Correspondence: Response of a gravimeter to an instantaneous step in gravity

Abstract

Montagner et al. presented evidence that a signal observed prior to P-waves on gravimeters operating in Japan during the 2011 Tohoku-Oki earthquake was caused by gravitational field changes propagating at light speed. In this comment, I explore the expected response of gravimeters attached to the Earth's surface and subjected to an instantaneous acceleration step, g(t)=g_0+ΔgH(t), where H(t) is a Heaviside step function. I argue that at the time of the step, inertial accelerations from the elastic response of the Earth exactly cancel the gravitational step; in order to observe the gravity step using a gravimeter, one must wait until the Earth deforms and begins to re-equilibrate. This conclusion is an application of the principle of equivalence between gravitational and inertial mass. In the case of the signal reported for Tohoku-Oki, I estimate that inclusion of the Earth's elastic response should significantly decrease the expected acceleration at the time of the P-wave.

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