Strong offshore site effect revealed by a broad-band seismometer installed on the Nice airport slope at 17 m water depth (South-East of France)
Résumé
Using earthquakes and seismic noise recordings on a broad-band station installed in October 2016 on the slope of the Nice airport at 17 m depth, we find a strong site amplification of the seismic waves (factor 10 around a frequency of 1Hz).
The effects of superficial layers on the amplitude, frequency and duration of seismic waves (often called site effect) has been extensively studied during the last two decades because it is responsible of large damages. Until now, site effect studies were only realized inland; the potential offshore site effect generated by sediment layers has therefore never been studied specifically. Nonetheless, a correct estimation of the amplification caused by offshore sediments is of great importance for three main applications:
-Landslides and tsunamis : seismic waves can trigger submarine landslides, that themselves can trigger tsunami waves. In numerical simulations and geotechnical experiments, it is then important to take into account the potential input wave amplifications.
-Offshore paleoseismology: strong vibrations generated by large earthquakes can trigger turbidite landslides. Turbidite stratigraphy is then a powerful tool often used to evidence and characterize the occurrence of ancient earthquakes. A correct amplitude of the seismic ground motion that takes into account site effects, has then to be estimated in models to correctly infer the magnitude of ancient earthquakes.
-Offshore infrastructures: in many highly populated zones, in order to gain new space, buildings and/or infrastructures are more and more often constructed on offshore areas.
In order to test and quantify these potential amplifications under the sea, we installed a broad-band seismometer “PRIMA” on the slope of the airport of the city of Nice (south-east of France) at a depth of 17 meters. The seismological station PRIMA belongs to the EMSO-Nice cabled observatory (EMSO: European Multidisciplinary Seafloor and water-column Observatory).
We then analyze the recordings of local and regional earthquakes and ambient seismic noise on this station, and compare them with the recordings of nearby stations inland (max 3 km away). We find a clear amplification of waves of a factor of ~10 (compared with inland station situated on rock) at a frequency of 0.8-1Hz, as well as smaller amplification peaks at higher frequencies for local earthquakes. These amplification will be compared with data from high resolution seismic profiles in order to better understand their origin.
This result is particularly important for the city of Nice. Indeed, the airport slope already experienced a large landslide in 1979, which triggered a tsunami wave that killed 10 persons and caused extensive damages all along the coast. It is them of upmost importance to be able to evaluate if a future earthquake could cause the same domino effect.