Water Well Resonance Induced by Preearthquake Signals

This study elucidates the water well resonance induced by preearthquake signals using an analytic approach. Weak pressure waves passing through a confined aquifer are considered as the incoming preearthquake signal. Owing to the hardness of the porous skeleton, the pressure waves are simply the limiting case of the second kind of dilatational waves of poroelasticity. Because the driving signals of a weak preearthquake pressure wave transmitting through a confined aquifer are too weak to significantly affect the solid skeleton, we believe that this phenomenon is attributed to the effect of resonant amplification of water levels in wells. The flow inside the well is assumed to be incompressible, inviscid, and irrotational, while that outside the well is treated as a porous-media flow with a rigid skeleton. The flows inside and outside the well are analyzed using the potential flow theory under the assumption of small-amplitude waves and Darcy’s law of porous-media flow, respectively. The well-posed boundary value problem is solved using the regular perturbation expansion based on a small gauge function $k_{0}R$ , where $k_0=\text{wave}$ number and $R=\text{well}$ radius. The results of this study demonstrate that the weak preearthquake longitudinal pressure wave is only a triggering mechanism for the resonance of transverse gravity surface water waves inside the well.