IMPROVED STABILITY OF A DEEPLY ANCHORED GEODETIC MONUMENT FOR DEFORMATION MONITORING

A test of two different monument designs used in geodetic networks sho ws that monuments installed to depths of 5 to 10 meters, and laterally braced, exhibit less environmentally caused displacement than those i nstalled to 2 meters depth. At Parkfield, California, we have been mon itoring the lengths of 17 baselines over the past decade with a 2 to 3 day interval between measurements using a two-color geodimeter with a nominal precision of 0.5mm over 5km long baselines. Significant varia tions are observed on many of these baselines which appear to be relat ed to the seasonally occurring rainfall, with the larger variations ap proaching 10 mm over the past decade. To test whether we could improve upon the measurements on some of the more susceptible lines, at two s ites we installed new monuments within about 30 meters of the original monuments. After 1.5 years of measurements it is evident that the new monuments significantly attenuate the seasonal displacements to less than 1 mm. The use of deeply anchored monuments should improve the abi lity of fault-scale geodetic monitoring arrays to detect small tectoni c displacements.