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−1 P = (0.0011±0.0008)f (0.36±0.32). Using only wave forms comprised in the area of seismic swarms, the dependence of attenuation on frequency increases (Q−1P = (0.0373±0.0011)f (−0.59±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
This work was undertaken as part of the Natural Environment Research Council (NERC) Centre for Doctoral Training (CDT) in Oil and Gas [grant number NEM00578X/1]. It is sponsored by University of Aberdeen whose support is gratefully acknowledged. We thank the Universit`a della Calabria for providing the dataset for this study. We thank Dr Cristina Totaro for providing the velocity model used for comparison with our results. We would also like to thank two anonymous reviewers whose constructive feedback greatly improved the original manuscript.
Waveforms of temporary stations used in this paper were provided by Universitá della Calabria and are available upon request. Seismograms recorded by INGV seismic stations (CUC, MTSN, MGR, SALB) were extracted by EIDA Data Archives at http://www.webdc.eu/webdc3/ (last access August 2017). Figures of this paper were produced using Voxler (Golden Software LLC, www.goldensoftware.com), Inkscape for MacOS X (version 1.0beta) and Adobe® Photoshop®.
- Seismic attenuation
- Seismic tomography
- Body Waves
- Fluid Storage