Borehole initiatives in France and Greece: The Grenoble and Corinth gulf vertical arrays.

F Lemeille (1), C. Berge-Thierry (1), D. Hatzfeld (2), P. Bernard (3) European Geophysical Society XXVII General Assembly - Nice, 21-26 avril 2002.

The near-surface geological site conditions in the upper tens of meters are one of the dominant factors in controlling the amplitude and variation of strong ground motion, and the damage patterns that result from large earthquakes. Our understanding of these site effects comes primarily from surface recordings. In recent years, however, the increase in the number of borehole instruments provides a significant step forward in directly measuring the effects of surface geology. In the last ten years, the IRSN has been involved in the Garner Valley Deep Accelerometer Project (GVDA) in Southern California in collaboration with the Nuclear Regulatory Commission (NRC) and the University of California Santa Barbara (UCSB) both in the USA. This project has produced valuable data to study site effects and it constituted a milestone in terms of managing experience of a vertical array. In this way in order to study site effects in the European Community, IPSN is also collaborating in two recent projects, one in Grenoble (France), and another in the Gulf of Corinth (Greece). In the Grenoble valley, a 556 m deep borehole has been drilled. This project is also supported by the LGIT in 1999-2000 with local and ministerial funding. The reference accelerometer is in the Mesozoic basement at 556 m deep. On the surface, the accelerometer is also part of the Permanent Accelerometric Network ruled by the LGIT. The borehole drilling was performed together with acoustic logging. One year of recording of accelerations is available on the Web. The LGIT, BRGM and INPG/3S carried out geophysical experiments on the site as well. In the framework of CORSEIS European program, IPSN together with ENS Paris, IPG Paris (France), AUTH and NKUA (Greece), are supporting a vertical array of accelerometers and pore pressure transducers dedicated to the study of liquefaction and nonlinearity in the Aigion harbor. These phenomena occurred during the June 15 1995 (M = 6.2) event. All transducers are located in soft sediment (holocene marine sand and clay related to the paleoshoreline). The deeper accelerometer is in a stiff layer of conglomerates at 200 m deep. Geotechnical measurements (SPT, Shelby, lab tests), and geophysical experiments (cross-hole, down-hole) also improve the knowledge of the physical parameters at the site. (1) Institut de Radioprotection et de Surete Nucleaire, (2) LGIT-Universite Joseph Fourier Grenoble, (3) IPG Paris