Ground Structure and Strong Motions in the Grenoble Basin

在这项研究中，我们想要澄清的是，日本近年来积极开发的强地面运动估计和破坏估计方案是否同样适用于欧洲，因为欧洲的地质环境与日本非常不同。为此，我们与法国格勒诺布尔大学合作，利用微震测量对格勒诺布尔盆地的地下结构进行了调查，该大学为我们提供了一项联合研究。然后，我们对格勒诺布尔市进行了地面运动预测研究。预计将表明，作为日本独创技术开发的强地震动预报技术具有通用性，适用于地质环境差异较大的地区。2005年格勒诺布尔盆地南部发生的Laffrey 2005级地震，观测到了极长持续时间的地震动场。我们首先提取持续时间和波能量信息，并与其他事件进行比较。我们还使用了格勒诺布尔盆地的三维有限差分方法的模拟结果。可以指出的是，如果我们假设周围岩层顶部的表层相当薄弱，可能会引入较长的持续时间。自然地，在沉积物顶部引入软层会使持续时间更长，并成功地在盆地内的大部分地点复制持续时间。然而，盆地东北部持续时间很长的现象只能通过考虑非常软的风化层来解释。另一方面，据了解，该地区地面测量的分析是先进的，在钻探和微震台阵测量中，地表存在约25m厚的软弱表面。因此，软表层沉积物在观测到的波形中产生很长持续时间的可能性很高。

What we would like to clarify in the present study is whether the estimation of strong ground motion and the estimation of damage scheme that Japan has developed aggressively in these years are similarly effective in Europe where the geological environment is different very much from Japan. To this end we investigated the underground structure of the Grenoble basin by using microtremor measurement in cooperation with the Grenoble University in France who offered us a joint research for the purpose. We then perform ground motion prediction study for the Grenoble city. It is expected to show that the strong ground motion prediction technology that has been developed as a Japanese original technology has the generality that can be applied to the region where the geological environment is quite different.The extremely long duration of the seismic ground motion were observed in the Laffrey 2005 event that had been occurred in the south of the Grenoble basin in 2005. We first extract information of duration and wave energy in comparison to the other events. We also use simulation results of the three-dimensional finite difference methods for the Grenoble basin. It was able to be pointed out that a long duration can be introduced if we assume considerably weak surface layers on top of the surrounding rock formations. As is quite natural an introduction of soft layers on top of sediments makes duration even longer and successfully reproduces durations at most of the site within the basin. However, very long duration in the northeastern part of the basin can only be explained by considering the very soft, weathered layer.On the other hand, it has been understood that the analysis of the ground survey in the region is advanced, and a weak surface of the layer thickness of about 25m exists in the surface in boring and the array measurement of microtremors. Therefore, the possibility of the soft surface sediments is high that create very long duration in the observed waveforms.