Supershear Speed Research Articles

Non‐Typical Supershear Rupture: Fault Heterogeneity and Segmentation Govern Unilateral Supershear and Cascading Multi‐Fault Rupture in the 2021 Mw ${M}_{w}$7.4 Maduo Earthquake

AbstractPrevious geodetic and teleseismic observations of the 2021 7.4 Maduo earthquake imply surprising but difficult‐to‐constrain complexity, including rupture across multiple fault segments and supershear rupture. Here, we present an integrated analysis of multi‐fault 3D dynamic rupture models, high‐resolution optical correlation analysis, and joint optical‐InSAR slip inversion. Our preferred model, validated by the teleseismic multi‐peak moment rate release, includes unilateral eastward double‐onset supershear speeds and cascading rupture dynamically triggering two adjacent fault branches. We propose that pronounced along‐strike variation in fracture energy, complex fault geometries, and multi‐scale variable prestress drives this event's complex rupture dynamics. We illustrate how supershear transition has signatures in modeled and observed off‐fault deformation. Our study opens new avenues to combine observations and models to better understand complex earthquake dynamics, including local and potentially repeating supershear episodes across immature faults or under heterogeneous stress and strength conditions, which are potentially not unusual.

Transonic and Supershear Crack Propagation Driven by Geometric Nonlinearities.

Linear elastic fracture mechanics theory predicts that the speed of crack growth is limited by the Rayleigh wave speed. Although many experimental observations and numerical simulations have supported this prediction, some exceptions have raised questions about its validity. The underlying reasons for these discrepancies and the precise limiting speed of dynamic cracks remain unknown. Here, we demonstrate that tensile (mode I) cracks can exceed the Rayleigh wave speed and propagate at supershear speeds. We show that taking into account geometric nonlinearities, inherent in most materials, is sufficient to enable such propagation modes. These geometric nonlinearities modify the crack-tip singularity, resulting in different crack-tip opening displacements, cohesive zone behavior, and energy flows towards the crack tip.

Along-Strike Variation of Rupture Characteristics and Aftershock Patterns of the 2023 Mw 7.8 Türkiye Earthquake Controlled by Fault Structure

Abstract On 6 February 2023, an Mw 7.8 earthquake occurred along the East Anatolian fault zone (EAFZ) in southeastern Türkiye, representing the strongest earthquake in the region in nearly 80 yr. We investigate rupture characteristics and aftershock patterns of the earthquake through focal mechanism calculation, backprojection analysis, and finite-fault inversion. The results show bilateral rupture propagation of the mainshock with transient supershear speed in the southwest portion of the EAFZ, as well as shallower coseismic slip and abundant normal-faulting aftershocks in the same portion. We attribute these earthquake behaviors to the along-strike variation of fault structure of the EAFZ, which features a more complex fault geometry accompanied by numerous short normal faults in the southwest portion. These results shed light on fault segmentation, rupture speed variation, and slip partitioning along the EAFZ, advancing our understanding of fault structural control on earthquake behaviors in a complex multisegment fault system.

Dynamics of episodic supershear in the 2023 M7.8 Kahramanmaraş/Pazarcik earthquake, revealed by near-field records and computational modeling

The 2023 M7.8 Kahramanmaraş/Pazarcik earthquake was larger and more destructive than what had been expected. Here we analyzed nearfield seismic records and developed a dynamic rupture model that reconciles different currently conflicting inversion results and reveals spatially non-uniform propagation speeds in this earthquake, with predominantly supershear speeds observed along the Narli fault and at the southwest (SW) end of the East Anatolian Fault (EAF). The model highlights the critical role of geometric complexity and heterogeneous frictional conditions in facilitating continued propagation and influencing rupture speed. We also constrained the conditions that allowed for the rupture to jump from the Narli fault to EAF and to generate the delayed backpropagating rupture towards the SW. Our findings have important implications for understanding earthquake hazards and guiding future response efforts and demonstrate the value of physics based dynamic modeling fused with near-field data in enhancing our understanding of earthquake mechanisms and improving risk assessment.

Kinematic Rupture Model of the 6 February 2023 Mw 7.8 Türkiye Earthquake from a Large Set of Near-Source Strong-Motion Records Combined with GNSS Offsets Reveals Intermittent Supershear Rupture

ABSTRACT The 2023 Mw 7.8 southeast Türkiye earthquake was recorded by an unprecedentedly large set of strong-motion stations very close to its rupture, opening the opportunity to observe the rupture process of a large earthquake with fine resolution. Here, the kinematics of the earthquake source are inferred by finite-source inversion based on strong-motion records and coseismic offsets from permanent Global Navigation Satellite Systems stations. The strong-motion records at stations NAR and 4615, which are the closest to the splay fault (SPF) where the rupture initiated and which were previously interpreted to contain the signature of supershear rupture speeds, are successfully modeled here by a subshear rupture propagating unilaterally to the northeast. Once the rupture on the SPF reaches the east Anatolian fault (EAF), it propagates on the EAF bilaterally, extending about 120 km northeast and 180 km southwest. To the south, the depth extent of the rupture decreases, as it passes a bend of the EAF. Although the rupture velocity remains globally subshear along the EAF, we identify three portions of the fault where the rupture is transiently supershear. The transitions to supershear speed coincide with regions of reduced fault slip, which suggests supershear bursts generated by the failure of local rupture barriers. Toward the southwest termination, the rupture encircles an asperity before its failure, which is a feature that has been observed only on rare occasions. This unprecedented detail of the inversion was facilitated by the proximity to the fault and the exceptional density of the accelerometric network in the area.

3D Dynamic Rupture Modeling of the 6 February 2023, Kahramanmaraş, Turkey Mw 7.8 and 7.7 Earthquake Doublet Using Early Observations

Abstract The 2023 Turkey earthquake sequence involved unexpected ruptures across numerous fault segments. We present 3D dynamic rupture simulations to illuminate the complex dynamics of the earthquake doublet. Our models are constrained by observations available within days of the sequence and deliver timely, mechanically consistent explanations of the unforeseen rupture paths, diverse rupture speeds, multiple slip episodes, heterogeneous fault offsets, locally strong shaking, and fault system interactions. Our simulations link both earthquakes, matching geodetic and seismic observations and reconciling regional seismotectonics, rupture dynamics, and ground motions of a fault system represented by 10 curved dipping segments and embedded in a heterogeneous stress field. The Mw 7.8 earthquake features delayed backward branching from a steeply branching splay fault, not requiring supershear speeds. The asymmetrical dynamics of the distinct, bilateral Mw 7.7 earthquake are explained by heterogeneous fault strength, prestress orientation, fracture energy, and static stress changes from the previous earthquake. Our models explain the northward deviation of its eastern rupture and the minimal slip observed on the Sürgü fault. 3D dynamic rupture scenarios can elucidate unexpected observations shortly after major earthquakes, providing timely insights for data-driven analysis and hazard assessment toward a comprehensive, physically consistent understanding of the mechanics of multifault systems.

Laboratory earthquakes decipher control and stability of rupture speeds

Earthquakes are destructive natural hazards with damage capacity dictated by rupture speeds. Traditional dynamic rupture models predict that earthquake ruptures gradually accelerate to the Rayleigh wave speed with some of them further jumping to stable supershear speeds above the Eshelby speed (~sqrt{2} times S wave speed). However, the 2018 Mw 7.5 Palu earthquake, among several others, significantly challenges such a viewpoint. Here we generate spontaneous shear ruptures on laboratory faults to confirm that ruptures can indeed attain steady subRayleigh or supershear propagation speeds immediately following nucleation. A self-similar analysis of dynamic rupture confirms our observation, leading to a simple model where the rupture speed is uniquely dependent on a driving load. Our results reproduce and explain a number of enigmatic field observations on earthquake speeds, including the existence of stable subEshelby supershear ruptures, early onset of supershear ruptures, and the correlation between the rupture speed and the driving load.

A Method to Generate Initial Fault Stresses for Physics-Based Ground-Motion Prediction Consistent with Regional Seismicity

ABSTRACTNear-field ground motion is the major blind spot of seismic hazard studies, mainly because of the challenges in accounting for source effects. Initial stress heterogeneity is an important component of physics-based approaches to ground-motion prediction that represents source effects through dynamic earthquake rupture modeling. We hypothesize that stress heterogeneity on a fault primarily originates from past background seismicity. We develop a new method to generate stochastic stress distributions as a superposition of residual stresses left by the previous ruptures that are consistent with regional distributions of earthquake size and hypocentral depth. We validate our method on Mw 7 earthquake models suitable for California by obtaining a satisfactory agreement with empirical earthquake scaling laws and ground-motion prediction equations. To avoid the excessive seismic radiation produced by dynamic models with abrupt arrest at preset rupture borders, we achieve spontaneous rupture arrest by incorporating a growth of fracture energy as a function of hypocentral distance. Our analyses of rupture and ground motion reveal particular signatures of the initial stress heterogeneity: rupture can locally propagate at supershear speed near the highly stressed areas; the position of high-stress and low-stress areas due to initial stress heterogeneity determines how the peak ground-motion amplitudes and polarization spatially vary along the fault, as low-stress areas slow down the rupture and decrease stress drop. We also find that the medium stratification in the fault zone amplifies fault slip and consequent ground motion, which requires understanding the interaction between site effects and rupture dynamics. Our approach advances our understanding of the relations between dynamic features of earthquake ruptures and the statistics of regional seismicity, and our capability to integrate information about regional seismicity into near-field ground-motion prediction.

Supershear shock front contribution to the tsunami from the 2018 Mw 7.5 Palu, Indonesia earthquake

SUMMARY Hazardous tsunamis are known to be generated predominantly at subduction zones. However, the 2018 Mw 7.5 Palu (Indonesia) earthquake on a strike-slip fault generated a tsunami that devastated the city of Palu. The mechanism by which this tsunami originated from such an earthquake is being debated. Here we present near-field ground motion (GPS) data confirming that the earthquake attained supershear speed, i.e. a rupture speed greater than the shear wave speed of the host medium. We subsequently study the effect of this supershear rupture on tsunami generation by coupling the ground motion to a 1-D non-linear shallow-water wave model accounting for both time-dependent bathymetric displacement and velocity. With the local bathymetric profile of Palu bay around a tidal station, our simulations reproduce the tsunami arrival and motions observed by CCTV cameras. We conclude that Mach (shock) fronts, generated by the supershear speed, interacted with the bathymetry and contributed to the tsunami.

Field observations of surface ruptures accompanying a tsunami and supershear earthquake along a plate boundary strike-slip fault

AbstractStrike-slip earthquakes near major subduction zones have received less attention than thrust or reverse earthquakes in subduction zone areas. The occurrence of the 2018 Palu Mw 7.5 earthquake in eastern Indonesia provides an unprecedented opportunity to investigate the characteristics of one of these events. The Palu earthquake occurred on the left-lateral, north–south-striking Palu–Koro fault, which is the main plate boundary structure accommodating the convergence between blocks in a triple junction area. It excited a significant tsunami, which unusually is associated with strike-slip earthquakes, and also ruptured at a supershear speed, which is mostly observed on strike-slip faults in continents. Based on our fieldwork, we speculate that the normal slip component of the offshore rupture section in Palu bay on the middle segment probably favours tsunami genesis. Our field investigation has revealed evidence of a simple geometry as well as slip partitioning of dip-slip and strike-slip motion on two subparallel strands on the main segment, both of which may have contributed to the supershear of the rupture propagation. Instead of only a transtensive behaviour of the middle segment, our results also illustrate the transpressional property of the northern and southern rupture segments, which shows more complex behaviour than that of a common continental strike-slip fault.

Fast rupture of the 2009Mw 6.9 Canal de Ballenas earthquake in the Gulf of California dynamically triggers seismicity in California

SUMMARYIn the Gulf of California, Mexico, the relative motion across the North America–Pacific boundary is accommodated by a series of marine transform faults and spreading centres. About 40 M> 6 earthquakes have occurred in the region since 1960. On 2009 August 3, an Mw 6.9 earthquake occurred near Canal de Ballenas in the region. The earthquake was a strike-slip event with a shallow hypocentre that is likely close to the seafloor. In contrast to an adjacent M7 earthquake, this earthquake triggered a ground-motion-based earthquake early warning algorithm being tested in southern California (∼600 km away). This observation suggests that the abnormally large ground motions and dynamic strains observed for this earthquake relate to its rupture properties. To investigate this possibility, we image the rupture process and resolve the slip distribution of the event using a P-wave backprojection approach and a teleseismic, finite-fault inversion method. Results from these two independent analyses indicate a relatively simple, unilateral rupture propagation directed along-strike in the northward direction. However, the average rupture speed is estimated around 4 km s−1, suggesting a possible supershear rupture. The supershear speed is also supported by a Rayleigh wave Mach cone analysis, although uncertainties in local velocity structure preclude a definitive conclusion. The Canal de Ballenas earthquake dynamically triggered seismicity at multiple sites in California, with triggering response characteristics varying from location-to-location. For instance, some of the triggered earthquakes in California occurred up to 24 hr later, suggesting that nonlinear triggering mechanisms likely have modulated their occurrence.

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

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.

Laboratory earthquakes along faults with a low velocity zone: Directionality and pulse-like ruptures

Low velocity zone (LVZ) is a common structural feature in geological faults. To examine the effect of LVZ on faulting, a laboratory fault model considering the LVZ is constructed and loaded to different levels. Subsequently ruptures are triggered on one of its contact boundaries with the host rock. The bilateral rupture propagation along this laboratory fault that separates different materials shows distinct directionality in both rupture speeds and rupture modes. The rupture in the positive direction with respect to the fault is always crack-like growing at the Generalized Rayleigh (GR) wave speed. However, in the negative direction both stable and unstable slip pulses are observed. Higher load and thinner LVZ facilitate the unstable slip pulses. The trailing tip for both types of pulses propagates at the GR wave speed, the leading tip of the stable pulse propagates with the GR wave speed while that of the unstable pulse features a supershear speed.

Back-propagating supershear rupture in the 2016 Mw 7.1 Romanche transform fault earthquake

How an earthquake rupture propagates strongly influences the potentially destructive ground shaking. Complex ruptures often involve slip along multiple faults, which masks information on the frictional behaviour of fault zones. Geometrically smooth ocean transform fault plate boundaries offer a favourable environment to study fault dynamics, because strain is accommodated along a single, wide fault zone that offsets the homogeneous geology. Here we present an analysis of the 2016 Mw 7.1 earthquake on the Romanche fracture zone in the equatorial Atlantic, using data from both nearby seafloor seismometers and global seismic networks. We show that this rupture had two phases: (1) upward and eastward propagation towards a weaker region where the transform fault intersects the mid-ocean ridge, and then (2) an unusual back-propagation westwards at a supershear speed towards the centre of the fault. We suggest that deep rupture into weak fault segments facilitated greater seismic slip on shallow locked zones. This highlights that even earthquakes along a single distinct fault zone can be highly dynamic. Observations of back-propagating ruptures are sparse, and the possibility of reverse propagation is largely absent in rupture simulations and unaccounted for in hazard assessments.

Inchworm-like source evolution through a geometrically complex fault fueled persistent supershear rupture during the 2018 Palu Indonesia earthquake

How does fault slip follow an earthquake rupture front propagating faster than the local shear-wave velocity (i.e., at supershear speed)? How does a supershear rupture front pass through a geometrically complex fault system? Resolving the evolution of such complex earthquake ruptures is fundamental to our understanding of earthquake-source physics, but these events have not been well captured by conventional waveform inversions of observational data. We applied a new framework of finite-fault inversion to globally observed teleseismic waveforms and resolved both the spatiotemporal evolution of slip and the fault geometry of the 2018 Palu earthquake (moment magnitude 7.6) in Sulawesi, Indonesia. We show that supershear rupture propagation for this event was sustained by transient slip stagnation and advancement as the rupture front passed through the geometrically complex fault system. This peculiar inchworm-like slip evolution was caused by the rupture front encountering fault bends with favorable and unfavorable orientations for rupture propagation. Our analysis also identified the possible existence of a fault junction beneath Palu Bay connecting an unmapped primary fault in northern Sulawesi with the Palu-Koro fault in the south.

Spatiotemporal Properties of Sub‐Rayleigh and Supershear Ruptures Inferred From Full‐Field Dynamic Imaging of Laboratory Experiments

AbstractMany earthquakes propagate at sub‐Rayleigh speeds. Earthquakes propagating at supershear speeds, though less common, are by far more destructive. Hence, it is important to quantify the motion characteristics associated with both types of earthquake ruptures. Here we report on the spatiotemporal properties of dynamic ruptures measured in our laboratory experiments using the dynamic digital image correlation technique. Earthquakes are mimicked by the frictional rupture propagating along the interface of two Homalite plates. Digital images of the propagating ruptures are captured by an ultrahigh‐speed camera and processed with digital image correlation in order to produce sequences of evolving displacement and velocity maps. Our measurements reveal the full‐field structure of the velocity components, bridge the gap between previous spatially sparse velocimeter measurements available only at two to three locations, and enable us to quantify the attenuation patterns away from the interface.

Strong ground motion over a large area in northern Taiwan caused by the northward rupture directivity of the 2019 Hualien earthquake

An Mw 6.2 earthquake struck Hualien on the eastern coast of Taiwan on April 18, 2019. Its focal mechanism was determined to be a thrust fault with a depth of 18.8 km. A wide range of strong ground shaking with an intensity value from 4 (25–80 Gal) to 7 (greater than 400 Gal) occurred in northern Taiwan, which is unusual for earthquakes of this size. Here we show a source model as determined by local seismic waveform inversion. Two asperities were found, the smaller one occurred at the hypocenter and the larger one was found 10 km north of the hypocenter with a depth of 18–25 km. The initial rupture started from the hypocenter and then rapidly propagated all the way to the large asperity in the north. A near supershear speed of close to 4.0 km/s was found to have occurred during this northward rupture process. The inverted source model was evaluated using forward three-dimensional ground motion simulations. The results show strong agreement with island-wide observed displacement waveforms and peak ground acceleration for periods from 3.0 to 20.0 s. The rapid northward rupture caused a strong directivity effect coupled with the specific source radiation pattern, resulting in a large area of strong ground shaking in northern Taiwan, even though the Hualien earthquake was a moderately-sized event.

Complex Triggering Supershear Rupture of the 2018 Mw 7.5 Palu, Indonesia, Earthquake Determined from Teleseismic Source Inversion

ABSTRACTAn Mw 7.5 earthquake struck Palu in the northern coast of Sulawesi island, Indonesia, on 28 September 2018. Its focal mechanism was determined to be a left‐lateral strike‐slip fault, which is generally expected to not produce a tsunami. However, a large tsunami with runup heights of more than 6 m was observed along the coast of Palu city. Here, we show a complex triggering supershear source model as determined by teleseismic waveform inversion. Three asperities with different slip characteristics were found on the 120‐kilometer‐long rupture zone. Significant triggering rupture with a supershear speed was observed south of the epicenter, which was just beneath Palu city. This special rupture process can cause a strong directivity effect that produced anomalously large ground shaking with nonlinear effects in Palu area. The coseismic deformation determined from the inverted source model showed large horizontal displacements. These horizontal movements combined with complex bathymetry and topography could have pushed seawater to generate a tsunami even though the Palu earthquake was a strike‐slip event.

Supershear tsunami disaster

A magnitude 7.5 strike-slip earthquake that struck Palu, Indonesia, in 2018 unexpectedly generated a devastating tsunami. Seismic data reveal that its rupture propagated fast, at supershear speed. Whether the two are connected remains to be seen.

Evidence of supershear during the 2018 magnitude 7.5 Palu earthquake from space geodesy

A magnitude 7.5 earthquake hit the city of Palu in Sulawesi, Indonesia on 28 September 2018 at 10:02:43 (coordinated universal time). It was followed a few minutes later by a 4–7-m-high tsunami. Palu is situated in a narrow pull-apart basin surrounded by high mountains of up to 2,000 m altitude. This morphology has been created by a releasing bend in the Palu-Koro fault, a rapidly moving left-lateral strike-slip fault. Here we present observations derived from optical and radar satellite imagery that constrain the ground surface displacements associated with the earthquake in great detail. Mapping of the main rupture and associated secondary structures shows that the slip initiated on a structurally complex and previously unknown fault to the north, extended southwards over 180 km and passed through two major releasing bends. The 30 km section of the rupture south of Palu city is extremely linear, and slightly offset from the mapped geological fault at the surface. This part of the rupture accommodates a large and smooth surface slip of 4–7 m, with no shallow slip deficit. Almost no aftershock seismicity was recorded from this section of the fault. As these characteristics are similar to those from known supershear segments, we conclude that the Palu earthquake probably ruptured this segment at supershear velocities. The devastating 2018 magnitude 7.5 earthquake in Palu, Indonesia, ruptured at supershear speeds according to evidence from space geodesy.