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. Among the best strainmeters are the 750 meter baseline laser interferometer systems at Pinon Flat that operate in evacuated tubes and have massive end monuments with "optical anchors" thru the decomposed granite of the surface to virgin rock 20 meters down. They have achieved stability of better than 10^-6 / year but at great expense and effort. GPS measurements are also used, but over longer distances, and expected tectonic deformations and fault creep have been seen. But even here the wide area data does not agree with the on-fault creep data because the fault is at depth and its movement is better integrated over a large area of the surface rather than being localized at a surface fault. A classical reference on the subject can be found in: Agnew, D.C., "Strainmeters and Tiltmeters", Reviews of Geophysics, vol 24, No 3, 579-634, 1986 _______________ Regards, Sean-Thomas __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>