The borehole seismic packages provide good signal to noise characteristics compared to the NHFN stations due to their relatively deep installation. The systems have the best signal to noise near their 2-Hz characteristic frequency. The deepest installation at OXMT has the lowest noise at higher frequencies (Figure 8.4). We are considering adding a pre-amplifier to these systems to improve their signal-to-noise characteristics, since microseismic noise typically found around 0.1 Hz is not evident on most of the sites. These stations currently sample at 100-Hz, so they miss some of the seismic energy at high frequencies that are observed on the 500-Hz Parkfield borehole stations. Analysis of GPS observations show that the short-term daily repeatabilities in the horizontal components are about 0.5-1 mm, and annual signals with about 1 mm amplitude. These values are similar to those obtained with more typical monuments, such as concrete piers or braced monuments.
The tensor strainmeters appear to faithfully record strain signals over a broad frequency range, except at the longest periods where the strains show a long-term exponential signal. This large signal is most likely due to cement hardening effects and re-equilibration of stresses in the surrounding rock in response to the sudden appearance of the borehole. These effects can last for many years and are the principal reason that borehole strainmeters cannot reliably measure strain at periods greater than a few months.
At periods around 1 day, tidally induced strains are the dominant strain signal. Since the response of the strainmeter volumes is difficult to estimate independently, theoretically predicted Earth tides are typically used to calibrate the strainmeters. Figure 8.5 shows the approximate microstrain of the OHLN strainmeter over a several month period interval, and some of the steps required to clean the data, including removing the tides and atmospheric pressure effects. The remaining signal is highly correlated with rainfall, indicating the extent that hydrologic events can affect strain. The proximity of the strainmeters to the San Francisco Bay complicates the determinations of theoretical tides due to ocean and bay loading effects, a problem that has not been resolved for all the strainmeters, particularly for OHLN, which exhibits a physically unrealistic negative M2 tide.
At higher frequencies, strains due to seismic events are also well recorded. The December 26, 2004 M=9 Great Sumatra earthquake excited free oscillations of the Earth that were measurable on the Mini-PBO strainmeters and nearby dilatometers in California. Figure 8.6 shows a preliminary comparison of observed and theoretical normal modes measured at 4 stations. In this band, a number of normal modes (S, S, S, S, S, S, S, and S) are clearly observed. We are improving our strainmeter data editing techniques to remove high-rate atmospheric effects over long time intervals, which should improve resolution of the normal mode peaks.
We are also examining the strain data for other types of transient behavior, such as episodic creep or slow earthquake displacements. These multiparameter stations are providing a prototype for more than 140 planned borehole strainmeter, seismometer, and GPS stations in Cascadia and along the San Andreas fault as part of the Plate Boundary Observatory.
Berkeley Seismological Laboratory
215 McCone Hall, UC Berkeley, Berkeley, CA 94720-4760
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