PSN-L Email List Message

Subject: Re: strainmeters/creepmeters
From: Ed Thelen ethelen@........
Date: Sat, 20 May 2000 21:53:43 -0700

re: following message - 20 years ago, creep measurements 
were plagued by many surprising (to clean handed city dwellers
and college students) problems.   

I visited an installation near Hollister, CA one weekend.  
The maintenance technician was there, and doing the usual:

  - cleaning out crickets, and even mice that had burrowed
     through the cracks and other real separations between
     various walls, tubes, holes for wires, ...

  - manually clearing out mud from the wires and strain gauge
    heads due to a "once in a hundred year storm", which seemed
    to come every year or so.

  - re-adjusting the time standard by comparing the phase 
    of the 400 wave from the on site time unit and the 
    400 cycle wave from the WWV receiver using a big
    vacuum tube osciloscope.

This was before GPS and similar technologies -
 - ah - for the good old days  ;-)

  Ed Thelen

S-T Morrissey wrote:
> Jim,
> The first task when considering strainmeters or tiltmeters is just
> what range of strain or displacement is expected over what period of
> time. If you have an active fault crossing a paved road, a linear
> line of nails makes a great creepmeter for movements of centimeters
> per year. For continuous measurements a taught wire (often of Invar
> or carbon fiber) is stretched across the fault at an oblique angle
> inside a large (eg 6") ABS pipe between buried concrete vaults (pre-
> cast septic tanks are used). Often several fiber creepmeters are installed
> in parallel using the same pipe, vaults, and monuments. Extra fibers
> are set aside as spares, and using different wires or fibers allows
> temperature compensation.
> One end of the wire or fiber is anchored to a large a monument in as deep
> a hole as you can afford, and the free end is coupled to a displacement
> transducer anchored to a similar monument. In hard rock sites, earth-
> tide resolution has been achieved at 10^-7 strain. Otherwise surface
> hydrology limits useful long term measurements, unless large movements
> are expected, such as 2 to 3 cm/year at Parkfield and Hollister in CA.
> Much work has been done trying to get reproducible data, especially
> in surface installations, over time periods of tectonic interest, like
> YEARS. After a number of wishful claims of success, surface strain
> or creep measurements have faded away except in locations with very
> active tectonics. There are no cheap or easy ways to do this.

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Larry Cochrane <cochrane@..............>