Can geodetic strain rate be useful in seismic hazard studies ?

Abstract

Geodetic measurements devoted to active tectonics studies are extensively carried out in Italy only since the early 2000s, the maximum effort in deploying the GPS networks in Italy dating back to about 2005, so that the observed deformations represent the instantaneous seismic cycle conditions in every area of the Italian region. If we assume a simple idealized seismic cycle model for earthquake recurrence, we can draw some tracks potentially useful in seismic hazard studies, concerning both the spatial mapping of hazardous areas and the time trend of active fault relaxation. We show how the background strain-rate (SR) estimated from GPS velocities in Italy and the comparison with seismicity have evidenced that larger earthquakes occur with higher probability in areas of lower SR, providing evidence that elastic energy accumulates in areas where faults are locked and SR is lower. In tectonically active areas, such as the Apennine subduction belt, SR lows and the knowledge of active faults can be used to identify areas more prone to release larger amount of energy with respect to adjacent zones characterized by higher SR. We have found and modeled an exponential decrease relationship between SR and the time elapsed since the last largest earthquake for normal and thrust faults in the Italian area and estimated the characteristic times of relaxation by a non-linear inversion showing that thrust events exhibit a characteristic time (~ 990 yr) about 3 times larger, and lower SR, than those with normal fault mechanism. Assuming standard rigidity and viscosity values we are able to infer an average recurrence time of about 600 yr for normal faults and about 2000 yr for thrust faults.