Strong Site Effect Revealed by a New Broadband Seismometer on the Continental Shelf Offshore Nice Airport (Southeastern France)

Abstract

A broadband seismological station (PRIMA) installed offshore Nice airport (southeastern France) reveals a strong amplification effect of seismic waves. PRIMA station was in operation for 2 years (9/2016 to 10/2018) on the outer shelf at a water depth of 18 m. Situated at the mouth of the Var River, this zone is unstable and prone to landslides. A catastrophic landslide and tsunami already occurred in 1979, causing 10 casualties. Given the level of seismicity of the area, it is important to infer the impact of an earthquake on this zone. We analyze the recordings of earthquakes and seismic noise at the PRIMA station by comparing them to nearby inland stations. We find that the seismic waves are strongly amplified at PRIMA at some specific frequencies (with an amplification factor greater than 10 at 0.9 Hz). Using geological and geophysical data, we show that the main amplification frequency peak (at 0.9 Hz) is due to the velocity contrast between the Pliocene sedimentary layer and fine-grained sediments dated from the Holocene, at about 100 m depth. This velocity contrast is also present along the Var valley, but the level of amplification detected on PRIMA station is larger. Using numerical simulations of seismic waves in a 2D model that accounts for the pinch-out geometry related to the termination of the Holocene sedimentary layer, we can partially explain this amplification. This offshore site effect could have a crucial impact on the triggering of a submarine landslide by an earthquake in this region. More generally, this effect should be taken into account for the modeling of landslides and induced tsunamis triggered by seismic waves.

On the Detection Capabilities of Underwater Distributed Acoustic Sensing

“…uation in the 2-20 Hz band compared with signals recorded outside the bay. This behavior is indicative of sedimentary basins, which are generally characterized by low seismic velocities (e.g., Courboulex et al, 2020;Pratt et al, 2003).…”

“…Earthquakes recorded inside Methoni Bay show significant amplification of up to a factor of 10 at frequencies of 1–2 Hz, and stronger attenuation in the 2–20 Hz band compared with signals recorded outside the bay. This behavior is indicative of sedimentary basins, which are generally characterized by low seismic velocities (e.g., Courboulex et al., 2020; Pratt et al., 2003).…”

Transient Groundwater Flow Through a Coastal Confined Aquifer and Its Impact on Nearshore Submarine Slope Instability

“…(a) ST5‐PZ2L‐02PZA/P5 and (b) PZC/P18 pore pressure data (depth 3.99 mbsf) compared to temperature measured by sensor PZA/T1 (0.79 mbsf) of ST5‐PZ2L‐02 and (b) SPF‐PZ3L‐01/P18 pore pressure data (depth 25.75 mbsf) compared to seabed temperature at SPF‐PZ3L‐01. Six earthquakes recorded by a broadband seismological station (PRIMA) installed offshore Nice airport (Courboulex et al, 2020) are shown in (b) as vertical lines (from left to right: Norcia‐Central Italy, Digne‐South of France, Chile, Solomon, Limone‐northern Italy, and Monaco‐offshore). Blue line in (b) indicates the local earthquake that occurred in offshore Monaco, 40 km from the study site (Courboulex et al, 2020) is indicated on both figures.…”

Assessing Spatio‐Temporal Variability of Free Gas in Surficial Cohesive Sediments Using Tidal Pressure Fluctuations

“…There is indeed a wide recognition that by increasing pore-fluid compressibility free gas reduces excess pore pressure build-up during earthquake loading (DeJong et al, 2014 and reference therein). Hence, despite the strong site effect revealed by Courboulex et al (2020), questions remain as to the possibility that the presence of gas mitigates the earthquake-induced liquefaction potential that Roesner et al (2019) has stressed.…”