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The inverse problem for Earth structure from normal mode data is strongly non-linear and can be inherently non-unique. Traditionally, the inversion is linearized by taking partial derivatives of the complex spectra with respect to the model parameters (i.e. structure coefficients), and solved in an iterative fashion. This method requires that the earthquake source model is known. However, the release of energy in large earthquakes used for the analysis of Earth's normal modes is not simple. A point source approximation is often inadequate, and a more complete account of energy release at the source is required. In addition, many earthquakes are required for the solution to be insensitive to the initial constraints and regularization. In contrast to an iterative approach, the autoregressive linear inversion technique conveniently avoids the need for earthquake source parameters, but it also requires a number of events to achieve full convergence when a single event does not excite all singlets well. To build on previous improvements, we develop a technique to estimate structure coefficients (and consequently, the splitting functions) using a derivative-free parameter search, known as neighbourhood algorithm (NA).We implement an efficient forward method derived using the autoregresssion of receiver strips, and this allows us to search over a multiplicity of structure coefficients in a relatively short time. After demonstrating feasibility of the use of NA in synthetic cases, we apply it to observations of the inner core sensitive mode 13S2. The splitting function of this mode is dominated by spherical harmonic degree 2 axisymmetric structure and is consistent with the results obtained from the autoregressive linear inversion. The sensitivity analysis of multiple events confirms the importance of the Bolivia, 1994 earthquake. When this event is used in the analysis, as little as two events are sufficient to constrain the splitting functions of 13S2 mode. Apart from not requiring the knowledge of earthquake source, the newly developed technique provides an approximate uncertainty measure of the structure coefficients and allows us to control the type of structure solved for, for example to establish if elastic structure is sufficient. [ABSTRACT FROM AUTHOR]

Dickman argues that our model of the Mf ocean tide is unrealistic and that this invalidates conclusions regarding mantle anelasticity. His evidence is based on comparison with his own ocean modelling experiments and on his physical intuition regarding near-equilibrium tides. That evidence is unconvincing. Simple physical arguments alone are enough to rebut his main points. Furthermore, we test here his suggestion of increasing bottom-friction dissipation by a factor of 100 over our preferred solution, and we show that this results in unacceptable polar motion and poor agreement with independent tide-gauge data. In contrast, tests against independent data lend support to the realism of our Mf tide model and give confidence that our estimates of mantle anelasticity are reliable within stated error bounds. [ABSTRACT FROM AUTHOR]

The article discusses geophysics research by N. Schmerr and colleagues within the issue. The author presents a brief overview of the theory of plate tectonics which posits that horizontal plates below the Pacific Ocean are moving laterally at a rate of approximately 10 centimeters (cm) per year above a weak layer called the asthenosphere. He notes that scientists are puzzled by how the plates, which are as large as 10 thousand kilometers (km), can maintain their rigidity while causing earthquakes at their edges. Topics include the use of high-resolution seismic maps by Schmerr's team to observe the process of plate tectonics, the role of the asthenosphere, which is a viscous layer below the lithosphere, in plate movement, and a diagram illustrating plate tectonics in the ocean.

The article discusses a series of studies carried out at the Institute of Dynamics of Geospheres of the Russian Academy of Sciences during the past 10 years. These studies concern the problem of the early Earth starting from the Earth's formation, its primordial state and subsequent evolution. It outlines a general picture for the past 300 years, passing to the 1950-1960s when the problem of the early Earth started to be systematically investigated.

The permeability of the upper mantle controls melt segregation beneath spreading centers. Reconciling contradictory geochemical and geophysical observations at ocean ridges requires a better understanding of transport properties in partially molten rocks. Using x-ray synchrotron microtomography, we obtained three-dimensional data on melt distribution for mantle peridotite with various melt fractions. At melt fractions as low as 0.02, triple junctions along grain edges dominated the melt network; there was no evidence of an abrupt change in the fundamental character of melt extraction as melt fraction increased to 0.2. The porosity of the partially molten region beneath ocean ridges is therefore controlled by a balance between viscous compaction and melting rate, not by a change in melt topology. [ABSTRACT FROM AUTHOR]

Cyclic sedimentary deposits link stratigraphic sequences that are now geographically distant but were once part of the same depositional environment. Some of these sequences occur at periods of 2 to 20 million years, and eustatic sea-level variations or regional tectonic events are likely causes of their formation. Using numerical modeling, we demonstrate that small-scale mantle convection can also cause the development of stratigraphic sequences through recurrent local and regional vertical surface movements. Small-scale convection-driven stratigraphic sequences occur at periods of 2 to 20 million years and correlate only at distances up to a few hundred kilometers. These results suggest that previous sequence stratigraphic analyses may contain erroneous conclusions regarding eustatic sea-level variations. [ABSTRACT FROM AUTHOR]

Bends in volcanic hotspot lineaments, best represented by the large elbow in the Hawaiian- Emperor chain, were thought to directly record changes in plate motion. Several lines of geophysical inquiry now suggest that a change in the locus of upwelling in the mantle induced by mantle dynamics causes bends in hotspot tracks. Inverse modeling suggests that although deep flow near the core-mantle boundary may have played a role in the Hawaiian-Emperor bend, capture of a plume by a ridge, followed by changes in sub-Pacific mantle flow, can better explain the observations. Thus, hotspot tracks can reveal patterns of past mantle circulation. [ABSTRACT FROM AUTHOR]

We use teleseismic body waveforms to explore S-wave layered velocity structures beneath 30 portable digital seismic stations deployed around western Yunnan Province. Results show that the Moho depth in this region is ∼40 km and decreases in general from north to south, consistent with previous geophysical studies. Associated with this lateral variation of the Moho depth, the lower crust above the Moho discontinuity has a 15–25 km thick zone with an S-wave velocity lower than that of the upper crust. This lower velocity zone might be interpreted as a lower crust weak channel, which may mechanically partially decouple the upper-crust deformation from the underlying mantle. Thus, the inverted S-wave velocity structure could provide new evidence for the lateral flow of lower crust in the build-up of the south-eastern Tibetan plateau. [ABSTRACT FROM AUTHOR]

We use numerical dynamos to investigate the possible role of magnetic diffusion at the top of the core. We find that the contribution of radial magnetic diffusion to the secular variation is correlated with that of tangential magnetic diffusion for a wide range of control parameters. The correlation between the two diffusive terms is interpreted in terms of the variation in the strength of poloidal flow along a columnar flow tube. The amplitude ratio of the two diffusive terms is used to estimate the probable contribution of radial magnetic diffusion to the secular variation at Earth-like conditions. We then apply a model where radial magnetic diffusion is proportional to tangential diffusion to core flow inversions of geomagnetic secular variation data. We find that including magnetic diffusion does not change dramatically the global flow but some significant local variations appear. In the non frozen-flux core flow models (termed ‘diffusive’), the hemispherical dichotomy between the active Atlantic and quiet Pacific is weaker, a cyclonic vortex below North America emerges and the vortex below Asia is stronger. Our results have several important geophysical implications. First, our diffusive flow models contain some flow activity at low latitudes in the Pacific, suggesting a local balance between magnetic field advection and diffusion in that region. Second, the cyclone below North America in our diffusive flows reconciles the difference between mantle-driven thermal wind predictions and frozen-flux core flow models, and is consistent with the prominent intense magnetic flux patch below North America in geomagnetic field models. Finally, we hypothesize that magnetic diffusion near the core surface plays a larger role in the geomagnetic secular variation than usually assumed. [ABSTRACT FROM AUTHOR]

Cosmochemical, geochemical, and geophysical studies provide evidence that Earth's core contains iron with substantial (5 to 15%) amounts of nickel. The iron-nickel alloy Fe09Ni0.1 has been studied in situ by means of angle-dispersive x-ray diffraction in internally heated diamond anvil cells (DACs), and its resistance has been measured as a function of pressure and temperature. At pressures above 225 gigapascals and temperatures over 3400 kelvin, Fe09Ni0.1 adopts a body-centered cubic structure. Our experimental and theoretical results not only support the interpretation of shockwave data on pure iron as showing a solid-solid phase transition above about 200 gigapascals, but also suggest that iron alloys with geochemically reasonable compositions (that is, with substantial nickel, sulfur, or silicon content) adopt the bcc structure in Earth's inner core. [ABSTRACT FROM AUTHOR]

Presents a seismological study of the mantle beneath the Bowie hotspot of west of Canada. Revellation of a zone of low seismic velocitites; Delay of 0.15 seconds to the travel time of seismic waves traversing a region approximately 150 kilometers in diameter; Suggestions that a mantle plume is present in the lower mantle with the amplitude of the anomaly implying a temperature contrast of 300 K.

The article discusses geophysics research by P. Poli et al within the issue. The author notes that the Earth's thermal and chemical structure has historically been determined via the analysis of seismic waves excited by earthquakes, arguing that the resolution of those methods is limited because earthquakes occur along tectonic plate boundaries, resulting in large areas being seismically quiet. Topics include an imaging study of deep-Earth structures by Poli et al using improved processing methods for seismic noise data, the observation of body waves in ambient seismic noise, and diagrams illustrating surface and body waves in ambient noise tomography.

The article discusses the composition of pallasite meteorites in connection with the geophysical dynamo theory, with a focus on the article "Evidence for a Dynamo in the Main Group Pallasite Parent Body," published elsewhere in the November 16, 2012 issue by J. A. Tarduno et al. Topics include molten metals within the Earth's core, olivine crystals in pallasites, and paleomagnetic measurements. Additional information is presented on isothermal metallic cores and the generation of magnetic fields in asteroids.

The article discusses the use of precision isotope analysis to reveal details about the dynamic processes involved in the formation of planet Earth's mantle. Topics include a belief among geologists that evidence of the first few hundred million years of Earth's history had been erased by mantle convection and plate tectonics, a study within the issue regarding geophysics research by M. Touboul and colleagues reporting measurements of the isotopic composition of tungsten (W) in 2.8-billion-year-old basaltic rock, called komatiite, which is known to be derived from source regions deep within Earth's mantle, and the abundance of an tungsten isotope 182W, which forms from the decay of an isotope of hafnium (182Hf).

The article discusses four reports within the issue regarding geophysics research, including one by Sato and colleagues, Simons and colleagues, Ide and colleagues, and Vigny and colleagues. The teams illustrate the use of a network of global positioning system (GPS) detectors to show how the Earth's surface deformed before and after the Tohoku-Oki magnitude 9 (M9) earthquake in Japan on March 11, 2011 and the M9 earthquake in central Chile (Maule) on February 27, 2010, in which thousands of lives were saved as a result of early warning systems for tsunamis.

The article discusses a report within the issue regarding geophysics research investigating global plate tectonics. A brief overview of how convention within planet Earth's mantle affects geological activity, such as tectonic motions on the surface of Earth, is presented. Topics include an overview of plate tectonics and the team's creation of a numerical model, which uses adaptive mesh refinement algorithms on parallel computers to observe global mantle flows through simulation methods, and the behavioral influence of the oceanic lithosphere on plate driving forces.

Mike Kendall, elected President of the British Geophysical Association in February, explains why we need a strong geophysics organization in the UK. [ABSTRACT FROM AUTHOR]

Randall S Perry and Mark A Sephton discuss a target material for Mars Sample Return: rocks coated with desert varnish, a mineral layer that records past surface climates on Earth, and probably on Mars too. With the prospect of a Mars Sample Return mission in the next two decades, attention is now focusing on the types of samples that would be most useful for returning to Earth for laboratory analysis. Recent data on desert varnish reveal that laminated coatings on rocks from extreme environments are a passive recorder of past and present environments. Martian desert varnish would contain a chronology of the martian setting, perhaps extending back to a wetter and more biologically active period. [ABSTRACT FROM AUTHOR]