GTSM at Parkfield, California
DonaLee - DLT
The collocated boreholes at DonnaLee site.
The Sacks Evertson dilatometer borehole (background) was considerably deeper. Both boreholes were artesian making grout procedures difficult.
1. 71.4 E of N
2. 11.4 E of N
3. 131.4 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 17 year record,(1986 - )..The variation in the DLT site is annual and directly related to the presence of an acquifer which traverses the borehole, and is seen on the other borehole instrument at this site.
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 1 , gauge 2, and gauge 3) of the instruments to be quantified.
Data plots of interest:
Parkfield Regional Anomaly 1993-1997: multi- instrument event detail See papers.
September, 2000 Strain Creep event across the Parkfield array : tensor strain, dilatometer volumetric strain,creep detail
Selection of Characteristic Strain/Creep events at FLT: detail of several events over the years
Preliminary processing of San Simeon M6.5 as seen at our GTSM sites at DonnaLee, and Frohlich in Parkfield, and for San Juan Bautista.
Coseismic steps are approximately:-
DLT FLT SJT
eadl 74ne, eafl 700ne,
gam1dl 15.5ne, gam1fl -130ne gam1sj -11ne
gam2dl 220ne, gam2fl 890ne gam2sj -26ne
The event was also observed at San Juan Bautista:
These instruments provide strain data on Long term strain accumulation. Data records from the from the three Parkfield sites EDT, FLT and DLT from 1987.
Medium term strain changes associated with earthquake activity and other fault processes.
Coseismic strain offsets useful for constraining earthquake source mechanisms,eg see San Simeon 2003, above.
High precision and high stability measurements of the horizontal strain field in a region are made in California using a Borehole Tensor Strain Measurement instrument (BTSM). These instruments are installed at a depth of 100-200 metres at 7 sites in California. The sites are shown in red in the accompanying map.