GFZ German research centre for geo sciences

Magnetotelluirc Experiments at the San Andreas Fault, California

In einer Reihe von drei großen Feldexperimenten (DeepRoot, TremorMT und ELSAF, 2005-2008) wurden mehr als 250 MT Stationen entlang von sieben Profilen über die San Andreas Verwerfung (SAF) installiert. Die Stationen befanden sich nahe dem Tiefenobservatorium der San Andreas Verwerfung (SAFOD) und überdeckten das Übergangssegment der SAF, wo sich die SAF von einer kriechend zu einer blockierten Verwerfung ändert und wo ein Quellgebiet für nicht-vulkanische Tremore (NVT) lokalisiert wurde. Diese regionalen Studien hatten zum Ziel, die tiefen strukturellen Änderungen entlang der Streichrichtung des Übergangssegments der SAF zu untersuchen und zum ersten Mal die Quellregion der NVT mit MT zu erforschen. Im Ganzen misst die überdeckte Fläche 60 km x 130 km.

Two-dimensional (2D) inversion of the MT data along three northern profiles across the transitional-to-creeping segment of the SAF reveals a sub-vertical channel connecting a high conductivity region in the upper mantle and lower crust with the upper-crustal, brittle deformation zone of the SAF. This high conductivity is interpreted as a zone where fluids can migrate into the SAF system (Becken et al., 2011). Interestingly, the crustal fluid channel is absent at the transitional-to-locked segment of the SAF. The three southern profiles across the transitional-to-locked segment consistently show that the mantle conductor is separated from the seismogenic SAF by a deep-reaching resistive zone, in sharp contrast to the situation at the transitional-to-creeping segment farther to the north. Along the locked segment, the zone of high conductivity correlates with the source region of NVT. 3D inversion and FWD modelling results of the data set (Tietze & Ritter, 2013) were in general consistent with the 2D results of Becken et al. (2008b, 2011) and could provide new insights into the southeastern part of the survey area, where strong 3D effects are present in the data. Our research activities onshore were extended offshore with the Deep San Andreas Fault Boundary Structure from Marine MT experiment conducted by Steven Constable, Kerry Key, and Brent Wheelock from Scripps Institution of Oceanography, UCSD.

Time Frame

  • 01/2005 - 12/2006 (DeepRoot)
  • 01/2007 - 12/2007 (TremorMT)
  • 01/2008 - 08/2012 (ELSAF)

Funding

  • German Science Foundation for DeepRoot and ELSAF
  • GFZ Potsdam for TremorMT

Principal Investigator 

  • PD Dr. Oliver Ritter (GFZ)

Personnel

  • Jun.-Prof. Dr. Michael Becken (GFZ Potsdam, now at Westfälische Wilhelms-Universität, Münster)
  • Dr. Kristina Tietze (GFZ Potsdam)

Cooperations 

  • Paul Bedrosian (U.S. Geological Survey, Denver)
  • Steven Constable (Scripps Institution of Oceanography, University of California, San Diego
  • Kerry Key (Scripps Institution of Oceanography, University of California, San Diego)
  • Stephen Park (University of California, Riverside)

Methods & Equipment  

  • Magnetotellurics, 2D and 3D inversion
  • EDL and Spam Mk III data logger
  • Induction-coil magnetometers (Metronix)
  • Flux-gate magnetometers (Geomagnet)
  • Non-polarisable Ag/AgCl electrodes (GFZ Potsdam)

Publications/Results

  • Becken, M., Ritter, O., Bedrosian, P. A. & Weckmann U., 2011. Correlation between deep fluids, tremor and creep along the central San Andreas fault, Nature, 480, 87—90.
  • Becken, M., Ritter, O., Park, S. K., Bedrosian, P. A., Weckmann, U. & Weber, M., 2008. A deep crustal fluid channel into the San Andreas Fault system near Parkfield, California, Geophys. J. Int., 173, 718732.
  • Becken, M., Ritter, O.,Weckmann, U. & Bedrosian, P., 2008. Recent and on-going MT studies of the San Andreas Fault zone in Central California. Proceedings of the 22nd Schmucker-Weidelt-Colloquium on Electromagnetic Depth Sounding, Decin, Czech Republic, 2007.
  • Tietze, K. & Ritter O., 2013. Three-dimensional magnetotelluric inversion in practice - the electrical conductivity structure of the San Andreas Fault in Central California, Geophysical Journal International 2013; doi: 10.1093/gji/ggt234.
  • Tietze, K., 2012. Investigating the 3D electrical conductivity structure of the San Andreas fault system in the Parkfield-Cholame region, central California, with 3D magnetotelluric inversion, PhD thesis, Free University Berlin.
  • Tietze, K. & Ritter O., 2012. Resolution of 3D elongated deep conductive bodies embedded in a 2D background conductivity structure by 3D and 2D magnetotelluric inversion, Proceedings of the 24th Schmucker-Weidelt-Colloquium on Electromagnetic Depth Sounding Neustadt / Weinstr., Germany, 2011.

Acknowledgments

  • Magnetotelluric instruments provided by the magnetotelluric component of the geophysical instrument pool Potsdam (GIPP-MT) and the instrument facility for Electromagnetic Studies of the Continents.

Additional Information

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