Study Reveals Large-Scale Movement Along San Andreas Fault

New areas of large-scale motion along the San Andreas fault system have been discovered, according to the University of Hawai'i.

Using data from an array of GPS instruments, the researchers found nearly 125 mile-wide “lobes” of uplift and subsidence along the Saint Andreas fault system. The lobes see about a few millimeters of movement annually.  

The researchers are from the University of Hawaiʻi at Mānoa, University of Washington, and Scripps Institution of Oceanography (SIO).

The GPS instruments work by detecting motion of the Earth’s crust near the San Andreas fault system. Sometimes movement is large and sudden—such as during an earthquake, other times, movement is more creeping and subtle.

Movement in the crust can be vertical and horizontal. Vertical movement is affected by many factors, such as tectonic motion of the crust, pumping of groundwater, and precipitation, so discerning broad regional motion from shorter-scale, local motion was a challenge.

In order to discern broad and short scale motion, doctoral student Samuel Howell and his team used a comprehensive statistical technique to extract patterns of large-scale vertical motions from the GPS.

“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 Howell. “Using this technique, we were able to break down the noisy signals to isolate a simple vertical motion pattern that curiously straddled the San Andreas fault.”

The large scale motion found was in accordance with motion found by previous earthquake studies.

“We were surprised and thrilled when this statistical method produced a coherent velocity field similar to the one predicted by our physical earthquake cycle models,” said study co-author Bridget Smith-Konter, associate professor at the University of Hawai'i.

“The powerful combination of a priori model predictions and a unique analysis of vertical GPS data led us to confirm that the buildup of century-long earthquake cycle forces within the crust are a dominant source of the observed vertical motion signal.”

The new findings were published in Nature Geoscience on June 20.