vS30, κ, regional attenuation and Mw from accelerograms: application to magnitude 3-5 French earthquakes

Abstract

International audienceWe investigate recordings from weak to moderate earthquakes, with magnitudes ranging between about 3 and 5, recorded by the French Accelerometric Network. S-wave spectra are modelled as a product of source, propagation and site terms. Inverting large data sets of multiple earthquakes recorded at multiple stations allows us to separate the three contributions. Source parameters such as moment magnitude, corner frequency and stress drop are estimated for each earthquake. We provide the first complete and homogeneous catalogue of moment magnitudes for France, for the events with magnitude greater than 3 that occurred between 1996 and 2006. Stress drops are found to be regionally dependent as well as magnitude dependent, and range from about 0.1 MPa (1 bar) to about 30 MPa (300 bars). The attenuation parameters show that, in France on a nationwide scale, variations of attenuation properties do exist. Site transfer functions are also computed, giving the level of amplification at different frequencies with respect to the response of a generic rock site (characterized by an average 30 m S-wave velocity, vs30, of 2000ms-1). From these site terms, we compute the high-frequency fall-off parameter κ [modelled as exp (-πκf), with f the frequency] for 76 stations. We also determine rock stations vs30 and we show the κ-vs30 relationship for 21 rock stations

Characterization of site conditions (soil class, VS30, velocity profiles) for 33 stations from the French permanent accelerometric network (RAP) using surface-wave methods

“…Another previous study was done by Drouet et al (2010) that have been aimed at estimation of V S30 using generalized inversion methods and by comparing site-transfer functions from the inversion to generic amplification curves derived from generic V S profiles (Boore and Joyner 1997;Cotton et al 2006). This study was focused on 21 presumed rock sites.…”

“…The choice of the accelerometric sites to be characterized was a compromise between the wish to characterize sites that produce the largest number of accelerograms, and the wish to characterize sites that were used as 'reference sites' in Drouet et al (2010). Finally, some sites were ruled out due to logistical constraints (e.g., difficulties in site access).…”

“…For example, Régnier et al (2010) gathered a wide range of pre-existing geophysical and geotechnical information in the vicinity of the RAP sites. Other studies (Drouet et al 2008(Drouet et al , 2010 have been aimed at estimation of V S30 using generalized inversion methods.…”

A Study on the Variability of Kappa ( ) in a Borehole: Implications of the Computation Process

“…Finally, κ 0 may be used as an indicator of local site conditions by comparing values at specific sites with typical values, especially given empirical correlations of κ 0 with V S30 such as those introduced by Silva et al (1998). Thus, it is also studied in the context of site characterization and site effect studies (e.g., Drouet et al, 2010). Recently it has been correlated also to other engineering parameters such as the Arias intensity and PGA (Mena et al, 2010).…”

“…This is still one of the most widely used methods despite the existence of alternative approaches, for example, use of displacement spectra (Biasi and Smith, 2001), fitting of observed response spectra (Silva et al, 1998), and full inversions for source, path, and site parameters (Drouet et al, 2010;Oth et al, 2011). We assume a linear dependence of κ with distance, which we justify a priori by its simplicity and a posteriori by the high coefficients of correlation computed for the lines.…”