Geomagnetic Anomalies caused by a Man-made Lake

Abstract

TECTONOMAGNETISM—the study of magnetic anomalies due to tectonic stresses—is attracting increasing interest as a method of monitoring local changes in Earth stress¹. The most important aspect is the search for the seismomagnetic effect; no convincing observation has yet been reported, although calculations^2,3 indicate that in many cases it must be of measurable magnitude, and observations of magnetic precursors to creep increments on the San Andreas fault⁴ encourage the view that magnetic effects of earthquakes themselves are observable. Local magnetic changes preceding and accompanying volcanic eruptions in New Zealand are very striking^5,6 and indicate that substantial stresses are associated with eruptions; a quite different pattern of volcanomagnetic changes in Hawaii (P. D., F. S., Jackson and Field, to be published) suggests that Kilauea volcano is highly fragmentednd a incapable of supporting a large scale stress pattern. But both volcanic eruptions and the San Andreas creep increments are very imperfectly understood so that a large scale control experiment which demonstrates quantitatively the role of the piezomagnetic effect in producing local magnetic anomalies is very desirable. The requirement is satisfied by the water loads imposed on underlying strata by man-made lakes, which provide large scale Earth stresses of sufficient intensities to produce measurable magnetic anomalies. Recent completion of a new dam at Talbingo in the Snowy Mountains of SE Australia and rapid filling of the reservoir provided an opportunity to observe this effect.