Strain rates in northwestern Italy from spatially smoothed seismicity

This work presents seismic strain rate maps for the Western Alps and Northern Apennines (northern Italy) as derived from an earthquake catalog collecting both historical and instrumental data. Strain rates are calculated on the basis of the rate of seismic moment release using the Anderson method. Unlike previous applications, which determined the total strain rate associated with specific seismogenic sources, we have employed an innovative zoneless approach based on a spatially smoothed seismicity method. In addition, a Monte Carlo simulation procedure is applied to allow for uncertainty in the input data (e. g., magnitude to moment conversion, seismogenic thickness, maximum earthquake magnitude). Strain rate maps are developed by summing the moments of the earthquakes reported in the catalog and by using two different earthquake recurrence relations. Our results indicate that deformation rates are quite high, ranging from about 2 to 12 x 10(-9) yr(-1) in the Northern Apennines and from 0.5 to 6 x 10(-9) yr(-1) in the Western Alps. These values, however, are similar to 1 order less than those derived from Global Positioning System measurements, suggesting that a portion of the recent deformation in northwestern Italy is related to aseismic processes. The discrepancies between seismic and geodetic strain rates may also indicate that the record of seismicity may not provide a sufficient time window for assessment of secular rates of moment release (or secular deformation rates) and rates of recurrence of large magnitude earthquakes in the study area.