Redistribution of dynamic stress during coseismic ruptures: Evidence for fault interaction and earthquake triggering

We investigate the spatiotemporal evolution of dynamic stress outside a rup turing extended fault. The dynamic stress variations caused by a coseismic rupture in a half space are computed by using the discrete wavenumber and r eflectivity methods. After a transient phase, the stress time history evolv es to the final static stress value. We compare the static stress changes r esulting from this model with those computed from a static dislocation mode l. We have applied this method to study the interactions between the first two normal faults which ruptured during the 1980 (M-S 6.9) Irpinia earthqua ke. These two subevents are separated in time by nearly 20 s, while the thi rd (and last) subevent occurred 40 s after the rupture onset. We compute th e dynamic stress changes caused by the rupture of the first subevent. Our m odeling results show that the dynamic stress peak on the second subevent fa ult plane is reached between 7 s and 8 s after the rupture initiation on th e main fault. On the average the static stress level on the second subevent (20 s) fault plane is reached nearly after 14 s. The dynamic rupture did n ot jump from a rupturing segment to the adjacent one immediately, but the t riggering of the 20 s subevent is delayed by roughly 10 s with respect to t he instant of occurrence of the dynamic stress peak induced by the 0 s even t. The dynamic stress pulse propagates along the strike direction of the se cond subevent fault plane at an average velocity of nearly 3.4 km/s. The de layed triggering of the second subevent can be interpreted in terms of the frictional properties of the faults. In particular, rate- and state-depende nt frictional law can explain a delayed instability after a sudden change i n stress. Using the estimated values of the subevent triggering delay and t he shear stress change, we attempt to constrain the parameter A sigma on th e 20 s fault. The values here inferred agree well with those resulting from previous studies.