GTSM at San Juan Bautista, California

 

SJT is close to San Juan Bautista to the south of San Francisco, in an array consisting presently of USGS installed creepmeters XSJ, XHR, CWC,CWN, and XMR.

 

 

Gauge angles

1. 327.4 E of N

2. 267.4 E of N

3. 207.4 E of N

 

 

 

These instruments provide 3 component strain data on

1. Long term strain accumulation. Long term data records at this site (SJT) are available.

2. Medium term strain changes associated with earthquake activity and other fault processes. The sample data shows a slow earthquake sequence recorded at SJT in 1992.

 

Coseismic strain offsets useful for constraining earthquake source mechanisms.

Raw, exponential models, and residual plots for each measured channel are shown for   gauge 1,  gauge 2, and for   gauge 3.

 

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 +/- 0.6 microstrain per year for the whole of the 20 year record. The residual linear borehole recovery strain rate over the total record is shown in microstrain/year as the "slope" on each plot.

 

Residual data is 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:

 October 17, 1989 Mag 7.1 Loma Prieta : event detail plot

 December 12, 1992 Mag 4.8 'Slow quake' : event detail plot

 August 12, 1998 Mag 5.5 'Slow quake' event 12km distant: short plot and long plot

 July 13, 2003 'Slow quake' of apparent magnitude 4.5 associated with a Mag 3 seismic  event about 3 km to the west of the site. Note that apart from a small co-seismic step, this event is present only in the shears, and is not seen at the surface by any of the regions creepmeters. A small creep event, possibly associated, occurred at CWN site on July 29.

 

Areal co seismic = +19.4ne

Areal aseismic = 0 ne

Gamma 1 co-seismic = -71.7ne

Gamma 1 aseismic = -410ne over 17 days

Gamma 2 co-seismic = -71.6ne

Gamma 2 aseismic = -310ne over 10 days

 

Comparison of this event with the 98 Slow quake indicates they are of similar magnitude.

 

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 and Gamma1 strain at CHT in San Francisco Bay Area 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.