Time–Space Evolution of Seismic Strain Release in the Area Shocked by the August 24– October 30 Central Italy Seismic Sequence SIMONE BARANI, 1 CLAUDIA MASCANDOLA, 2 ENRICO SERPELLONI, 3 GABRIELE FERRETTI, 1 MARCO MASSA, 2 and DANIELE SPALLAROSSA 1 Abstract—In this study, we analyze the space–time evolution of the seismic strain release in the area shocked by the still ongoing Italian Central Apennines seismic crisis started on August 24, 2016 and culminated with the October 30 main shock of M w 6.5. Specifically, we examine the variation in time and space of the seismic strain release rate with the aim of identifying the presence of peculiar seismicity patterns, such as seismic gaps, according to the seismic cycle theory. To this end, seismic strain rates are checked for consistency with strain rates from GPS measurements to possibly adjust them for missing events due to limited seismic catalog extension or incompleteness at large magnitudes. Our results has revealed that the seismic crisis followed a long-term quiescence of about 310 years, characterized by the absence of M6.5? earthquakes, and marked by an almost steady release of seismic deformation. Such temporal gap started after the occur- rence of two nearby strong events in 1703 (Valnerina and L’Aquila earthquakes with magnitudes of 6.9 and 6.7, respectively) and terminated with the beginning of the current Central Apennines seismic crisis. Key words: Central Apennines, seismic sequence, quiescence, seismic gap, seismic strain rate, geodetic strain rate. 1. Introduction On August 24, 2016 at 1:36 UTC, a strong earthquake of moment magnitude M w 6 (green star in Fig. 1a) struck a vast area between Amatrice and Norcia in the Italian Central Apennines, causing severe damage and casualties (300 people died). After about 1 h, the earthquake was followed by an M w 5.3 aftershock located to the north, near the city of Norcia. Since then, the seismic crisis mainly evolved to the north where a large number of earth- quakes of lower magnitude were recorded (http://cnt. rm.ingv.it). On October 26 at 19:18 UTC and October 30 at 6:40 UTC, other two strong shocks occurred north of Norcia with M w of 5.9 and 6.5, respectively. The latter is the strongest shock generated during the entire seismic crisis (orange star in Fig. 1a), which is currently ongoing. While writing other four strong shocks (with M w between 5.2 and 5.8) frightened the population south of Amatrice, near Montereale. However, the present study focuses on the first stages of the crisis, until the end of November 2016. In this period, the study area was affected by more than 8.000 events with M [ 2(http://cnt.rm.ingv.it). The moment tensor solutions (http://cnt.rm.ingv.it/tdmt) of the main shocks, along with deep and surface geological information and geodetic data (e.g., Pierantoni et al. 2013; Bigi et al. 2011; Cheloni et al. 2016; INGV Working Group ‘‘GPS Geodesy’’ 2016), point out a normal kinematic, with an NNW–SSE (*150°) direction and a dip ranging from 45° to 50°. The seismic crisis, which occurred north of the area shocked by the 2009 L’Aquila seismic sequence (e.g., Anzidei et al. 2009; Barani and Eva 2011), evolved over an area of approximately 50 km length and 15 km width, throughout the Sibillini mountain chain and the Laga massif. The entire region has been experiencing chain-normal extension since the late Pliocene, with extensional faults (with a general NW–SE trend) that crosscut previous compressional structures, originating several intra-mountain basins. The extensional tectonics is still ongoing, as mea- sured by geodetic data (e.g., Anderlini et al. 2016). 1 Dipartimento di Scienze della Terra dell’Ambiente e della Vita, Universita ` di Genova, Genoa, Italy. E-mail: barani@dipteris.unige.it 2 Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Milano, Milan, Italy. 3 Centro Nazionale Terremoti, Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy. Pure Appl. Geophys. Ó 2017 Springer International Publishing DOI 10.1007/s00024-017-1547-5 Pure and Applied Geophysics