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| Titel |
The Pollino Seismic Sequence: Activated Graben Structures in a Seismic Gap |
| VerfasserIn |
Dirk Rößler, Luigi Passarelli, Aladino Govoni, Dino Bindi, Simone Cesca, Sebastian Hainzl, Francesco Maccaferri, Eleonora Rivalta, Heiko Woith, Torsten Dahm |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250112494
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| Publikation (Nr.) |
 EGU/EGU2015-12654.pdf |
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| Zusammenfassung |
| The Mercure Basin (MB) and the Castrovillari Fault (CF) in the Pollino range (Southern
Apennines, Italy) represent one of the most prominent seismic gaps in the Italian seismic
catalogue, with no M>5.5 earthquakes during the last centuries. In historical times several
swarm-like seismic sequences occurred in the area including two intense swarms within the
past two decades. The most energetic one started in 2010 and has been still active in 2014.
The seismicity culminated in autumn 2012 with a M=5 event on 25 October. The range hosts
a number of opposing normal faults forming a graben-like structure. Their rheology and their
interactions are unclear. Current debates include the potential of the MB and the CF to
host large earthquakes and the style of deformation. Understanding the seismicity
and the behaviour of the faults is necessary to assess the tectonics and the seismic
hazard.
The GFZ German Research Centre for Geosciences and INGV, Italy, have jointly monitored
the ongoing seismicity using a small-aperture seismic array, integrated in a temporary seismic
network. Based on this installation, we located more than 16,000 local earthquakes that
occurred between November 2012 and September 2014. Here we investigate quantitatively
all the phases of the seismic sequence starting from January 2010. Event locations along with
moment tensor inversion constrain spatially the structures activated by the swarm and the
migration pattern of the seismicity. The seismicity forms clusters concentrated within the
southern part of the MB and along the Pollino Fault linking MB and CF. Most
earthquakes are confined to the upper 10 km of the crust in an area of ~15x15
km2. However, sparse seismicity at depths between 15 and 20 km and moderate
seismicity further north with deepening hypocenters also exist. In contrast, the CF
appears aseismic; only the northern part has experienced micro-seismicity. The spatial
distribution is however more complex than the major tectonic structures mapped for the
area. Consistent with mapped faults, the seismicity interested both eastwards and
westwards dipping normal faults that define the geometry of seismically active
graben-like structures. At least one cluster shows an additional spatio-temporal
migration with spreading hypocentres similar to other swarm areas with fluid-triggering
mechanisms.
The static Coulomb stress change transferred by the largest shock onto the swarm area and on
the CF cannot explain the observed high seismicity rate. We study the evolution of the
frequency-size distribution of the events and the seismicity rate changes. We find that the
majority of the earthquakes cannot be justified as aftershocks (directly related to the tectonics
or to earthquake-earthquake interaction) and are best explained by an additional forcing
active over the entire sequence. Our findings are consistent with the action of fluids
(e.g. pore-pressure diffusion) triggering seismicity on pre-loaded faults. Additional
aseismic release of tectonic strain by transient, slow slip is also consistent with our
analysis. Analysis of deformation time series may clarify this point in future studies. |
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