Strong Hydro‐Related Localized Long‐Period Crustal Deformation Observed in the Plate Boundary Observatory Borehole Strainmeters

Strong localized hydro‐related long‐period strain variations, with amplitude up to about 2,000 ns (nanostrain) (1–2 orders of magnitude larger than the tidal deformation), are observed in some Plate Boundary Observatory borehole strainmeters in Parkfield and Anza, California. The long‐period strain signals exhibit compression from December 2010 to January 2011, extension till May 2011, and compression till 2014 to 2015. The observed strain signals are accompanied by similar pore pressure change at some strainmeters and no pore pressure change at other strainmeters. Similar long‐period signals are also observed in rainfall, groundwater, and soil moisture in the same regions. Poroelastic simulations suggest that the observed long‐period strains and their relationships with rainfall and pore pressure could be explained by a rainfall–hydrological diffusion–poroelastic deformation mechanism. Our study indicates that poroelastic response to hydrological water would generate significant underground deformation, which is intimately related to local hydrological properties and settings.