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San Andreas Fault Mysteries Begin to Unravel

San Andreas Fault Mysteries Begin to Unravel

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.

The article may be read in full online.

USGS Newsroom

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Parameter Value Description
Magnitude mb The magnitude for the event.
Longitude ° East Decimal degrees longitude. Negative values for western longitudes.
Latitude ° North Decimal degrees latitude. Negative values for southern latitudes.
Depth km Depth of the event in kilometers.
Place Textual description of named geographic region near to the event. This may be a city name, or a Flinn-Engdahl Region name.
Time 1970-01-01 00:00:00 Time when the event occurred. UTC/GMT
Updated 1970-01-01 00:00:00 Time when the event was most recently updated. UTC/GMT
Timezone offset Timezone offset from UTC in minutes at the event epicenter.
Felt The total number of felt reports
CDI The maximum reported intensity for the event.
MMI The maximum estimated instrumental intensity for the event.
Alert Level The alert level from the PAGER earthquake impact scale. Green, Yellow, Orange or Red.
Review Status Indicates whether the event has been reviewed by a human.
Tsunami This flag is set to "1" for large events in oceanic regions and "0" otherwise. The existence or value of this flag does not indicate if a tsunami actually did or will exist.
SIG A number describing how significant the event is. Larger numbers indicate a more significant event.
Network The ID of a data contributor. Identifies the network considered to be the preferred source of information for this event.
Sources A comma-separated list of network contributors.
Number of Stations Used The total number of Number of seismic stations which reported P- and S-arrival times for this earthquake.
Horizontal Distance Horizontal distance from the epicenter to the nearest station (in degrees).
Root Mean Square sec The root-mean-square (RMS) travel time residual, in sec, using all weights.
Azimuthal Gap The largest azimuthal gap between azimuthally adjacent stations (in degrees).
Magnitude Type The method or algorithm used to calculate the preferred magnitude for the event.
Event Type Type of seismic event.
Event ID Id of event.
Event Code An identifying code assigned by, and unique from, the corresponding source for the event.
Event IDS A comma-separated list of event ids that are associated to an event.


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