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From analyses using near-field and teleseismic data, we estimate the rupture speed of the 2010 Qinghai, China, earthquake (M-w 6.9). Comparisons of model calculations of the initiations to the observed near-field seismogram at station YUS indicate a fast supershear rupture propagation of about 5.0 km/s. From the teleseismic analyses using an envelope deconvolution method with an empirical Green's function event, two high-frequency pulses are identified at 6.5 km and 41.8 km southeast of the epicenter, indicating two subevents. The location and timing of the high-frequency events also show a supershear rupture velocity of 4.7 to 5.8 km/s. The supershear speed of the rupture likely contributed to the severe damage caused in the forward direction of the rupture toward the town of Yushu.
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... In this study, we examined a near-fault continuous recording in the last few hours before the 13 April 2010 M w 6.7 Yushu earthquake in Qinghai, China. The mainshock ruptured primarily toward the SE along the Yushu segment of the left-lateral Ganzi-Yushu fault in eastern Tibet (e.g., Guo et al., 2012), and was likely a supershear rupture (D. Wang & Mori, 2012). This event was preceded by foreshocks near the mainshock epicenter, with the largest foreshock having a moment magnitude M w 4.6 and occurring about 2 hours before the mainshock. ...
... The sampling rate is 100 Hz. Although the mainshock signal is clipped, the M w 4.6 foreshock and subsequent events were recorded on scale (D. Wang & Mori, 2012). While some large aftershocks were recorded by three other permanent stations that are about 125 km (ZAD and QML) and 240 km (MAD) kilometers away from the Yushu mainshock epicenter (Figure 1 and Figure S1 in Supporting Information S1), the majority of foreshocks were only recorded at station YUS. ...
We conduct a detailed study of the foreshock sequence preceding the 2010 Mw 6.7 Yushu, Qinghai earthquake in the Tibetan plateau by examining continuous waveforms recorded at a seismic station near the mainshock rupture zone. By using a deep learning phase picker—EQTransformer and a matched‐filter technique, we identify 120 foreshocks with magnitude ranging from −0.7 to 1.6, starting with a Mw 4.6 foreshock approximately 2 hr before the Mw 6.7 Yushu mainshock. Our analyses show that the foreshock sequence follows a typical Omori's law decay with a p‐value of 0.73 and the Gutenberg‐Richer frequency‐magnitude b‐value of 0.66. We do not find any evidence of accelerating events leading up to the Yushu mainshock. Hence, they could be considered as aftershocks of the Mw 4.6 earthquake. We further invert for the focal mechanisms and rupture directions for both the largest foreshock and the mainshock. The Mw 4.6 foreshock likely occurred on a NE‐SW trending fault conjugating to the NW‐SE trending fault of the mainshock. Coulomb stress analysis suggests the Mw 4.6 foreshock induces negative stress on the mainshock source area. These observations do not support either the pre‐slip or the cascade triggering model for foreshock generation. The occurrence of the foreshock, mainshock and large aftershocks appear to be modulated by the Earth's tidal forces, likely reflecting the role of high pore‐fluid pressures. Our observations, together with other recent studies, suggest that extensional step‐overs and conjugate faults along major strike‐slip faults play an important role in generating short‐term foreshock sequences.
... Y.Jiang et al., 2013;C. Y. Li et al., 2019;Wang & Mori, 2012;C. Xu & Xu, 2014), have enriched our knowledge of seismic hazards linked to such faults. ...
The tectonic deformation on the eastern margin of the Qaidam Basin, which has preserved complete sedimentary records, significantly influences the evolutionary model of the northeastern margin of the Tibetan Plateau. However, the deformation history in this area during the Holocene remains unclear. This study is based on the high‐precision digital elevation model obtained through drone mapping technology, which identifies three active faults on the eastern margin of the Qaidam Basin: the Xiariha Fault (XRHF) and Yingdeerkang Fault Yingdeerkang Fault (YKF) are NW‒SE‐orientated dextral faults, whereas the Reshui‐Taosituohe Fault (RTF) is a nearly east‒west‐orientated sinistral fault. Based on the optically stimulated luminescence dating of the landform surfaces, the rates of strike‐slip offset are as follows: those of the XRHF range from 1.12 ± 0.07 to 1.68 ± 0.12 mm/yr and those of the YKF are from 0.99 ± 0.06 to 2.29 ± 0.13 mm/yr. Recent paleoseismic events occurred along the RTF at approximately 714–1,792 years BP and at 700 ± 18 years BP, implying a recurring millennial pattern. Together, these faults possibly form a complex cross‐fault system along the southeastern edge of the basin, heightening seismic risk. Deformation in the western part of the northeastern Tibetan Plateau is driven by slip on the Altyn Tagh Fault and compression in the Qaidam Basin. The central part experiences slip on the East Kunlun Fault, along with secondary faults, shortening, and block rotation. The eastern part primarily experiences slip along the Haiyuan Fault.
... In summary, although only large strike-slip earthquakes have well-documented cases with supershear rupture propagation (e.g., Wang and Mori, 2012;Wang et al., 2016;Hu et al., 2019), the occurrence of a supershear microseismic event cannot be disregarded. We have shown that our data prefer supershear rupture with a pronounced directivity effect, although strictly speaking, fast yet subshear rupture cannot be ruled out. ...
Rupture directivity is a fundamental effect well known mainly for large natural earthquakes. Its observation for microseismic events is difficult due to small rupture size and short duration, usually insufficient coverage of monitoring array and attenuation along wave propagation paths. Here, we detect the rupture directivity for an induced micro-seismic event (M w ∼ 1:2) recorded by a dense surface starlike array during hydraulic fracturing of a shale reservoir in China. We use durations of initial P-wave arrivals as a proxy to peak frequency content. The observed directional and offset dependence of the peak frequencies can be explained by superimposed effects of the rupture direc-tivity of fast, possibly supershear rupture propagation and attenuation, permitting the determination of the event's fault plane orientation. Furthermore, we implement a simple statistical correction to the amplitudes, proving the inverted source mechanism to be stable, only with a slightly lower, yet unreliable nonshear component.
... In summary, although only large strike-slip earthquakes have well-documented cases with supershear rupture propagation (e.g., Wang and Mori, 2012;Wang et al., 2016;Hu et al., 2019), the occurrence of a supershear microseismic event cannot be disregarded. We have shown that our data prefer supershear rupture with a pronounced directivity effect, although strictly speaking, fast yet subshear rupture cannot be ruled out. ...
Rupture directivity is a fundamental effect well known mainly for large natural earthquakes. Its observation for microseismic events is difficult due to small rupture size and short duration, usually insufficient coverage of monitoring array and attenuation along wave propagation paths. Here, we detect the rupture directivity for an induced microseismic event (Mw∼1.2) recorded by a dense surface starlike array during hydraulic fracturing of a shale reservoir in China. We use durations of initial P-wave arrivals as a proxy to peak frequency content. The observed directional and offset dependence of the peak frequencies can be explained by superimposed effects of the rupture directivity of fast, possibly supershear rupture propagation and attenuation, permitting the determination of the event’s fault plane orientation. Furthermore, we implement a simple statistical correction to the amplitudes, proving the inverted source mechanism to be stable, only with a slightly lower, yet unreliable nonshear component.
... effects 13 and teleseismic source inversions 14 are challenged by the trade-off between source size and rupture speed 15 , the usage of relative low-frequency (<0.1 Hz) contents and smoothing constraints 16 . Those based on near-field recordings, such as array techniques 17 and early S-arrival identification 18,19 , are limited by the scarcity of such data. On the other hand, teleseismic or regional back-projection (BP) rupture imaging 20 has the potential to constrain the rupture speed of large earthquakes globally, especially if travel time errors due to three-dimensional (3D) path effects 21 are mitigated by the recently developed slowness-enhanced BP (SEBP) approach 4 . ...
Earthquakes are supershear when their rupture speed is faster than that of the seismic shear waves produced. These events are rare, but they can be highly destructive owing to the associated strong ground shaking, and understanding why they occur may provide insights into fault mechanics. Only a few supershear earthquakes have been reported previously, most of which were continental. Here we perform a systematic global search for supershear earthquakes by analysing seismic data from all large (Mw ≥ 6.7) shallow strike-slip earthquakes occurring between 2000 and 2020. Based on the rupture speeds determined by slowness-enhanced back-projection, and the identification of Rayleigh Mach waves, we identify four oceanic earthquakes consistent with supershear events. We find that at least 14.0% of large earthquakes during the study period were supershear, with oceanic events occurring as frequently as continental ones. We further observe a wider range of stable rupture speeds during supershear events than predicted by two-dimensional fracture mechanics theory, which we attribute to the presence of fault damage zones or slip obliqueness. The transition to and propagation of supershear earthquakes may be promoted in oceanic settings due to the thicker crustal seismogenic zones and the material contrast at oceanic–continental boundaries.
... However, theoretical and numerical studies (e.g., Burridge, 1973;Andrews, 1976;Das and Aki, 1977;Freund, 1990;Broberg, 1994Broberg, , 1995Gao et al., 2001;Shi et al., 2008;Liu et al., 2014) and laboratory experiments (e.g., Rosakis et al., 1999;Xia et al., 2004;Passelègue et al., 2013) demonstrate that rupture speed can exceed the shear wave speed and even reach the compressional wave speed. There is also growing evidence of these "supershear" earthquakes observed from large strike-slip faults in nature (e.g., Archuleta, 1984;Bouchon et al. 2001;Bouchon and Vallee 2003;Dunham and Archuleta, 2004;Wang and Mori, 2012;Yue et al., 2013;Socquet et al., 2019). In addition, experimental work suggests that subshear rupture might not produce high enough strain rates at sufficient distance to explain the width of pulverization around natural faults (e.g., Aben et al., 2017a;Xu and Ben-Zion, 2017;Griffith et al., 2018). ...
Although the mechanics of continental, seismogenic strike-slip faults have been primarily studied around active faults near Earth’s surface, large earthquakes on these faults commonly extend to depths between 10 and 20 km. At the base of seismogenic strike-slip faults, interaction and feedback between coseismic brittle fracturing and post- and interseismic viscous flow affect transient and long-term changes in stress cycling, fluid and heat transport, fault strength, and associated strain localization and deformation mechanisms. A primary goal of my dissertation is to explore the deeper structures of damage zones near the base of the seismogenic zone and to better understand the influence of the damaged rocks on rupture dynamics, by examining microstructures of exhumed fault rocks. My study area, the Sandhill Corner shear zone that is the longest strand of the Paleozoic Norumbega fault system in Maine, USA, represents large-displacement, seismogenic strike-slip faults at frictional-to-viscous transition depths (corresponding to temperatures of ~400–500 °C). The shear zone contains mutually overprinting pseudotachylyte and mylonite, and juxtaposes quartzofeldspathic mylonites and mica-rich schists. I analyzed fractured and fragmented garnet grains using particle size distributions, microfracture patterns, and electron backscatter diffraction fabrics. Microstructural studies of fragmented garnets reveal asymmetric distribution of dynamic pulverization with a width of ~70 m in the Sandhill Corner shear zone, and these results imply that the same damage processes observed around active seismogenic strike-slip faults operate at the base of the seismogenic zone. Garnet microstructures formed during earthquake cycles at the frictional-viscous transition can also provide evidence for dynamic pulverization even though the particle size distribution is modified by quasi-static fragmentation during post- and interseismic shearing. Elastic and seismic properties of the quartzofeldspathic rock and the mica-rich schist are quantified using the Thermo-Elastic and Seismic Analysis (TESA) numerical toolbox. The results illustrate how elastic contrast across bimaterial faults separating two different anisotropic materials affects preferred rupture propagation and asymmetric damage distribution. Strong anisotropy occurs in fault zones where preferentially aligned phyllosilicate minerals are a major component of the modal mineralogy. My findings suggest that the orientation and proportion of preferentially aligned phyllosilicates, or other highly anisotropic minerals, should be considered when investigating fault ruptures in anisotropic rocks.
Variations in fault zone maturity have intermittently been invoked to explain variations in some seismological observations for large earthquakes. However, the lack of a unified geological definition of fault maturity makes quantitative assessment of its importance difficult. We evaluate the degree of empirical correlation between geological and geometric measurements commonly invoked as indicative of fault zone maturity and remotely measured seismological source parameters of 34 MW ≥ 6.0 shallow strike‐slip events. Metrics based on surface rupture segmentation, such as number of segments and surface rupture azimuth changes, correlate best with seismic source attributes while the correlations with cumulative fault slip are weaker. Average rupture velocity shows the strongest correlation with metrics of maturity, followed by relative aftershock productivity. Mature faults have relatively lower aftershock productivity and higher rupture velocity. A more complex relation is found with moment‐scaled radiated energy. There appears to be distinct behavior of very immature events which radiate modest seismic energy, while intermediate mature faults have events with higher moment‐scaled radiated energy and very mature faults with increasing cumulative slip tend to have events with reduced moment‐scaled radiated energy. These empirical comparisons establish that there are relationships between remote seismological observations and fault system maturity that can help to understand variations in seismic hazard among different fault environments and to assess the relative maturity of inaccessible or blind fault systems for which direct observations of maturity are very limited.
We used seismic data recorded at local and teleseismic distances to analyze the May 22, 2021 Mw 7.3 Madoi, China earthquake sequence, which occurred along a low‐slip‐rate (0.3–1.0 mm/yr) and small‐cumulative‐displacement (4–5 km) fault—the Kunlun Mountain Pass‐Jiangcuo Fault (KMPJF). We first restored the clipped waveforms recorded at local distances, and obtained more accurate phase arrivals for relocating the earthquake sequence. The relocated earthquakes illuminate a ∼170 km‐long complex northwest‐southeast (NW‐SE) striking ruptured fault with apparent bifurcated ends at both tips and variations in dip angles along strike. We further performed backprojection analyses to image the rupture process of the mainshock using seismic data recorded at four teleseismic arrays (Europe, Australia and Southeast Asia, Japan, and Alaska). The results show that the earthquake propagated bilaterally in the NW and SE directions with maximum rupture speeds of ∼4.0 km/s. This observation was further refined by multiple array backprojections onto the fault that was determined by the fine relocations of the earthquake sequence. Synthetic and realistic tests validated the resolutions of the backprojection analyses (within 5–20 km). The fast supershear rupture speeds were only transient but exhibited spatial correlation with the geometric complexities of the KMPJF. This suggests that the transient supershear ruptures are due to the varying geometry of the KMPJF, which is likely an immature fault. Our results suggest that the potential for supershear ruptures along immature strike‐slip faults should be emphasized in earthquake hazard assessment.
On 19 October 2020, an Mw 7.6 earthquake occurred within the Shumagin Islands, Alaska, which is the largest strike-slip earthquake occurred in the shallow subducted plates with abundant seismic observations. Here, we relocated the earthquake sequence, implemented back-projection analyses, and finite-fault inversion to investigate the source processes of the mainshock, and calculated mainshock focal mechanisms using the polarities of P waves and W-phase inversion, respectively. Our results show that the faulting of the mainshock can be divided into two segments with the initial rupture along a steep plane (strike = 15°, dip = 81°) and propagation southeastward along a more shallowly dipping plane (strike = 344°, dip = 48°). The inferred strikes of the mainshock faults are similar to the orientations of preexisting structures in the source region, likely indicating that the 2020 Mw 7.6 earthquake ruptured along the preexisting plate fabric in the downgoing plate. The fabrics are located at the boundary with significant variations of the plate coupling, indicating that these structures within the subducting plate may affect the interplate coupling or as a result of the varying interplate coupling in subduction zones.
Strong-motion records of the 1968 Tokachi-Oki earthquake were examined, and two very high stress drop subevents were identified. The first subevent had been previously located by Nagamune (1969), and the second subevent was located in this study using P waves recorded on short-period WWSSN records. Estimates of source parameters revealed small source dimensions (<1 per cent of the aftershock area) and very high dynamic and static stress drops in the kilobar range for both of the subevents. It is suggested that these subevents are important in driving the main rupture of this earthquake.
The two subevents also produced the dominant accelerations on the strong-motion records, and it is shown that high-peak accelerations (150 to 200 cm/sec2) were recorded even at relatively large distances (100 to 200 km).
[1] We study how heterogeneous rupture propagation affects the coherence of shear and Rayleigh Mach wavefronts radiated by supershear earthquakes. We address this question using numerical simulations of ruptures on a planar, vertical strike-slip fault embedded in a three-dimensional, homogeneous, linear elastic half-space. Ruptures propagate spontaneously in accordance with a linear slip-weakening friction law through both homogeneous and heterogeneous initial shear stress fields. In the 3-D homogeneous case, rupture fronts are curved owing to interactions with the free surface and the finite fault width; however, this curvature does not greatly diminish the coherence of Mach fronts relative to cases in which the rupture front is constrained to be straight, as studied by Dunham and Bhat (2008a). Introducing heterogeneity in the initial shear stress distribution causes ruptures to propagate at speeds that locally fluctuate above and below the shear wave speed. Calculations of the Fourier amplitude spectra (FAS) of ground velocity time histories corroborate the kinematic results of Bizzarri and Spudich (2008a): (1) The ground motion of a supershear rupture is richer in high frequency with respect to a subshear one. (2) When a Mach pulse is present, its high frequency content overwhelms that arising from stress heterogeneity. Present numerical experiments indicate that a Mach pulse causes approximately an ω−1.7 high frequency falloff in the FAS of ground displacement. Moreover, within the context of the employed representation of heterogeneities and over the range of parameter space that is accessible with current computational resources, our simulations suggest that while heterogeneities reduce peak ground velocity and diminish the coherence of the Mach fronts, ground motion at stations experiencing Mach pulses should be richer in high frequencies compared to stations without Mach pulses. In contrast to the foregoing theoretical results, we find no average elevation of 5%-damped absolute response spectral accelerations (SA) in the period band 0.05–0.4 s observed at stations that presumably experienced Mach pulses during the 1979 Imperial Valley, 1999 Kocaeli, and 2002 Denali Fault earthquakes compared to SA observed at non-Mach pulse stations in the same earthquakes. A 20% amplification of short period SA is seen only at a few of the Imperial Valley stations closest to the fault. This lack of elevated SA suggests that either Mach pulses in real earthquakes are even more incoherent that in our simulations or that Mach pulses are vulnerable to attenuation through nonlinear soil response. In any case, this result might imply that current engineering models of high frequency earthquake ground motions do not need to be modified by more than 20% close to the fault to account for Mach pulses, provided that the existing data are adequately representative of ground motions from supershear earthquakes.
On April 14th, 2010, an Ms7.1 earthquake occurred in Yushu Tibetan Autonomous State, Qinghai province, China. It resulted in a great loss of life and property, and especially, the Yushu, a city southeastern to the epicenter, was most seriously damaged. We calculated the ground motion, including displacement and velocity, near the causative fault using the dynamic model of rupture process from the inversion of teleseismic waveform data and the crust model of Crust2. 0, reconstructing an image of the fault-near ground motion at the time of quaking. The image shows that there appeared the evident Doppler's effect, generated by the unilateral rupture process of the source. Seismic waves radiated out wards from the epicenter, but those propagating toward southeast along the fault had much larger amplitudes, and were much more crowded. The Yushu city happens to lie on not only where static displacement was larger also where the stronger velocity waves and displacement waves ran through. There should be no doubt that the Doppler's effect caused by the unilateral rupture process was the main reason why the most disastrous damage happened in the Yushu city, in spite the estimated ground motion was not accurate enough compared with the real one.
As revealed by field investigations, the co-seismic surface rupture zone of the 2010 M
S7.1 Yushu earthquake, Qinghai is a characteristic sinistral strike-slip feature consisting of three distinct sinistral primary ruptures, with an overall strike of 310°–320° and a total length of 31 km. In addition, an approximately 2-km-long en-echelon tensile fissure zone was found east of Longbao Town; if this site is taken as the north end of the rupture zone, then the rupture had a total length of ∼51 km. The surface rupture zone is composed of a series of fissures arranged in an en-echelon or alternating relationship between compressive bulges and tensile fissures, with a measured maximum horizontal displacement of 1.8 m. The surface rupture zone extends along the mapped Garzê-Yushu Fault, which implicates it as the seismogenic fault for this earthquake. Historically, a few earthquakes with a magnitude of about 7 have occurred along the fault, and additionally traces of paleoearthquakes are evident that characterize the short-period recurrence interval of large earthquakes here. Similar to the seismogenic process of the 2008 Wenchuan earthquake, the Yushu earthquake is also due to the stress accumulation and release on the block boundaries resulting from the eastward expansion of Qinghai-Tibet Plateau. However, in contrast with the Wenchuan earthquake, the Yushu earthquake had a sinistral strike-slip mechanism resulting from the uneven eastward extrusion of the Baryan Har and Sichuan-Yunnan fault blocks.
An idealized rupture, propagating smoothly near a terminal rupture velocity, radiates energy that is focused into a beam. For rupture velocity less than the S-wave speed, radiated energy is concentrated in a beam of intense fault-normal velocity near the projection of the rupture trace. Although confined to a narrow range of azimuths, this beam diverges and attenuates. For rupture velocity greater than the S-wave speed, radiated energy is concentrated in Mach waves forming a pair of beams propagating obliquely away from the fault. These beams do not attenuate until diffraction becomes effective at large distance. Events with supershear and sub-Rayleigh rupture velocity are compared in 2D plane-strain calculations with equal stress drop, fracture energy, and rupture length; only static friction is changed to determine the rupture velocity. Peak velocity in the sub-Rayleigh case near the termination of rupture is larger than peak velocity in the Mach wave in the supershear case. The occurrence of supershear rupture propagation reduces the most intense peak ground velocity near the fault, but it increases peak velocity within a beam at greater distances.
A numerical method to deconvolve complex body waves into a multiple shock sequence is developed. With the assumption that all the constituent events of a multiple shock have identical fault geometry and depth, the far-field source time function is obtained as a superposition of ramp functions. The height and the onset time of the ramp functions are determined by matching the synthetic waveforms with the observed ones in the least-square sense.
The individual events are then identified by pairs of ramp functions or discrete trapezoidal pulses in the source time sequence. The method can be used for the analysis of both single and multi-station data. Teleseismic long-period P waves from the 1976 Guatemala earthquake are analyzed as a test of our method. The present method provides a useful tool for a systematic analysis of multiple event sequences.