Integration and Analysis of Seismic, Pore Pressure, and Strain Signals at the PBO Borehole Stations.

摘要

The borehole stations of the Earthscope Plate Boundary Observatory (PBO) are providing unprecedented observations of co-located pore pressure and strain measurements over time scales of seconds to years, and over wide geographic and tectonic environments. Water level fluctuations in response to tides are well documented and understood, however the response to earthquakes, especially at the dynamic co-seismic, and short-term post-seismic time scales is less well understood. Observations of strain and water level steps due to earthquakes are presented, and we hypothesize the mechanism responsible for these changes based on the co-located strain observations. For each earthquake, the water table change is consistent with the co-seismic areal strain change, suggesting at least in part the influence of a static strain mechanism in each event. Two methods, one using Fourier transforms and the other cross correlation, are developed for calculating phase and amplitude differences of tidal forces before and after the earthquakes. Temporary phase and amplitude differences in the tidal signal were observed for the gradual post-seismic pore pressure increase at PBO 84 Piñon Flats during the El Mayor-Cucapah earthquake, implying a change in hydrological parameters. A smaller amplitude difference is also detected for the gradual decrease observed at PBO 88 Sky Oaks during the 2010 M5.4 Collins Valley earthquake. Using information from geophysical logs, the fracture orientation in boreholes was correlated with the principal strain directions of the strain field. Of the three tested earthquakes, two had ideal configurations for the observed pore pressure change. Lastly, the developed methods were tested for a pore pressure record in which there was no co-located strain.