Earthquakes are among the most dangerous natural hazards in Southern California, home to millions of people. A web of fractures crisscrosses the region, including the well-known San Andreas Fault and a series of faults in the Transverse Ranges north of Los Angeles. The crustal movement caused by this geologic jigsaw means that potentially destructive earthquakes are inevitable.
The Transverse Ranges, a collection of mountains oriented east to west, are particularly concerning to earthquake experts. Across these ranges, Earth’s crust contracts, or shortens, by about 10–15 millimeters per year. Past research into this motion suggests that the accumulating strain will eventually result in a massive earthquake. Despite this hazard, a lack of accurate geodetic measurements has hidden the complete seismic picture.
In a new study, Johnson et al. used precise measurements of Earth’s surface to investigate the Transverse Ranges’ vertical motion along faults. The researchers applied what’s known as a kinematic model to explain the rates, patterns, and directions of active crustal deformation. Most previous modeling efforts used 2D or simplified 3D fault geometries, but in this work the authors integrated vertical and horizontal motion of the Transverse Ranges with additional data like offshore geologic and fault maps in a 3D model.
The authors found differing rates and characteristics of fault slip potential in the Transverse Ranges. Whereas inland movement results in lasting uplift, coastal deformation could be transformed by future seismic events. For example, the next big earthquake could relift the subsiding Santa Barbara coastline. The accumulated crustal motion in both the Santa Barbara Channel and the San Fernando Valley–Los Angeles Basin regions is equivalent to two magnitude 7.0 earthquakes every 100 years, the researchers report.
The results improve understanding of earthquake hazards in Southern California and constrain a model for fault slip and elastic strain in the western Transverse Ranges. The authors note that the findings also help resolve critical questions about Earth’s surface along the Santa Barbara coast. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1029/2020JB019672, 2020)
—Aaron Sidder, Science Writer