Nowcasting Earthquakes in Southern California with Machine
Learning:Bursts, Swarms and Aftershocks May Reveal the Regional Tectonic
Stress
Abstract
Seismic bursts in Southern California are sequences of small earthquakes
strongly clustered in space and time, and include seismic swarms and
aftershock sequences. A readily observable property of these events, the
radius of gyration (), allows us connect the bursts to the temporal
occurrence of the largest ³7 earthquakes in California since 1984. In
the Southern California earthquake catalog, we identify hundreds of
these potentially coherent space-time structures in a region defined by
a circle of radius 600 km around Los Angeles. We compute for each
cluster, then filter them to identify those bursts with large numbers of
events closely clustered in space, which we call “compact” bursts. Our
basic assumption is that these compact bursts reflect the dynamics
associated with large earthquakes. Once we have filtered the burst
catalog, we apply an exponential moving average to construct a time
series for the Southern California region. We observe that the of these
bursts systematically decreases prior to large earthquakes, in a process
that we might term “radial localization.” The then rapidly increases
during an aftershock sequence, and a new cycle of “radial
localization” then begins. These time series display cycles of recharge
and discharge reminiscent of seismic stress accumulation and release in
the elastic rebound process. The complex burst dynamics we observe are
evidently a property of the region as a whole, rather than being
associated with individual faults. This new method allows us to improve
earthquake nowcasting in a seismically active region.