Pore pressure sensitivities to dynamic strains: Observations in active tectonic regions

Abstract

Triggered seismicity arising from dynamic stresses is often explained by the Mohr‐Coulomb failure criterion, where elevated pore pressures reduce the effective strength of faults in fluid‐saturated rock. The seismic response of a fluid‐rock system naturally depends on its hydromechanical properties, but accurately assessing how pore fluid pressure responds to applied stress over large scales in situ remains a challenging task; hence, spatial variations in response are not well understood, especially around active faults. Here I analyze previously unutilized records of dynamic strain and pore pressure from regional and teleseismic earthquakes at Plate Boundary Observatory (PBO) stations from 2006 to 2012 to investigate variations in response along the Pacific/North American tectonic plate boundary. I find robust scaling response coefficients between excess pore pressure and dynamic strain at each station that are spatially correlated: around the San Andreas and San Jacinto fault systems, the response is lowest in regions of the crust undergoing the highest rates of secular shear strain. PBO stations in the Parkfield instrument cluster are at comparable distances to the San Andreas Fault (SAF), and spatial variations there follow patterns in dextral creep rates along the fault, with the highest response in the actively creeping section, which is consistent with a narrowing zone of strain accumulation seen in geodetic velocity profiles. At stations in the San Juan Bautista (SJB) and Anza instrument clusters, the response depends nonlinearly on the inverse fault‐perpendicular distance, with the response decreasing toward the fault; the SJB cluster is at the northern transition from creeping‐to‐locked behavior along the SAF, where creep rates are at moderate to low levels, and the Anza cluster is around the San Jacinto Fault, where to date there have been no statistically significant creep rates observed at the surface. These results suggest that the strength of the pore pressure response in fluid‐saturated rock near active faults is controlled by shear strain accumulation associated with tectonic loading, which implies a strong feedback between fault strength and permeability: dynamic triggering susceptibilities may vary in space and also in time.

Sensitivity of stress inversion of focal mechanisms to pore pressure changes

“…Inversion of earthquake focal mechanisms to determine the stress orientation and the stress ratio is a well-established technique [e.g., Angelier, 1984;Gephart and Forsyth, 1984;Michael, 1984]. Here we apply the approach developed by Michael [1984Michael [ , 1987 and adopted in the SATSI [Hardebeck and Michael, 2006] and MSATSI [Martínez-Garzón et al, 2014b] software packages.…”

“…The pore fluid pressure has also been suggested to play an important role in unlocking fault patches in the Nazca subduction interface resulting in the M w 8.8 Chile earthquake [ Moreno et al ., ]. Additionally, the pore pressure is coupled in poroelastic media with stress field, e.g., through fluid diffusion over long‐time scales [e.g., Townend and Zoback , ] or by shear strain accumulation near active faults at shorter time scales [ Barbour , ], and thus, it may locally modify the stress regime [ Segall and Fitzgerald , ; Altmann et al ., ]. Stress changes related to fluids are highlighted in fluid‐induced seismicity, where injection and production activities modify the pore pressure in the vicinity of the wells [e.g., Terakawa et al ., ; Martínez‐Garzón et al ., ; Schoenball et al ., ].…”

Impact of fluid injection on fracture reactivation at The Geysers geothermal field

“…Recently, there has been an increase in the seismicity induced by human activities such as injection of wastewater in disposal wells, hydraulic fracturing, or geothermal production [e.g., Ellsworth , ; Weingarten et al ., ]. The analysis of induced seismicity allows studying deformation in geological reservoirs related to stimulation and production [e.g., Chen and Shearer , ; Martínez‐Garzón et al ., ; Barbour , ; Kwiatek et al ., ]. Induced earthquakes from reactivated faults and fractures with varying orientation are commonly related to perturbations of reservoir fluid pressures and temperatures [ Barton et al ., ; Shapiro et al ., ; Rutqvist et al ., ; Boyle and Zoback , ].…”

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

“…Near‐surface hydrological process is known to be an important process in generating significant deformation and stress change in the crust, and those related stress changes are recognized to may have played an important role in the triggering and modulation of many types of tectonic events. For example, crustal deformation caused by hydrological process has been reported to be significant in observations of GPS (e.g., Fu & Freymueller, ; Ji & Herring, ; King et al, ; Larson et al, ; Tsai, ; Zhan et al, ), InSAR (e.g., Jónsson et al, ; King et al, ), and borehole strainmeters (e.g., Barbour, ; Barbour & Wyatt, ; Evans & Wyatt, ; Segall et al, ; C.‐Y. Wang & Barbour, ).…”