Report of the panel on earth structure and dynamics, section 6

The panel identified problems related to the dynamics of the core and mantle that should be addressed by NASA programs. They include investigating the geodynamo based on observations of the Earth's magnetic field, determining the rheology of the mantle from geodetic observations of post-glacial vertical motions and changes in the gravity field, and determining the coupling between plate motions and mantle flow from geodetic observations of plate deformation. Also emphasized is the importance of support for interdisciplinary research to combine various data sets with models which couple rheology, structure and dynamics

Multiple CMT source analysis of the 2004 Sumatra earthquake

While it is agreed that the great Sumatra earthquake of December 26, 2004 was among the largest earthquakes of the past century, there has been disagreement on how large it was, which part of the fault ruptured, and how the rupture took place. We present a centroid-moment-tensor (CMT) analysis of the earthquake in which multiple point sources are used in the inversion to mimic a propagating slip pulse. The final model consists of five point sources, with the southernmost sources accounting for the majority of the moment release. The presumed fault planes of the southern sources strike northwest, while those in the north strike northeast, consistent with the geometry of the subduction trench. Slip on the fault is found to be more oblique in the north than in the south. The inversion with five sources leads to a moment magnitude for the Sumatra earthquake of M_W = 9.3, consistent with estimates from long-period normal-mode amplitudes

Travel times and station corrections for P waves at teleseismic distances

Approximately 3300 shallow focus earthquakes and 1000 seismic stations have been used in a study of P wave travel times and station residuals, including azimuthal effects. The events were selected from a catalog containing 160,000 earthquakes, and those having uniform distance and azimuthal coverage were systematically relocated and used to refine P wave travel times and station corrections. Station corrections are provided for 994 seismic stations. The station corrections involve three terms: the static effect and two cosine terms with appropriate phase shifts. They exhibit general consistency over broad geographic areas and, where coverage is dense, often show abrupt changes from one geological province to another. The cos 2θ terms appear to be due to upper mantle anisotropy, and they correlate with the stress direction in the crust

Upper mantle anisotropy: evidence from free oscillations

Isotropic earth models are unable to provide uniform fits to the gross Earth normal mode data set or, in many cases, to regional Love-and Rayleigh-wave data. Anisotropic inversion provides a good fit to the data and indicates that the upper 200km of the mantle is anisotropic. The nature and magnitude of the required anisotropy, moreover, is similar to that found in body wave studies and in studies of ultramafic samples from the upper mantle. Pronounced upper mantle low-velocity zones are characteristic of models resulting from isotropic inversion of global or regional data sets. Anisotropic models have more nearly constant velocities in the upper mantle. Normal mode partial (Frediét) derivatives are calculated for a transversely isotropic earth model with a radial axis of symmetry. For this type of anisotropy there are five elastic constant. The two shear-type moduli can be determined from the toroidal modes. Spheroidal and Rayleigh modes are sensitive to all five elastic constants but are mainly controlled by the two compressional-type moduli, one of the shear-type moduli and the remaining, mixed-mode, modulus. The lack of sensitivity of Rayleigh waves to compressional wave velocities is a characteristic only of the isotropic case. The partial derivatives of the horizontal and vertical components of the compressional velocity are nearly equal and opposite in the region of the mantle where the shear velocity sensitivity is the greatest. The net compressional wave partial derivative, at depth, is therefore very small for isotropic perturbations. Compressional wave anisotropy, however, has a significant effect on Rayleigh-wave dispersion. Once it has been established that transverse anisotropy is important it is necessary to invert for all five elastic constants. If the azimuthal effect has not been averaged out a more general anisotropy may have to be allowed for

Presentation of the Arthur L. Day Medal to Don L. Anderson

I am honored to present to you Dr. Don L. Anderson, this year's recipient of the Arthur L. Day Medal. Note the immediate link between the two: the reversed initials

Seismic Tomography of the Earth's Interior

The concept of plate tectonics, introduced nearly twenty years ago, unified many fields of earth science by providing a global framework for large-scale geological processes. Seismology played an important role in the formulation of this theory by supplying maps of global seismicity, delineating plate boundaries, producing evidence for deep subduction of the lithosphere, and determining the direction and nature of forces acting at plate boundaries

Preliminary reference Earth model

A large data set consisting of about 1000 normal mode periods, 500 summary travel time observations, 100 normal mode Q values, mass and moment of inertia have been inverted to obtain the radial distribution of elastic properties, Q values and density in the Earth's interior. The data set was supplemented with a special study of 12 years of ISC phase data which yielded an additional 1.75 × 10^6 travel time observations for P and S waves. In order to obtain satisfactory agreement with the entire data set we were required to take into account anelastic dispersion. The introduction of transverse isotropy into the outer 220 km of the mantle was required in order to satisfy the shorter period fundamental toroidal and spheroidal modes. This anisotropy also improved the fit of the larger data set. The horizontal and vertical velocities in the upper mantle differ by 2–4%, both for P and S waves. The mantle below 220 km is not required to be anisotropic. Mantle Rayleigh waves are surprisingly sensitive to compressional velocity in the upper mantle. High S_n velocities, low P_n velocities and a pronounced low-velocity zone are features of most global inversion models that are suppressed when anisotropy is allowed for in the inversion. The Preliminary Reference Earth Model, PREM, and auxiliary tables showing fits to the data are presented

Structure, elastic and rheological properties and density of the earth's interior, gravity and pressure

The elastic and anelastic properties of the earth are determined from the travel times and amplitudes of body waves and the periods and decay of normal modes. The periods of the normal modes also depend on density. The mass and moment of inertia provide additional constraints on the radial variation of density. The pressure and gravity as a function of radius are determined from the density by integration. In the presence of anelastic dispersion the seismic velocities and elastic moduli are frequency dependent. For seismic data which are sensitive to the upper 200 km of the mantle a better fit is achieved by assuming that this region is anisotropic. The viscosity of the mantle can be estimated from postglacial rebound and other considerations but is known only approximately and the effect of stress is uncertain. This chapter gives tables for the radial variation of the elastic constants, density, and seismic quality factor Q. Pressure and gravity throughout the earth are also tabulated. Q is the best known anelastic property of the earth but its variation with frequency is uncertain. In the present model the quality factors are assumed to be independent of frequency

Seismic Tomography

The outer layer of the earth, the lithosphere, consists of a dozen rigid, crust-bearing plates that ride on the underlying mantle, rearranging continents, forming mountains, creating and destroying oceans. What drives this constant remodeling? Ultimately the motive force is the convective circulation of the mantle