Effects of Off-Fault Inelasticity on Near-Fault Directivity Pulses

Near-fault motion is often dominated by long-period, pulse-like particle velocities with fault-normal polarization that, when enhanced by directivity, may strongly excite middle- to high-rise structures. We assess the extent to which plastic yielding may affect amplitude, frequency content, and distance scaling of near-fault directivity pulses. Dynamic simulations of 3-D strike-slip ruptures reveal significant plasticity effects, and these persist when geometrical fault roughness is added. With and without off-fault yielding, these models (similar to M 7) predict fault-normal pulse behavior similar to that of observed pulses (periods of 2-5 s, amplitudes increasing with rupture distance until approaching a limit), but yielding systematically reduces pulse amplitude and increases the dominant period. Yielding causes near-fault (< similar to 2 km) peak ground velocity (PGV) to saturate with respect to increases in both stress drop and epicentral distance, and, in that distance range, yielding may contribute significantly to the observed magnitude saturation of PGV. The results support the following elements for functional forms in empirical pulse models: (i) a fault-normal distance saturation factor, (ii) a period-dependent and along-strike distance-dependent factor representing directivity, and (iii) an along-strike saturation factor to truncate growth of the directivity factor. In addition to the foregoing effects on long-period fault-normal pulses, the model with off-fault plasticity is very efficient in suppressing the high-frequency fault-parallel acceleration pulses that otherwise develop when rupture breaks free surface. This effect is likely to inhibit the initiation of a sustained supershear rupture triggered by a strong free surface breakage.