Charles A. Hurich

Seismic reflectivity of mylonite zones in the crust

Deep fault zones are characterized by mylonitic rocks with strong preferred orientation of constituent minerals and retrograde mineral assemblages. Sonic velocities are consequently lower (for propagation directions normal to mylonite foliation) than in surrounding unmylonitized rocks. Synthetic seismograms computed for low-velocity, near-horizontal, thick mylonite zones of complex, laminated geometry show multicyclic reflections with amplitudes up to twice those generated by a single interface. Seismic characteristics of mylonite zones are sufficient to produce reflections in crystalline rocks, provided noise levels are relatively low.

Nature of the Wind River thrust, Wyoming, from COCORP deep-reflection data and from gravity data

Critical data for the interpretation of Laramide structure, a major tectonic problem bearing on the formation of the Rocky Mountains, have been obtained by the Consortium for Continental Reflection Profiling (COCORP) in the form of deep crustal seismic-reflection profiles. The Wind River Mountains in Wyoming have been uplifted by the Wind River thrust, which can be traced on COCORP seismic-reflection profiles to at least 24-km depth with an average dip of 30° to 35°. This Laramide uplift is thus the result of extensive horizontal compression with a minimum horizontal displacement of 21 km and a minimum vertical displacement of 13 km. The crust appears to have deformed essentially as a rigid plate. Gravity anomalies across the uplift can be modeled by a thrust, with the same geometry as indicated by the seismic-reflection profiles.

Seismic evidence of mylonite reflectivity and deep structure in the Kettle dome metamorphic core complex, Washington

The first direct test of the seismic reflectivity of mylonites is provided by a seismic reflection profile over a mylonite zone exposed in the Kettle dome metamorphic core complex, northeastern Washington. The mylonite zone developed at mid-crustal levels and has undergone a minimum of retrogression and cataclasis during uplift. Downdip projection of the exposed mylonite zone into the seismic profile indicates that the compositionally layered mylonite zone is reflective and traceable to at least 5-km depth as a complex zone of multicyclic reflections. The seismic data also show that the crust beneath the Kettle dome is characterized by abundant, strongly reflective, moderately dipping layers down to at least 18 km. These layers can be interpreted in terms of compressional or extensional orogenic events or a combination of both. Models for the structure and genesis of metamorphic core complexes must take these data into account.

Gravity interpretation of the southern Wind River Mountains, Wyoming

The Wind River range is the largest Laramide uplift in Wyoming and is cored by Precambrian rocks thrust over sedimentary rocks of the Green River Basin to the southwest. The origin of these uplifts is in dispute; it has been ascribed to horizontal compression along thrusts or vertical movement along high‐angle reverse faults. Therefore, the attitude of the bounding fault is critical to understanding the mechanics of deformation. A gravity study based on 1800 stations has been carried out to attack this question and to complement a COCORP deep crustal reflection study. Smoothed free‐air gravity anomalies are highly positive over the Wind River range and near zero over the adjacent margins. Bouguer gravity anomalies range from −252 and −225 mgal in the Green River basin and Wind River basin, respectively, to −150 mgal in the basement core of the Wind River range. The bulk density of sedimentary rocks in the adjacent basins is the critical parameter for gravity interpretation of the deep structure and ranges...

Geotechnical Site Investigation Using S-waves with Implications for Ground Motion Analysis

Abstract Evaluation results of shear wave attenuation-based ground motion restricted by fracture orientation and rheology, from among those of an extended experimental study, are presented herein. The issues of competence of fractured bedrock dynamically disturbed multilaterally are assessed. Disturbance is primarily modelled by Sh and Sv stimulation, given fracture orientation, while subjected to direct fracture stress regime conditions varying in time. Hence, directionalities of polarisation and stress are taken into consideration simultaneously following simple site-specific non-erodetic approach. Comparison of spectral curves and spectral ratio curves of attenuation with respect to variations of direction and stress emphasise the amplification of the ‘seismic response’ in one direction compared to the other, i.e. vertical vs. horizontal, in terms of weighing possibilities of or predicting structural integrity against failure. The composite analyses of multiple spectral curves not only enable determination of the orientation of the fracture set/s in space but also allow inferring the nature of more amplified response perpendicular to the crack surface compared to that of a response parallel to the crack surface.

Practical implementation of Seismic Interferometry for imaging a steeply dipping dyke

We present a simple, heuristic analysis of seismic interferometry from a ray tracing perspective. The analysis allows for maximum insight into the acquisition and processing parameters of a walk-away VSP survey utilising the virtual source technique for imaging a steeply-dipping dyke. We demonstrate that by understanding which image rays are captured by the survey we can adjust the processing sequence to utilise only that data which contains the virtual source information, and therefore obtain a better final image.