Publication Date:
2021-11-29
Description:
The geological evolution of the western Mediterranean
exhibits complicated interactions between orogenic processes and
widespread extensional tectonics. The region is located in a
convergent plate margin separating Africa and Europe, and
consists of marine basins – the Alboran Sea, the Algerian-
Provençal Basin, the Valencia trough, the Ligurian Sea and the
Tyrrhenian Sea- which formed as back-arc basins since the
Oligocene. In most reconstructions, it has been stressed that
back-arc extension led to drifting of continental blocks and to
large-scale block rotations. The opening of the Ligurian Sea. is in
fact the result of counterclockwise rotation of Corsica and
Sardinia.
From the point of view of seismicity, the south western Alps
and northern part of the Ligurian basin are subject to frequent
earthquakes of low to moderate magnitudes. However
significantly destructive events are known to have occurred in the
past (e.g. 1564 and 1887). Apart from these rare large events,
regional studies agree in concluding that the important local
microseismicity appears to be poorly focused (e.g., COURBOULEX
et alii, 2007) and that, if some tectonic lines are documented
onland (COURBOULEX et alii, 2001), the active structures at sea
remain unknown. It is therefore an essential prerequisite to gain
better insight into the deep seismogenic structures along the
North Ligurian margin and even farther offshore, in the identified
oceanic domain. The fact that some of these structures can
undergo ruptures of Mw~6.5, such as the 1887 event (BAKUN &
SCOTTI, 2006), suggests that, at least to some extent, instrumental
insufficiencies in the detection and location of microseismicity is
a limit to identify active faults that have not experienced large
instrumented ruptures to date.
The irregular coverage provided by regional seismic networks
produces a bias in the recording of local seismicity. Permanent
stations are naturally limited to land areas and fail to properly
constrain seismicity offshore. Taking into consideration the
peculiarities of regional dynamics (low strain rates, rare large
events and a regular seismic activity limited to small events with
M 〈 3-4), even onshore seismicity is insufficiently covered by
permanent networks and requires dense temporary instrumenting
by mobile stations.
Considering the potential threat of strong offshore
earthquakes, it is of first importance to characterize faults that are
prone to rupture in order to quantify associated seismic and
tsunami hazards. Assuming some weak seismicity exists along
these faults and remains undetected by onland networks, some
marine stations are necessary to address instrumental remoteness
and help delineate active structures.
Moreover, since the velocity models used for locations are
obtained by inverting seismic data and the reliability of their
locations depend, in turn, from the quality of the velocity model
used for their hypocentral parameters, the constraints on the
seismic path provided by a more dense seismic network may
contribute to a more accurate reference model.
In this study, we profited from the recent developments in sea
bottom seismic instrumentation to deploy OBSs above the zones
of the North Ligurian to perform seismic shots and obtain the
distribution of seismic velocities with 3D active tomography. We
also took the opportunity of the long term (6 months) OBSs
reduced array to decrease both the detection threshold and
recording distances so as to obtain more complete catalogs and
better localisations.
Description:
Published
Description:
789-791
Description:
1T. Struttura della Terra
Description:
N/A or not JCR
Keywords:
04.01. Earth Interior
;
04.06. Seismology
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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