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


The seismicity pattern along the San Andreas fault near Parkfield and Cholame, California, varies distinctly over a length of only fifty kilometres. Within the brittle crust, the presence of frictionally weak minerals, fault-weakening high fluid pressures and chemical weakening are considered possible causes of an anomalously weak fault northwest of Parkfield1,2,3,4. Non-volcanic tremor from lower-crustal and upper-mantle depths5,6,7 is most pronounced about thirty kilometres southeast of Parkfield and is thought to be associated with high pore-fluid pressures at depth8. Here we present geophysical evidence of fluids migrating into the creeping section of the San Andreas fault that seem to originate in the region of the uppermost mantle that also stimulates tremor, and evidence that along-strike variations in tremor activity and amplitude are related to strength variations in the lower crust and upper mantle. Interconnected fluids can explain a deep zone of anomalously low electrical resistivity that has been imaged by magnetotelluric data southwest of the Parkfield–Cholame segment. Near Cholame, where fluids seem to be trapped below a high-resistivity cap, tremor concentrates adjacent to the inferred fluids within a mechanically strong zone of high resistivity. By contrast, subvertical zones of low resistivity breach the entire crust near the drill hole of the San Andreas Fault Observatory at Depth, northwest of Parkfield, and imply pathways for deep fluids into the eastern fault block, coincident with a mechanically weak crust and the lower tremor amplitudes in the lower crust. Fluid influx to the fault system is consistent with hypotheses of fault-weakening high fluid pressures in the brittle crust.

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Figure 1: Magnetotelluric resistivity sections across the central San Andreas fault.
Figure 2: Two distinct electrical resistivity models.
Figure 3: Relation between electrical resistivity and tremor.


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This research was supported by the German Science Foundation (DFG) and the Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences. Instruments were provided by the Geophysical Instrument Pool Potsdam and the instrument facility for Electromagnetic Studies of the Continents. Seismicity data are taken from the catalogue provided by the Northern California Earthquake Data Center, Northern California Seismic Network, US Geological Survey, Menlo Park ( Fault zone locations are from the California Geological Survey, Geologic Data Map Series No. 6. H. Zhang and D. Shelly provided their tremor source location estimates. This work would not have been possible without field support and the cooperation of numerous landowners.

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All authors participated in the various field experiments. M.B. processed the time series, analysed and inverted the magnetotelluric data, and validated the resistivity models. All authors contributed to the interpretation and the manuscript. O.R. was principal investigator of the DFG/GFZ-funded research proposals.

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Correspondence to Michael Becken.

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Becken, M., Ritter, O., Bedrosian, P. et al. Correlation between deep fluids, tremor and creep along the central San Andreas fault. Nature 480, 87–90 (2011).

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