A recent study published in Nature Geoscience reveals large-scale vertical motion across the San Andreas Fault System. The research team, headed by Samuel Howell at the University of Hawai’i, was able to pick apart subtle signals in an noisy data set consisting of vertical velocities along one of the most studied and famous faults in the world.
Given the dense population straddled across the San Andreas Fault System, it is a site of active monitoring through an array of GPS instruments, accelerometers, and seismograms. Studies have previously measured and tracked lateral (horizontal) movement of plates along the Fault System, but vertical movement has thus far remained largely uncharacterized.
By systematically removing noise from the vertical velocity data set, the scientists were able to extract a massive spatial vertical movement. The vertical movement spans 200 km with ±2 mm per year movement. Although the vertical movement is not significant to human detection, the fact that it trends over hundreds of kilometers provides a fingerprint for how the fault continues to evolve and where we may see future earthquakes.
San Andreas Fault System vertical velocities (Credit: Nature Geoscience)
The vertical velocities imaged above represent GPS movement data combined with both a statistical and physical deformation model to simulate vertical crust deformation across the San Andreas Fault System. The presence of scissoring between rising and falling crust is often seen in strike-slip faults and represents the crust’s response to the regional stress field.
Subtle vertical changes over large geographic regions, with some areas sinking and others rising, gives clues as to how the fault system is transferring stress in some instances for more than 300 years. Through understanding how both vertical and horizontal movement varies across the San Andreas Fault System, scientists will be able to better constrain their fault models in predicting future earthquakes.
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Geographic representation of vertical crust movement along San Andreas Fault (Credit: Nature Geoscience)
Above is the unfiltered vertical movement calculated from various GPS instruments across southern California and the San Andreas Fault. Color-coded for rising vs. falling crust, there is significant variability across the fault in movement magnitude and direction. This was part of the reason the vertical movement was so difficult to decipher for so long.
Despite the San Andreas Fault being a strike-slip fault with primarily lateral movement, all strike-slip faults have a vertical component, albeit very small in comparison. Hence, the vertical movement does not demonstrate fundamental changes in the way we believe the San Andreas Fault works, but further characterizes its movement in hopes to predictively model future movement. This is a monumental and vital task given the dense population along this fault system and significant length of time since an earthquake in most sections of the fault. For instance, the southern section has not relieved built up stress since the 1857 earthquake of magnitude 7.9. Thankfully there are many early warning messaging apps that can notify you of any danger or threat of an earthquake.
http://www.forbes.com/sites/trevornace/2016/06/22/large-movement-detected-along-san-andreas-fault/#32365b661437
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