Determination of geomechanical site effects in France from macroseismic intensities and reliability of macroseismic magnitude of historical events.

Bossu, R; Scotti, O; Cotton, F; Cushing, M; Levret, A. TECTONOPHYSICS, 324: (1-2) 81-110.

Results from a detailed analysis of the French macroseismic database SIRENE96 show that geomechanical site effects (i.e. site effects due to superficial geology) can be statistically detected and quantified from macroseismic data. Our results are used to derive a predictive map of potential site effects in France and to check if site effects induce a significant overestimation of macroseismic magnitude of historical events. The key hypothesis of our work is that geological formations of similar age produce similar site effects because one expects the younger sediments to be less compacted, and therefore more prone to cause site effects. A geological formation is associated with each intensity using a Geographic Information System (GIS) and the digital geological map of France. Two approaches are developed in order to characterise the intensity increase associated with geological formations having the same age. The first approach is based on the intensities that differ significantly from the average pattern, intensities which are labelled as anomalies. In the second approach, a theoretical intensity attenuation law is computed for each selected event only from observations located on formations older than 2 My. According to the results of the first approach, these formations are unlikely to cause site effects, and for the sake of simplicity they are labelled as rocks (younger formations are defined as sediments). The derived attenuation laws are then used to compute the residuals for each observation. Residuals are classified as a function of the age of the formations on which they are located. The results of the two approaches are in good agreement. No intensity amplification is detected for formations older than 1 My. For younger formations, the younger the formations, the larger the average intensity increase. The average intensity increase reaches 0.6+/-0.2 degree on the MSK scale for 0 My sediments. The results are better expressed in terms of probability: for 0 My sediments, there is an estimated 83% probability of observing an intensity increase; for 0.01 My and 0.75 My sediments, this probability is about 52%. The magnitude of the intensity increase is variable, but it is generally small: it has a 26% probability to reach I degree or above on the MSK scale for 0 My sediments and this probability falls to about 5% for 0.01 and 0.75 My formations. A predictive map of geomechanical site effects is produced by extrapolating the characteristics of intensity increases to all geologic formations of similar age in France. Glacial and fluvial valleys, already known as being subject to site effects, clearly appear on this map, as well as less expected areas such as the South Atlantic coast. Lastly, new estimations of macroseismic magnitude that exclude intensities potentially affected by geomechanical site effects are realised. A magnitude overestimation due to site effects can possibly exist, but it remains limited (<0.2). This result tends to demonstrate that geomechanical site effects in France do not significantly distort our knowledge of historical seismicity.