Three-dimensional wavefield imaging of data from the USArray: New constraints on the geometry of the Farallon slab

Abstract

This paper presents preliminary results from direct imaging of P to S conversion data from the Earthscope Transportable Array (TA). Input data are receiver function estimates from the Earthscope Automated Receiver Function Survey (EARS). The waveforms from EARS were first stacked to produce composite events from 60 different source regions defined by a radial grid with a center in the western United States (U.S.). The composite waveforms were imaged with a three-dimensional, prestack migration method using a plane wave decomposition and an inverse generalized Radon transform algorithm to yield estimates of radial and transverse component scattering potential from each composite event. Data deficiencies limited this processing to 50 of the composite events. The migrated results from these 50 events were stacked to produce a final threedimensional image volume examined in this paper through three-dimensional visualization techniques. The crust-mantle boundary, the 410 km, and 660 km discontinuities are imaged. An erratic negative conversion at the depth that has been suggested as the seismic lithosphere-asthenosphere boundary is observed in these results, but I avoid interpreting this feature as it may be biased by interference with first-order, free-surface multiples from the Moho. The most significant new feature imaged by this technique is a series of east-dipping conversions directly above the 410 km discontinuity. This feature is imaged in the region immediately above the transition zone across the entire western U.S., although it becomes partly obscured in Arizona by interference with what I interpret as sediment reverberations that contaminate some stations in southern California and the Rio Grande Rift. Overlaying these results with published tomography models suggests that this feature is linked to the Juan de Fuca-Farallon slab. I follow this hypothesis to build a three-dimensional model of the top of the Farallon slab that honors the new data and results from published tomography models. The model clarifies that these dipping features are consistent with a bending plate model of the slab. Using a simple kinematic model of the geometry of the slab window linked to the widening of the San Andreas system, I argue that the same dipping features seen under Nevada are consistent with those seen to the north, suggesting that there is a continuous, relatively smooth slab-related boundary layer under most of the western U.S.

Imaging Upper‐Mantle Structure Under USArray Using Long‐Period Reflection Seismology

“…Tomography models differ in their details, including the position of the high‐velocity upper‐mantle anomalies in the WUS (e.g., Burdick et al, ; James et al, ; Obrebski et al, , ), and we do not always image reflectors at the top of fast “blobs” in the models, including those seen in other cross‐sections through the Schmandt and Lin () model. Indeed, our slab does not agree in position with the inferred slab location in the Pavlis () migrated receiver‐function study or the slab position in the tomography/receiver‐function synthesis of Pavlis et al (). In addition, our images are unmigrated, and dipping reflectors may not be imaged in the correct position using CRP stacking, even without additional horizontal smoothing.…”

“…Our common‐conversion‐point (CRP) approach for teleseismic SH reverberation analysis is simple to implement and appears to give robust results for the USArray data set. However, our method likely could be improved in a number of ways: Bootstrap resampling of the waveforms would provide a measure of the statistical significance of any observed reflections and could be used to replace our requirement of at least 200 seismograms to display the result. In principle, migration approaches, such as those described by Pavlis (), Shang et al (), Shragge et al (), and Burdick et al () for receiver functions and/or free‐surface multiples, could be applied to better image dipping structures. However, migration methods work best with uniform data coverage, so the very uneven distribution of earthquake sources may present challenges. More comprehensive testing of corrections for 3‐D velocity structure would help to better understand their sensitivity to specific tomography models and how they can change the coherence of the image.…”

Detailed crustal thickness variations beneath the Illinois Basin area: Implications for crustal evolution of the midcontinent

“…Epicenters of the teleseismic events in (a–c) OIINK data set and in (d–f) CUS data set, colored by source depths. Figures 3a and 3d show the events in the raw data sets, Figures 3b and 3e are the events after removing low‐quality data through quality control using the method of Yang et al [], and Figures 3c and 3f are the composite earthquakes after applying source‐side stacking following Pavlis [].…”

Roughness of the mantle transition zone discontinuities revealed by high‐resolution wavefield imaging

“…Details on the mathematical and computational background for this approach can be found in Poppeliers and Pavlis [, ] and Pavlis []. Our data processing flow is similar to that of Pavlis []. Like most migration methods in exploration seismic imaging, PWMIG also requires a known velocity model.…”

“…Our results are possible because of the deployment of large arrays of broadband seismometers and a new class of seismic imaging techniques, which we will refer to collectively as scattered wave imaging [ Kind et al ., ; Dueker and Sheehan , , ; Bostock et al ., ; Ravi Kumar et al ., ; Poppeliers and Pavlis , , ; Kumar et al ., , , ; Levander and Nolet , ; Angus et al ., ; Pavlis , , ]. This technology was originally applied to image d 410 and d 660 using P to S scattered wavefield recorded by local or regional arrays [ Dueker and Sheehan , ; Gao et al ., ; Blum and Shen , ; Ai et al ., ; Lee et al ., ; Mohamed et al ., ; Cottaar and Deuss , ].…”