Strain rate relaxation of normal and thrust faults in Italy

Abstract

We find that geodetic strain rate (SR) integrated with the knowledge of active faults points out that hazardous seismic areas are those with lower SR, where active faults are possibly approaching the end of seismic cycle. SR values estimated from GPS velocities at epicentral areas of large historical earthquakes in Italy decrease with increasing elapsed time, thus highlighting faults more prone to reactivation. We have modelled an exponential decrease relationship between SR and the time elapsed since the last largest earthquake, differencing historical earthquakes according to their fault rupture style. Then, we have estimated the characteristic times of relaxation by a non-linear inversion, showing that events with thrust mechanism exhibit a characteristic time (∼ 990 yr) about three times larger than those with normal mechanism. Assuming standard rigidity and viscosity values we can infer an average recurrence time of about 600 yr for normal faults and about 2000 yr for thrust faults.