Cratonic mantle roots, remnants of a more chondritic Archean mantle?

The Earth's continents are cored by Archean cratons underlain by seismically fast mantle roots descending to depths of 200⁺ km that appear to be both more refractory and colder than the surrounding asthenospheric mantle. Low-temperature mantle xenoliths from kimberlite pipes indicate that the shallow parts of these cratonic mantle roots are dominated by refractory harzburgites that are very old (3⁺ Ga). A fundamental mass balance problem arises, however, when attempts are made to relate Archean high-Mg lavas to a refractory restite equivalent to the refractory lithospheric mantle roots beneath Archean cratons. The majority of high-Mg Archean magmas are too low in Al and high in Si to leave behind a refractory residue with the composition of the harzburgite xenoliths that constitute the Archean mantle roots beneath continental cratons, if a Pyrolitic primitive mantle source is assumed. The problem is particularly acute for 3⁺ Ga Al-depleted komatiites and the Si-rich harzburgites of the Kaapvaal and Slave cratons, but remains for cratonic harzburgites that are not anomalously rich in orthopyroxene and many Al-undepleted komatiites. This problem would disappear if fertile Archean mantle was richer in Fe and Si, more similar in composition to chondritic meteorites than the present Pyrolitic upper mantle of the Earth. Accepting the possibility that the Earth's convecting upper mantle has become poorer in Fe and Si over geologic time not only provides a simpler way of relating Archean high-Mg lavas to the lithospheric mantle roots that underlie Archean cratons, but could lead to new models for the nature Archean magmatism and the lower mantle sources of modern hot-spot volcanism.     

Source model of the 11th July 2004 Zhongba earthquake revealed from the joint inversion of InSAR and seismological data

We use interferometric synthetic aperture radar (InSAR) and broadband seismic waveform data to estimate a source model of the 11th July, 2004 M W 6.2 Zhongba earthquake, Tibet of China. This event occurred within the seismically active zone of southwestern Tibetan Plateau where the east-west extension of the upper crust is observed. Because of limitations in one pair of InSAR data available, there are trade-offs among centroid depth, rupture area and amount of slip. Available seismic data tightly constrain ...     

Sewers as a Source and Sink of Chlorinated‐Solvent Groundwater Contamination,Marine Corps Recruit Depot,Parris Island,South Carolina

Groundwater contamination by tetrachloroethene and its dechlorination products is present in two partially intermingled plumes in the surficial aquifer near a former dry‐cleaning facility at Site 45, Marine Corps Recruit Depot, Parris Island, South Carolina. The northern plume originates from the vicinity of former above‐ground storage tanks. Free‐phase tetrachloroethene from activities in this area entered the groundwater. The southern plume originates at a nearby, new dry‐cleaning facility, but probably was the result of contamination released to the aquifer from a leaking sanitary sewer line from the former dry‐cleaning facility. Discharge of dissolved groundwater contamination is primarily to leaking storm sewers below the water table. The strong influence of sanitary sewers on source distribution and of storm sewers on plume orientation and discharge at this site indicates that groundwater‐contamination investigators should consider the potential influence of sewer systems at their sites.     

Magnitude and rupture duration from high frequency teleseismic P wave with projected landweber deconvolution

Earthquake magnitude and rupture duration are among the most important parameters characterizing an earthquake for the purpose of early tsunami warning. While they can be routinely determined from broadband P waveforms with iterative inver- sion procedures, the inversion procedures may fail when the rupture either lasts longer than the interval between P and later arrivals or requires too much time or human intervention. Little contaminated by later arrivals, high frequency P waves are useful for modeling earthquake source processes, though the envelope waveform is affected by strong scattering in lithosphere. With high frequency envelopes from aftershocks as Empirical Green’s Function (EGF), the coda effects can be removed and more accurate relative source time function (RSTF) of the main shock can be obtained. Assuming that RSTFs cannot be negative, we use the projected Landweber deconvolution method (PLD) to obtain high frequency RSTFs because PLD method has the advantage of non-negativity, causality, and compactness (finite duration). We are able to determine rupture durations of four large earthquakes: the 2004 Sumatra-Andaman earthquake, the 2005 Nias event, the 2006 Java event, and the 2011 Tokuko earthquake. The rupture durations of the Sumatra-Andaman, Nias, and Tohuko events are found to be around 550, 110, and 120 s respectively, consistent with previous studies. The rupture duration of the Java event is about 130 s, supporting that the Java event is a tsunami earthquake. The magnitudes of these earthquakes are found to depend on both the amplitude and the duration of the deconvolved waveforms, and can be approximated by integrating these waveforms.     

Reflection point sideslip and smear in imaging below dipping anisotropic media

Dipping anisotropic clastic strata are ubiquitous in fold and thrust belts. Geological structures below these strata will be mispositioned laterally and vertically on seismic images if we do not properly correct for seismic anisotropy during migration. The magnitude of this lateral mispositioning of a target structure varies with source‐receiver offset, so reflection points will be smeared in the final stacked image. Raytracing demonstrates the lateral‐position and smear phenomena when imaging structures below tilted transversely isotropic media. Analysis of the raytracing results predicts the quantity of lateral‐position error and reflection‐point smear on a seismic image. We created numerical‐model seismic data to show reflection‐point smear on synthetic seismic images and to evaluate the accuracy of the predictions from raytracing.     

Tomographic measurements of pore filling at infiltration fronts

For certain porous media and initial conditions, constant flux infiltrations show a saturation profile which exhibits overshoot. This overshoot is the cause of gravity driven fingering, cannot be described by standard models of unsaturated flow, and is likely controlled by the exact nature of the pore filling at the initial front. Here we report synchrotron X-ray micro-tomography measurements of the porous medium and measure which pores are filled by water and air at the initial wetting front as a function of flux. We find that at high fluxes all the pores are filled with water; for intermediate fluxes, the pores along the edge of the column remain unsaturated; and for low fluxes the pores in the bulk of the experimental column remain unsaturated. This suggests that the unsaturated overshoot conditions observed at higher fluxes are primarily an edge effect of the column. The results can help delineate the correct continuum models that can capture overshoot and gravity driven fingering.     

Simulating Gravity Changes in Topologically Realistic Driven Earthquake Fault Systems: First Results

Currently, GPS and InSAR measurements are used to monitor deformation produced by slip on earthquake faults. It has been suggested that another method to accomplish many of the same objectives would be through satellite-based gravity measurements. The Gravity Recovery and Climate Experiment (GRACE) mission has shown that it is possible to make detailed gravity measurements from space for climate dynamics and other purposes. To build the groundwork for a more advanced satellite-based gravity survey, we must estimate the level of accuracy needed for precise estimation of fault slip in earthquakes. We turn to numerical simulations of earthquake fault systems and use these to estimate gravity changes. The current generation of Virtual California (VC) simulates faults of any orientation, dip, and rake. In this work, we discuss these computations and the implications they have for accuracies needed for a dedicated gravity monitoring mission. Preliminary results are in agreement with previous results calculated from an older and simpler version of VC. Computed gravity changes are in the range of tens of μGal over distances up to a few hundred kilometers, near the detection threshold for GRACE.     

Oscillating pulsar polar gaps

An analytical model for oscillating pair creation above the pulsar polar cap is presented in which the parallel electric field is treated as a large amplitude, superluminal, electrostatic wave. An exact formalism for such wave is derived in one dimension and applied to both the low-density regime in which the pair plasma density is much lower than the corotating charge density and the high-density regime in which the pair plasma density is much higher than the corotating charge density. In the low-density regime, which is relevant during the phase leading to a pair cascade, a parallel electric field develops resulting in a rapid acceleration of particles. The rapid acceleration leads to bursts of pair production and the system switches to the oscillatory phase, corresponding to the high-density regime, in which pairs oscillate with net drift motion in the direction of wave propagation. Oscillating pairs lead to a current that oscillates with large amplitude about the Goldreich–Julian current. The drift motion can be highly relativistic if the phase speed of large amplitude waves is moderately higher than the speed of light. Thus, the model predicts a relativistic outflow of pairs, a feature that is required for avoiding overheating of the pulsar polar cap and is also needed for the pulsar wind.