California's earthquake 'swarm' triggered by fluid, scientists say

A strange'swarm' of small earthquakes in California that lasted nearly four years was triggered by fluid spilling into the fault system from underground reservoirs, scientists say. The naturally occurring injection of underground fluid drove the earthquake swarm near Cahuilla in Southern California, which occurred in bursts around the region from early 2016 to late 2019. US scientists have made their conclusions based on earthquake detection algorithms that catalogued more than 22,000 individual seismic events that made up the'swarm'. Using machine learning to plot the location, depth and size of the tremors, the researchers generated a 3D representation of the underlying fault zone. The results suggested dynamic pressure changes from natural fluid injections deep below the surface largely controlled the prolonged evolution of the Cahuilla swarm.