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San Andreas Fault

Large-scale motion along the San Andreas Fault was predicted in computer models for years. But the predicted movements were so tiny it was never actually witnessed and documented – until now.

Newly-interpreted satellite data gave researchers the evidence that the plates are moving millimeters per year, vertically, the University of Hawaii team reports in the latest issue of Nature Geoscience.

The trick was focusing in on the infinitesimal, but also huge, dynamics.

“While the San Andreas GPS data has been publicly available for more than a decade, the vertical component of the measurements had largely been ignored in tectonic investigations because of difficulties in interpreting the noisy data,” said Samuel Howell, UH – Manoa doctoral student, and the lead author. “Using this technique, we were able to break down the noisy signals to isolate a simple vertical motion pattern that curiously straddled the Sand Andreas fault.”

The creeping motion is incredibly slow – but also wide-ranging. While the fault height was just 2 millimeters per year of the analysis, it also showed that change was spread out over 200 kilometers.

Horizontal motion of faults has been well-observed, especially through earthquakes. But this observation of the vertical motion is a new breakthrough, the scientist contend. It could add a whole new dimension to understanding seismic activity, contend the authors, also from the University of Washington and the Scripps Institution of Oceanography.

“We suggest that these motions reveal the subtle, but identifiable, tectonic fingerprint of far-field flexure due to more than 300 years of fault locking and creeping depth variability,” they write. “Understanding this critical component of interseismic deformation at a complex strike-slip plate boundary will better constrain regional mechanics and crustal rheology, improving the quantification of seismic hazards in southern California and beyond.”

California has been relatively free of disastrous earthquakes so far this century – though one in Baja topped out at 7.2 on the Richter scale in 2010, causing more than $1 billion in damages.

The geological discoveries of the complex seismic dynamics underground continue. Last month the U.S. Geological Survey announced it had found that Mount St. Helens was gradually recharging its magma supplies in huge subterranean chambers through a “swarm” of small earthquakes.

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