Revealing the buried structure of Apennines intermontane basins through dense nodal microtremor surveys: the case of Colfiorito and Annifo basins (central Italy).

Extensive microtremor surveys can provide valuable constraints on the deep structure of seismically active Quaternary intramontane basins. This study investigates two study areas, namely the Colfiorito and Annifo basins, which were affected by a seismic sequence during the years 1997–1998  [1]. Within the framework of the First ILGE Transnational Access (TNA) and National Open Access (NOA) Call (within the PNRR MEET WP3 project), a dense microtremors dataset was acquired in 2024 to improve the geological characterization of the two Quaternary basins. A total of 160 single-station microtremor measurements were collected over six days using 48 seismic nodes equipped with 4.5 Hz triaxial sensors, with recording time windows ranging from a few hours to two days. Two helicoidal nodal arrays were deployed in the northern Colfiorito plain and an additional one was installed in the southern area of the Annifo basin, to derive detailed shear-wave velocity profiles [2]. Moreover, two temporary stations equipped with 5-s Lennartz sensors and Reftek dataloggers were also operated for a few days. H/V spectral ratio analysis was carried out and revealed contrasting behaviors between the two basins. In Annifo, H/V peaks exceed 1.0 Hz, with the main depocenter located in the southern part of the basin. In Colfiorito, two main H/V frequency ranges have been observed: one is characterized by low-frequency peaks, between 0.6 and 1.0 Hz, located between the central and northeastern sectors of the basin, whilst a second, with frequencies between 1.0 and 6.0 Hz, characterizes the rest of the basin. In Colfiorito, the spatial distribution of resonant frequencies is consistent with a recent and independent gravimetric survey results [3], which identifyed two significant gravity minima in the central sector of the basin.

Acknowledgments

This publication results from work carried out under transnational     /national open access  (TNA/NOA) action under the support of WP3 ILGE - MEET project, PNRR - UE Next Generation Europe program, MUR grant number D53C22001400005.

References

[1] Messina, P.; Galadini, F.; Galli, P.; Sposato, A. Quaternary Basin Evolution and Present Tectonic Regime in the Area of the 1997–1998 Umbria–Marche Seismic Sequence (Central Italy). Geomorphology 2002, 42, 97–116, doi:10.1016/S0169-555X(01)00077-0.

[2] Di Giulio, G.; Cornou, C.; Ohrnberger, M.; Wathelet, M.; Rovelli, A. Deriving Wavefield Characteristics and Shear-Velocity Profiles from Two-Dimensional Small-Aperture Arrays Analysis of Ambient Vibrations in a Small-Size Alluvial Basin, Colfiorito, Italy. Bulletin of the Seismological Society of America 2006, 96, 1915–1933.

[3] Di Filippo, M.; Mancinelli, P.; Cavinato, G.P.; Pauselli, C.; Sabatini, A.; Mirabella, F.; De Franco, R.; Barchi, M.R. Bouguer Gravity Anomaly in the Colfiorito Quaternary Continental Basin, Northern Apennines, Central Italy. Journal of Maps 2025, 21, 2503244, doi:10.1080/17445647.2025.2503244.