Small magnitude earthquake sequences and seismic swarms in the Northern Apennines (central Italy): new observations from the analysis of Montefeltro seismicity

In this work we examine the seismicity of Montefeltro, a historical region of central Italy located between the northern Apennines and the Republic of San Marino towards the northern Adriatic coast. The seismicity of the northern Apennines region is mainly concentrated along the chain within a seismogenic layer that extends from about 5 to 15 km depth. Recently, important seismic sequences occurred in the central-northern Apenninic chain with magnitudes exceeding 5.5 (Colfiorito 1997, Mw 6.0; L’Aquila 2009 Mw 6.3; Emilia 2012, Mw 6.1; Amatrice 2016, Mw 6.5). Instead, the seismicity of Montefeltro, as recorded in the past three decades, appears quite widespread and discontinuous over time, except in some phases of intense activity. The magnitudes detected so far vary essentially from low to moderate, lower than 4.5 for the largest events. Unfortunately, the region that includes the Montefeltro is still not adequately monitored, therefore a detailed analysis of the background seismicity, the calculation of focal mechanisms for low magnitude events, and a more reliable estimate of the b-value is limited. To better define these parameters, a temporary seismic network has been set up starting from December 2018 to integrate the instrumental coverage provided by the Italian National Seismic Network (RSN). The analysis of the seismic activity occurred in the period January 2005 – up to now shows a sporadic, rather diffuse background seismicity marked by small magnitude seismic sequences strongly clustered in time and space. The most important sequences occurred in September-October 2005 (M=3.2) and August-September 2006 (M=3.7), nearby the towns of Macerata Feltria and Casteldelci, respectively. The spatio-temporal evolution of these main episodes highlights an energy release consistent with swarm-like activity. Others clusters of less intensity and duration were observed in January 2011 (ML 2.2, 15 events), September 2012 (ML 2.8, 6 events), and July 2015 (ML 2.3, 10 events). These bursts are confined in the upper crust contrary to the swarm-like seismicity which extends up to ~25 km depth. Another major feature of the seismicity pattern is represented by the occurrence of events even in the lower crust and in the upper mantle down to at least 50 km depth.