High‐resolution, two‐dimensional vertical tomography of the central Pacific mantle using ScS reverberations and frequency‐dependent travel times

Abstract

Abstract. A vertical tomogram is constructed for the mantle between Tonga and Hawaii. Using a complete Gaussian-Bayesian approach, we inverted a data set comprising 304 ScS reverberation travel times with Fr6chet kernels computed by the paraxial ray approximation and 1122 frequencydependent phase delays of turning and surface waves with Fr6chet kernels calculated by a normalmode coupling algorithm. The model parameters include shear speed variations, perturbations to shear wave radial anisotropy in the uppermost mantle, and the topographies of the 410-and 660-km discontinuities. Tests demonstrate that the data set resolves lateral structures in the upper mantle with scale lengths of several hundred kilometers and greater. The most significant feature in our model is a well resolved regular pattern of high and low shear velocities in the upper mantle. These variations have a horizontal wavelength of 1500 km, a vertical dimension of 700 km, and an amplitude of about 3%, and they show a strong positive correlation with seafloor topography and geoid height variations. The geoid highs correspond to a series of northwest trending swells associated with the major hotspots of the Society, Marquesas, and Hawaiian Islands. Where these swells cross the corridor, they are underlain by high shear velocities throughout the uppermost mantle, so it is unlikely that their topographies are supported by thermal buoyancy. Although chemical buoyancy generated by basaltic volcanism and the formation of its low-density peridotitic residuum can explain the positive correlation between topography and velocity variation at the shallow depth, an additional mechanism is needed to account for the shear velocity pattern at depths greater than 200 km. It is therefore hypothesized that the central Pacific is underlain by a system of Richter-type convective rolls that are oriented subparallel to the absolute plate velocity, have unit aspect ratio transverse to this orientation, and are confined above the 660-kin discontinuity. This convection pattern appears to be strongly correlated with the locations of the Tahitian, Marquesan, and Hawaiian hotspots, which raises interesting questions for Morgan's hypothesis that these hotspots are the surface manifestations of deep mantle plumes.

Upper mantle velocity structure in the western Pacific rim estimated from short-period recordings at Matsushiro Seismic Array System

“…6(b), (c)). Slow upper mantle for the Philippine Sea is consistent with the findings in the low-frequency seismology (Katzman et al, 1998;Kato and Jordan, 1999) as well as global tomographic models with high-frequency phase delays (e.g., Fukao et al, 1992). High velocities in the transition zone beneath the Philippine Sea have been reported in the one-dimensional model as well as in three-dimensional tomographic models (Suetsugu and Nakanishi, 1987;van der Hilst et al, 1991;Fukao et al, 1992).…”

New evidence for dislocation creep from 3-D geodynamic modeling of the Pacific upper mantle structure

“…3) The TBI provides an explanation for seismically observed small-scale structures (i.e., smaller than the plate scale) in the upper mantle [45]. We also found that the TBI structure in the upper mantle from non-Newtonian rheology is rather irregular and is significantly different from the roll structure reported in early studies with uniform viscosity structure [36].…”

Full‐3‐D tomography for crustal structure in Southern California based on the scattering‐integral and the adjoint‐wavefield methods

“…On the 2 km depth S wave velocity map (Figure 3a), the most significant low-velocity anomaly is associated with the Southern San Joaquin Basin (SSJB) [Bailey et al, 1970;Ingersoll, 1982], which is mostly missing in both CVM-S4 and CVM-H11.9 but well imaged in our model. To the west of the SSJB, our model shows low-velocity anomalies associated with the Santa Maria Basin [e.g., Hall, 1991], the Cuyama Basin [e.g., Davis et al, 1988], and an intermediate-to high-velocity anomaly associated with surface exposures of the Salinian basement rocks [e.g., Jennings, 1977;Ducea et al, 2009]. To the east of the SSJB, our model shows a high-velocity belt in the western Sierra Nevada foothills, which is correlated with magnetic and gravity anomalies ( Figure 3b).…”

“…F3DT accounts for the nonlinearity of the structural inverse problem through iterated cycles of forward simulation, data measurement, kernel calculation, and inversion. iterations using two complementary methods for F3DT inversion: the adjoint-wavefield (AW) technique [Tarantola, 1984[Tarantola, , 1988Pratt, 1990;Pratt et al, 1998;Akcelik et al, 2002Akcelik et al, , 2003Tromp et al, 2005;Liu and Tromp, 2006] and the scattering-integral (SI) technique Katzman et al, 1998;Zhao et al, 2005Zhao et al, , 2006Chen et al, 2007aChen et al, , 2007b.…”