Bulletin of the National Speleological Society - ISSN 0146-9517
Volume 27 Number 4:133-142 - October 1965

A publication of the National Speleological Society

Semidiurnal Movement Along a Bedrock Joint in Wool Hollow Cave, California
Stanley N. Davis and George W. Moore


A 22-hour record of strain has been obtained from three mutually perpendicular transducing cells across a joint in the wall of Wool Hollow Cave, Calaveras County, California, in the hope of determining the causes of strain. The rock cover at the instrument station is 25 meters thick, and the air temperature at the insturments ranged from 11.25-11.42°C during the observations. The joint, which strikes N 50° E and dips 27° NW, cuts recent travertine as well as the bedrock. The slippage showed two maxima and two minima roughly conincident in time with the theoretical earth tides. At high tide the hanging wall of the joint moved southeastward up the dip 0.4 micron with respect to the footwall, and just before high tide, 0.6 micron northeastward along the strike of the joint. A seismic distrubance with an amplitude of 0.01 micron from the Moluccan earthquake of March 21, 1965, was superimposed on the slower tidal fluctuations during a five-hour period, and it appears to have caused abrupt deflections as large as 0.03 micron, indicated by steps on the strain record.

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