Response of an aquifer near Ottawa to tidal forcing and the Alaskan earthquake of 1964

Abstract

Coincident with the Alaskan event the water level in a deep well near Ottawa showed a small initial rise then a fall of 28 cm followed by a decay back to the original level over a period of several days. This is suggestive of a residual dilatation with subsequent fluid diffusion. It is shown that this explanation is consistent with the diffusion and elastic properties of the aquifer determined independently from a slug test and from the response to earth tides. Assuming a simple unconfined saturated porous half-space, the results indicate that the upper 200 m (approximately) is effectively decoupled from horizontal stresses whereas below this depth a permanent dilatation of approximately 2.9 × 10−7 occurred. This is much too large and of the wrong sign to be accounted for by the residual strain field of the earthquake. As an alternative, dilatancy induced by strong local ground motion is suggested.