Seismology at Antares Site. Anne Deschamps, Diane Rivet, Géoazur, UCA,CNRS,IRD,OCA

Seismologists question : earthquakes understanding Location from land based instrumentation

Not very precise Shift in location to the SW reaching ~100 km ! THALES WAS RIGHT EU project Offshore EQs located by 5 OBS and ~10 land-stations (6 months) LOC WITHOUT OBS (Permanent network, NOA catalogues) Shift in location to the SW reaching ~100 km ! Previously located “Aegean Sea events” are in fact : “Ionian Sea subduction events” LOC WITH OBS

Seismicity is mainly moderate In Mediterannean area Last 14 days of seismicity Seismicity is mainly moderate But, for a large part, just offshore along coasts. Installing permanent seismological observations is important for some specific studies

At Antares Site in 2005/2006: Comparaison de la densité spectrale de l’accélération du sol sur une station large bande (50hz à 360 sec) : modèle moyen admis pour les stations terrestres, 2 stations à terre ARBF et RUSF et la station sous marine en bleu : on voit une bande dans laquelle le bruit est similaire (entre 5sec et 20sec , mais en dehors le bruit est nettement plus important) Les courbes représentes une moyenne sur l’année de courbes journalières. Les périodes pendant les quelles il y a de gros séismes ont été enlevées. Variation en cours d’année du bruit du bruit sur la composante EW : on voit une variation saisonnière importante dans différentes bandes de fréquence. La barre violette marque la limite en fréquence de la courbe précédente. On a là l’ensemble du spectre utilisé dans une station sismologique standard. Dans la partie basse fréquence (inférieure à 0.1 Hz) qui nous intéresse, on note une véritable amélioration en couvrant le capteur: l’opération a été réalisée en avril 2006 et le capteur a mis un certain temps a se stabiliser. Ce n’est qu’en comparant les niveaux de bruit en janvier 2006 et décembre 2006 que l’on peut se faire une idée de l’amélioration. Durant l’été le niveau de bruit des plus faible de façon général. A haute fréquence le signal est perturbé par des résonances que l’on peut associer aux propriétés des sédiments directement sous le capteur Current speed influence on seismic noise. Variation of the DSP of the EW acceleration of sea bottom (12 hours periods) Burying the sensor improve the signal/noise ratio at low frequency Noise is lower during summer * DSP = density power spectra

Absolute Pressure Sensor Differential Pressure Gauge At Antares site 2010/2016 ASEAF station. Velocimeter, Accelerometer Absolute Pressure Sensor Velocimeter sensors response function of frequency Protection bell Differential Pressure Gauge Velocimeter sensors response as function of frequency Electronic Container Interlink Cable

Large earthquakes observations Earth normal modes observed at ASEAF (Antares) and SSB (Massif Central, less than 300km away) after the M=8.5 Sumatra event April 2012: the very good agreement shows the quality of the instrument. Detail on each pick contribue to the knowledge of earth internal properties. 0S3 period 36 mn frequency 0.47mHz 0S3 period 26 mn frequency 0.65mHz 0S5 period 20 mn frequency 0.84mHz 0S0 period 21 mn frequency 0.81mHz

Not working anymore Pressure measurement Absolute pressure : tides and high frequency signal are present. Ground acceleration (black), pressure gauge (red) and absolute pressure (green) signal during Tohoku-Oki march 2011 event. In the frequency range 0.01 and 0.1 signals are equivalent. Absolute pressure is more noisy.

Tsunami observation : a pressure gauge Real time observation needed

At Antares site 2010/2016 ASEAF station: Data send continously to EPOS eida nodes to be integrated in European seismological data base where the quality control is performed. winter ASEAF CALF summer

Regional sismicity Contribue to 20% of evnts detected by Géoazur network Neries-esonet Workshop, Paris february 2010

Regional sismicity Contribue to 20% of evnts detected by Géoazur network Neries-esonet Workshop, Paris february 2010

Regional seismicity Waveform modeling allows to determine location, magnitude and focal mechanism of the regional seismicity. After a period of extension during which Corsica went away from the continent, the present deformation of the Earth crust shows a shortening of the distance. How is distributed this deformation, what are the largest potentialy seismic faults?

Structure of the Earth at different scales Regional scale: P and S waves velocities beneath North American plate Farrallon plate beneath North America USARRAY data

Contribution to AlpArray program European effort 2012-2018 French participation : on land : 80 stations on seafloor 40 OBS But we did not suceed to synchronize with Antares recording period.

EMSO- Ligure East

EMSO- Ligure East

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