Preseismic Velocity Changes Observed from Active Source Monitoring at the Parkfield SAFOD Drill Site

Preseismic Velocity Changes Observed from Active Source Monitoring at the Parkfield SAFOD Drill Site Figure 1
(a) Map of the experiment site showing the SAFOD drill site and the seismicity (circles). (b) Depth distribution of earthquakes that occurred in the experimental period. Red square, red and green circles indicate the SAFOD experiment site, the M3 and M1 earthquake, respectively. (c) Predicted coseismic stress changes at SAFOD for earthquakes occurring between December 22 of 2005 (day 50) and January 1 of 2006 (day 60). Note velocity changes (arrows) started a few hours before the two earthquakes (solid lines).
It is well known from laboratory experiments that seismic velocities vary with the level of applied stress [e.g., Birch, 1960; Nur and Simmons, 1969]. Such dependence is attributed to the opening and closing of microcracks due to changes in the stress normal to the crack surface [e.g., O’Connell & Budiansky, 1974]. In principle, this dependence constitutes a stress meter, provided that the induced velocity changes can be measured precisely and continuously. Indeed, there were several attempts in the 1970s to accomplish this goal using either explosive or non-explosive surface sources [e.g., Leary et al., 1979]. However, source repeatability and the precision of travel- time measurements only recently became adequate to observe the effect of tidal and barometric stress changes on seismic velocities in the field [Yamamura et al., , 2003; Silver et al., 2007]. The SAFOD pilot and main holes provided an unprecedented opportunity for a continuous active-source cross-well experiment to measure velocity changes at seismogenic depth. A specially designed 18-element piezoelectric source and a three-component accelerometer were deployed inside the pilot and main holes, respectively, at ~1 km depth. Over a two months period, we found a 0.3% change in the average S-wave velocity along the ~10 m baseline between the pilot and main holes. The velocity change showed an excellent anti-correlation with the barometric pressure. We attributed this anti-correlation to stress sensitivity of seismic velocity and the stress sensitivity is estimated to be 2.4x10-7 /Pa. Our results thus indicate that substantial cracks and/or pore spaces exist even at seismogenic depths and may thus be used to monitor the subsurface stress field. We also observed two large excursions in the travel-time data that are coincident with two earthquakes that are among those predicted to produce the largest coseismic stress changes at SAFOD (Figure 1) [Niu et al, 2008]. The two excursions started approximately 10 and 2 hours before the events, respectively, suggesting that they may be related to pre-rupture stress induced changes in crack properties, as observed in early laboratory studies [e.g., Scholz, 1968].
</p><p>Birch, F. (1960), The velocity of compressional waves in rocks to 10 kilobars, part 1, J. Geophys. Res., 65, 1083–1102.
</p><p>Leary, P.C., P.E. Malin, R.A. Phinny, T. Brocher, and R. Voncolln (1979), Systematic monitoring of millisecond travel time variations near
Palmdale, California, J. Geophys. Res., 84, 659-666. F. </p><p>Niu, P. Silver, T. Daley, X. Cheng, E. Majer, 2008, Preseismic velocity changes observed from active source monitoring at the Parkfield
SAFOD drill site, Nature, 454, doi:10.1038/nature07111.
</p><p>Nur, A., and G. Simmons (1969), The effect of saturation on velocity in low porosity rocks, Earth Planet. Sci. Lett., 7, 183-193.
</p><p>Scholz, C.H. (1968), Microfracturing and the inelastic deformation of rock I: compression, J. Geophys. Res., 73, 1417-1432.
</p><p>Silver, P.G., T.M. Daley, F. Niu, and E.L. Majer
</p><p>(2007), Active Source Monitoring of Cross-Well Seismic Travel Time for Stress-Induced Changes, . Bull. Seismol. Soc., 97, 281 - 293.
</p><p>Yamamura, K., O. Sano, H. Utada, Y. Takei, S. Nakao, and Y. Fukao (2003), Long-term observation of in situ seismic velocity and attenuation, J. Geophys. Res., 108, 10.1029/2002JB002005.
Acknowledgements: We would like to thank the NSF funded SAFOD program and all the people involved for providing the experiment site, Rob Trautz of LBNL for supplying the barometric pressure logger, Dr. Mark Zumberge of University of California San Diego for providing the SAFOD strainmeter data, Don Lippert and Ramsey Haught of LBNL for helping the field work.</p>


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