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Summary The Pollino range is a region of slow deformation where earthquakes generally nucleate on low-angle normal faults. Recent studies have mapped fault structures and identified fluid-related dynamics responsible for historical and recent seismicity in the area. Here, we apply the coda-normalization method at multiple frequencies and scales to image the 3D P-wave attenuation (QP) properties of its slowly-deforming fault network. The wide-scale average attenuation properties of the Pollino range are typical for a stable continental block, with a dependence of QP on frequency of $Q_P^{-1}=(0.0011\pm 0.0008) f^{(0.36\pm 0.32)}$. Using only waveforms comprised in the area of seismic swarms, the dependence of attenuation on frequency increases ($Q_P^{-1}=(0.0373\pm 0.0011) f^{(-0.59\pm 0.01)}$), as expected when targeting seismically-active faults. A shallow very-low-attenuation anomaly (max depth of 4-5 km) caps the seismicity recorded within the western cluster 1 of the Pollino seismic sequence (2012, maximum magnitude MW = 5.1). High-attenuation volumes below this anomaly are likely related to fluid storage and comprise the western and northern portions of cluster 1 and the Mercure basin. These anomalies are constrained to the NW by a sharp low-attenuation interface, corresponding to the transition towards the eastern unit of the Apennine Platform under the Lauria mountains. The low-seismicity volume between cluster 1 and cluster 2 (maximum magnitude MW = 4.3, east of the primary) shows diffuse low-to-average attenuation features. There is no clear indication of fluid-filled pathways between the two clusters resolvable at our resolution. In this volume, the attenuation values are anyway lower than in recognized low-attenuation blocks, like the Lauria Mountain and Pollino Range. As the volume develops in a region marked at surface by small-scale cross-faulting, it suggests no actual barrier between clusters, more likely a system of small locked fault patches that can break in the future. Our model loses resolution at depth, but it can still resolve a 5-to-15-km-deep high-attenuation anomaly that underlies the Castrovillari basin. This anomaly is an ideal deep source for the SE-to-NW migration of historical seismicity. Our novel deep structural maps support the hypothesis that the Pollino sequence has been caused by a mechanism of deep and lateral fluid-induced migration.

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SUMMARY In volcanoes, topography, shallow heterogeneity and even shallow morphology can substantially modify seismic coda signals. Coda waves are an essential tool to monitor eruption dynamics and model volcanic structures jointly and independently from velocity anomalies: it is thus fundamental to test their spatial sensitivity to seismic path effects. Here, we apply the Multiple Lapse Time Window Analysis (MLTWA) to measure the relative importance of scattering attenuation vs absorption at Mount St Helens volcano before its 2004 eruption. The results show the characteristic dominance of scattering attenuation in volcanoes at lower frequencies (3–6 Hz), while absorption is the primary attenuation mechanism at 12 and 18 Hz. Scattering attenuation is similar but seismic absorption is one order of magnitude lower than at open-conduit volcanoes, like Etna and Kilauea, a typical behaviour of a (relatively) cool magmatic plumbing system. Still, the seismic albedo (measuring the ratio between seismic energy emitted and received from the area) is anomalously high (0.95) at 3 Hz. A radiative-transfer forward model of far- and near-field envelopes confirms this is due to strong near-receiver scattering enhancing anomalous phases in the intermediate and late coda across the 1980 debris avalanche and central crater. Only above this frequency and in the far-field diffusion onsets at late lapse times. The scattering and absorption parameters derived from MLTWA are used as inputs to construct 2-D frequency-dependent bulk sensitivity kernels for the S-wave coda in the multiple-scattering (using the Energy Transport Equations—ETE) and diffusive (AD, independent of MLTWA results) regimes. At 12 Hz, high coda-attenuation anomalies characterize the eastern side of the volcano using both kernels, in spatial correlation with low-velocity anomalies from literature. At 3 Hz, the anomalous albedo, the forward modelling, and the results of the tomographic imaging confirm that shallow heterogeneity beneath the extended 1980 debris-avalanche and crater enhance anomalous intermediate and late coda phases, mapping shallow geological contrasts. We remark the effect this may have on coda-dependent source inversion and tomography, currently used across the world to image and monitor volcanoes. At Mount St Helens, higher frequencies and deep borehole data are necessary to reconstruct deep volcanic structures with coda waves.

The Amatrice–Visso–Norcia seismic sequence struck Central Italy across the Apenninic normal fault system in 2016. Fluids likely triggered the sequence and reduced the stability of the fault network following the first earthquake (Amatrice, Mw 6.0), with their migration nucleating the Visso (Mw 5.9) and Norcia (Mw 6.5) mainshocks. However, both spatial extent and mechanisms of fluid migration and diffusion through the network remain unclear. High fluid content, enhanced permeability, and pervasive microcracking increase seismic attenuation, but different processes contribute to different attenuation mechanisms. Here, we measured and mapped peak delay time and coda attenuation, using them as proxies of seismic scattering and absorption before and during the sequence. We observed that the structural discontinuities and lithology control the scattering losses at all frequencies, with the highest scattering delineating carbonate formations within the Gran Sasso massif. The Monti Sibillini thrust marks the strongest contrasts in scattering, indicating a barrier for northward fracture propagation. Absorption does not show any sensitivity to the presence of these main geological structures. Before the sequence, low-frequency high-absorption anomalies distribute around the NW-SE-oriented Apennine Mountain chain. During the sequence, a high-absorption anomaly develops from SSE to NNW across the seismogenic zone but remains bounded north by the Monti Sibillini thrust. We attribute this spatial expansion to the deep migration of CO2-bearing fluids across the strike of the fault network from a deep source of trapped CO2 close to the Amatrice earthquake. Fluids expand SSE-NNW primarily during the Visso sequence and then diffuse across the fault zones during the Norcia sequence.