- Experiment Name: Observational Constraints on Persistent Water Level
Changes Generated by Seismic Waves
- PI: Brodsky, Roeloffs
- Email: brodsky@seismo.berkeley.edu
- Sponsor: USGS
- Inst: Berkeley, USGS
- Start_Date: 12/1/2001
- End_Date: 12/31/2002
- Lat: 42.5
- Long: -123.37
- Chan6: 1
- Broadband: 1
- Descr: Large well-water level changes (>10 cm) in response to distant
(>100s km) earthquakes have proven enigmatic for over 30 years [Bower
and Heaton, 1978]. The redistribution of pore pressure can generate
crustal deformation and perhaps even trigger seismicity [Hill, 1993]. However,
the mechanism by which small cyclic stresses induce persistent pore
pressure changes has remained uncertain. We propose to constrain the
mechanism for coseismic water level steps by using both high-sample
rate water level data and seismic data at a well-studied site in Grants
Pass, OR. The transfer function from the ground velocity to the water
level in the well is sensitive to the aquifer properties [Cooper, 1965].
Therefore, if we measure both the water level and the seismic ground
velocity, we can measure the aquifer permeability. A preliminary study
shows that we can measure permeability changes induced by seismic waves
using this method. For the preliminary study, we used seismic data
from the broadband Berkeley Digital Seismic Network station Yreka Blue
Horn Mine (YBH) in Yreka, CA (41.732N 122.71039W). The 100 km separation
between well and seismometer makes precise analysis of the phase response
impossible and does not allow us to account for local site effects.
We therefore request a seismometer and datalogger to install at Grants
Pass next to the well (42.5N 123.37W). Since we are studying the frequency
response to regional and teleseismic events, the seismometer must be
broadband. The well typically has oscillatory responses to 5 teleseisms
per year and produces a quasi-permanent offset once every two years.
It is therefore crucial to maintain continuous monitoring over as long
a time as possible. We expect to acquire a permanent seismometer in
late 2002 or early 2003. We therefore request one instrument from PASSCAL
to cover the site for the next year. Bower, D.R. and K.C. Heaton,
Response of an aquifer near Ottawa to tidal forcing and the Alaskan
earthquake of 1964, Canadian Journ. of Earth Sci., 15, 331-340, 1978.
Cooper, H.H., J.D. Bredehoeft, I.S. Papdopulos and R.R. Bennett, The
Response of Well-Aquifer Systems to Seismic Waves, Jour. Geophys. Res.,
70, 3915-3926, 1965. Hill, D.P. et al., Seismicity Remotely Triggered
by the Magnitude 7.3 Landers, California, Earthquake, Science, 260,
1617-1623, 1993.
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