D.A. Lockner
A.V. Ponomarev
S. A. Stanchits
2003
<p>Fluid<span> </span>infiltration<span> and pore </span>fluid<span> pressure </span>changes<span> are known to have a significant effect on the occurrence of earthquakes. Yet, for most damaging earthquakes, with nucleation zones below a few kilometers depth, direct measurements of </span>fluid<span> pressure variations are not available. Instead, pore </span>fluid<span> pressures are inferred primarily from seismic-</span>wave<span> propagation characteristics such as V</span><sub>p</sub><span>/V</span><sub>s</sub><span> ratio, </span>attenuation<span>, and reflectivity contacts. We present laboratory measurements of </span>changes<span> </span>in<span> </span>P<span>-</span>wave<span> </span>velocity<span> and </span>attenuation<span> during the injection of water into a </span>granite<span> sample as it was loaded to failure. A cylindrical sample of Westerly </span>granite<span> was deformed at constant confining and pore pressures of 50 and 1 MPa, respectively. Axial load was increased </span>in<span> discrete steps by controlling axial displacement. </span>Anisotropic<span> </span>P<span>-</span>wave<span> </span>velocity<span> and </span>attenuation<span> fields were determined during the experiment using an array of 13 piezoelectric transducers. At the final loading steps (86% and 95% of peak stress), both spatial and temporal </span>changes<span> </span>in<span> </span>P<span>-</span>wave<span> </span>velocity<span> and peak-to-peak amplitudes of </span>P<span> and S waves were observed. </span>P<span>-</span>wave<span> </span>velocity<span> anisotropy reached a maximum of 26%. Transient increases </span>in<span> </span>attenuation<span> of up to 483 dB/m were also observed and were associated with diffusion of water into the sample. We show that </span>velocity<span> and </span>attenuation<span> of </span>P<span> waves are sensitive to the process of opening of microcracks and the subsequent resaturation of these cracks as water diffuses </span>in<span> from the surrounding region. Symmetry of the orientation of newly formed microcracks results </span>in<span> </span>anisotropic<span> </span>velocity<span> and </span>attenuation<span> fields that systematically evolve </span>in<span> response to </span>changes<span> </span>in<span> stress and influx of water. With proper scaling, these measurements provide constraints on the magnitude and duration of </span>velocity<span> and </span>attenuation<span> transients that can be expected to accompany the nucleation of earthquakes </span>in<span> the Earth's crust.</span></p>
application/pdf
10.1785/0120020101
en
Seismological Society of America
Anisotropic changes in P-wave velocity and attenuation during deformation and fluid infiltration of granite
article