The Deformational Environment of the Haleakala Lunar Laser Ranging Observatory

Abstract

The University of Hawaii is initiating a geodetic- geophysical program in support of the Lunar Laser Ranging at Haleakala, Maui, Hawaii. The program is aimed at measuring and understanding possible motion of the observatory site with respect to its surroundings, so that local and regional movement can be separated from the more general plate motion, as referred to other lunar ranging or VLBI stations.

Preliminary studies of ocean tide-gage data together with geological evidence indicate a long-term downward flexure of the Hawaiian island chain at its eastern end of about four mm per year with respect to Honolulu. However, depending on the epoch being considered, opposite trends lasting 10 or more years are recognizable. Short-term differential variations in the late 1950’s and early 1960’s (between islands) correlate with the positional shift of South America (as obtained from the annual average position of three astronomical observatories) preceding and in response to the large, magnitude 8.3, 1960 Chile and other circum-Pacific earthquakes. The corresponding tide level differences (from 12-month running means) exceed the lunar ranging accuracy (of 3 cm). The p-p difference was 6 cm between Honolulu and Maui, and 7 cm between Honolulu and the island of Hawaii (Hilo). The corresponding p-p astronomical longitudinal shift of South America reached about 60 cm, which compares to a distributed (over the whole Nazca plate edge) seismically determined slip of about 20 cm. The Hawaiian Islands seem to respond to the stress variation by lithospheric buckling. Further investigation of the seismically determined cumulative slip for the period 1900 to 1964 along major Pacific plate boundaries indicates a rather jerky motion of up 1 m over a year or two, and much longer quiescent times over which stress and strain are again accumulated.

The implication is that tectonic plate motion is not a smooth process. Short-term rather large variations, of the order of tens of centimeters, should be expected in the three-dimensional position of the Haleakala Lunar Ranging station.