Relationship between anisotropy of P and S wave velocities and anisotropy of attenuation in serpentinite and amphibolite

Abstract

Abstract. Velocities and Q values of P and S waves as functions of pressure and temperature (at 100 arid 600 MPa) are presented for a serpentinite and an amphibolite.Both rocks exhibit a strong lattice preferred orientation (LPO) of the major mineral phases antigorite and hornblende, respectively. Velocities and Q values increase with pressure; the rate of increase is different in the three orthogonal directions (normal and parallel to foliation and lineation) and closely related to progressive closure of microcracks. Increasing temperature decreases velocities and Q values only slightly as long as thermal cracking is prevented by the applied confining pressure. Substantial anisotropy of velocities and Q in P and S waves is observed in both rocks but is found to be different in origin. Anisotropy of P and S wave velocities is highest at low pressure and basically caused by constructive interference of effects related to oriented microcracks and to the LPO of major minerals. Increasing confining pressure decreases velocity anisotropy at a smaller and smaller rate. The residual anisotropy of P and S wave velocities (shear wave splitting) at high confining pressure is mainly a result of preferred mineral orientation. By contrast, anisotropy of Q is very low at low confining pressure and markedly enhanced as pressure is increased. At high confining pressure, substantial anisotropy of Q in P waves is apparent but reversed from that of P wave velocities' Q is highest in the direction normal to the foliation pl.ane P whereas Vp (and Vs) is lowest in this direction. The generation of a pronounced anisotropy of Qp by increasing pressure is due to a directionally dependent increase of contact areas on the oriented grain boundaries of the platy minerals defining the foliation. The increase of Q with pressure in the direction normal to foliation is mainly caused by the decrease of energy loss due to compressive strain relative to shear strain. The reverse is true for the X and Y directions (serpentinite) and X direction (amphibolite) parallel to the foliation plane.

Evolution of dilatancy and permeability in rock salt during hydrostatic compaction and triaxial deformation

“…The accuracy of the ultrasonic measurements is better than 1%. In some few runs attenuation of P and S wave velocities was determined by the spectrum-amplitude ratio method (details of the method are reported by Kern et al [1997]). …”

Voids and Rock Friction at Subseismic Slip Velocity

“…13), the maximum temperature on the sliding surface was less than 50°C. Taking into account the previous experimental results that the change in Q -1 due to the above heating is less than a few percent (e.g, Jackson et al 1992;Kern et al 1997), we think the temperature rise effect might not be significant in this case. For the case of the SHRA049-06 (Fig.…”

“…For the case of the SHRA049-06 (Fig. 14), by referring to the experimental results (Jackson et al 1992;Kern et al 1997), the change in attenuation is not significant in that temperature change. The variation of Q -1 value is still less than a few tens of percent, which is much smaller than that due to the voids.…”

“…Finally, it could be interesting to consider the effect of heating on the sliding surfaces because frictional force generates heat and thus temperature of the specimen increases during the experiments. In addition, elastic wave attenuation is related to the temperature (e.g., Jackson et al 1992); Kern et al 1997). Therefore, to investigate the effect of heating, we conducted finite element simulations to estimate the distribution of the temperature as shown in Appendix B.…”

Effect of pore and confining pressures on VP in thermally pre‐cracked granites

“…In particular, compressional waves velocity Vp was measured as a function of pressure (Birch, 1960;Kern et al, 1997;Spencer and Nur, 1976). Todd and Simmons (1972) show from Vp measurements as a function of pore pressure in lowporosity rocks that Vp is controlled by the effective pressure Pelt defined as Pefr= Pc-bPp, where Pc is the confining pressure, P?…”