GTSM at San Francisco East Bay
Chabot - CHT
1. 1.8 E of N
2. 121.8 E of N
3. 61.8 E of N
4. 31.8 E of N
Note that the gauge residuals have a Y axis scale range of +/- 4 microstrain for each component, and show that excluding known tectonic events, all variation in gauge strain rates have been less than +/- 0.5 microstrain per year for the whole of the 12 year record,(1992 - 2004). The variation in the CHT site is annual and is considered directly related to the topographic exposure of the site to the varying water storage in nearby Lake Chabot, and is seen on all these borehole instruments in this array.
The residual linear borehole recovery strain rate over the total record is shown in microstrain/year as the "slope" on each plot.
Residual data are recovered from the raw data by extracting least squares fitted exponential(s) and a linear function which describe the processes of curing of the grout and recovery of the borehole, after drilling and installation disturbance of the virgin stress field.
Regions used in the least squares fitting are marked in red on the X axis, and the fit parameters are printed in the upper panel of the figure and are also tabulated elsewhere.
In August 2003, the borehole recovery exponentials were updated from values determined in the early 1990's to incorporate the additional 10 years of data length now available. Changes of strain rates calculated from these residuals are unaffected by the exponential removal procedure and allow the very long term stability(see gauge 2 , gauge 3, and gauge 4) of the instruments to be quantified.
Data are provided for channels 2, 3, and 4. Anomalous tidal response was identified on channel 1 early in the life of this instrument. All post 1986 instruments include a redundant channel (here channel 4) to be used for this purpose when necessary.
OBSERVED STRAIN and UPPER SAN LEANDRO RESERVOIR
Chabot site (CHT) shows large seasonal strains associated with the water levels/storage volume in the nearby Upper San Leandro Reservoir. The correlation is shown, where the direct reservoir effect is dominantly on the areal strain with minimal perturbation on the shears. These effects are exacerbated by the strong topography between the reserve and the site.
LARGE SCALE OBSERVED STRAIN and TILT
There are accumulating strain rate and tilt rate changes which began in 2000 and continued since then). This change of strain rate appears to be over an extensive area. Data for the 11 year period ending October 2005 for the two stations SJT and CHT and the Sacks-Evertson dilatometer at Garin (cyan trace).
These sites are far apart and totally independent, yet the time signatures and amplitude scaling of the anomalous responses in areal strain at SJT in the Monterey Bay area and Gamma1 strain at CHT are remarkably similar. If it continues, it represents an anomaly at a spatial scale not previously identified in this project. The change in strain rate is approximately 0.6 microstrain per year at SJT and nearly 1 microstrain per year in Gamma1 at CHT, and should be detectable in GPS data for the time period. The effect is confirmed at CHT by the independent tilt channel NS. At both sites the changes are almost undisturbed by the SJT slowquake of March 2004 or the Parkfield event in September,2004.
The component data from all the miniPBO sites in San Francisco Bay area have also been residualised and examined for trend changes. However, all of these sites are under 4 years since installation, and corroborative signals would not be expected to be seen, especially as the changes seen in the East bay sites CHT and GAT began just before the miniPBO installation.
Data plots of interest in East Bay:
· Feb 1996 Strain : event detail plot
· April 27, 2000 Strain Tilt Creep: event detail plot with tilt measured on the GTSM on-board tiltmeter.
· May 11 - 17, 2001 Strain Tilt Creep: event detail plot with tilt measured on the GTSM on-board tiltmeter.
· June 1 - 2, 2001 Strain Tilt Creep: event detail plot with tilt measured on the GTSM on-board tiltmeter.
· ALL data to May 2003 Strain Tilt : detail plot with tilt measured on the GTSM on-board tiltmeter.