The Near‐Fault Ground Motion Characteristics of Sustained and Unsustained Free Surface‐Induced Supershear Rupture on Strike‐Slip Faults

Abstract

The free surface is shown to be one of the key factors that may promote supershear rupture propagation on strike‐slip faults even if its initial shear stress is not larger enough as predicted by the Burridge‐Andrews mechanism. However, previous study has shown the free surface‐induced supershear rupture may be unsustained, which turns to sub‐Rayleigh rupture itself as the rupture propagates. We study the near‐field ground motion of sustained and unsustained supershear ruptures using the finite difference method based on the dynamic rupture processes of three vertical strike‐slip faults with the same initial stresses and different hypocenter depths. Both the unsustained supershear ruptures with shallower hypocenter depths show the sub‐Rayleigh characteristics in the peak ground velocity distribution, that is, strong amplitude is noticed beyond the end of the fault without any observed Mach cone. We observe that the arrival time differences between the maximum fault‐perpendicular and fault‐parallel velocity reveal the Mach cone clearly for the sustained supershear rupture. A distinct supershear phase in the high‐frequency seismograms is observed. We also compare the 70° dip strike‐slip case in which unsustained supershear rupture may still present sub‐Rayleigh characteristics in the near‐field ground motion, and the break of symmetry perturbs the normal stress as rupture propagates, which has a key influence in the rupture propagation and ground motion. Our work provides a new insight to understand the supershear rupture in nature earthquakes and the relationship between the ground motion and the rupture process on the fault.

The Effect of Depth‐Dependent Stress in Controlling Free‐Surface‐Induced Supershear Rupture on Strike‐Slip Faults

“…To obtain a better understanding of ground motion characteristics of the three rupture scenarios, Figure 5 compares the particle velocities and their normalized frequency amplitudes at five stations with a 1 km interval spanned along the FN direction (Figure 1). For the Scenario A (Figure 5a), the strong peak FP peaks arrive earlier than the weaker FN peaks, which is a distinguishing feature of supershear feature (Aagaard & Heaton, 2004; Hu et al., 2020). The shear and Rayleigh Mach fronts can be seen in both the FP and FN components, although the time differences between the shear and Rayleigh arrivals are small.…”

Supershear Rupture, Daughter Cracks, and the Definition of Rupture Velocity

“…Hu et al. (2019, 2020) have shown that the existence of a daughter crack in strike‐slip faults cannot guarantee a sustained supershear rupture, since the daughter crack is possible to be absorbed by the following main crack, which turns into an unsustained supershear rupture. Our simulations show even if the “supershear” (in the 1D gradient view) daughter crack can propagate to the fault bottom, its near field ground motion could still present sub‐Rayleigh characteristics as revealed in the 2D gradient.…”

“…Our interpretation, in contrast, is that the diagonal rupture fronts are a consequence of the supershear rupture right at the surface leading the subshear rupture at depth; the diagonal rupture fronts in our interpretation are a symptom, rather than a cause, of the subshear rupture at depth. Hu et al (2019Hu et al ( , 2020 have shown that the existence of a daughter crack in strike-slip faults cannot guarantee a sustained supershear rupture, since the daughter crack is possible to be absorbed by the following main crack, which turns into an unsustained supershear rupture. Our simulations show even if the "supershear" (in the 1D gradient view) daughter crack can propagate to the fault bottom, its near field ground motion could still present sub-Rayleigh characteristics as revealed in the 2D gradient.…”

“…As shown in Figure 4, we compare the 1D and 2D rupture velocity distributions of the three rupture scenarios HU ET AL. V on the faults of the cases presented in Figure 1 of Hu et al (2020). The black lines mean the rupture front contours with a 1.0 s time interval.…”

“…There are many physical mechanisms by which rupture may transition to supershear speed (Andrews, 1976; Bhat et al., 2007; Bruhat et al., 2016; Burridge, 1973; Das & Aki, 1977; Day, 1982; Dunham, 2007; Dunham et al., 2003; Fukuyama & Olsen, 2002; Hu et al., 2016; Liu & Lapusta, 2008; Madariaga & Olsen, 2000; Ryan & Oglesby, 2014; Weng et al., 2015). In the present work, we focus on supershear rupture generated via interactions with the Earth's free surface (Day et al., 2008; Hu et al, 2019, 2020; Kaneko et al., 2008; Kaneko & Lapusta, 2010; Olsen et al., 1997; Xu et al., 2015; H. Zhang & Chen, 2006), which is mediated primarily by a critically converted S to P wave along the surface of the Earth (Kaneko & Lapusta, 2010; Olsen et al., 1997).…”