Accurate measurements of the Earth’s surface’s vertical movement help clarify the risks of faults in an earthquake-prone area.
Santa Barbara Channel is seen in the foreground of Ventura, California. New research shows that vertical motion in the Earth’s crust differs between the Santa Barbara Channel, the foreground here near Ventura, California, and the inland Transverse Ranges. Future earthquakes could suddenly lift the coast, unlike its current regular subsidence.
Earthquakes are one of the most dangerous natural hazards in Southern California, where millions of people live. A network of fractures crisscrosses the area, including the famous San Andreas Fault and a series of faults in the transverse chains north of Los Angeles. The Earth’s crust movement caused by this geological puzzle means that potentially destructive earthquakes are inevitable.
The Transverse Ranges, a collection of east-west-oriented mountains, are of particular concern to earthquake experts. In these areas, the Earth’s crust contracts or shortens by about 10 to 15 millimeters per year. Previous research on this move suggests that the accumulated tension will eventually cause a significant earthquake. Despite this danger, the lack of precise geodetic measurements obscured the complete seismic picture.
In a new study, Johnson et al. used precise measurements of the Earth’s surface to study transverse ranges’ vertical motion along faults. The researchers applied a kinematic model to explain the speeds, patterns, and directions of active crustal deformation. Most previous modeling efforts used simplified 2D or 3D fault geometries. Still, in this work, the authors incorporated the transverse ranges’ vertical and horizontal movement with additional data such as geological maps and offshore faults into one—3d model.
The authors found different rates and characteristics of fault slip potential in the transverse ranges. As movement inland results in sustained buoyancy, coastal deformation can be transformed by future earthquakes. For example, the next major earthquake could lift the collapse of the Santa Barbara coast. The crustal movement accumulated in the Santa Barbara Canal and San Fernando Valley and Los Angeles Basin areas is equivalent to two 7.0 magnitude earthquakes every 100 years.
The results improve the understanding of seismic hazards in southern California and constrain a model for fault slip and elastic deformation in the western transverse ranges. The authors note that the findings also help resolve critical land surface issues along the coast of Santa Barbara. (Journal of Geophysical Research: Solid Earth,
In Southern California, the landscape is fragmented into the shape of a colossal letter Z. The upper arm consists of a twisting series of cracks responsible for the earthquakes that shook the town of Ridgecrest last year. The diagonal section is an ancient fault called the Garlock that runs to the west. And along the bottom lies the mighty San Andreas.
Earthquakes along this long fault line, which runs more than 800 miles across California, are a looming concern, and a new study suggests a significant earthquake near the bustling city of Los Angeles could be three to five times in the next year. It could be that big. Probably than previously thought. The research, published in the Bulletin of the Seismological Society of America, found that the Ridgecrest earthquakes in 2019 made a future quake along the nearby Garlock Fault more likely. If a big enough quake could also trigger the San Andreas Fault, researchers say a series of events has a roughly 1 in 87 chance of occurring in the next year.
However, the overall probability of such an event remains low. The research team estimates there is a 2.3% chance that a 7.7 magnitude earthquake will occur on the Garlock fault in the next year, and a 1.15 percent chance of a similar quake will hit San Andreas.
“So, the sky is not falling,” says study co-author Ross Stein, CEO of Temblor, Inc., a company that assesses the risks of hazards such as earthquakes. “But it is significantly higher, in our judgment, than what it would have been had the Ridgecrest earthquake not occurred.”
Estimating the probability of earthquakes is notoriously tricky. Scientists have increasingly recognized the profound flaws they cause are complex networks of cracks and chasms. “They’re fractal. They’re grungy. They have bends and breaks,” Stein says.
Failures can interact too: motion along one could increase stress on another, triggering a sequence of quakes, “like a domino effect,” says Alessandro Verdecchia, a geologist at McGill University. The latter was not part of the study. The new model is the latest attempt to assess the likelihood of this potentially fatal scenario.