Effect of the Earthquake East off Chiba on a Kashima-Tsukuba Baseline

Abstract

Highly accurate geodetic VLBI measurements has been repeated once a year since 1984 on the same baseline between Kashima and Tsukuba, about 55-km in length. Formal errors of five independent measurements were 6-mm on the average. The formal error of each experiment was studied in detail and found to be the same size as was expected theoretically. The sample standard deviation, however, was much larger than the expected error. The large deviation from the theoretical estimation, especially seen in the horizontal components, was investigated from the view of a coseismic surface displacement due to an earthquake occurred at east off Chiba, in December 1987, the epicenter of which was very near to Kashima station. The surface displacement of strikeslip type was calculated with a simple model but found to be too small, one fifth of the observed slip, suggesting that a more complicated model shall be established to explain the observed large horizontal slip, or otherwise, unknown sources of an error should be searched and formulated for a theoretical error estimation. © 1989, The Geodetic Society of Japan. All rights reserved.