A mechanism for sustained groundwater pressure changes induced by distant earthquakes

TLDR

Large, sustained well‑water level changes (>10 cm) in response to distant earthquakes have remained enigmatic for over 30 years. The study uses high‑sampling‑rate measurements at a well near Grants Pass, Oregon, to simultaneously analyze dynamic water‑level responses and sustained steps, and proposes a new model for coseismic pore‑pressure steps. The proposed model attributes the steps to a temporary groundwater‑flow barrier that is entrained and removed by rapid seismic‑wave‑induced flow, as revealed by the high‑frequency data. The authors observed a 40‑fold increase in the water‑level amplitude to ground‑velocity ratio during a coseismic step, and estimate that the mechanism could generate ~0.04 MPa pressure changes and trigger seismicity in hydrothermal areas.

Abstract

Large, sustained well water level changes (>10 cm) in response to distant (more than hundreds of kilometers) earthquakes have proven enigmatic for over 30 years. Here we use high sampling rates at a well near Grants Pass, Oregon, to perform the first simultaneous analysis of both the dynamic response of water level and sustained changes, or steps. We observe a factor of 40 increase in the ratio of water level amplitude to seismic wave ground velocity during a sudden coseismic step. On the basis of this observation we propose a new model for coseismic pore pressure steps in which a temporary barrier deposited by groundwater flow is entrained and removed by the more rapid flow induced by the seismic waves. In hydrothermal areas, this mechanism could lead to 4 × 10 −2 MPa pressure changes and triggered seismicity.