Correlation between deep fluids, tremor and creep along the central San Andreas fault

Abstract

Magnetotelluric data sense the electrical resistivity of the Earth, a physical parameter particularly sensitive to the presence of low resistivity phases such as aqueous fluids, partial melts or metallic compounds. Fluid phases have electrical resistivities orders of magnitude lower than the rock matrix, and thus relatively small amount of fluids, when interconnected, can decrease bulk rock resistivity by several orders of magnitude 9 .Fluids additionally have a significant weakening effect on the rheology of rocks, even more so if fluids form an interconnected network 10 . Measurements of electrical resistivity can therefore be used to constrain the volume of subsurface fluids, their interconnectivity, and the rheology of the crust and mantle.We collected magnetotelluric data along seven profiles across the SAF near Parkfield and Cholame, covering the tremor concentration zone near Cholame and the transition from locked to creeping behaviour (Fig. 1a).The most prominent structure revealed by the MT data is a deep low-resistivity zone (1-5 ohm-m) centred 30-40 km southwest of the SAF below 15-20 km depth (Fig. 1b). This anomaly occupies a broad region in the lower crust and upper mantle between the surface traces of the Rinconada fault, a former strand of the SAF, and the modern-day SAF. Along profiles 4-7, which cross the SAF at the tremor concentration zone near the northern end of the locked segment 5,7 , crustal resistivities are in excess of 500 ohm-m for the profiles 5-7 across the tremor concentration zone near Cholame, the inversion reveals a deep low resistivity region which is bounded from above and laterally by resistive formations (Figs. 1b, 2b). Here, the tremor source region appears to coincide with the high resistivity rocks, adjacent to the less resistive and potential fluid source area. Inversions of the MT data confirm the resistive cap as a very robust feature of the models.Any low resistivity connection into the upper crust (as observed northeast of Parkfield) results in a significant increase of data misfit (see supplementary information).Tremor near Cholame separates spatially into a southwestern zone with periodic tremor episodes and a northeastern zone with aperiodic episodes 7 . Our resistivity models suggest that the fluid source (i.e. low resistivity) is located to the southwest of the periodic tremor zone. Ongoing fluid generation by mantle dehydration reactions 4 could result in high fluid pressures in the low resistivity zone, and consequently, in fluids continuously driven through fracture systems towards and into the more resistive tremor regions.Observations of high seismic reflectivity 21 (cf. Fig. 2b) at tremor source depths between the low resistivity fluid source and the more resistive tremor zone are consistent with deformed and fractured material.Lateral migration of fluids could be responsible for elevated fluid pressures in the tremor source region Tremor amplitudes along the central SAF vary by a factor of seven 6 and correlate with variations of resistivity by a factor ...

A large mantle water source for the northern San Andreas fault system: a ghost of subduction past

“…They showed, using realistic fluid transport properties of the crust, that the fluid discharge rates from our model are sufficient to produce high fluid pressures in the seismogenic zone of the SAFS that approach lithostatic pressures. Becken et al (2011) show that electrical resistivity surveys measured across the SAF near Parkfield, California are consistent with vertical zones of low resistivity. Citing the Fulton and Saffer paper, Becken et al (2011) interpret these zones as representing fluid pathways where high-pressure fluids reduce resistance to frictional sliding.…”

“…Becken et al (2011) show that electrical resistivity surveys measured across the SAF near Parkfield, California are consistent with vertical zones of low resistivity. Citing the Fulton and Saffer paper, Becken et al (2011) interpret these zones as representing fluid pathways where high-pressure fluids reduce resistance to frictional sliding. The present paper documents the modeling results that are, at least in part, the basis of interpretations in these later studies, and, in particular, the present paper documents the conceptual and numerical formulation of the model, and the rates of fluid discharge and the longevity of this source of pressurized water from our modeling results.…”

Crustal metamorphic fluid flux beneath the Dead Sea Basin: constraints from 2-D and 3-D magnetotelluric modelling

“…High conductivities in active tectonic zones have often been associated with fluids and hydrous minerals (Boerner et al 1998;Unsworth & Bedrosian 2004;Jones et al 2005;Ritter et al 2005;Wannamaker 2005;Becken et al 2011;Meqbel et al 2014). For instance, Becken et al (2011) interpreted a subvertical conductive anomaly extending through the entire crust northwest of Parkfield in California as a migration path for deep (partly mantle derived) fluids into the San Andreas Fault system. Signatures of mantle derived fluids in surface waters have been reported from major strike-slip faults around the world (e.g.…”

“…Ritter et al 2003Ritter et al , 2005Unsworth & Bedrosian 2004;Tank et al 2005;Thiel et al 2009;Becken et al 2011;Desissa et al 2013). The geometry and the overall conductance of FZCs depend on local geological, hydrogeological conditions and the geodynamic (tectonic) setting of a fault (see Unsworth & Bedrosian 2004;Ritter et al 2005, and references therein.).…”

Heterogeneous mantle anisotropy and fluid upwelling: implication for generation of the 1891 Nobi earthquake

“…A review of intraplate earthquakes in Japan found clear imaging of lowvelocity zones and/or high-Vp/Vs zones near the rupture sources of many major earthquakes (Hasegawa et al 2009), presumably related to the presence of lower crustal fluid; however, velocity structure alone is not sufficient to infer the presence of fluids. Geomagnetic surveys have found regions of low resistivity near many active faults, which could relate to the presence of fluids in the source area (Gupta et al 1996;Tank et al 2005;Wannamaker et al 2009;Becken et al 2011;Becken and Ritter 2012). In Japan, some close relationships between low-resistivity regions and earthquake faults have been proposed (Mitsuhata et al 2001;Yamaguchi et al 2001;Uyeshima et al 2005;Yoshimura et al 2008).…”