Signature of transition to supershear rupture speed in the coseismic off-fault damage zone

Abstract

Most earthquake ruptures propagate at speeds below the shear wave velocity within the crust, but in some rare cases, ruptures reach supershear speeds. The physics underlying the transition of natural subshear earthquakes to supershear ones is currently not fully understood. Most observational studies of supershear earthquakes have focused on determining which fault segments sustain fully grown supershear ruptures. Experimentally cross-validated numerical models have identified some of the key ingredients required to trigger a transition to supershear speed. However, the conditions for such a transition in nature are still unclear, including the precise location of this transition. In this work, we provide theoretical and numerical insights to identify the precise location of such a transition in nature. We use fracture mechanics arguments with multiple numerical models to identify the signature of supershear transition in coseismic off-fault damage. We then cross-validate this signature with high-resolution observations of fault zone width and early aftershock distributions. We confirm that the location of the transition from subshear to supershear speed is characterized by a decrease in the width of the coseismic off-fault damage zone. We thus help refine the precise location of such a transition for natural supershear earthquakes.

A Travel‐Time Path Calibration Strategy for Back‐Projection of Large Earthquakes and Its Application and Validation Through the Segmented Super‐Shear Rupture Imaging of the 2002 Mw 7.9 Denali Earthquake

“…Here we divide the DF and TF into four segments based on the BP result and fault segmentation (Figures 13a and 13b). Segment 1 is corresponding to the super-shear transition and is associated with very few aftershocks, consistent with the proposed signature of a shrunk off-fault damage zone in the transition from sub-Rayleigh to super-shear rupture (e.g., Jara et al, 2021;Okubo et al, 2019). Segments 2 and 3 show many more aftershocks and are featured with asymmetric distribution, with more aftershocks located to the southwest of the DF (the extensional side of the rupture), suggesting that more off-fault structures were activated there.…”

“…Besides an explanation that there are more remote active faults to the northeastern side, an alternative interpretation is that the southwestern side aftershocks were mainly activated by the stress associated with the rupture tip while the further northeastern side aftershocks were induced by the intense Mach cone as it sweeps that area. This conclusion has been supported by recent modeling the off‐fault damage generated by super‐shear ruptures (Bhat et al., 2007; Jara et al., 2021). However, further numerical simulations, laboratory experiments, field observations, as well as aftershocks studies may be needed to better understand whether such aftershock distribution is a feature primarily associated with super‐shear rupture.…”

The 2021 Mw 7.3 Madoi, China Earthquake: Transient Supershear Ruptures on a Presumed Immature Strike‐Slip Fault

“…Xu and Ben-Zion (2013) attribute such co-seismic damage gap to the less persistent dynamic loading during sudden rupture acceleration and further suggest that it may represent a quite general feature. Therefore, it is possible that the releasing bend could account for part of the observed aftershock gap, if we can treat early aftershocks as a proxy of co-seismic damage, as assumed by Jara et al (2021). Alternatively, the observed aftershock gap may be attributed to the special properties (e.g., relatively uniform frictional properties) of the fault segment that favor the occurrence of supershear rupture (Bouchon & Karabulut, 2008).…”

Revisiting the 1959 Hebgen Lake Earthquake Using Optical Image Correlation; New Constraints on Near‐Field 3D Ground Displacement

“…OFD represents distributed inelastic strain accommodated via secondary fracturing in the regions of increased structural complexity (e.g., fault bends) (Dolan & Haravitch, 2014; Gold et al., 2015) (Figure 3c). Factors such as rupture speed (Aben et al., 2020; Jara et al., 2021), and variable lithology (Zinke et al., 2014) also contribute to the off‐fault component of slip, but to a lesser extent. Due to the distributed fracturing, the total displacement is accommodated across a wide fault zone, which extends across 47.2 m (1 σ = 18 m), 56.2 m (1 σ = 19 m), and 65.4 m (1 σ = 20 m) for fault‐parallel, fault‐perpendicular, and vertical measurements, respectively.…”