Mantle shear-wave velocity structure beneath the Hawaiian hot spot.

Defining the mantle structure that lies beneath hot spots is important for revealing their depth of origin. Three-dimensional images of shear-wave velocity beneath the Hawaiian Islands, obtained from a network of sea-floor and land seismometers, show an upper-mantle low-velocity anomaly that is elongated in the direction of the island chain and surrounded by a parabola-shaped high-velocity anomaly. Low velocities continue downward to the mantle transition zone between 410 and 660 kilometers depth, a result that is in agreement with prior observations of transition-zone thinning.

Seismic reflection images of the Moho underlying melt sills at the East Pacific Rise.

The determination of melt distribution in the crust and the nature of the crust-mantle boundary (the 'Moho') is fundamental to the understanding of crustal accretion processes at oceanic spreading centres. Upper-crustal magma chambers have been imaged beneath fast- and intermediate-spreading centres but it has been difficult to image structures beneath these magma sills. Using three-dimensional seismic reflection images, here we report the presence of Moho reflections beneath a crustal magma chamber at the 9 degrees 03' N overlapping spreading centre, East Pacific Rise.

Excitation of T-phases by seafloor scattering.

T-phases excited by suboceanic earthquakes are classified into two types: abyssal phases which are excited near the earthquake epicenter at seafloor depths far below the SOFAR velocity channel, and slope T-phases which are excited at continental, or ocean island slopes and ridges at distances up to several hundreds of kilometers from the epicenter. In this article, it is demonstrated that approximate time-frequency characteristics of both classes of T-phase can be synthesized under the assumption that T-phases are excited by scattering from a rough seafloor. Seafloor scattering at shallow depths preferentially excites low order acoustic modes that propagate efficiently within the ocean sound channel minimum.

Evidence from three-dimensional seismic reflectivity images for enhanced melt supply beneath mid-ocean-ridge discontinuities.

Quantifying the melt distribution and crustal structure across ridge-axis discontinuities is essential for understanding the relationship between magmatic, tectonic and petrologic segmentation of mid-ocean-ridge spreading centres. The geometry and continuity of magma bodies beneath features such as overlapping spreading centres can strongly influence the composition of erupted lavas and may give insight into the underlying pattern of mantle flow. Here we present three-dimensional images of seismic reflectivity beneath a mid-ocean ridge to investigate the nature of melt distribution across a ridge-axis discontinuity.

Mantle seismic structure beneath the MELT region of the east pacific rise from P and S wave tomography.

Relative travel time delays of teleseismic P and S waves, recorded during the Mantle Electromagnetic and Tomography (MELT) Experiment, have been inverted tomographically for upper-mantle structure beneath the southern East Pacific Rise. A broad zone of low seismic velocities extends beneath the rise to depths of about 200 kilometers and is centered to the west of the spreading center. The magnitudes of the P and S wave anomalies require the presence of retained mantle melt; the melt fraction near the rise exceeds the fraction 300 kilometers off axis by as little as 1%.

Off-axis crustal thickness across and along the east pacific rise within the MELT area.

Wide-angle seismic data along the Mantle Electromagnetic and Tomography (MELT) arrays show that the thickness of 0.5- to 1. 5-million-year-old crust of the Nazca Plate is not resolvably different from that of the Pacific Plate, despite an asymmetry in depth and gravity across this portion of the East Pacific Rise.

Seismic Images of Active Magma Systems Beneath the East Pacific Rise Between 17{degrees}05' and 17{degrees}35'S.

Seismic reflection data from the East Pacific Rise between 17 degrees 05' and 17 degrees 35'S image a magma lens that varies regularly in depth and width as ridge morphology changes, confirming the notion that axial morphology can be used to infer ridge magmatic state. However, at 17 degrees 26'S, where the ridge is locally shallow and broad, the magma lens is markedly shallower and wider than predicted from regional trends. In this area, submersible dives reveal recent volcanic eruptions.

Crustal Thickness on the Mid-Atlantic Ridge: Bull's-Eye Gravity Anomalies and Focused Accretion.

Spreading segments of the Mid-Atlantic Ridge show negative bull's-eye anomalies in the mantle Bouguer gravity field. Seismic refraction results from 33 degrees S indicate that these anomalies can be accounted for by variations in crustal thickness along a segment. The crust is thicker in the center and thinner at the end of the spreading segment, and these changes are attributable to variations in the thickness of layer 3.

Seismic structure of the southern East pacific rise.

Seismic data from the ultrafast-spreading (150 to 162 millimeters per year) southern East Pacific Rise show that the rise axis is underlain by a thin (less than 200 meters thick) extrusive volcanic layer (seismic layer 2A) that thickens rapidly off axis. Also beneath the rise axis is a narrow (less than 1 kilometer wide) melt sill that is in some places less than 1000 meters below the sea floor. The small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge-crest thermal models.

Results of a cleanup and treatment test at the Nevada test site: evaluation of vacuum removal of Pu-contaminated soil.

We have conducted experiments to evaluate the effectiveness of removing contaminated soils from the Nevada Test Site with a large truck-mounted vacuum cleaner. Our results show that this method is effective, relatively easy, and safe for equipment operators. With four passes of the truck-mounted vacuum, 92% of the 241Am (and the accompanying 239 + 240Pu) was removed and resuspension rates were reduced by more than 99%.