Microswarms: a lens into fault structure and aseismic processes deep in Southern California’s crust

Earthquake swarms are clusters of seismic events occurring in a given area within a relatively short period of time. They are distinct from standard aftershock sequences – series of small earthquakes following large earthquakes - in that they appear to be driven by non-tectonic factors. Recent analysis suggests that the most likely processes triggering earthquake swarms are fluid migration within Earth’s crust, and slow fault creep events.

Here, the researchers examined data collected across Southern California between 2008 and 2020. They used machine learning algorithms which allow extracting small earthquake signals from the seismic noise. They discovered hundreds of unknown swarms that lasted 6 months to several years in duration.

The team now analyze these newly identified sequences to quantify their collective spatial and temporal patterns. One goal is to identify if there are generalizable characteristics. Another goal is to characterize the underlying driving processes and document where and how often these swarms occur.

The study aims to provide a more comprehensive regional context for the role of non-tectonic factors in driving earthquake activity. One of its outcomes is a publicly searchable high-resolution catalog of recent earthquake swarms in Southern California. The project also provide support for one graduate student and outreach to K-12 students and the public. It is funded by both NSF Geophysics and Geoinformatics programs.

Earthquake swarms are believed to be driven mainly by aseismic processes. The spatiotemporal evolution of these sequences therefore encodes unique information: the structural and permeability architecture of fault zones, the aseismic processes responsible, and the coupling between fault properties and earthquake physics. Here, the researchers leveraged recent advances in earthquake monitoring capabilities with deep learning algorithms.

They showed that during the last decade, Southern California has experienced hundreds of previously unknown swarm episodes that persist for months to years. These earthquake microswarms are now used to map the potential origins of fluids deep within the seismogenic crust and to quantify the frequency and duration of these transient episodes. The timing of the initiation and arrest of microswarms will be examined systematically for connections to the largest regional events.

Furthermore, the large collection of microswarms will be used to provide constraints on the geometrical, structural, and permeability architecture of the fault zones at depth. These data are critical to understanding the complex evolution of earthquake swarms in space and time.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.