A new seismological study found that a naturally-occurring injection of underground fluids triggered an earthquake swarm near Cahuilla, California, that lasted for nearly four years. The research shows an evolving understanding of how fault architecture drives earthquake patterns.
"We used to think of faults more in terms of two dimensions: like giant cracks extending into the earth," said lead author Zachary Ross, also an assistant professor of geophysics.
"What we're learning is that you really need to understand the fault in three dimensions to get a clear picture of why earthquake swarms occur."
Known as the Cahuilla swarm, it is a sequence of small tremors that happened between 2016 and 2019 near Mt. San Jacinto in Southern California.
Ross, along with his colleagues from Caltech, USGS, and the University of Texas used earthquake-detection algorithms with deep neural networks to generate a highly-detailed catalog of over 22 000 seismic events in the area, with magnitudes ranging from 0.7 to 4.4.
The compilation unveiled a complex but narrow fault zone, just 50 m (164 feet) wide, with steep curves. Ross said plotting those curves was crucial to understanding why the seismic activity lasted for years.
Faults are typically believed to either act as conduits for or barriers to the flow of underground fluids. While Ross's study generally supports that, the team discovered that the architecture of the fault produced complex conditions for underground fluids.
Image credit: Caltech
The team noted that the fault zone had undulating subterranean channels that linked with an underground reservoir of fluid, which was initially sealed off from the fault.
When the seal broke, fluids were injected into the fault zone and dispersed through the channels, prompting earthquakes. The team found that this natural injection process was sustained over around four years.
"These observations bring us closer to providing concrete explanations for how and why earthquake swarms start, grow, and terminate," said Ross.
The team's next plan is to build off these new insights and determine the role of this type of process throughout entire Southern California.
"3D fault architecture controls the dynamism of earthquake swarms" - Ross, Z. E. et al. - Science - DOI: 10.1126/science.abb0779
The vibrant evolutionary patterns made by earthquake swarms are incompatible with standard, effectively two-dimensional (2D) models for general fault architecture. We leverage advances in earthquake monitoring with a deep-learning algorithm to image a fault zone hosting a 4-year-long swarm in southern California. We infer that fluids are naturally injected into the fault zone from below and diffuse through strike-parallel channels while triggering earthquakes. A permeability barrier initially limits up-dip swarm migration but ultimately is circumvented. This enables fluid migration within a shallower section of the fault with fundamentally different mechanical properties. Our observations provide high-resolution constraints on the processes by which swarms initiate, grow, and arrest. These findings illustrate how swarm evolution is strongly controlled by 3D variations in fault architecture.
Featured image credit: Unsplash