Aseismic transient driving the swarm-like seismic sequence in the Pollino range, Southern Italy

Abstract

S U M M A R YTectonic earthquake swarms challenge our understanding of earthquake processes since it is difficult to link observations to the underlying physical mechanisms and to assess the hazard they pose. Transient forcing is thought to initiate and drive the spatio-temporal release of energy during swarms. The nature of the transient forcing may vary across sequences and range from aseismic creeping or transient slip to diffusion of pore pressure pulses to fluid redistribution and migration within the seismogenic crust. Distinguishing between such forcing mechanisms may be critical to reduce epistemic uncertainties in the assessment of hazard due to seismic swarms, because it can provide information on the frequency-magnitude distribution of the earthquakes (often deviating from the assumed Gutenberg-Richter relation) and on the expected source parameters influencing the ground motion (for example the stress drop). Here we study the ongoing Pollino range (Southern Italy) seismic swarm, a long-lasting seismic sequence with more than five thousand events recorded and located since October 2010. The two largest shocks (magnitude M w = 4.2 and M w = 5.1) are among the largest earthquakes ever recorded in an area which represents a seismic gap in the Italian historical earthquake catalogue. We investigate the geometrical, mechanical and statistical characteristics of the largest earthquakes and of the entire swarm. We calculate the focal mechanisms of the M l > 3 events in the sequence and the transfer of Coulomb stress on nearby known faults and analyse the statistics of the earthquake catalogue. We find that only 25 per cent of the earthquakes in the sequence can be explained as aftershocks, and the remaining 75 per cent may be attributed to a transient forcing. The b-values change in time throughout the sequence, with low b-values correlated with the period of highest rate of activity and with the occurrence of the largest shock. In the light of recent studies on the palaeoseismic and historical activity in the Pollino area, we identify two scenarios consistent with the observations and our analysis: This and past seismic swarms may have been 'passive' features, with small fault patches failing on largely locked faults, or may have been accompanied by an 'active', largely aseismic, release of a large portion of the accumulated tectonic strain. Those scenarios have very different implications for the seismic hazard of the area.

Aseismic transient during the 2010–2014 seismic swarm: evidence for longer recurrence of M ≥ 6.5 earthquakes in the Pollino gap (Southern Italy)?

“…2b). In particular, between June and August 2012, about 3–4 months before the occurrence of the M W 5.1 main shock, the rate of seismicity increased faster than in the previous 2011–2012 bursts of activity 27 , which unfortunately were not covered by the geodetic measurements. The aseismic slip rate shows a synchronous increment in almost the same time period, culminating in a dramatic increase just before the 25 October 2012  M W 5.1 earthquake (Fig.…”

“…Paleoseismological trenching studies 42, 43 on the normal faults at the southern border of the Pollino range suggest the occurrence of at least four M W  ≥ 6.5 events within the last 10,000 yr. Recent preliminary estimates of tectonic loading show that the Pollino range is actively deforming, probably slower than the Southern Apennines (the latter characterized by deformation rates up to 2–2.5 mm/yr 44, 45 ), with geodetic directions of active deformation consistent with the focal mechanisms of the largest events of the 2010–2014 Pollino swarm 27 .…”

“…The epicentre of the largest shock ( M W 5.1) is shown as a red star. The green stars indicate the location of the M W  > 3.5 events 27 . The mechanisms of these events are taken from time domain moment tensor (TDMT) catalogue (red and green beach-balls, http://cnt.rm.ingv.it/tdmt) and from Passarelli et al .…”

Fluid‐faulting evolution in high definition: Connecting fault structure and frequency‐magnitude variations during the 2014 Long Valley Caldera, California, earthquake swarm

“…Other studies of seismic swarms have also shown b value variations with time, in both natural [e.g., Wiemer et al ., ; Hainzl and Fischer , ; Passarelli et al ., ] and induced [e.g., Huang and Beroza , ] environments; however, the sense of these fluctuations varies. Wiemer et al .…”

Magmatic or Not Magmatic? The 2015–2016 Seismic Swarm at the Long-Dormant Jailolo Volcano, West Halmahera, Indonesia

“…The seismicity rate during swarms fluctuates over time and the largest earthquakes are usually delayed with respect to the swarm onset (Utsu, 2002;Hainzl, 2004;Vidale and Shearer, 2006;Passarelli et al, 2015aPassarelli et al, , 2018. They are thought to originate in response to stress histories more complex than a sudden stress change, e.g., due to a large earthquake, producing typical aftershock sequences (Mogi, 1967).…”