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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 (RG), allows us to connect the bursts to the temporal occurrence of the largest M ≥ 7 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 RG 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 RG of these bursts systematically decreases prior to large earthquakes, in a process that we might term "radial localization." The RG 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, which is a technique to evaluate the current state of hazard in a seismically active region. Plain Language Summary: Earthquake nowcasting is a method to evaluate the current state of seismic hazard from large earthquakes. In this paper, we connect the temporal occurrence of the largest and most potentially destructive earthquakes in California since 1984 with a readily observable property of small earthquake seismicity in the region. Our method involves the calculation of the time history of the average radius (horizontal size or extent) of "bursts" of small earthquakes, in the time leading up to and following major earthquakes in the region. We observe that the radius systematically and gradually decreases leading up to major earthquakes, increasing suddenly and discontinuously following the event. This observable pattern resembles the long‐hypothesized cycle of regional tectonic stress buildup and release, or elastic rebound, associated with large destructive earthquakes. We propose that the radius of these bursts might be considered to be a proxy variable for the changing state of regional stress in Southern California. Key Points: A new nowcasting method using earthquake swarms and aftershocks reveals the hazard from large M ≥ 7 earthquakes in Southern CaliforniaMachine learning defines a time series of bursts using an exponential moving average filtered and optimized to reveal temporal structureCycles of recharge and discharge associated with M ≥ 7 earthquakes are revealed, reminiscent of stress accumulation and release [ABSTRACT FROM AUTHOR]

The Southern Patagonian Icefield (SPI) lies above an area of slow convergence between Antarctic and South-America plates, where limited seismicity is recorded by global and regional seismic networks. To understand the seismic behavior of this zone, we analyze two years of continuous broad-band data recorded by 27 seismometers, deployed around the SPI. Substantial ice loss coupled with the unusually low viscosity of the underlying mantle is causing a rapid uplift. Our findings indicate that most of the seismicity occurs in the upper crust , likely associated with the (re)activation of regional compressive structures. However, earthquakes immediately beneath the SPI generally are shallower and show normal or strike-slip faulting. We suggest that this activity is promoted as a response to the crustal relaxation after rapid ice removal of SPI. The almost complete absence of interplate and intraslab events is consistent with a locked megathrust fault interface, highlighting the similarity of this region with the Cascadia subduction zone. • Shallow seismicity is observed in the majority in the area of southern Patagonia Icefield. • The inferred stress suggests the solid earth response to mass loss from the icefield as the major cause. • The almost complete absence of interplate earthquakes suggests that the megathrust may be locked. [ABSTRACT FROM AUTHOR]