Stephane Rondenay

Multiparameter two‐dimensional inversion of scattered teleseismic body waves 1. Theory for oblique incidence

This is the first paper in a three-part series that examines formal inversion of the teleseismic P wave coda for discontinuous variations in elastic properties beneath dense, three-component, seismic arrays. In this paper, we develop the theoretical framework for a migration method that draws upon the tenets of inverse scattering theory and is amenable to practical implementation. The forward problem is formulated for two-dimensional (2-D) heterogeneity in observance of formal sampling requirements and currently accessible instrumentation. A ray theoretic Greens function, corresponding to a line source with axial component of forcing, is employed within the 2-D Born approximation to accommodate planar, incident wave fields at arbitrary back azimuths. Both the forward scattered response generated by the upgoing incident wave field and the backscattered response created by its reflection at the free surface are included within the formulation. In accordance with the high-frequency and single-scattering approximations employed in the forward problem the inverse problem is cast as a generalized Radon transform. The resulting back projection operator is well suited to the teleseismic context in several respects. It is tolerant of irregularities in array geometry and source distribution and allows a full complement of global seismicity to be utilized through its accommodation of oblique incidence. By permitting both independent and simultaneous treatment of different scattering modes (reflections, transmissions, conversions) the inversion formula facilitates a direct appraisal of individual mode contributions to the recovery of structure. In particular, it becomes evident that incorporation of backscattered modes leads to (1) a better localization of structure than possible using forward scattered energy and (2) the imposition of complementary constraints on elastic properties.

Multiparameter two‐dimensional inversion of scattered teleseismic body waves 3. Application to the Cascadia 1993 data set

This is the third paper in a three-part series that examines formal inversion of the teleseismic P wave coda for discontinuous, two-dimensional (2-D) variations in elastic properties beneath dense arrays of three-component, broadband seismometers. In this paper, the method is applied to data from the Incorporated Research Institutions for Seismology-Program for Array Seismic Studies of the Continental Lithosphere (IRIS-PASSCAL) Cascadia 1993 experiment undertaken across central Oregon. Two major features are imaged in the resulting model. The continental Moho becomes evident � 150 km from the coast beneath the Western Cascades and extends through the eastern end of the profile at 35-40 km depth. In the western portion of the model, oceanic crust of the subducting Juan de Fuca plate dips shallowly (12� ) at the coast and more steeply (27� ) below the Willamette Valley and is evident to depths of >100 km beneath the High Cascades. The abrupt increase in plate dip at � 40 km depth coincides with an apparent thickening of the oceanic crust followed by a diminution in its signature. Building on previous work, we argue that these results are consistent with the consequences of prograde metamorphic reactions occurring within the oceanic crust. Progressive dehydration at lower-grade facies conditions culminates in the transformation to eclogite, producing a pronounced increase in the seismic velocity, density and dip of the subducting plate, and structural complexity in the overlying wedge.

Multiparameter two‐dimensional inversion of scattered teleseismic body waves 2. Numerical examples

In this paper, we investigate the formal inversion of synthetic teleseismic P coda waves for subsurface elastic properties using a ray theoretic approach which assumes single scattering [Bostock et al., this issue]. We consider a model comprising an idealized lithospheric suture zone whose geometrical configuration is drawn from previous deep crustal seismic studies. Two-dimensional, pseudospectral synthetic seismograms representing plane waves propagating through this model are preprocessed to extract an estimate of the scattered wave field generated by short-wavelength structure. These data are employed in a series of numerical simulations which examine the dependence of multiparameter inversion results on a range of input parameters. In particular, we demonstrate (1) the contrasting sensitivity which forward and backscattered waves display to structural recovery, (2) the diminution of the problem null-space accompanied by increased source coverage, (3) improvements in model reconstruction achieved through simultaneous treatment of multiple scattering modes, and (4) the robustness of the method for data sets with noise levels and receiver geometries that approach those of field experiments.

Constraints on localized core-mantle boundary structure from multichannel, broadband SKS coda analysis

[1]xa0In recent years, a wide range of geophysical results have offered evidence that Earths lowermost mantle is characterized by strong lateral variations in material properties. Among the structures of particular interest are intermittent ultralow-velocity zones (ULVZs), located directly above the core-mantle boundary (CMB), which were originally inferred from the distortion of teleseismic SPdKS phases. ULVZs have been modeled as layers with sharp boundaries and seismic velocity reductions ≥10% and interpreted as regions of partial melt. In this study, we further constrain local ULVZ structure beneath North America by signal processing and waveform modeling of the SKS coda recorded at broadband seismic arrays. Secondary phases in the SKS coda are effectively isolated by eigenimage processing. Residual (i.e., SKS-less) data sections from various western Pacific events display clear SPdKS arrivals, followed by a secondary phase whose timing and slowness are consistent with CMB origins. One-dimensional modeling of these phases by reflectivity and generalized ray synthetics favors an asymmetric model, with ULVZ present at only one of the CMB intercepts. The preferred ULVZ is characterized by reductions in P and S velocities of 18% and 50%, respectively, and a diffuse upper boundary. These characteristics are consistent with local production and gravitational sinking of dense (e.g., iron rich) partial melt above the CMB. We postulate that a gradational ULVZ beneath North America may mark a lateral transition domain between regions of mantle upwelling, where more uniform ULVZs exist, and regions of downwelling, where ULVZs are either nonexistent or imperceptibly thin.