Changes in Physical Properties of the Nankai Trough Megasplay Fault Induced by Earthquakes, Detected by Continuous Pressure Monitoring

Abstract

One primary objective of Integrated Ocean Drilling Program Expedition 365, conducted as part of the Nankai Trough Seismogenic Zone Experiment, was to recover a temporary observatory emplaced to monitor formation pore fluid pressure and temperature within a splay fault in the Nankai subduction zone offshore SW Honshu, Japan. Here we use a 5.3 year time series of formation pore fluid pressure, and in particular the response to ocean tidal loading, to evaluate changes in pore pressure and formation and fluid elastic properties induced by earthquakes. Our analysis reveals 31 earthquake‐induced perturbations. These are dominantly characterized by small transient increases in pressure (28 events) and decreases in ocean tidal loading efficiency (14 events) that reflect changes to formation or fluid compressibility. The observed perturbations follow a magnitude‐distance threshold similar to that reported for earthquake‐driven hydrological effects in other settings. To explore the mechanisms that cause these changes, we evaluate the expected static and dynamic strains from each earthquake. The expected static strains are too small to explain the observed pressure changes. In contrast, estimated dynamic strains correlate with the magnitude of changes in both pressure and loading efficiency. We propose potential mechanism for the changes and subsequent recovery, which is exsolution of dissolved gas in interstitial fluids in response to shaking.

Hikurangi Subduction Margin Coring, Logging, and Observatories

“…• Structural analyses to characterize deformation mechanisms and style and fracture and fault orientations (e.g., Byrne et al, 2009); • Analysis of wellbore failures (breakouts and tensile fractures) to characterize contemporary maximum and minimum stress orientations and, with constraints on rock strength, potentially constrain absolute stress magnitudes; • Laboratory measurements of fault and wall rock rheology to test hypotheses linking fault constitutive properties to slip behavior (e.g., Saffer and Wallace, 2015;Leeman et al, 2016); • Geomechanical and thermal properties measurements to define poroelastic, strength, and heat transport properties of the formation and to guide interpretation of observatory data (e.g., Wang, 2004;Sawyer et al, 2008;Davis et al, 2009;Kinoshita et al, 2018); and • Strength, permeability, and elastic moduli measurements to provide context for the interpretation of borehole failures as indicators of in situ stress magnitude, parameterization of deformation and hydrologic models, and core-log-seismic integration.…”

Site U1518

“…• Structural analyses to characterize deformation mechanisms and style, fracture and fault orientations, and wellbore failures (e.g., Byrne et al, 2009); • Experimental investigation of rheology and friction to test hypotheses linking fault constitutive properties to slip behavior (e.g., Saffer and Wallace, 2015;Leeman et al, 2016); • Geomechanical and thermal properties measurements to define poroelastic, strength, and heat transport properties of the formation to guide the interpretation of observatory data (e.g., Davis et al, 2009;Kinoshita et al, 2018); and • Strength, permeability, and elastic moduli measurements to provide context for the interpretation of borehole failures as indicators of in situ stress magnitude, parameterization of hydrological models, and core-log-seismic integration.…”

In Situ Permeability and Scale Dependence of an Active Accretionary Prism Determined From Cross‐Borehole Experiments

“…After removal of tidal loading and other oceanographic signals, disturbances associated with both regional and teleseismic earthquakes (Kinoshita et al, ) and with slow slip events on the plate interface below (Araki et al, ) are evident in the pressure records. The pressure data also document clear responses in formation pressure to drilling and coring operations in a pair of nearby boreholes in late 2012 (Holes C0002H and C0002I), located ~100 m to the northeast (Figure b).…”