A Study on the Long Duration of Ground Motion in The Grenoble Basin,French Alps

Abstract eng:
The city of Grenoble in the French Alps is settled on top of hundreds of meters of post-glacial sediments which cause ground motion to be significantly increased in terms of amplitude and duration. Here, we present analyses focused on the ground motion duration observed in the Grenoble valley for 3 well recorded local events with magnitudes M2.8, M2.9 and one regional event with magnitude M4.4. The analysis is performed in terms of energy duration (based on ground velocity) and Arias duration (based on ground acceleration) for frequencies between 0.5 Hz and 15 Hz. We find that the observed duration is controlled by low frequencies, i.e. less than 3 Hz for local (d<20 km) small (M2.8, M2.9) events and less than 2 Hz for the regional (d=150 km) larger (M4.4) event. For the regional event we observe a slight correlation between duration and sediment thickness on the vertical component. Assuming a simplified 3D model of the valley derived from gravimetric studies and borehole measurements, numerical simulation of those local events with the spectral element method successfully explains the level of observed amplification for frequencies up to 2 Hz but fail at predicting such values of lengthening of duration, unless considering unrealistic quality factors in quaternary sediments. The reason why the available model of the Grenoble valley does not explain the observed duration is further discussed in terms of weathered rock formation, intrinsic attenuation, and shallow velocity structures.

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