Data on Northern California’s Creeping Faults: The Decades-Running Alinement Array Network and its Role in Plate Boundary Science

Austin J. Elliott, Forrest McFarland, John Caskey, & James J. Lienkaemper

Published September 10, 2023, SCEC Contribution #13254, 2023 SCEC Annual Meeting Poster #057

All the major faults in northern CA creep to some degree along some of their lengths. Understanding earthquake hazards thus requires knowing the distribution of aseismic slip in space and time. Here we present the current state of a foundational creep-monitoring network across the region that employs surveys of alinement arrays—sets of monuments installed straddling creeping faults that permit high-precision recurring measurement of relative ground position. Near-field measurement and monitoring of creep is also supported by an increasing number of complementary surveying and imaging methods: SAR has been instrumental in detecting fault creep; ground-based lidar permits change detection; creepmeters provide high-sample-rate monitoring spanning widths limited to several tens of meters. Among these, the exceptionally precise measurements of geometric change across 10^2-m-wide fault zones afforded by alinement arrays have conventionally remained the “ground truth” of near-surface creep rates.

We maintain a network of 79 alinement arrays in northern CA, most of which are surveyed annually. Many of these have been measured annually since the early 1990s, though original measurements may date from the 1960s, making these data the longest-running continuous monitoring of fault creep in the world. Measurements from the alinement arrays have established shallow creep rates on the San Andreas, Hayward, Calaveras, Concord, Rodgers Creek, Maacama, Greenville, Green Valley, and Bartlett Springs faults. They have revealed multi-year transients associated with regional earthquakes. Variability of creep rate along strike informs knowledge of the distribution of locking and interseismic loading that is essential for seismic hazard assessment. Ease and speed of deployment mean that alinement arrays constitute a critical method of rapid data acquisition immediately after a large earthquake; they will be essential for detecting and forecasting afterslip, as demonstrated after the 2014 South Napa quake.

We have made recent improvements in the digital accessibility of creep data to provide the community access to updated and legacy measurements. Survey data are updated yearly via a USGS data release. New Google Earth files are available that show the locations of sites and graphs of historical data. We are working on further accessibility improvements including a web-based GIS dashboard to dynamically convey updated measurements from continued surveys of our alinement arrays.

Key Words
aseismic slip, creep, northern california, hayward fault

Citation
Elliott, A. J., McFarland, F., Caskey, J., & Lienkaemper, J. J. (2023, 09). Data on Northern California’s Creeping Faults: The Decades-Running Alinement Array Network and its Role in Plate Boundary Science. Poster Presentation at 2023 SCEC Annual Meeting.

Related Projects & Working Groups
Tectonic Geodesy