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