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We analyze the clustered microseismicity recorded over more than one decade in the Irpinia region, an active seismic area in Southern Italy. We studied the spatiotemporal properties of rate, source parameters and slipping mechanisms of the clustered seismicity in relationship with local medium properties and the deformation signal. Spatial pattern of both medium properties and source parameters indicates a segmentation along strike of the fault network with evidence of a high- and a low-coupling segments separated by a region with diffused microseismicity and high b-value. The high-coupling segment shows high fraction of clustered seismicity, low b-value, high relative coseismic slip, low Vp/Vs (1.7–1.75) and near-repeating earthquakes. We find that the seismicity in this high-coupling segment is triggered by non-tectonic loading, such as the recharge of a karst aquifer, suggesting a system close to critical conditions. The high-coupling segment can be identified as a potential strong motion area for future earthquakes, while the intermediate segment can mark the edges of an extensional transfer zone, which can act as a discontinuity for slip propagation. The low-coupling segment is the same where the Ms. 6.9, 1980 Irpinia earthquake nucleated. This segment shows today both relatively high b-values and high coseismic slip, but above all it shows temporal dependencies in its properties, which leads us to pay attention to it for the future spatiotemporal evolution of microseismicity in this area. • Fraction of clustered microseismicity on the Irpinia fault (Italy) highlights segmentation. • A low- and a high-coupling segments have been identified along the Apennines direction. • High-coupling segment hosts two domains with different fault mechanisms and rheologies. • Near-repeating earthquakes nucleate only in one domain of the high-coupling segment. • High-coupling segment can slip more during large earthquakes, as indeed occurred in the past. [ABSTRACT FROM AUTHOR]

On 1 April 2014, Northern Chile was struck by a magnitude 8.1 earthquake following a protracted series of foreshocks. The Integrated Plate Boundary Observatory Chile monitored the entire sequence of events, providing unprecedented resolution of the build-up to the main event and its rupture evolution. Here we show that the Iquique earthquake broke a central fraction of the so-called northern Chile seismic gap, the last major segment of the South American plate boundary that had not ruptured in the past century. Since July 2013 three seismic clusters, each lasting a few weeks, hit this part of the plate boundary with earthquakes of increasing peak magnitudes. Starting with the second cluster, geodetic observations show surface displacements that can be associated with slip on the plate interface. These seismic clusters and their slip transients occupied a part of the plate interface that was transitional between a fully locked and a creeping portion. Leading up to this earthquake, the b value of the foreshocks gradually decreased during the years before the earthquake, reversing its trend a few days before the Iquique earthquake. The mainshock finally nucleated at the northern end of the foreshock area, which skirted a locked patch, and ruptured mainly downdip towards higher locking. Peak slip was attained immediately downdip of the foreshock region and at the margin of the locked patch. We conclude that gradual weakening of the central part of the seismic gap accentuated by the foreshock activity in a zone of intermediate seismic coupling was instrumental in causing final failure, distinguishing the Iquique earthquake from most great earthquakes. Finally, only one-third of the gap was broken and the remaining locked segments now pose a significant, increased seismic hazard with the potential to host an earthquake with a magnitude of >8.5. [ABSTRACT FROM AUTHOR]