Using geophysical data inversion to constrain earthquake dynamics: a study on dynamically consistent source time functions.

Abstract

Earthquake kinematic models are often used to retrieve the main parameters of the causative dynamic rupture process. These models are usually obtained through the inversion of seismograms and geodetic data and they can be used as boundary conditions in dynamic modeling to calculate the traction evolution on the fault. Once traction and slip time histories are inferred at each point on the fault plane, it is feasible to estimate the dynamic and breakdown stress drop, the strength excess and the slip weakening distance (Dc). However the measure of these quantities can be biased by the adopted parametrization of kinematic source models. In this work we focus our attention on the importance of adopting source time functions (STFs) compatible with earthquake dynamics to image the kinematic rupture history on a finite fault. First, we compute synthetic waveforms, through a forward modeling, to evaluate the effects of STFs on the ground motion and on the radiated energy. Therefore, adopting different STFs, we perform kinematic inversion of strong motion and GPS data, using a new non linear two-stages search algorithm (Piatanesi et al., 2007) . We have quantitatively verified that the chioce of STFs affects ground motion time histories within the frequency band commonly used in kinematic inversion and that the inferred peak slip velocity and rise time strongly change among the inverted models. These differences has a dramatic impact when kinematic models are used to infer dynamic traction evolution. The shape of the slip weakening curve, the ratio between Dc and the final slip and the dynamic stress drop distribution are remarkably affected by the assumed STFs. We recommend the adoption in kinematic inversions of source time functions that are compatible with earthquake dynamics.