Strategy for Geophysical Observations at Laser Sites — McDonald Observatory

Abstract

Measurements of geoid height and of convergent or divergent velocities between segments of the Earth’s lithosphere are objectives of a world-wide laser ranging program. However, substantial geological and geophysical evidence indicates that rigid horizontal motion of major lithospheric plates is an imperfect approximation, especially near plate boundaries and continental margins. Therefore, continuous geophysical surveillance must be maintained in the vicinity of laser sites to estimate local anomalies in tectonic movement. Except for first order geodetic methods, an indirect approach combining tilt, strain and seismic measurements must be used. Regarding tilt and strain, the secular deformations to be measured are larger than the tidal or teleseismic transients observed with conventional instruments, but a higher standard of long-term stability is required in the new application. This will probably be achieved at some sacrifice in sensitivity. In addition to stability, low-cost installation and maintenance are important considerations for a world network of sites. A local seismic network can delineate active faults by detection of microearthquakes and can aid in estimating the near- field displacements from macroseismic events. At McDonald Observatory, we installed a permanent seismograph station (MOT) and also operated a temporary network of four short-period seismographs for three months in 1975. With these network data, about 15 earthquakes were located within 200 km of the Observatory. We plan to expand the program in 1976 by installing a permanent network of seismograph stations linked to the Observatory by radio telemetry. Presently, we are also evaluating the sensitivity and stability of a biaxial bubble tiltmeter. Further instrument development work will be done at McDonald and in the Hockley Mine Observatory. It is hoped that the measurement strategy developed for the McDonald Observatory will be a useful model for world-wide mobile laser ranging stations.

Contribution No. 100.