Earth-tilt around the Great Lakes

Saturday, November 23, 1996

Earthquake Damage and Earthquake Insurance, pp.139-142, John R. Freeman, 1932

It has recently been proved by careful comparisons of about 50 years of continuous records by several different pairs of recording gauges on opposite sides of the Great Lakes, that throughout the entire Great Lakes region, a tilting upward from a hinge line running W-18deg.N, located somewhere south of Chicago and Cleveland, has, during the past fifty years, been causing land to rise toward the north at the rate of about one-half foot per 100 miles per century.1 This must cause earth-stress to accumulate (unless perhaps it is a sign of the slow relief of existing earth-stress accumulated 25,000+/- years ago in the glacial epoch, and for lack of other explanation may be regarded as delayed isostatic readjustment).

A drawing of the great lakes from a bird's eye view

The epicenter of the earthquake distinctly felt in many parts of western New York State on August 12, 1929, was not far from one of the probably hinge lines of this earth-tilt.

This earth-tilt is deepening the harbours at Chicago and Cleveland at the rate of more than a foot per century and causing shoaling over the lower sills of the navigation locks at the Sault Ste. Marie at about the same rate.2 The uplift shown by ancient beach lines at the northern end of Lake Huron increases in steepness toward the north, and since glacial times now amounts to about 500 feet.3

The changing earth-tilts now being observed by the new clinometers in Japan are irregular and are of from more than 10 times to 100 times this magnitude of rate of earth-tilt in the Great Lakes region.


1 This is equivalent to the extremely small quantity of one sixteenth part of one inch per year in 100 miles of distance, or to less than one-thousandth part of an inch per mile per year, or to an inclination of about 1/300 of a second of arc. Nevertheless it has been measured with undoubted certainty by the half century of continuous records of the several pairs of gauge records.

2 Report on the Regulation of the Great Lakes, by John R. Freeman, 1929, published by the Chicago Sanitary District.

3 U.S. Geol. Survey Monograph, Vol LIII, 1915, by Leverett and Taylor, on The Pleistocene of Indiana and Michigan and the History of the Great Lakes.