Stochastic simulation of strong-motion records from the 26 September 1997 (M-w 6), Umbria-Marche (Central Italy) earthquake

We simulated the strong-motion time histories recorded during the main shoc k (M-w 6.0) of the Umbria-Marche seismic sequence (Central Italy) of Septem ber-October 1997. Ground-motion waveforms were computed using the stochasti c modeling technique proposed by Beresnev and Atkinson (1997, 1998) for fin ite faults. In this approach, the high-frequency amplitudes are simulated a s a summation of stochastic point sources. We used the FINSIM code (Beresne v and Atkinson, 1998), which incorporates regional attenuation and frequenc y-dependent site-amplification factors. We divided the fault plane into 60 elements whose length and width are 1.2 km and 1.5 km, respectively. The re sulting subfault corner frequency and rise time are 0.91 Hz and 0.5 sec, We found that the site-amplification functions play an important role in the simulation process, improving the fit to the observed time histories and sp ectra, The strong-motion waveforms recorded at the Nocera (NOC) station, lo cated at the northern end of the causative fault, show an important directi vity effect, Thus, to fit the observed ground motions, we used an inhomogen eous slip distribution, weighting the slip on the fault heavier toward the north. We tested two models: one that: simulates a fault rupture with two m ain slip patches and a second model that ruptures northward with a less het erogeneous slip distribution where slip is mostly concentrated near the rup ture nucleation at the southern edge of the fault plane. The simulated low- frequency amplitudes at NOC, however, require an additional frequency-depen dent directivity correction (e.g., Bernard et al., 1996). In conclusion, we found that stochastic finite-fault simulations calculated using adequate s ite amplification functions and crustal attenuation reproduce reasonably we ll the ground motions from the M-w = 6.0 Umbria-Marche earthquake.