km. Most of the inferred slip lies to the north of the hypocenter, consistent with northward rupture propagation. Stress drop estimates are in the range of 2-4 MPa. In addition, predicted Coulomb stress increases correlate remarkably well with the distribution of aftershock hypocenters; most of the aftershocks occur in areas for which the mainshock rupture produced stress increases larger than about 0.1 MPa. In contrast, predicted stress changes are near zero at the hypocenter of the Mw 7.3, June 28, 1992, Landers earthquake which nucleated about 20 km beyond the northernmost edge of the Joshua Tree rupture. Based on aftershock migrations and the predicted static stress field, we speculate that redistribution of Joshua Tree-induced stress perturbations played a role in the spatio-temporal development of the earthquake sequence culminating in the Landers event. Introduction The Mw 6.1, April 23, 1992, Joshua Tree, California, earthquake resulted from right-lateral rupture along a previously unmapped north trending late Quaternary fault located about 20 km south of the Pinto Mountain fault and about 10 km northeast of the MissionCreek branch of the San Andreas fault system (Figures 1 and 2). This and other subparallel late Quaternary faults, identified following the earthquake, offset an older northwest trending system [Rymer, 1992]. Seismicity here is characterized by frequent earthquake swarms suggesting that faults in the area are immature [Hauksson et al., 1993]. It is also the location of a sequence of moderate earthquakes occurring between 1940 and 1948 [Richter et al., 1958; Sykes and Seebet, 1985] which included the 1940 M5 5.3 Covington Flat, 1947 M5 5.4 Morongo Valley, and 1948 M5 6.5 Desert Hot Springs earthquakes. The Covington Flat earthquake likely involved rupture along one of these north trending faults, possibly the same fault ruptured during the Joshua Tree earthquake. These north trending faults lie in what is known as the Eastern California Shear Zon...

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