Dessa, Jean-Xavier

To constrain seismic anisotropy under and around the Alps in Europe, we study SKS shear wave splitting from the region densely covered by the AlpArray seismic network. We apply a technique based on measuring the splitting intensity, constraining well both the fast orientation and the splitting delay. Four years of teleseismic earthquake data were processed, from 723 temporary and permanent broad-band stations of the AlpArray deployment including ocean-bottom seismometers, providing a spatial coverage that is unprecedented. The technique is applied automatically (without human intervention), and it thus provides a reproducible image of anisotropic structure in and around the Alpine region. As in earlier studies, we observe a coherent rotation of fast axes in the western part of the Alpine chain, and a region of homogeneous fast orientation in the Central Alps. The spatial variation of splitting delay times is particularly interesting though. On one hand, there is a clear positive correlation with Alpine topography, suggesting that part of the seismic anisotropy (deformation) is caused by the Alpine orogeny. On the other hand, anisotropic strength around the mountain chain shows a distinct contrast between the Western and Eastern Alps. This difference is best explained by the more active mantle flow around the Western Alps. The new observational constraints, especially the splitting delay, provide new information on Alpine geodynamics. © 2021 The Author(s) 2021. Published by Oxford University Press on behalf of The Royal Astronomical Society.

The dense AlpArray network allows studying seismic wave propagation with high spatial resolution. Here we introduce an array approach to measure arrival angles of teleseismic Rayleigh waves. The approach combines the advantages of phase correlation as in the two-station method with array beamforming to obtain the phase-velocity vector. 20 earthquakes from the first two years of the AlpArray project are selected, and spatial patterns of arrival-angle deviations across the AlpArray are shown in maps, depending on period and earthquake location. The cause of these intriguing spatial patterns is discussed. A simple wave-propagation modelling example using an isolated anomaly and a Gaussian beam solution suggests that much of the complexity can be explained as a result of wave interference after passing a structural anomaly along the wave paths. This indicates that arrival-angle information constitutes useful additional information on the Earth structure, beyond what is currently used in inversions.

The AlpArray programme is a multinational, European consortium to advance our understanding of orogenesis and its relationship to mantle dynamics, plate reorganizations, surface processes and seismic hazard in the Alps–Apennines–Carpathians–Dinarides orogenic system. The AlpArray Seismic Network has been deployed with contributions from 36 institutions from 11 countries to map physical properties of the lithosphere and asthenosphere in 3D and thus to obtain new, high-resolution geophysical images of structures from the surface down to the base of the mantle transition zone. With over 600 broadband stations operated for 2 years, this seismic experiment is one of the largest simultaneously operated seismological networks in the academic domain, employing hexagonal coverage with station spacing at less than 52 km. This dense and regularly spaced experiment is made possible by the coordinated coeval deployment of temporary stations from numerous national pools, including ocean-bottom seismometers, which were funded by different national agencies. They combine with permanent networks, which also required the cooperation of many different operators. Together these stations ultimately fill coverage gaps. Following a short overview of previous large-scale seismological experiments in the Alpine region, we here present the goals, construction, deployment, characteristics and data management of the AlpArray Seismic Network, which will provide data that is expected to be unprecedented in quality to image the complex Alpine mountains at depth.

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.

The deep structure of the North Ligurian margin and its contiguous Ligurian basin as well as the seismicity recorded in these zones are neither well understood nor precisely constrained. In order to better address these questions, there is a need for offshore instrumenting, which was realised for a duration of nearly 6 months during the GROSMarin (Grand Réseau d’Observation Sous-Marin) experiment. An array of 21 ocean bottom seismometers was deployed over the most active area of the margin and was complemented on land by mobile seismological stations that densified existing permanent networks. We also realised the acquisition of deep refraction seismic shots at sea in order to get a 3D distribution of velocities along the margin through travel time tomography. We present here a preliminary analysis of the seismicity recorded during this experiment and a tomographic model of the margin structures obtained using data from the offshore network only. Our results support the existence of a high velocity zone at the base of a domain interpreted as transitional between continental and oceanic ones, on the northern part of the deep basin. A very similar pattern is observed across the neighbouring margin of the Gulf of Lions and is most likely related to serpentinisation of the underlying mantle during late rifting and continental break-up. North of this transition zone, we observe the basinward crustal thinning of the continental crust beneath the margin that seemingly narrows eastward. To the south, our results hint at transition to the oceanic domain. In contrast, our velocity distribution does not reveal a transition along strike between transitional and oceanic domains, as previous works suggest. Some microseismic activity was recorded throughout the duration of the experiment, on land and at sea. The number of detected events and precision of location were both improved by our considering French and Italian permanent networks. The detection capabilities of our dense network still need to be fully exploited

The North-Ligurian rifted margin is singular in that it lies immediately next to the Alpine orogenic arc. It is furthermore seismically active and can experience destructive earthquakes such as in 1887 in the region of Imperia—an event that resulted in a tsunami and more than 600 casualties in spite of a coastal area that was much less densely populated than today. Out of such rare large events, the area undergoes a limited and diffuse seismic activity that can remain undetected and is generally poorly located. This results in a poor knowledge of active structures, especially at sea. Such knowledge is however required towards a quantification of the seismic hazard along the French Riviera and the Ligurian region. To this end, the GROSMarin project was undertaken with a dual objective: (1) to characterize the North-Ligurian margin from a structural standpoint—mode and degree of crustal stretching prior to oceanic accretion, segmentation along strike, subsequent evolution in an orogenic context— and (2) to identify zones of active crustal deformation at sea that are likely to generate earthquakes. The programme is a collaborative work between GeoAzur and Dip.Te.Ris (University of Genova), with some support from INGV, IFM-GEOMAR and IFREMER. It took place from April to October 2008 and consisted in the deployment of 21 ocean-bottom seismometers (OBS) on a grid spanning 50 km along strike and 25 km across, located between Nice, France, and Imperia, Italy, and ranging from mid-slope to the deep basin. This array was extended on land by the permanent stations of the French and Italian regional networks, temporarily densified by 13 portable stations. These instruments recorded the shots of a marine seismic source towed from R/V l’Atalante and were left for more than 5 months for passive surveying. The active part of the programme aims at characterizing the main structures of the margin through crustal 3D tomography; the objective of the passive part is to decrease the detection threshold of marine microseismicity and to reach a precise location of events in order to map active faults. Some of the sea and land instruments were fitted with broadband sensors to allow for teleseismic imaging of deep lithospheric discontinuities. We present the preliminary results of this experiment—in particular a first 3D tomographic model obtained from 31.500 travel times derived from our recording of active seismic shots by the OBS’s. Passive data analysis is being under progress and first relocations have been obtained. These results give an insight into the variability of the crustal structure, both along and across strike.

The GROSMARIN (which stands for GrandROSMARIN) cruise is proposed by UMR Géosciences Azur (with fellow french and italian research groups). Its goals are to better characterize active structures along this zone and to assess the resulting seismic hazard in a sort of continuation with respect to the MALISAR experiment, which has already surveyed some active structures through shallow observations. The GROSMARIN cruise is in fact the necessary counterpart to characterize them at depth.

(English Abstract) The Ligurian margin, that is the junction area located between the Ligurian basin and the Southwestern Alps, is a passive margin, seismically active and subjected to gravitative movements. The active deformation in this sector is among the strongest ever experienced in Western Italy and Southern France. The current geodynamics of the basin is not completely understood yet, and somewhat under interest and debate of the scientific community. The latest results on the recent evolution of the Alps-Mediterranean system suggest that the area under study lay close to a domain under extension. The interest for the area is reinforced by its seismic activity that, although of low to moderate energy, acts in an area of high vulnerability. Some historical events involved in fact dramatic social and material damages. The growth of population (that now accounts for more than 2.500.000 inhabitants between Cannes and Genoa), the setting of numerous industries and the tourist business of the area are additional motivation for monitoring the area from the seismic point of view and especially to make specific studies on the seismogenic structures of this sector. Events with magnitude greater than 4.5 to 5.0 are in fact recorded every 5 years, but the area undergoes a rather weak microseismicity that often remains undetected and always poorly located by land seismic networks. The natural risks associated to this sector cannot neglect the presence of steep canyons that incise the offshore margin and favour gravitative slopes. The sediment masses accumulate on top of these canyons and may slip even after an earthquake of moderate magnitude. The GROSMARIN (which stands for GrandROSMARIN) cruise is proposed by UMR Géosciences Azur (with fellow french and italian research groups). It aims at (1) studying the microseismicity along a part of the northern margin of the Ligurian Basin, offshore France and Italy and (2) to realise a 3D tomography by wide-angle seismics. The goal is to better characterize active structures along this zone and to assess the resulting seismic hazard.