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Measurements of tectonic deformation depend on both accurate instrumentation and adequate coupling of the apparatus to the earth's crust. Existing techniques, capable of resolving the signals of interest (strain rates of 10 n∈/yr), are mainly observatory based. The limitation in the baselength of these instruments (∼1 km) results in a requirement that the reference monuments be exceptionally stable (10 μm/yr). However, records from Piñon Flat Observatory, California, show that the actual horizontal displacements for massive near surface monuments, emplaced in competent, weathered granite, are of the order of 50 μm/yr. The low noise level of the strain measurements at this site indicates that this magnitude of monument displacement is abnormally small. Until high‐accuracy geodetic techniques are developed, sophisticated monuments (or monument monitoring devices) will be necessary to record faithfully continuous crustal deformation.
Frank K. Wyatt (Wed,) studied this question.