Ground Structure and Strong Motions in the Grenoble Basin

在本研究中，我们想弄清楚的是，日本近年来大力发展的强地面运动估计和损害估计方案在地质环境与日本大不相同的欧洲是否同样有效。为此，我们调查了格勒诺布尔盆地的地下结构，通过与法国格勒诺布尔大学合作，为我们提供了一个联合研究。然后，我们进行地面运动预测研究的格勒诺布尔市。作为日本原创技术开发的强震预测技术，在地质环境差异较大的地区也具有通用性。2005年发生在格勒诺布尔盆地南部的Laffrey 2005地震中，观测到了持续时间极长的强震。我们首先提取信息的持续时间和波能量相比，其他事件。我们还使用三维有限差分法的格勒诺布尔盆地的模拟结果。可以指出的是，如果我们假设周围岩层顶部的表面层相当薄弱，则可以引入较长的持续时间。由于沉积物顶部软层的引入使得持续时间更长，并且成功地再现了盆地内大部分场地的持续时间，这是很自然的。然而，盆地东北部的持续时间很长，只能通过考虑非常软的风化层来解释。另一方面，据了解，该地区地面调查的分析是先进的，在钻孔和微震阵列测量中，地表存在层厚约25 m的弱面。因此，软表层沉积物的可能性很高，在观测到的波形中产生很长的持续时间。

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.