Crustal Structure of the Seismogenic Volume of the 2010–2014 Pollino (Italy) Seismic Sequence From 3D P- and S-Wave Tomographic Images

Abstract

A tomographic analysis of Mt. Pollino area (Italy) has been performed using earthquakes recorded in the area during an intense seismic sequence that occurred between 2010 and 2014. 870 local earthquakes with magnitude ranging from 1.8 to 5.0 were selected considering the number of recording stations, the signal quality, and the hypocenter distribution. P- and S-wave arrival times were manually picked and used to compute 3D velocity models through tomographic seismic inversion. The resulting 3D distributions of VP and VS are characterized by high resolution in the central part of the investigated area and from surface to about 10 km below sea level. The aim of the work is to obtain high-quality tomographic images to correlate with the main lithological units that characterize the study area. The results will be important to enhance the seismic hazard assessment of this complex tectonic region. These images show the ductile Apennine platform (VP = 5.3 km/s) overlaying the brittle Apulian platform (VP = 6.0 km/s) at depth of around 5 km. The central sector of the area shows a clear fold and thrust interface. Along this structure, most of the seismicity occurred, including the strongest event of the sequence (MW 5.0). High VP (>6.8 km/s) and high VP/VS (>1.9) patterns, intersecting the southern edge of this western seismogenic volume, have been interpreted as water saturated rocks, in agreement with similar geological context in the Apennines. These fluids could have played a role in nucleation and development of the seismic sequence. A recent study revealed the occurrence of clusters of earthquakes with similar waveforms along the same seismogenic volume. The hypocenters of these cluster events have been compared with the events re-located in this work. Jointly, they depict a 10 km × 4 km fault plane, NW-SE oriented, deepening towards SW with a dip angle of 40–45°. Instead, the volume of seismicity responsible for the ML 4.3 earthquake developed as a mainshock-aftershock sequence, occurring entirely within the average-to-low VP/VS Apennine platform. Our results agree with other independent geophysical analyses carried out in this area, and they could significantly improve the actual knowledge of the main lithologic units of this complex tectonic area.

Fast Changes in Seismic Attenuation of the Upper Crust due to Fracturing and Fluid Migration: The 2016–2017 Central Italy Seismic Sequence

“…In fault networks like those ruptured by the AVN, Napolitano et al (2020) found that high-scattering and highabsorption patterns obtained by increasing frequencies map the migration of the historical seismic events from the 16th century until the Pollino seismic sequence. This work supported the view that lateral fluid diffusion drove the sequence (De Matteis et al, 2021;Napolitano et al, 2021). Focal mechanism tomography highlighted the excess of pore pressure in the hypocentral area, which affected the seismicity distribution during the seismic sequence (De Matteis et al, 2021).…”

Seismic imaging of fluid-filled inherited structures of the Northern Thessaly (Greece) seismic gap

“…The V P /V S ratio maps (Figures 3, 4; Supplementary Figure S7) have been obtained by dividing the value of V P by V S at each node of the inversion grid. We centered the color map to the average V P /V S value (1.78) computed by De Novellis et al (2021) and set the lower and the upper limit of the color scale at 1.5 and 2.1, respectively. This allows us to visually identify relatively high and low V P /V S anomalies if compared with the average value.…”

“…The 1,500 events selected from the starting dataset have been located using the Non-LinLoc software (Lomax et al, 2000), considering the new minimum 1D P-wave model (Supplementary Figure S1) and the V P /V S ratio of 1.78 provided by De Novellis et al (2021). We excluded all the events having less than 6 direct wave arrival times, azimuthal gap higher than 220 °and horizontal and vertical location error larger than 1 km.…”

“…The earthquakes of the sequence have been recorded by the broad-band and strong motion stations of the Hellenic Unified Seismological Network (HUSN; http://eida.gein.noa.gr/). Mainshocks and aftershocks have been previously located by De Novellis et al (2021) by using a 1D velocity model proposed for an area to the South-East of Thessaly (Papadimitriou et al, 2010). In order to obtain an accurate tomographic image of the crustal volume affected by the sequence, in the present study we selected the events and the stations that fall within a volume extending 40×41×20 km 3 around the mainshock epicentral area.…”

“…Early geodetic models (Ganas et al, 2021;Papadopoulos et al, 2021;Kontoes et al, 2022) was caused by unmapped blind normal fault segments. De Novellis et al (2021) and Yang et al (2022) proposed four seismogenic faults for the seismic sequence, consisting of two NE-dipping faults (one of which is responsible for the Mw 6.3), another NE-dipping fault for Mw 6.0 and a SW-dipping fault for Mw 5.6. Based on field investigations, among the activated structures, only the Vlachogianni fault segment (Figure 1) provided clear evidence of linear morphogenic deformation along the surface (Ganas et al, 2021;Koukouvelas et al, 2021;Kassaras et al, 2022;Kontoes et al, 2022).…”