Velocity structure and lithospheric age of the Gamburtsev Subglacial Mountains

第2回極域科学シンポジウム/第31回極域地学シンポジウム 11月16日（水） 国立国語研究

Shear velocity structure of central Antarctica from teleseismic Rayleigh waves

第2回極域科学シンポジウム/第31回極域地学シンポジウム 11月16日（水） 国立国語研究

Upper Mantle Seismic Anisotropy Beneath the West Antarctic Rift System and Surrounding Region from Shear Wave Splitting Analysis

We constrain azimuthal anisotropy in the West Antarctic upper mantle using shear wave splitting parameters obtained from teleseismic SKS, SKKS and PKS phases recorded at 37 broad-band seismometres deployed by the POLENET/ANET project. We use an eigenvalue technique to linearize the rotated and shifted shear wave horizontal particle motions and determine the fast direction and delay time for each arrival. High-quality measurements are stacked to determine the best fitting splitting parameters for each station. Overall, fast anisotropic directions are oriented at large angles to the direction of Antarctic absolute plate motion in both hotspot and no-net-rotation frameworks, showing that the anisotropy does not result from shear due to plate motion over the mantle. Further, the West Antarctic directions are substantially different from those of East Antarctica, indicating that anisotropy across the continent reflects multiple mantle regimes. We suggest that the observed anisotropy along the central Transantarctic Mountains (TAM) and adjacent West Antarctic Rift System (WARS), one of the largest zones of extended continental crust on Earth, results from asthenospheric mantle strain associated with the final pulse of western WARS extension in the late Miocene. Strong and consistent anisotropy throughout the WARS indicate fast axes subparallel to the inferred extension direction, a result unlike reports from the East African rift system and rifts within the Basin and Range, which show much greater variation. We contend that ductile shearing rather than magmatic intrusion may have been the controlling mechanism for accumulation and retention of such coherent, widespread anisotropic fabric. Splitting beneath the Marie Byrd Land Dome (MBL) is weaker than that observed elsewhere within the WARS, but shows a consistent fast direction, possibly representative of anisotropy that has been ‘frozen-in’ to remnant thicker lithosphere. Fast directions observed inland from the Amundsen Sea appear to be radial to the dome and may indicate radial horizontal mantle flow associated with an MBL plume head and low upper mantle velocities in this region, or alternatively to lithospheric features associated with the complex Cenozoic tectonics at the far-eastern end of the WARS

Upper Mantle Structure of Central and West Antarctica from Array Analysis of Rayleigh Wave Phase Velocities

The seismic velocity structure of Antarctica is important, both as a constraint on the tectonic history of the continent and for understanding solid Earth interactions with the ice sheet. We use Rayleigh wave array analysis methods applied to teleseismic data from recent temporary broadband seismograph deployments to image the upper mantle structure of central and West Antarctica. Phase velocity maps are determined using a two–plane wave tomography method and are inverted for shear velocity using a Monte Carlo approach to estimate three-dimensional velocity structure. Results illuminate the structural dichotomy between the East Antarctic Craton and West Antarctica, with West Antarctica showing thinner crust and slower upper mantle velocity. West Antarctica is characterized by a 70–100 km thick lithosphere, underlain by a low-velocity zone to depths of at least 200 km. The slowest anomalies are beneath Ross Island and the Marie Byrd Land dome and are interpreted as upper mantle thermal anomalies possibly due to mantle plumes. The central Transantarctic Mountains are marked by an uppermost mantle slow-velocity anomaly, suggesting that the topography is thermally supported. The presence of thin, higher-velocity lithosphere to depths of about 70 km beneath the West Antarctic Rift System limits estimates of the regionally averaged heat flow to less than 90 mW/m2. The Ellsworth-Whitmore block is underlain by mantle with velocities that are intermediate between those of the West Antarctic Rift System and the East Antarctic Craton. We interpret this province as Precambrian continental lithosphere that has been altered by Phanerozoic tectonic and magmatic activity

The 3 May 2006 (M<inf>w</inf> 8.0) and 19 March 2009 (M<inf>w</inf> 7.6) Tonga earthquakes: Intraslab compressional faulting below the megathrust

The Tonga subduction zone is among the most seismically active regions and has the highest plate convergence rate in the world. However, recorded thrust events confidently located on the plate boundary have not exceeded Mw 8.0, and the historic record suggests low seismic coupling along the arc. We analyze two major thrust fault earthquakes that occurred in central Tonga in 2006 and 2009. The 3 May 2006 Mw 8.0 event has a focal mechanism consistent with interplate thrusting, was located west of the trench, and caused a moderate regional tsunami. However, long-period seismic wave inversions and finite-fault modeling by joint inversion of teleseismic body waves and local GPS static offsets indicate a slip distribution centered ~65km deep, about 30km deeper than the plate boundary revealed by locations of aftershocks, demonstrating that this was an intraslab event. The aftershock locations were obtained using data from seven temporary seismic stations deployed shortly after the main shock, and most lie on the plate boundary, not on either nodal plane of the deeper main shock. The fault plane is ambiguous, and investigation of compound rupture involving coseismic slip along the megathrust does not provide a better fit, although activation of megathrust faulting is responsible for the aftershocks. The 19 March 2009 Mw 7.6 compressional faulting event occurred below the trench; finite-fault and W-phase inversions indicate an intraslab, ~50km deep centroid, with ambiguous fault plane. This event also triggered small megathrust faulting. There continues to be a paucity of large megathrust earthquakes in Tonga

Investigating Dam Seepage using Geophysical Methods: Providing Hands-On Geophysical Experience to Geological Sciences and Engineering Students

An electrical resistivity survey was carried out at an existing leaking dam site in Salem, Missouri. The purpose of the study was to investigate possible seepage pathways and to provide students at the University of Missouri-Rolla hands-on experience in the application of geophysics to engineering problems. The electrical resistivity data were acquired using the dipole-dipole and pole-dipole arrays. The data suggest two geoelectric layers: an upper conductive layer interpreted to be surficial clay deposits and a lower more resistive layer interpreted to be the dolomite bedrock characteristic of this part of Missouri. The results further suggest that the bedrock is laterally discontinuous, which we suggest to be the result of fracturing and solution activity. We also observed an anomalously sub-circular conductive zone within the bedrock, which we interpret as a subsurface solution cavity filled with water or finer materials such as silt or clay. This anomalous area was laterally extensive and occurred beneath the spillway of the dam. We suggest this subsurface cavity may have provided potential pathways for the water seepage which emptied the dam. This study demonstrates the usefulness of electrical resistivity measurements in detecting seepage zones under dams. In addition, the study also provided students at the University of Missouri Rolla hands-on experience and an appreciation of geophysical methods as an investigative tool for engineering problems

Seismic evidence for lithospheric foundering beneath the southern Transantarctic Mountains, Antarctica

The 3000-km-long Transantarctic Mountains (TAMs), which separate cratonic East Antarctica from tectonically active West Antarctica, remain one of the least understood of Earth’s major mountain ranges. The tectonic mechanism that generates the high elevation, as well as the processes that produce major differences between various sectors of the TAMs, are still uncertain. Here we present newly constructed seismic images of the crust and uppermost mantle beneath central Antarctica derived from recently acquired seismic data, indicating ongoing lithospheric foundering beneath the southern TAMs. These images reveal an absence of thick, cold cratonic lithosphere beneath the southern TAMs. Instead, an uppermost-mantle slow seismic anomaly extends across the mountain front and 350 km into East Antarctica, beneath a high plateau near the South Pole. Under the slow anomaly, a relatively high-wave-speed root is found at ∼200 km depth, connected with the East Antarctic lithosphere, suggesting that sinking lithosphere has been replaced at shallow depths by warm, slow-velocity asthenosphere. A mantle lithosphere foundering model is proposed to interpret these images, which best explains the present large area of high elevation and the uplift of the TAMs, as well as Miocene-age volcanism in the Mount Early region