Geology topics

G. Hirth

Publications and source records attributed to G. Hirth.

4 recordsLinked to original sources

USGS2019

Azimuthal seismic anisotropy of 70 Ma Pacific‐plate upper mantle

Plate formation and evolution processes are predicted to generate upper mantle seismic anisotropy and negative vertical velocity gradients in oceanic lithosphere. However, predictions for upper mantle seismic velocity structure do not fully agree with the results of seismic experiments. The strength of anisotropy observed in the upper mantle varies widely. Further, many refraction studies observe a fast direction of anisotropy rotated several degrees with respect to the paleospreading direction, suggesting that upper mantle anisotropy records processes other than 2D corner flow and plate‐driven shear near mid‐ocean ridges. We measure 6.0 ± 0.3% anisotropy at the Moho in 70 Ma lithosphere in the central Pacific with a fast direction parallel to paleospreading, consistent with mineral alignment by 2D mantle flow near a mid‐ocean ridge. We also find an increase in the strength of anisotropy with depth, with vertical velocity gradients estimated at 0.02 km/s/km in the fast direction and 0 km/s/km in the slow direction. The increase in anisotropy with depth can be explained by mechanisms for producing anisotropy other than intrinsic effects from mineral fabric, such as aligned cracks or other structures. This measurement of seismic anisotropy and gradients reflects the effects of both plate formation and evolution processes on seismic velocity structure in mature oceanic lithosphere, and can serve as a reference for future studies to investigate the processes involved in lithospheric formation and evolution.

Journal of Geophysical Research B: Solid Earth↗

USGS2018

On the depth extent of co-seismic rupture

We investigate the implications of deformation experiments for the coseismic down‐dip extent of rupture in quasi‐dynamic, whole‐cycle earthquake models of a fault for which the depth of the transition between seismic and aseisimic fault slip depends on strain rate. The calculations use a dislocation fault model from Tse and Rice (1986) with a vertical strike‐slip orientation, mode III rupture, and variable along‐strike length. Our reference calculation is the original rheological representation of Tse and Rice with a strain‐rate‐independent transition. The primary calculations use two different representations of a strain‐rate‐dependent transition: (1) between rate‐weakening friction and dislocation creep and (2) between rate‐weakening and rate‐strengthening frictions. For both these cases, when fault strength is high (friction between 0.5 and 0.6) and the transition is sharp, coseismic slip extends a small distance (1–2 km) below the fixed temperature (depth) that is commonly used to define the rheological transition at the plate‐motion rate. Thus, coseismic slip occurs below the depth assumed in seismic hazard models using microseismicity or a chosen fixed‐temperature contour. Though significant coseismic slip occurs below the plate‐rate transition depth, the added moment is < 10 % "> < 10 % of the total. The deep extension is a region that is rheologically distinct; for example, deep coseismic slip can produce a stress increase rather than a stress drop. If friction is smaller, the deepening effect and its contribution to moment are larger. For all representations of the transition, average and surface slip increase with the along‐strike rupture length in a manner consistent with the limited data from natural observations. However, this property is not controlled by the assumed fault rheology; instead, it arises because the stiffness of the fault decreases weakly with fault length, an intrinsic and unrealistic property of the particular crustal scale fault model used.

Bulletin of the Seismological Society of America↗

USGS2007

Olivine friction at the base of oceanic seismogenic zones

We investigate the strength and frictional behavior of olivine aggregates at temperatures and effective confining pressures similar to those at the base of the seismogenic zone on a typical ridge transform fault. Triaxial compression tests were conducted on dry olivine powder (grain size ???60 ??m) at effective confining pressures between 50 and 300 MPa (using Argon as a pore fluid), temperatures between 600??C and 1000??C, and axial displacement rates from 0.06 to 60 ??m/s (axial strain rates from 3 ?? 10-6 to 3 ?? 10-3 s-1). Yielding shows a negative pressure dependence, consistent with predictions for shear enhanced compaction and with the observation that samples exhibit compaction during the initial stages of the experiments. A combination of mechanical data and microstructural observations demonstrate that deformation was accommodated by frictional processes. Sample strengths were pressure-dependent and nearly independent of temperature. Localized shear zones formed in initially homogeneous aggregates early in the experiments. The frictional response to changes in loading rate is well described by rate and state constitutive laws, with a transition from velocity-weakening to velocity-strengthening at 1000??C. Microstructural observations and physical models indicate that plastic yielding of asperities at high temperatures and low axial strain rates stabilizes frictional sliding. Extrapolation of our experimental data to geologic strain rates indicates that a transition from velocity weakening to velocity strengthening occurs at approximately 600??C, consistent with the focal depths of earthquakes in the oceanic lithosphere. Copyright 2007 by the American Geophysical Union.

Journal of Geophysical Research B: Solid Earth↗

USGS2007

Thermal structure of oceanic transform faults

We use three-dimensional finite element simulations to investigate the temperature structure beneath oceanic transform faults. We show that using a rheology that incorporates brittle weakening of the lithosphere generates a region of enhanced mantle upwelling and elevated temperatures along the transform; the warmest temperatures and thinnest lithosphere are predicted to be near the center of the transform. Previous studies predicted that the mantle beneath oceanic transform faults is anomalously cold relative to adjacent intraplate regions, with the thickest lithosphere located at the center of the transform. These earlier studies used simplified rheologic laws to simulate the behavior of the lithosphere and underlying asthenosphere. We show that the warmer thermal structure predicted by our calculations is directly attributed to the inclusion of a more realistic brittle rheology. This temperature structure is consistent with a wide range of observations from ridge-transform environments, including the depth of seismicity, geochemical anomalies along adjacent ridge segments, and the tendency for long transforms to break into small intratransform spreading centers during changes in plate motion.

Geology↗