By mapping the more than 22,000 tremblors, researchers composed a detailed, three-dimensional image of the complex fault structure below southern California's Cahuilla Valley. According to a new study, the four-year-long earthquake swarm that rocked the region was likely triggered by the dynamic interaction between the fault's intricate architecture and natural subterranean fluids, revealing new insights into how these enigmatic seismic events evolve. Despite being known as highly complex three-dimensional structures, earthquake faults are often simplified into two-dimensional features in most standard models of general fault architecture. However, these idealized representations generally fail to explain the dynamic seismicity of earthquake swarms - prolonged periods of localized seismic activity that can occasionally persist over several years. While many of a swarm's tremblors are small, the overall length of the phenomenon and the potential severity of individual seismic events cannot be predicted, thus making them a public safety concern. According to the authors, understanding 3D fault geometry is essential to understanding the complex seismic evolution of earthquake swarms. Zachary Ross and colleagues used advanced earthquake detection algorithms to catalog more than 22,000 individual seismic events during the 2016-2019 Cahuilla swarm in southern California. Using machine learning to plot the location, depth and size of the tremblors, Ross et al. generated a high-resolution, 3D representation of the underlying fault zone structure. The results reveal a complex yet permeable fault structure and suggest that dynamic pressure changes from natural fluid injections from below largely controlled the evolution of Cahuilla swarm. The methods offer a new way to characterize other similar faults and seismic events around the world.