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Preseismic Velocity Changes Observed from Active Source
Monitoring at the Parkfield SAFOD Drill Site
Fenglin Niu (Department of Earth Science, Rice University), Paul G. Silver (Department of Terrestrial Magnetism,
Carnegie Institution of Washington), Thomas M. Daley (Earth Sciences Division, Lawrence Berkeley National Laboratory),
Xin Cheng (Department of Earth Science, Rice University), Ernest L. Majer (Earth Sciences Division, Lawrence Berkeley
National Laboratory)
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 traveltime
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].
36 o 02'
36 o 00'
35 o 58'
35 o 56'
35 o 54'
NW
(b)
Depth (km)
ms
5
10
(a)
SAFOD
M3 earthquake
M1 earthquake
km
0 5
120 o 36' 120 o 30' 120 o 24'
-10 -5 0 5 10
Along fault distance (km)
4 (c)
12/24/05 10:10
2
12/29/05 01:32
12/23/05 23:24
0
12/28/05 23:00
-2
52
56
Elapse time (day) from 11/02/05
SE
0.4
0.2
0.0
-0.2
60
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).
(KPa)
References
Birch, F. (1960), The velocity of compressional waves in rocks to 10 kilobars, part 1, J. Geophys. Res., 65, 1083–1102.
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. 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.
Nur, A., and G. Simmons (1969), The effect of saturation on velocity in low porosity rocks, Earth Planet. Sci. Lett., 7, 183-193.
Scholz, C.H. (1968), Microfracturing and the inelastic deformation of rock I: compression, J. Geophys. Res., 73, 1417-1432.
Silver, P.G., T.M. Daley, F. Niu, and E.L. Majer
(2007), Active Source Monitoring of Cross-Well Seismic Travel Time for Stress-Induced Changes, . Bull. Seismol. Soc., 97, 281 - 293.
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.
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