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Seismic Characteristics of Supershear and Sub-Rayleigh Earthquakes: Implication from Simple Cases, Geophys

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Geophysical Research Letters RESEARCH LETTER Seismic characteristics of supershear and sub-Rayleigh 10.1002/2017GL074158 earthquakes: Implication from simple cases Key Points: • The common conclusion that Zhenguo Zhang1 , Jiankuan Xu1 , Hanqing Huang2 , and Xiaofei Chen1 supershear earthquake transmits farther distance than subshear one is 1Department of Earth and Space Sciences, Southern University of Science and Technology, Shenzhen, China, 2School of confirmed by numerical simulations • Shaking of subshear earthquake Earth and Space Sciences, University of Science and Technology of China, Hefei, China is more intensive than that of supershear one at short distance to the fault plane Abstract Numerous investigations of supershear earthquakes make a conclusion that a supershear • The supershear earthquake has earthquake produces a seismic shock wave on the ground that may increase the resulting destruction. relatively quiet aftershock potential based on Coulomb We investigate a supershear rupture promoted by the free surface and find out that although the seismic failure stress analysis energy of a supershear earthquake can be transmitted further with large amplitudes, the peak slip velocity on a fault near the free surface is smaller than that caused by a subshear rupture earthquake. Our results Correspondence to: show that the free-surface-induced supershear rupture mitigates the amplitudes of ground motions near X. Chen, the fault plane compared with the subshear rupture. The Coulomb failure change derived from dynamic [email protected] modeling further suggests that this free-surface-induced supershear reduces aftershock potential compared to a subshear rupture. Both ground motion at near-fault and aftershock possibility show low Citation: risk for the free-surface-induced supershear rupture earthquake than subshear earthquake, contrary to the Zhang, Z., J. Xu, H. Huang, and traditional concept. X. Chen (2017), Seismic characteristics of supershear and sub-Rayleigh earthquakes: Implication from simple cases, Geophys. Res. Lett., 44, 6712–6717, doi:10.1002/ 1. Introduction 2017GL074158. The rupture speed of an earthquake is an important parameter that results in high frequencies and strong seis- mic radiation, particularly the transition from a speed slower than Rayleigh wave speeds (subshear rupture) Received 14 MAY 2017 to that faster than shear wave speeds (supershear rupture) of the surrounding medium [Madariaga, 1983]. Accepted 26 JUN 2017 Earthquake simulations with specific rupture velocities indicate that supershear rupture, compared with the Accepted article online 28 JUN 2017 Published online 11 JUL 2017 subshear case, radiates S waves as a Mach front that causes extensive shaking at places far away the fault [Aagaard and Heaton, 2004; Bernard and Baumont, 2005; Dunham and Archuleta, 2005]. This Mach cone effect is further supported by dynamic modeling [Andrews, 2010], laboratory earthquakes [Rosakis et al., 1999; Xia et al., 2004], and supershear earthquake observations [Vallée et al., 2008]. Theoretical analysis and numerical experiments have demonstrated that the rupture pattern of an earthquake depends primarily on the rela- tionship between the shear stress and strength of the fault, which can be denoted by the quantitative seismic ratio S. For a planar fault embedded in a homogeneous medium, a sufficiently small S can accelerate a rup- ture to a supershear one [Burridge, 1973; Andrews, 1976; Das and Aki, 1977; Dunham, 2007]. This mechanism of supershear transition is usually called the Burridge-Andrews mechanism (BAM). Moreover, recent numer- ical simulations also prove the existence of another rupture type that increases the speed to supershear [Liu et al., 2014]. It has been identified that the rupture speeds of a few crustal earthquakes exceed the shear wave velocities [Archuleta, 1984; Bouchon et al., 2000, 2001; Bouchon and Vallée, 2003; Dunham and Archuleta, 2004; Wang and Mori, 2012; Wang et al., 2012; Yueetal., 2013]. These crustal supershear earthquakes are all strike-slip and reach the Earth’s surface, which is consistent with the theoretical studies that the flat free surface will always induce the supershear transition given a long enough propagation distance for a strike-slip rupture [Xu et al., 2015]. This free-surface-induced (FSI) supershear rupture, which is caused by the SV-P phase conversion at the free surface [Kaneko and Lapusta, 2010], behaves quite differently compared to those induced by BAM and can be prevented by inhomogeneities such as low velocity media [Kaneko and Lapusta, 2010], an irregular surface [Zhang et al., 2016], or a barrier [Xu et al., 2016]. It is necessary to evaluate the seismic hazard characteristics of a FSI supershear rupture because it is common for faults to reach the Earth’s surface. Is the ground motion caused by a FSI supershear earthquake stronger than that caused by a subshear earthquake? We report the ©2017. American Geophysical Union. characteristics of a FSI supershear rupture in this study and find that the answer for this question is negative All Rights Reserved. from both near-source ground motion and aftershock triggering. ZHANG ET AL. SUPERSHEAR AND SUB-RAYLEIGH EARTHQUAKES 6712 Geophysical Research Letters 10.1002/2017GL074158 2. Models and Method This work focuses on the seismic hazards caused by a free-surface-induced supershear earthquake. For a strike-slip fault, a supershear rupture is promoted from subshear rupture at the free surface due to SV-P wave conversion [Kaneko and Lapusta, 2010; Xu et al., 2015]. To discuss seismic hazards, a subshear rupture, with initial conditions and dynamic parameters identical to those of the supershear rupture, must be compared and analyzed. Then, the free-surface-induced supershear rupture must be prevented without changing the initial conditions and parameters. Numerical experiments indicate that an irregular topography can prevent the supershear rupture transition [Zhang et al., 2016]. In this work, this supershear transition is prevented by a topographical perturbation, causing a subshear on the entire fault plane. The supershear and subshear have the same initial stress, dynamic stress drop, and other parameters, but they have different rupture velocities. Note that the model of earthquake faulting is relative simple, it considers a vertical planar fault surrounded by a homogeneous media and driven by uniform stress field except for the nucleation patch. The condi- tions that trigger an earthquake might have much complexity in stress, media, fault geometry, and so on. However, to differentiate the seismic radiations between supershear and subshear rupture earthquakes, the simplified model is chosen to run simulation and analyze the associated phenomena. Moreover, all the supershear earthquakes observed in the upper crust are all the strike-slipping faults reaching Earth’s surface. Three-dimensional (3-D) numerical simulations indicate that a long enough fault in the strike direc- tion is required to accelerate rupture from subshear to supershear speed. With these considerations, we choose the following 3-D models to compare the seismic dangers between subshear and supershear rupture earthquakes. We investigate supershear rupture induced by interaction with Earth’s surface using 3-D rupture dynamics modeling. A vertical planar fault with 110 km length and 20 km width is modeled under the slip-weakening . friction law [Ida, 1972]. In the dynamic modeling, the critical distance d0 = 0 4 m, the static and dynamic . . frictions are set to s = 0 677 and d = 0 525, respectively. The medium is homogeneous with P wave speed 3 vp = 6000 m/s, S wave speed vs = 3464 m/s, and density = 2670 kg/m . A homogeneous initial stress field (the normal and shear stresses are 120 MPa and 70.0 MPa, respectively) is employed over the whole fault plane except for the nucleation patch with a size of 3 km × 3 km, within which a stress slightly higher than the strength of the fault is used to trigger the dynamic rupture. The square nucleation zone, located 10 km away from the left, bottom, and flat free-surface boundaries, initializes the dynamic rupture. The subsequent rupture spontaneously propagates over the entire fault plane until it is stopped by surrounding barriers with sufficiently high strength. For the supershear rupture model, a vertical planar fault of 110 km length and 20 km width is considered. The flat surface is used to generate FSI supershear rupture. A canyon, defined by z(r)=1000 exp(−r2/15002), where r is the distance to the canyon center, is added to prevent the free-surface-induced supershear rupture. The fault plane crosses the center of canyon. We use the curved grid finite-difference method (CG-FDM) [Zhangetal., 2014] to simulate the dynamic rupture dynamic on a fault with flat and canyon-shaped surfaces. This method was verified by modeling benchmarks released by the Southern California Earthquake Center [Harris et al., 2009]. By using curvilinear grids that can fit the irregular interfaces and adopting of split nodes to represent the discontinuous condition across the two sides of a fault, the CG-FDM can model the rupture dynamics on a generic fault with complex geometry, including one with irregular surface topographies. 3. Results Previous investigations have indicated that a strike-slip earthquake rupture with a flat free surface would be accelerated into a supershear rupture, given a long enough propagating distance [Xu et al., 2015]. However, when the free surface is topographically irregular, the same strike-slip
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