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.

中文翻译：

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新的宽带地震仪在尼斯大陆架离岸机场（法国东南部）上显示出强烈的场地效应

安装在尼斯机场（法国东南部）近海的宽带地震台站 (PRIMA) 显示出强烈的地震波放大效应。PRIMA 站在水深 18 m 的外陆架上运行了 2 年（9/2016 至 10/2018）。该地区位于瓦尔河河口，地势不稳定，容易发生滑坡。1979 年已经发生了灾难性的山体滑坡和海啸，造成 10 人伤亡。鉴于该地区的地震活动水平，推断地震对该地区的影响非常重要。我们通过将 PRIMA 站的地震和地震噪声记录与附近的内陆站进行比较来分析它们。我们发现地震波在某些特定频率（在 0.9 Hz 时放大系数大于 10）在 PRIMA 处被强烈放大。利用地质和地球物理数据，我们表明，主要放大频率峰值（0.9 Hz）是由于上新世沉积层与全新世细粒沉积物之间的速度对比造成的，深度约为 100 m。这种速度对比也存在于 Var 谷，但在 PRIMA 站检测到的放大水平更大。使用 2D 模型中地震波的数值模拟，该模型解释了与全新世沉积层终止相关的尖灭几何形状，我们可以部分解释这种放大。这种近海场地效应可能对该地区地震引发海底滑坡产生至关重要的影响。更一般地说，在模拟由地震波引发的滑坡和诱发海啸时，应考虑这种影响。