Reappraisal of magnitude of 20th century earthquakes in Switzerland

We reappraise the material needed to assess the 20th-century seismicity of Switzerland and of the adjacent areas of the Alps in terms of magnitude. For this we make use of macroseismic reports and literature and by calculating the surface-wave magnitude from the Prague formula of all significant earthquakes in the region. No attempt is made to relocate earthquake positions; instead their reliability is ranked using existing solutions and macroseismic observations. We find that for small earthquakes (M-S < 4.5), which constitute the bulk of the events in the region, the calculation of MS observed at relatively short distances, requires station and distance corrections which can be significant. Also we find that recomputed Ms estimates differ from those reported in other earthquake catalogues. From this reappraisal of MS and from a uniform re-evaluation of the associated macroseismic data, we derive a stable correlations between MS and felt areas that can be used to assess the magnitude of historical, pre-instrumental, events for which only isoseismal radii (T-i) and the associated intensities (I-i) are available. We examined the conversion of surface-wave magnitude into moment magnitude, a conversion that presents some interesting problems for relatively small events for which the M-S-log(M-0) scaling changes. We conclude that the rate of moment release derived from events of M-S > 4.0 is small, and that it should be associated with horizontal and vertical velocity rates of less than 1 mm/yr, too small to be confirmed by GPS measurements over short periods of time so that can be used to constrain hazard assessment.