Geology topics

Ze’ev Reches

Publications and source records attributed to Ze’ev Reches.

5 recordsLinked to original sources

USGS2019

The susceptibility of Oklahoma’s basement to seismic reactivation

Recent widespread seismicity in Oklahoma is attributed to the reactivation of pre-existing, critically stressed and seismically unstable faults due to decades of wastewater injection. However, the structure and properties of the reactivated faults remain concealed by the sedimentary cover. Here, we explore the major ingredients needed to induce earthquakes in Oklahoma by characterizing basement faults in the field, in seismic surveys and via rock-mechanics experiments. Outcrop and satellite mapping reveal widespread fault and fracture systems with trends that display a marked similarity to the trends of recent earthquake lineaments. Our three-dimensional seismic analyses show steeply dipping basement-rooted faults that penetrate the overlying sedimentary sequences, representing pathways for wastewater migration. Experimental stability analysis indicates that Oklahoma’s basement rocks become seismically unstable at conditions relevant to the dominant hypocentral depths of the recent earthquakes. These analyses demonstrate that the geometry, structure and mechanical stability of Oklahoma’s basement make it critically susceptible to seismic reactivation.

Oklahoma↗

USGS2015

Moment tensors and other source parameters of mining‐induced earthquakes in TauTona Mine, South Africa

Induced seismicity exhibits diverse source mechanisms that are often difficult to constrain for small events. Here, we use data from the in‐mine seismic network, the Natural Earthquake Laboratory in South African Mines network, and a temporary Program for the Array Seismic Studies of the Continental Lithosphere deployment in TauTona Mine, South Africa, to determine full moment tensors of 100 mining‐induced earthquakes in the magnitude range −2.7< M w <2.5. Ground displacement derived from velocity and acceleration data show clear near‐field effects, indicating that the lowest frequencies are well resolved. Phase amplitudes of between 11 and 77 picks per event were inverted to obtain the six independent moment tensor components. The quality of each moment tensor solution is quantified using (1) the misfit between observed and synthetic waveforms, (2) bootstrap resampling to estimate uncertainties, and (3) the F ‐test to determine the need for including an isotropic component with an extra degree of freedom in the solution. The results indicate 82% of the events have well‐constrained solutions, and 45% of the well‐constrained events require an isotropic source term. Throughout the magnitude range, both deviatoric and implosive mechanisms are observed, with implosive ratios of volume change to shear deformation (ΔV/Σ τ ) of −1.03 to −0.15. Two explosive events are observed at M w −0.5 and −0.2, withΔV/Σ τ =0.15 and 0.51, respectively. For the largest events, we determine maximum slip and apparent stress ( τ a ) and find values consistent with those of natural tectonic earthquakes, with 0.1≤ τ a ≤9.2 MPa. Our results support previous speculation on the nature of isotropic components of mining‐induced earthquakes, in which events of all sizes begin as shear failure that may intersect a void (tunnel or stope) and cause collapse, whereas only small events result in explosive sources.

TauTona Mine↗

USGS2011

The role of adsorbed water on the friction of a layer of submicron particles

Anomalously low values of friction observed in layers of submicron particles deformed in simple shear at high slip velocities are explained as the consequence of a one nanometer thick layer of water adsorbed on the particles. The observed transition from normal friction with an apparent coefficient near μ = 0.6 at low slip speeds to a coefficient near μ = 0.3 at higher slip speeds is attributed to competition between the time required to extrude the water layer from between neighboring particles in a force chain and the average lifetime of the chain. At low slip speeds the time required for extrusion is less than the average lifetime of a chain so the particles make contact and lock. As slip speed increases, the average lifetime of a chain decreases until it is less than the extrusion time and the particles in a force chain never come into direct contact. If the adsorbed water layer enables the otherwise rough particles to rotate, the coefficient of friction will drop to μ = 0.3, appropriate for rotating spheres. At the highest slip speeds particle temperatures rise above 100°C, the water layer vaporizes, the particles contact and lock, and the coefficient of friction rises to μ = 0.6. The observed onset of weakening at slip speeds near 0.001 m/s is consistent with the measured viscosity of a 1 nm thick layer of adsorbed water, with a minimum particle radius of approximately 20 nm, and with reasonable assumptions about the distribution of force chains guided by experimental observation. The reduction of friction and the range of velocities over which it occurs decrease with increasing normal stress, as predicted by the model. Moreover, the analysis predicts that this high-speed weakening mechanism should operate only for particles with radii smaller than approximately 1 μm. For larger particles the slip speed required for weakening is so large that frictional heating will evaporate the adsorbed water and weakening will not occur.

Pure and Applied Geophysics↗

USGS1996

The Loma Prieta, California, Earthquake of October 17, 1989: Main shock characteristics

The October 17, 1989, Loma Prieta, Calif., earthquake (0004:15.2 G.m.t. October 18; lat 37.036º N., long 121.883º W.; 19-km depth) had a local magnitude (M L ) of about 6.7, a surface-wave magnitude (M S ) of 7.1, a seismic moment of 2.2x10 19 N-m to 3.5x10 19 N-m, a source duration of 6 to 15 s, and an average stress drop of at least 50 bars. Slip occurred on a dipping fault surface about 35 km long and was largely confined to a depth of about 7 to 20 km. The slip vector had a large vertical component, and slip was distributed in two main regions situated northwest and southeast of the hypocenter. This slip distribution caused about half of the earthquake's energy to be focused toward the urbanized San Francisco Bay region, while the other half was focused toward the southeast. Had the rupture initiated at the southeast end of the aftershock zone, shaking in the bay region would have been both longer and stronger. These source parameters suggest that the earthquake was not a typical shallow San Andreas-type event but a deeper event on a different fault with a recurrence interval of many hundreds of years. Therefore, the potential for a damaging shallow event on the San Andreas fault in the Santa Cruz Mountains may still exist.

California↗

USGS1992

Microcrack interaction leading to shear fracture

In this paper we draw upon recent laboratory results concerning the nucleation and growth of shear fractures in brittle rock. In homogeneous, crystalline rock such as granite, fault nucleation occurs rapidly and with only subtle changes in precursory microcrack patterns. Once nucleated, the fault grows rapidly, restricting microcrack damage to a small region near the advancing fracture front. Observations of acoustic emission locations during fault nucleation and growth combined with determinations of microcrack densities in the fractured samples have led to a conceptual model in which brittle fracture is controlled by microcrack interactions.

Conference Paper↗