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

Abstract

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 (~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 (< ~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.

Physics-based dynamic rupture models, fault interaction and ground motion simulations for the segmented Húsavík-Flatey Fault Zone, Northern Iceland

“…This suggests that for such large earthquakes, a small portion of the ruptured faults can locally dominate near field ground shaking. Scenario A2, of similar magnitude as scenarios B4 and B5, generates weaker ground shaking in Húsavík, possibly due to smaller peak slip rates on the eastern section of the HFFZ, combined with weaker directivity e ects associated with shorter fault segments (Wang & Day, 2020). However, scenarios based on Model-A result in stronger ground shaking than Model-B and Model-C in other towns further away from the fault system, especially in Dalvík, Ólafsfjörur and Grenivík.…”

Revisiting the Source Rupture Process of the Mainshock of the 2016 Kumamoto Earthquake and Implications for the Generation of Near-Fault Ground Motions and Forward-Directivity Pulse

“…Larger slip-weakening distance (DC) and small strength excess in the shallow layer zone are possible factor making the slip-velocity function smooth and long (e.g.,Dalguer et al, 2020). The free surface effect will also take additional role in generating long rise time (e.g.,Wang and Day, 2020). On the other hand, the slip velocity functions of the rest of the eight depth bins, which are thought to be within the typical depth of the seismogenic layer of Japanese crustal earthquakes, are temporally asymmetric and have a sharp peak.…”

Integration of rupture directivity models for the US National Seismic Hazard Model

“…Rupture directivity effects have long been noted in the literature to cause variations in ground-motion amplitudes not explainable with typical azimuth-independent predictor variables (to name a few: Aagaard et al, 2004; Archuleta and Hartzell, 1981; Bray and Rodriguez-Marek, 2004; Somerville, 2003; Wang and Day, 2020). This is evidenced in the seismograms recorded from large magnitude events (such as Landers: Gomberg et al, 2001, Chi-Chi: Hwang et al, 2001, Kobe: Somerville et al, 1996, Denali: Velasco et al, 2004; and Ridgecrest: Ahdi et al, 2020) that exhibit both forward and backward directivity features caused by the rupture propagation, the source radiation pattern, and polarization of seismic waves (as noted in the work by Somerville et al, 1997).…”