When is the strain in the meter the same as the strain in the rock?

Abstract

Borehole dilatometers are emplaced in porous fluid saturated rock. Pore‐fluid flow induces strain, however there is no fluid exchange with the dilatometer. Thus, the strainmeter response is the same as the strain in the rock only when the rock remains undrained. Otherwise the instrumental strain Δ inst is given by Δ inst = C 1 (Δ ∞ − C 2 p ∞ ), where Δ ∞ and p ∞ are strain and pore pressure far from the borehole, and C 1 and C 2 depend on poroelastic rock properties. Postseismic strain in the rock is expected to increase as the induced pore pressure gradients relax. However, a dilatometer ∼3 km from a M w 6.5 earthquake in south Iceland shows a postseismic strain change opposite in sign to the coseismic response. The theory developed here for a homogeneous, isotropic medium can only partly explain this discrepancy. Fracture dominated poroelastic response may yield an improved fit to the data.