Central San Andreas crawls without a major earthquake

Between Los Angeles and San Francisco, the San Andreas fault releases tension through gradual movement. Scientists calculate slip fault from compensation terrain and discuss seismic hazards.

Written by Chelsea Scott, Ph.D., Assistant Research Scientist, Arizona State University (ChelseaPScott)

Quote: Scott, C.E., 2021, central San Andreas area crawls out without major earthquake, Templore, http://doi.org/10.32858/temblor.152

The crack “creep” in Hollister, California, replaced this sidewalk and created a small break across the road (orange arrows). In places like these, error compensation can be measured with a ruler. When roads and fences do not cross the wrong way, measuring this disparity is more difficult. Credit: Chelsea Scott

The San Andreas Fault, California, is known for causing large and destructive earthquakes – including the San Francisco earthquake of 1906 with a magnitude of 7.8. Perhaps less well known is the fact that some parts of the San Andreas Fault seldom erupt in large earthquakes. Between Los Angeles and San Francisco, for example, a portion of the San Andreas Fault is constantly slipping, or “creeping,” at about three quarters of an inch (20 mm) a year. A creeping fault can cause small earthquakes, but the vibration is generally imperceptible or only felt near the fault. Sometimes, the crawler fault can slip suddenly in a large earthquake.

In a new study published in Geophysical Research Letters, my colleagues and I show where and how much of this creeping error. We use airborne instruments to measure changes in terrain that are so small that they occur so slowly that our eyes cannot see them. These new observations reveal the active effect of the creeping San Andreas fault and show the extent to which the active motion embraces a narrowing fault trace. These limitations are essential for scientists to understand the processes that control fault creep in the crust and to determine seismic risk.

Crawling errors are a risk

Because crawler faults slide gradually and almost continuously, they do not build up as much pressure as lock faults, which slip infrequently and release most of the pressure in large earthquakes. However, creeping faults cause some stress, which often is not fully released, which means that these faults can host large earthquakes and pose a risk. Measurement of fault activity or creep rate is essential to know the amount of stress accumulated along these faults and the probability of a medium to large earthquake in the future.

Map showing the creeping section of the San Andreas Rift between Los Angeles and San Francisco. The rate of creep along the San Andreas fault and the Calaveras fracture close to this study are shown in the purple to yellow colored line. Credit: modified from Scott et al. (2020).

Measurement of motion error of changing landscapes

Measuring false slip rate is difficult. Offset baffles like the one pictured above in Hollister, California, give scientists a way to directly monitor the amount of slip that has occurred at a particular site, but these types of structures do not cross the error in the spacing required to accurately measure the wide range error proposition. There are a variety of methods for measuring fault slip rate, but most of them do not capture spatially dense slip rate which is critical for assessing overall fault activity.

Light detection and rangefinding, known as lidar, is a technique used to create a 3D representation of a target based on a laser signal. Although geologists are known for using it in self-driving cars, they use this technology to measure elevation across landscapes. When the elevation surrounding the fault is measured at two different times – often the distance from months to years – a comparison between the two sets of topographic data reveals how the fault has moved (Nissen et al. 2012; Oskin et al. 2012).

My colleagues and I were the first to measure the rate of creep along the entire crawling portion of the central San Andreas fault from airborne lidar datasets. This work resulted in the most spatially dense slip rate measurements along the available error. We measured the right lateral crawl rate every 1,300 feet (400 meters) along the fault from airborne LIDAR topographic data sets obtained with a difference of just over a decade. Applying this method to a crawler error was difficult because the error crawl nodes produced a relatively low amount of slip relative to the noise in the data. To reduce noise and better resolve creep, we removed vegetation from the lidar data because changes in vegetation cover significantly contribute to noise.

Airborne lidar point cloud in elevation along the San Andreas Fault from EarthScope data set collected in 2008 (EarthScope, 2008). Black arrows indicate the San Andreas Fault. Red colors indicate higher elevation than blue colors. Credit: Chelsea Scott

Not all tectonic movements occur along a separate fault

The movement between the two sides of the fault can either be determined on a narrow surface or distributed over a wider area in the shallow crust of the Earth. Earthquake research showed that older faults that absorbed more slip tend to locate the movement in a narrow fault. The San Andreas fault is very old and about 90% of the movement should be put into the trace of the error (Dolan & Haravitch, 2014). However, we show that only 50-80% of the movement along the creeping San Andreas Fracture occurs in a narrowing trail. We suggest that this unexpected behavior reflects a much slower rate of slip during crawl events relative to earthquakes.

Right lateral surface displacement surrounding the San Andreas Fracture was measured between 2007 and 2018 from an airborne lidar. The red colors indicate a movement toward the northwest, while the blue colors indicate a movement toward the southeast. In the plot on the left, the sharp change in red to blue across the fault indicates that the tectonic motion is centered on a narrow fault. On the right, 37 miles (60 km) to the southeast, a white-colored low displacement zone is grouped along the fault curve, indicating that the movement is distributed behind the fault trail. Credit: Chelsea Scott

What is active error?

Our results show an image of motion in the tilt range surrounding the error tracer, allowing scientists to clearly see faults in shallow Earth’s crust that adapt to the motion. In the right figure above, we tracked the active San Andreas fault in black based on the error movement over the past decade. This active fault tracking is about half a mile from where the U.S. Geological Survey identified the defect based on the shape of the hills and valleys that indicated an older fault site. The various fault sites show that the active tracking of the San Andreas fault has changed over time. Crucially, our result reveals the location of the fault active over the past decade, and is an important input into assessing earthquake risk.

Earthquake hazards

The stress accumulation over the length of the rift depends on the difference between the long-term rate of tectonic plate boundaries and the rate of creep. One of the main challenges in measuring earthquake risk is knowing how fault slip rates change from the surface as they can be measured to the deeper portions of the fault where slip rates are more difficult to measure. To infer the slip rate in depth, scientists look at surface movement over a larger area, indicating a deeper fault slippage. These displacements measure fault slip on a scale between very near and far field instruments. In the future, we plan to use surface motion measurements over a range of regions to infer fault slip over the entire subsurface range of the fault. This analysis will indicate pressure build-up and the likelihood of a future earthquake along the fault.

Further reading

Dolan, GF, and Haravitch, BD (2014). How well do surface slip measurements slide deep into large striking earthquakes? Importance of fault structural maturity in controlling fault-induced slip versus surface deformation outside the fault. Earth and Planetary Sciences Letters, 388, 38-47. https://doi.org/10.1016/j.epsl.2013.11.043

EarthScope (2008). EarthScope, Northern California Lidar Project. NSF OpenTopography attachment. https://doi.org/10.5069/G9057CV2

Nissen, E, Krishnan, AK, Arrowsmith, JR, and Saribale, S. (2012). Surface 3D displacement and rotation processes from LiDAR point clouds variation before and after the earthquake. Geophysical Research Letters, 39 (16). https://doi.org/10.1029/2012GL052460

Oskin, ME, Arrowsmith, JR, Corona, AH, Elliott, AJ, Fletcher, JM, Fielding, EJ, et al. (2012). Near field deformation detected by the El Mayor-Cucapah earthquake by differential LIDAR. Science, 335 (6069), 702-705. https://doi.org/10.1126/science.1213778

Scott, CB, DeLong, SP, & Arrowsmith, JR (2020). Seismic distortion distribution along the San Andreas and Calaveras faults from the repeated variation of airborne lidar. Geophysical Research Letters. https://doi.org/10.1029/2020GL090628

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