Differences in seismic activity along the San Andreas fault appear to be related to strength variations in the lower crust and upper mantle, as suggested by new findings in the Dec. 1 edition of Nature.
U.S. Geological Survey scientist Paul Bedrosian, along with colleagues Michael Becken, Oliver Ritter, and Ute Weckmann from the GFZ German Research Centre for Geosciences, Potsdam, Germany, used an electromagnetic geophysical method to image subsurface conductivity within the crust.
“Segmentation of the San Andreas fault was first identified more than 40 years ago based on distinct patterns of seismicity. This work links mantle fluids, possibly resulting from ancient subduction along the California coast, and their interaction with the crust, as the driver behind the observed differences. This is really exciting as it illustrates how past structure and tectonics effects present-day dynamics along the San Andreas fault,” said Bedrosian.
Fluid influx is implicated as a driving force behind the processes that ultimately define seismic segmentation. The findings may help to explain why motion along the fault results in earthquakes on some segments and less harmful creep on others.
“Decades ago USGS researchers explored the strong dependence of water on the strength of the rocks in the deep crust and upper mantle, with the firm conviction that this effect would be key to understanding fault mechanics,” said USGS Director Marcia McNutt. “Now finally with new technology available to map the in situ distribution of water at depths inaccessible to geologic observation, we have an excellent example of how an investment in basic research will pay off in a very practical understanding of a long-standing mystery that affects lives and property.”
The area studied is a transition zone between segments of locked and creeping behavior along the San Andreas fault, and includes a zone of pronounced seismic tremor. The data provide evidence of fluids migrating into the creeping section that appear to originate from a region that is also responsible for stimulating tremors. The results are consistent with the hypothesis that high fluid pressures play a crucial role in the weakening of faults.
“Understanding how large and possibly dangerous fault systems, like the San Andreas fault, work in all their complexity is a grand challenge. The San Andreas fault is a key natural laboratory for studying large transform faults, as many geo-scientific methods are tested here to provide different pieces of the puzzle. I hope that our results will trigger similar research along other major active fault systems around the world,” said Weckmann.
Contact Information:
U.S. Department of the Interior, U.S. Geological Survey
Office of Communications and Publishing
12201 Sunrise Valley Dr, MS 119
Reston, VA 20192 Heidi Koontz 
FULL ARTICLE:
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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