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Stability Research in Tectonically “quiet” Australia

Australian Stability Data

The instrument has long term stability of better than one nanostrain per year with good de-stressing procedures.  An instrument in a standard inclusion has a long term stability limited by the stability of the inclusion rather than the instrument.   Typically this is better than 100 nano strain per year, so that observation of long term rates in tectonic regions larger than this will be robust.

Strain data from Hillcrest site in eastern Australia – a tectonically inactive region.

This performance claim has been demonstrated by installation of a standard instrument in a relatively stable tectonic region in Australia to directly address this issue. The instrument was installed in a granite environment north of Canberra in 1991 and the record is shown above.  The shear strains over the entire record show approximately 200 nanostrain drift/variation over the 11 years or less than 20 nanostrain per year even during the grout curing sequence (1991 to 1993).  Areal strain fluctuations over that period vary by up to 600 nanostrain, and as expected show  mild correlated with the monthly averaged rainfall plotted at the bottom of the figure.  The offset in areal strain during 1993 is an artifact produced by an experiment with an alternate uphole measurement system.  The associated rainfall record demonstrates well the general insensitivity of the GTSM to rainfall, with the increased annual average rainfall in the late nineties influencing  the areal strain record by a few hundred nanostrain over five years.

 

Californian Stability Data       
Even in the tectonically active zones in California, the stability of the system is also well documented using the longest deployments.  At the San Juan,  for example, 18 years of strain data observed at the San Juan Bautista site in northern California in the figure below, show a stability in areal strain (red trace) at better than five microstrain  over that 18 year period despite the significant changes of shear strain rate associated with the Loma Prieta event and after the 1992 slow earthquake.   The shear strain signals in the intervening periods between these events show a stability at better than 100 nanostrain per year.  The absolute values of in particular the gamma 1 shear rate is not significant in stability studies, being referenced to exponential and  linear drift rates determine before 1985.   Absolute values of long term rates are much better determined by GPS solutions- the borehole strain meters are best suited to high resolution infill.  The GPS data has NOT been used to constrain these plots.

A second example is shown in the figure below by direct comparison with the Pinon Flat Laser Strainmeter.   Only one component (NW-SE) of the LSM is optically anchored (the other two components are insufficiently stable for the comparison). A NW-SE strain is computed from the measured GTSM data.  This GTSM inferred NW-SE record (red trace below) mirrors the laser strain record (black trace) after the grout curing is complete, from 1988 to 1998.  This is a clear indicator that the borehole strain is comparable in stability to the optically anchored laser system.  Note significant offset for Landers earthquake.  The shift in 1993 was identified by Wyatt as caused by pumping at a local water well..  The planned comparison between the GTSM and the Sacks–Evertson dilatometer at Pinon could never been performed because all three dilatometers at the site failed within a few years of installation.

 

In the figure to the right, Strain  data(red) from Pinon Flat GTSM directly compared with LSM(black).  Beginning 1988 after the grout cure was complete,  for the ten years to 1998 when one of the GTSM gauges  was damaged by  lightning,  the two systems track to within a total of a few hundred nano-strain in the presence of a significant imposed gamma 1 strain despite the large water level changes at the CIC well.

GTSM Technologies is located in:

Brisbane

Queensland

Australia

Phone: +61 7 3376 4848

E-mail: GTSM