Teleseismic long-period P waves recorded at the World-Wide Standard Seismograph Network station LON (Longmire, Washington) are shown to exhibit strong anomalous particle motion not attributable to instrument miscalibration or malfunction. In particular, a large and azimuthally smoothly varying tangential component is observed after vector rotation of horizontal P waves into the ray direction and after application of a deconvolution technique which equalizes effective source time functions and removes the instrument response. These tangential waves attain amplitudes comparable to the radial component and demonstrate wave form antisymmetry about a NNE azimuth. A model which contains a single high-contrast interface dipping toward the NNE at a depth of 15-20 km can explain most of the characteristics of the long-period P wave data, provided dips are greater than about 10 ø and only the interference of P and Ps generated at the interface is considered. The model breaks down for later arrivals which are presumably multiples or scattered waves. Examination of long-period S waves from several deep teleseisms shows a prominent Sp arrival 18 s before S. The timing of this phase conversion suggests an interface at about 145-km depth, and its sense of polarity suggests that the velocity contrast is from higher to lawer velocities as depth decreases. This interface may correspond to the bottom of the upper mantle low-velocity zone in the area.

Group velocity dispersion curves of surface waves extracted from ambient seismic noise are inverted to find 3‐D shear wave structure of the crust beneath eastern North America. The 3‐D model consists of one sediment layer and another six layers with fixed depths at 5, 7.5, 10, 15, 25, and 43 km. Velocities of the seven layers together with the thickness of the sediment layer are determined by the simulated annealing method. We found that almost all failed ancient rifting events (e.g., the Reelfoot rift, Ouachita triple junction, and the Midcontinent rift) and rifting related events (e.g., the Ozark uplift and the Nashville dome) are associated with high‐velocity bodies in the middle and lower crust. Our results also suggest the existence of a triple junction‐like high‐velocity body centered around the New Madrid and the Wabash Valley seismic zones with the Reelfoot rift, the Ozark uplift, and the Nashville dome being on its southwestern, northwestern, and southeastern arms, respectively. We also found that the western limb of the Midcontinent rift (MCR) extends southwestward to western Oklahoma and Texas, and the eastern limb of the MCR extends southeastward into western Ohio. The Appalachian Mountains are characterized by high‐velocity upper crust underlain with relatively low velocity middle and lower crust. All major seismic zones are associated with either divergent or convergent events. The New Madrid seismic zone and Wabash Valley seismic zone are clearly associated with the failed Reelfoot rift. Both the eastern Tennessee seismic zone and Ouachita Orogen seismic zone are located along convergent boundaries.

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I incorporate the spatial gradient of the wave field recorded from onedimensional arrays into a processing method that yields the horizontal-wave slowness and the change of geometrical spreading with distance. In general, the model for seismic-wave propagation is enough to be appropriate for body and surface waves propagating from nearby seismic sources but can be simplified into a plane-wave model. Although computation of the spatial gradient requires that array elements be closer than 10% of the horizontal wavelength, seismic-array apertures, in the usual sense, may extend over many horizontal wavelengths and illuminate changes within the wave field. Array images of horizontal slowness and the relative geometricalspreading changes of seismic waves are derived using filter theory and used to interpret observed array wave fields. Errors in computing finite-difference spatial gradients from array nodes are explicitly considered to avoid spatial aliasing in the estimates. I apply the method to interpret waves in strong ground motion and smallscale refraction data sets. Use of the wave spatial gradient accentuates spatial differences in the wave field that can be theoretically exploited in fine-scale tomographic studies of structure and is complementary to frequency/wavenumber or beamforming array-processing techniques.

Teleseismic receiver functions for structure under Pasadena, California (PAS) are derived from azimuthally distributed teleseismic P waves recorded on Benioff 1–90 instrumentation. The broadband three‐component Benioff 1–90 system is peaked at a 1‐s period and allows resolution of major crustal interfaces from large Ps conversions seen in the receiver function data. The observed body wave data are quite complex, showing exceptionally large Ps conversions and scattered waves on horizontal components. Radial and tangential motions are of equal magnitude and show major off‐azimuth converted Ps waves, suggesting large‐scale crustal heterogeneity beneath the station. Stochastic simulations of one‐dimensional plane layered structure show that geologically unreasonable one‐dimensional models are required to fit the data. The observed coda decay yields a scattering Q estimate of 239 at a 2‐s period using an energy flux model for a propagating plane wave interacting with a scattering layer over a homogeneous half‐space. Observed and synthetic coda decay follows the theoretical exponential decay predicted by the model and is due entirely to diffusion of coda energy out of the layer into the half‐space. PAS coda is compared to coda from deep teleseisms recorded at State College, Pennsylvania, and it is seen that scattering is more severe at PAS, as reflected in higher coda levels and slower decay rate. Consideration of energy partitioning and coda amplitude suggests that much of the coda consists of scattered surface waves. Analysis of a major Ps conversion arriving 3 s after direct P indicates that a major crustal discontinuity at about 20 km depth dips at moderate angles to the north under the San Gabriel Mountains. This interface probably represents the crustal tectonic boundary between the Transverse Ranges and the Los Angeles Basin.