HR: 0830h
AN: G31B-0717    [PDF]
TI: Local fluid flow and borehole strain in the South Iceland Seismic Zone
AU: * J\'onsson, S
EM: sj@eps.harvard.edu
AF: Harvard University, Department of Earth and Planetary Sciences, 20 Oxford Street, Cambridge, MA 02138 United States
AU: Segall, P
EM: segall@pangea.stanford.edu
AF: Department of Geophysics, Stanford University, Stanford, CA 94305-2215 United States
AU: \'Ag\'ustsson, K
EM: kri@vedur.is
AF: The Icelandic Meteorological Office, B\'ustadavegur 9, Reykjav\'ik, 150 Iceland
AU: Agnew, D
EM: dagnew@ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, 9500 Gilman Drive, La Jolla, CA 92093-0225 United States
AB: Installation of 175 borehole strainmeters is planned for PBO. It is therefore vital to understand the behavior of existing strainmeter installations. We investigate signals recorded by three borehole dilatometers in the south Iceland seismic zone following two $M_w6.5$ earthquakes in June 2000. Poroelastic relaxation has been documented following these events based on InSAR and water level data [{\it J\'{o}nsson et al., 2003, Nature}]. According to poroelastic theory for a homogeneous isotropic (unfractured) medium, the anticipated post-seismic volumetric strain has the same sign as the coseismic strain step. For example, coseismic compression results in pore-pressure increases; post-earthquake fluid drainage causes additional compression. However, we find that observed strain changes vary considerably between different instruments after the earthquakes. One instrument (HEL) behaves as expected with transient strain increasing with the same sign as the coseismic strain step. Another instrument (SAU) shows partial strain relaxation, opposite in sign to the coseismic signal. The third (BUR) exhibits complete strain relaxation by 3-4 days after the earthquakes ({\it i.e.,} BUR does not record any permanent strain). BUR has responded in the same fashion to three different earthquakes and two volcanic eruptions, demonstrating conclusively that the transient response is due to processes local to the borehole. Fluid drainage from cracks can explain these observations. Rapid straining results in compression (extension) of the rock and strainmeter. Fluid filled fractures near the borehole transmit normal stress, due to the relative incompressibility of water. Thus, at short time scales the instrument records a coseismic strain step. With time, however, fluid flows out of (in to) the fractures, and the normal stress transmitted across the fractures decreases (increases). As the stress relaxes the strainmeter expands (contracts), reversing the coseismic strain. Barometric responses are consistent with this model. HEL shows shows high coherence and zero phase lag at periods of 1-8 days. SAU and especially BUR show substantial phase lags at periods longer than 4 days. Our results highlight the need to limit (or at least calibrate the effects of) fracture dominated flow in the near field of borehole strainmeters.
DE: 1242 Seismic deformations (7205)
DE: 1294 Instruments and techniques
SC: Geodesy [G]
MN: 2003 Fall Meeting