ALBERT

Description: We derive equations for HTI and orthorhombic symmetries to analyze fluid substitution effects in porous fractured media. The derivations are based on the anisotropic Gassmann equation and linear slip theory. We assess the influence of fluid substitution (gas, brine, and oil), on elastic moduli, velocities, anisotropy, and azimuthal amplitude variations. We find that in the direction normal to fractures, P-wave moduli increase as much as 56% and P-wave velocity increases up to 19% for gas-to-brine substitution. For the direction parallel to fractures, P-wave velocity remains almost constant when porosity is low (5%), but can increase up to 4% if porosity is high (25%). Since P-waves in two different directions have different sensitivities to fluids and fractures, the Thomsen's parameters (defined for HTI and orthorhombic symmetries), ε and δ , are sensitive to fluid types and fractures. We also found that δ is sensitive to porosity for liquid saturation, but insensitive to porosity for the case of gas saturation. Gassmann assumes (and as has been observed) that shear modulus does not depend on fluids. And we observe no changes in shear-wave splitting ( γ ) for different fluids. The azimuthal amplitude variation is dependent on fluid types, fractures and porosity. We observe up to 12% increase in azimuthal amplitude variation for low porosity gas sands after brine saturation, and 6% decrease for high porosity gas sands. We find that the percentage changes in gas-to-oil substitution are about half that of the gas-to-brine case. The equations we have derived provide a useful tool to quantitatively evaluate the effects of fluid substitution on seismic anisotropy.

Description: In the Ursa Basin, Gulf of Mexico, in situ mudstone permeability near the seafloor declines from 1.1 × 10−16 to 5.8 × 10−19 m2 over a depth of 578 m. We can reproduce this in situ permeability-porosity behavior through consolidation experiments in the laboratory. We use uniaxial constant-rate-of-strain consolidation experiments to measure permeability-porosity relationships and derive in situ permeabilities of 31 mudstone samples collected at Integrated Ocean Drilling Program (IODP) Sites U1324 and U1322. Although these mudstones have similar grain-size distributions, permeability at a given porosity varies significantly between the samples due to small variations in composition or fabric. We calculate an upscaled permeability relationship based on the observed permeability variation in the samples and determine a resultant large-scale permeability anisotropy of around 30. Based on this upscaled relationship and observations of in situ pressure, we calculate upward fluid flow rates of 0.5 mm/yr. We find that given the observed compressibility, permeability, and the geologic forcing at Ursa, overpressures are predicted as observed in the subsurface. The primary mechanism for overpressure generation at Ursa is sediment loading due to rapid burial. Low vertical permeabilities, accompanied by high sedimentation rates, can cause severe overpressure near the seafloor, which controls fluid flow and can reduce slope stability as observed in the Mississippi Canyon region. Such flow systems, especially at intermediate depths on passive margins, are important due to their control over macroscale behavior such as topographic gradient of continental slopes and submarine landslides, but have been largely understudied in the past.

Description: [1]  Antigorite, the high temperature form of serpentinite, is believed to play a critical role in various geological processes of subduction zones. We have measured P- and S-wave velocities (V p and V s ), anisotropy and shear-wave splitting of 17 serpentinite samples containing 〉90% antigorite at pressures up to 650 MPa. The new results, combined with data for low temperature lizardite and/or chrysolite, reveal distinct effects of low and high temperature (LT and HT) serpentinization on the seismic properties of mantle rocks. At 600 MPa, V p  = 5.10 and 6.68 km/s, V s  = 2.32 and 3.67 km/s, and V p /V s  = 2.15 and 1.81 for pure LT and HT serpentinites, respectively. Above the crack-closure pressure (~150 MPa), the velocity ratio of antigorite serpentinites displays little dependence on pressure or temperature. Serpentine contents within subduction zones and forearc mantle wedges where temperature is 〉300 °C should be at least twice that of previous estimates based on LT serpentinization. The presence of seismic anisotropy, high-pressure fluids or partial melt is also needed to interpret HT serpentinized mantle with V p  〈 6.68 km/s, V s  〈 3.67 km/s and V p /V s  〉 1.81. The intrinsic anisotropy of the serpentinites (3.8-16.9% with an average value of 10.5% for V p , and 3.6-18.3% with an average value of 10.4% for V s ) is caused by dislocation creep-induced lattice-preferred orientation (LPO) of antigorite. Three distinct patterns of seismic anisotropy correspond to three types of antigorite fabrics (S-, L-, and LS-tectonites) formed by three categories of strain geometry (i.e., coaxial flattening, coaxial constriction, and simple shear), respectively. Our results are thought to provide a new explanation for various anisotropic patterns of subduction systems observed worldwide.

Description: We develop a theory of using difference wavefields of repeated sources to locate and quantify temporal medium change and apply the theory to locate temporal change of seismic properties beneath the Japan subduction zone using repeated earthquakes. Our theory states the difference wavefields of two repeated sources in a temporally changed medium can be equivalently treated as wavefields propagating from conceptual sources, with their location at the place of temporal change and their strengths equal to the product of magnitude of medium property change and magnitude of the initial wavefields from the repeated sources. When the medium change extends to a finite region, the conceptual sources become volumetric sources distributed over the region of the medium change and propagating in the direction of the initial wave. The conceptualization establishes a theoretical framework for possible applications of using difference wavefields to locate and quantify temporal medium changes in geological sciences, ultrasonic experiments, civil engineering and medical imaging. We search repeating earthquakes occurring in the Japan subduction zone, formulate an empirical procedure to extract the difference wavefields between repeating earthquakes and determine temporal change of seismic properties using a back-projection method. We locate the temporal change of seismic properties beneath the Japan subduction zone to be at (37.2°N, 142°E), and estimate the magnitude of the conceptual body force associated with the temporal change to be 1.15 × 1010N, or as a reference, a 0.87% density change for an assumed volume of temporal change of 103 km3.

Description: The Longitudinal Valley Fault (LVF) in Eastern Taiwan is a high slip rate fault (about 5 cm/yr), which exhibits both seismic and aseismic slip. Deformation of anthropogenic features shows that aseismic creep accounts for a significant fraction of faultslip near the surface, whereas a fraction of the slip is also seismic, since this fault has produced large earthquakes with five M w  〉 6.8 events in 1951 and 2003. In this study, we analyze a dense set of geodetic and seismological data around the LVF, including campaign-mode Global Positionnig System(GPS) measurements, time series of daily solutions for continuous GPS stations (cGPS), leveling data, and accelerometric records of the 2003 Chenkung earthquake. To enhance the spatial resolutionprovided by these data, we complement them with Interferometric Synthetic Aperture Radar (InSAR) measurements produced from a series of Advanced Land Observing Satellite (ALOS) images processed using a persistent scatterer (PS) technique. The combined dataset covers the entire LVF and spans the period from 1992 to 2010. We invert this data to infer the temporal evolution of fault slip at depth using the Principal Component Analysis-based Inversion Method (PCAIM). This technique allows the joint inversion of diverse data, taking the advantage of the spatial resolution given by the InSAR measurements and the temporal resolution afforded by the cGPS data. We find that (1) seismic slip during the 2003 Chengkung earthquake occurred on a fault patch whichhad remained partially locked in the interseismic period; (2) the seismic rupture propagated partially into a zone of shallow aseismic interseismic creep but failed to reach the surface; (3) that aseismic afterslip occurred around the area that ruptured seismically. We find consistency between geodetic and seismological constraints on the partitioning between seismic and aseismic creep. About 80-90% of slip on the LVF in the 0-26 km, seismogenic depth range is actually aseismic. We infer that the clay-rich Lichi Mélange is the key factor promoting aseismic creep at shallow depth.

Description: Seismic anisotropy provides essential constraints on mantle dynamics and continental evolution. One particular question concerns the depth distribution and coherence of azimuthal anisotropy, which is key for understanding force transmission between the lithosphere and asthenosphere. Here, we reevaluate the degree of coherence between the predicted shear wave splitting derived from tomographic models of azimuthal anisotropy and that from actual observations of splitting. Significant differences between the two types of models have been reported, and such discrepancies may be due to differences in averaging properties or due to approximations used in previous comparisons. We find that elaborate, full waveform methods to estimate splitting from tomography yield generally similar results to the more common, simplified approaches. This validates previous comparisons and structural inversions. However, full waveform methods may be required for regional studies, and they allow exploiting the back-azimuthal variations in splitting that are expected for depth-variable anisotropy. Applying our analysis to a global set of SKS splitting measurements and two recent surface wave models of upper-mantle azimuthal anisotropy, we show that the measures of anisotropy inferred from the two types of data are in substantial agreement. Provided that the splitting data is spatially averaged (so as to bring it to the scale of long-wavelength tomographic models and reduce spatial aliasing), observed and tomography-predicted delay times are significantly correlated, and global angular misfits between predicted and actual splits are relatively low. Regional anisotropy complexity notwithstanding, our findings imply that splitting and tomography yield a consistent signal that can be used for geodynamic interpretation.

Description: A detailed study of seismic properties (P and S wave velocities, hysteresis, anisotropy and shear wave splitting) has been carried out on a unique suite of deep borehole core samples from the Chinese Continental Scientific Drilling (CCSD) project, which penetrated 5158 m into the Sulu ultrahigh-pressure (UHP) metamorphic terrane (China). Seismic velocities of the deep core samples are more and less sensitive to pressure in the low pressure ( 200–300 MPa) linear regimes, respectively, than samples from the surface. The comparison suggests that the high pressure data from the core samples are much more reliable for extrapolation to deeper crust than the data from surface analogs that have been subjected to long histories of weathering and alteration along intergranular and transgranular cracks. The significant increases in the pressure sensitivity of seismic velocities for the core samples in the nonlinear regime indicate that drilling-induced and stress-relief microcracks with small aspect ratios are fresh and clean without secondary mineral in-fillings, and are thus easy to close completely under the applied hydrostatic pressure conditions of the laboratory. The data also elucidate that the velocity-pressure data can successfully provide important hints about the preferred orientation of microcracks that causes P wave velocity anisotropy and shear wave splitting in cracked rocks, and that the effect of compression on the Vp/Vs ratios is negligible for crack-free compacted rocks. The seismic velocities of equivalent isotropic (fabric-free) and crack-free crystalline aggregates calculated from room pressure single crystal elastic constants using the Voigt average are in good agreement with the laboratory data at ∼200 MPa. Comparison of the seismic reflection image from the vicinity of the borehole with the normal-incidence reflection coefficient profile computed from the laboratory-measured velocities and densities infers that the seismic reflections originate from mafic (eclogite and retrograde eclogite) or ultramafic units within dominantly felsic rocks.

Description: Recently, a number of source-side shear wave splitting measurements that directly constrain anisotropy in the upper mantle beneath subducting slabs have been published. Such measurements have yielded an observational foundation on which to base our understanding of the dynamics of the sub-slab mantle. Here, we compile measurements from recent studies of source-side splitting beneath slabs that employ identical measurement and processing techniques. We use this compilation to test the predictions of a number of recently proposed conceptual models for the dynamics of the sub-slab mantle, including those that invoke three-dimensional return flow beneath slabs, strong radial anisotropy in suboceanic asthenosphere that is entrained via subduction, and a model based on the correlation between sub-slab splitting behavior and the age of the subducting lithosphere. We find that a model in which fast splitting directions are determined by slab age matches the observations better than either the 3D-return flow or radial anisotropic models. Based on this observation, we propose that the sub-slab mantle is characterized by two distinct anisotropic and mantle flow regimes. Beneath younger lithosphere (〈95 Ma), we propose the sub-slab mantle is characterized by 2-D entrained flow resulting in an entrained mantle layer. Beneath older lithosphere (〉95 Ma), the entrained layer is thin and effectively serves as decoupling layer; the dynamics of the sub-slab region beneath old lithosphere is therefore dominated by three-dimensional return flow. This variation in the amount of mechanical coupling may be facilitated by the onset of small-scale convection beneath older lithosphere.

Description: Fractional crystallization and crystal segregation controlled by settling or floating of minerals during the cooling of magma can lead to layered structures in mafic and ultramafic intrusions in continental and oceanic settings in the lower crust. Thus the seismic properties and fabrics of layered intrusions must be calibrated to gain insight into the origin of seismic reflections and anisotropy in the deep crust. To this end, we have measured P- and S-wave velocities and anisotropy in 17 plagioclase-rich mafic igneous rocks such as anorthosite and gabbro at hydrostatic pressures up to 650 MPa. Anorthosites and gabbroic anorthosites containing 〉80 vol% plagioclase and gabbros consisting of nearly equal modal contents of plagioclase and pyroxene display distinctive seismic anisotropy patterns: V p (Z)/ V p (Y) ≥ 1 and V p (Z)/ V p (X) ≥ 1 for anorthosites while 0.8 〈  V p (Z)/ V p (Y) ≤ 1 and 0.8 〈 V p (Z)/V p (X) ≤ 1 for gabbros. Amphibolites lie in the same domain as gabbros, but show a significantly stronger tendency of V p (X) 〉  V p (Y) than the gabbros. Laminated anorthosites with V p (X) ≈  V p (Y) ≪  V p (Z) display a strong crystal preferred orientation (CPO) of plagioclase whose (010) planes and [100] and [001] directions parallel to the foliation. For the gabbros and amphibolites characterized by V p (X) ≈  V p (Y) 〉  V p (Z) and V p (X) 〉  V p (Y) 〉  V p (Z), respectively, pyroxene and amphibole play a dominant role over plagioclase in the formation of seismic anisotropy. The Poisson's ratio calculated using the average P- and S-wave velocities from the three principal propagation-polarization directions (X, Y and Z) of a highly anisotropic anorthosite cannot represent the value of a true isotropic equivalent. The CPO-induced anisotropy enhances and decreases the foliation-normal incidence reflectivity at gabbro-peridotite and anorthosite-peridotite interfaces, respectively.