Supershear Earthquakes Research Articles

One of the Scenarios for Supershear Earthquakes

One of the Scenarios for Supershear Earthquakes

Investigating seismic performance of SMRF buildings under subshear and supershear rupture conditions: Assessment of storey-level internal damage

The source effect of earthquake ruptures causes spatial-temporal variability in ground motion characteristics in the vicinity of a fault. This research employs a dynamic rupture-based method with SPECFEM3D to simulate two rupture types: subshear and supershear. While the former depends on the rupture length of the fault, the latter depends on the rupture velocity. Synthetic earthquake ground motion data are generated across the entire domain, encompassing both near-fault and far-fault locations. Ground motions exhibit multidimensionality, with subshear effects evident in the fault normal component and supershear effects distinctly reflected in the fault parallel component. These ground motions have been employed to excite low (S1L), mid (S1M), and high-rise (S1H) steel special moment resisting frame (SMRF) buildings modelled using the modified Ibarra-Medina-Krawinkler (IMK) analogy. Understanding the far-field effects of supershear earthquakes and the near-field effects of directivity in subshear earthquakes on the structural response of SMRF buildings has been aided by several engineering demand parameters (EDPs), including floor acceleration, interstorey-drift ratio, and roof drift ratio.

Influence of rupture velocity on risk assessment of concrete moment frames: Supershear vs. subshear ruptures

The possibility of ground motions propagating at speeds greater than the Rayleigh wave speed was accepted not long ago, and thus studies on them are limited. These supershear motions are believed to damage specific kinds of infrastructure, and thus studying the risk associated is important. As a part of this investigation, we use state-of-the-art tools from the Natural Hazards Engineering Research Infrastructure to estimate normalized economic losses and injuries, by considering one particular building type and occupancy category listed in HAZUS-MH. Faults of similar dimensions are shown to replicate subshear and supershear earthquake instances with four groups of station points parallel to the fault and four more perpendicular to the fault. Both fault parallel and fault normal components of ground motion are stimulated. The economic loss % and injury is standardized in metrics of repair cost and population respectively and is estimated along with the associated uncertainty for 3, 6 and 9-story commercial concrete moment frame structures. Inclusion of complex phenomena such as supershear earthquake events in evaluating economic losses and risk will contribute to the United Nation Office for Disaster Risk Reduction (UNDRR) vision of disaster-resilient development goals.

28th September 2018 Mw 7.5 Sulawesi Supershear Earthquake, Indonesia: Ground effects and macroseismic intensity estimation using ESI-2007 scale

The 28th September 2018 Sulawesi Supershear earthquake (MW 7.5) was one of the deadliest earthquakes in the recent history of Indonesia causing ∼4000 causalities. The earthquake caused a ∼ 177 km long surface rupture along the Palu-Karo fault. Apart from surface rupture, the earthquake caused extensive earthquake environmental effects (EEEs) around the Palu-Donggala area of Central Sulawesi, Indonesia, which includes tsunami, coastal landslide, liquefaction, ground cracks and more than 7300 landslides in hilly areas. Initial post-event analysis and reports assigned a Modified Mercalli Intensity (MMI) of VII to VIII in Palu City and the surrounding area. Building damage and ground effects caused by the earthquake suggested that seismic intensity was understated. Here we applied the EEEs information from field survey data, published reports, and remote sensing tools to determine macroseismic intensity using the Environmental Seismic Intensity (ESI-07) Scale. The ESI-07 intensity derived from the ground effects suggests the maximum intensity of X-XI, which is 3–4° higher than the traditional intensity estimated by the United States Geological Survey (USGS) and the Indonesian Agency for Meteorology, Climatology, and Geophysics (BMKG). ShakeMaps were generated considering the ESI-07 values. The ShakeMap was compared with the instrumentally derived ShakeMap for the Palu earthquake, which proves that the ShakeMap prepared from the instrumental data or structural damage data is underrated. We argue that proper documentation of the EEEs is necessary for such damaging earthquakes for future earthquake hazard mapping and planning in the study area and other earthquakes in Indonesia. In addition, this will help in defining the on-fault and off-fault damage zone towards reducing the seismic risk of the Palu Donggala area.

The role of pore fluids in supershear earthquake ruptures

The intensity and damage potential of earthquakes are linked to the speed at which rupture propagates along sliding crustal faults. Most earthquakes are sub-Rayleigh, with ruptures that are slower than the surface Rayleigh waves. In supershear earthquakes, ruptures are faster than the shear waves, leading to sharp pressure concentrations and larger intensities compared with the more common sub-Rayleigh ones. Despite significant theoretical and experimental advances over the past two decades, the geological and geomechanical controls on rupture speed transitions remain poorly understood. Here we propose that pore fluids play an important role in explaining earthquake rupture speed: the pore pressure may increase sharply at the compressional front during rupture propagation, promoting shear failure ahead of the rupture front and accelerating its propagation into the supershear range. We characterize the transition from sub-Rayleigh to supershear rupture in fluid-saturated rock, and show that the proposed poroelastic weakening mechanism may be a controlling factor for intersonic earthquake ruptures.

The Paucity of Supershear Earthquakes on Large Faults Governed by Rate and State Friction

AbstractSupershear earthquakes are rare compared to their subshear counterparts, but the cause for their paucity remains to be understood. We investigate for the first time the prevalence of supershear ruptures across multiple earthquake cycles on long faults using rate‐and‐state friction and a 2.5D approximation that accounts for the finite seismogenic width W. We find supershear events occur only in a narrow range of friction parameters that is not commonly observed in laboratory experiments, which may explain its rarity in nature. Particularly, the ratio between direct and evolution effects of rate‐and‐state friction needs to be low (a/b < 0.4) and the nucleation length has to be sufficiently large compared to W, but not too large that it causes a transition from seismic to aseismic slip. Our numerical and analytical developments contribute fundamentally to understanding the state of stress on long faults over multiple earthquake cycles and their potential for hosting supershear earthquakes.

Global frequency of oceanic and continental supershear earthquakes

Global frequency of oceanic and continental supershear earthquakes

Chasing supershear earthquakes

Chasing supershear earthquakes

Spatial variation of structural fragility due to supershear earthquakes

Collapse fragility of structures is studied using equipotent subshear and supershear earthquakes. Supershear earthquakes propagate at a speed faster than the shear-wave speed of the medium, thereby producing significantly different ground motion characteristics than subshear ruptures. In particular, fault parallel component dominance is established in the literature for supershear events. By considering a set of stations parallel to the strike of the fault, and another set perpendicular to the strike of the fault at various distances, fragility curves are derived as a function of distance from the fault in the near-field. In addition, the fault-parallel and fault-normal components in the near-field are applied separately to single-degree-of-freedom systems with peak-oriented modified Ibarra–Medina–Krawinkler hysteretic behaviour to characterize the component-wise differences across the subshear and supershear cases. A comparison of the median fragilities with HAZUS suggested values of similar structural systems is also presented.

Offshore wind turbine subjected to supershear earthquake ruptures

Supershear earthquakes, analogous to supersonic speeds of sounds, show the presence of a Mach front in the top view. This feature was observed in several past earthquakes, including the 1999 Izmit earthquake, 2001 Kokoxili earthquake, to name a few. The ground motion from supershear ruptures can be destructive to structures due to dominant fault-parallel components. In this study, we demonstrate the effect of supershear ruptures on wind turbines by modelling them in OpenSees. Earthquake ruptures are simulated using SPECFEM3D, another open-source software. By considering arrays at different offsets along strike and away from the strike, the distance effect on serviceability limit states of wind turbines is studied using supershear and subshear earthquakes.

Effect of real-world frictional strengthening layer near the Earth's free surface on rupture characteristics with different friction laws: Implication for scarcity of supershear earthquakes

More and more supershear rupture earthquakes, with rupture propagation velocity larger than the local shear wave speed, have been identified in recent years. However, the number of supershear events is scarce compared with large strike-slip earthquakes. This problem remains unresolved although several previous workers have offered some explanations. In this study, we utilize finite element method to simulate spontaneous dynamic rupture propagations in three dimensional half-space model with and without velocity strengthening friction layer (VSFL) near the Earth's free surface employing different frictional constitutive relations, including linear slip weakening law and rate- and state-dependent relations with aging form and slip form. The simulated results showed that the VSFL suppresses supershear rupture transition at the Earth's free surface occurring in the absence of a VSFL. In particular, we found that the minimum thickness of VSFL to prevent supershear rupture transition is ~1.4-km. Meanwhile, we reviewed a great deal of previous work on thickness of VSFL along mature faults in nature, and its thickness generally exceeds ~3-km, which is enough to suppress the transition to supershear rupture in the real-world. The presence of VSFL may be the primary reason why the supershear rupture earthquakes in nature are so scarce. In addition, the modelling results indicated that spatial heterogeneity of VSFL on the surface along fault strike affects the supershear rupture transition to some extent. Therefore, this study contributes to understanding the earthquake source dynamics and seismic hazard mitigation.

Supershear Rupture During the 2021 MW 7.4 Maduo, China, Earthquake

AbstractCombined with the multi‐array back‐projection and finite‐fault joint inversion methods, we investigate the detailed spatiotemporal rupture complexity of the 2021 MW 7.4 Maduo earthquake using seismic and geodetic data. Our results reveal that this earthquake ruptured bilaterally 170 km long fault segments with a duration of 38 s. The obtained back‐projection results, specific aftershock signature of the supershear earthquake, and observations of far‐field Love Mach waves consistently validate that this earthquake is a supershear event. Furthermore, by analyzing global large strike‐slip earthquakes, we find that the moment‐scaled radiated energies of previously well confirmed supershear earthquakes with relatively simple fault geometries are similar to that of the 2021 MW 7.4 Maduo earthquake, which are significantly smaller than those of subshear strike‐slip events. This new finding provides additional supporting evidence to validate the supershear rupture during the 2021 MW 7.4 Maduo earthquake.

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.

InSAR data reveal that the largest hydraulic fracturing-induced earthquake in Canada, to date, is a slow-slip event

For tectonic earthquakes, slip rate spans a continuum from creep to supershear earthquakes, where slow slip events (SSEs) are important in releasing stress without radiating damaging seismic energy. Industrial-scale subsurface fluid injection has caused induced earthquakes, but the role of SSEs in fault activation is currently unclear. Ground-deformation observations, measured by satellite radar, show that SSEs up to magnitude 5.0 occurred during hydraulic fracturing (HF) operations in northwestern Canada, corroborated by reported deformation of the steel well casing. Although the magnitude 5.0 SSE exceeded the magnitude of the largest induced earthquake in this region (magnitude 4.55), it was undetected by seismograph networks. The observed SSEs occurred within a buried thrust belt and their magnitude and duration are consistent with scaling behavior of SSEs in unbounded natural systems, e.g. slab interfaces in subduction zones.

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.

Landslides triggered by the 2018 Mw 7.5 Palu supershear earthquake in Indonesia

On September 28th, 2018, a supershear earthquake of moment magnitude (Mw) 7.5 struck the Palu area in central Sulawesi, Indonesia. To gain insight into landslides triggered by the Palu earthquake, a detailed landslide interpretation project was carried out in the meizoseismal area. To establish a co-seismic landslide inventory, we collected high-resolution images, including Worldview pre- and post- earthquake images, Sentinel pre- and post- earthquake images and Google Earth pre- and post- earthquake images. The results show that the Palu event triggered at least 7063 co-seismic landslides, including 8 flowslides. The total landslide area is 29.7 km2, with the highest density being 96 landslides/km2. The rock type (Kambuno granite) is a dominant factor influencing the landslide distribution; the average landslide H/L (H is the fall height and L is the horizontal length of an entire landslide) is 0.56. The flowslides that occurred in the Palu Basin are seismic liquefaction-induced landslides that failed along very gentle slopes (generally, slopes that are <3°) and traveled hundreds to thousands of meters, and the average H/L was 0.021.

SIMULATION OF PEAK GROUND ACCELERATION AND PSEUDO SPECTRAL ACCELERATION OF PALU EARTHQUAKE SEPTEMBER 28TH 2018

The devastating earthquake Mw 7.4 hit Palu City, Central Sulawesi on September 28th, 2018, at 17:02:44 WIB. A high tsunami followed it. More than 2000 people died as a result of the earthquake and tsunami disaster. The latest evidence shows that the earthquake was categorized as a rare super shear earthquake. The earthquake shaking that hit Palu City is relatively large. Acceleration data are not available at the study site due to the lack of instrumentation in the area. The Authors present a simulation of peak ground acceleration (PGA) and pseudo-spectral acceleration (PSA) due to the earthquake in three locations Tatura Mall, Roa-Roa Hotel, and Antapura Hospital. PGA describes the maximum acceleration on the ground, while pseudo-spectral acceleration describes the acceleration of earthquake shaking from buildings with various floor numbers. Simulation of PGA and PSA to the three sites used three different Ground Motion Prediction Equation (GMPE) functions, BSSA14, CB14, and CY14, with the weighting of each Next Generation Attenuation (NGA) GMPE functions. The results of PGA simulation is about 0.22-0.23 g and show that in the three study site, it is more vulnerable to spectral acceleration period T=0.3 s or building with three floors or about 1-15 floors. These correlate with the level of damage caused by earthquakes which is more impact to relatively higher buildings.

A Transient in Surface Motions Dominated by Deep Afterslip Subsequent to a Shallow Supershear Earthquake: The 2018 Mw7.5 Palu Case

AbstractThe 2018 Palu earthquake is a remarkable strike‐slip event due to its nature as a shallow supershear fault rupture across several segments and a destructive tsunami that followed coseismic deformation. GPS offsets in the wake of the 2018 earthquake display a transient in the surface motions of northwest Sulawesi. A Bayesian approach identifies (predominantly aseismic) deep afterslip on and below the coseismic rupture plane as the dominant physical mechanism causing the cumulative, postseismic, surface displacements whereas viscous relaxation of the lower crust and poro‐elastic rebound contribute negligibly. We confirm a correlation between shallow supershear rupture and postseismic surface transients with afterslip activity in the zone below an interseismically locked fault plane where the slip rate tapers from zero to creeping.

A self-similar dynamic rupture model based on the simplified wave-rupture analogy

SUMMARYThe investigation of stresses, faults, structure and seismic hazards requires a good understanding and mapping of earthquake rupture and slip. Constraining the finite source of earthquakes from seismic and geodetic waveforms is challenging because the directional effects of the rupture itself are small and dynamic numerical solutions often include a large number of free parameters. The computational effort is large and therefore difficult to use in an exploratory forward modelling or inversion approach. Here, we use a simplified self-similar fracture model with only a few parameters, where the propagation of the fracture front is decoupled from the calculation of the slip. The approximative method is flexible and computationally efficient. We discuss the strengths and limitations of the model with real-case examples of well-studied earthquakes. These include the Mw 8.3 2015 Illapel, Chile, megathrust earthquake at the plate interface of a subduction zone and examples of continental intraplate strike-slip earthquakes like the Mw 7.1 2016 Kumamoto, Japan, multisegment variable slip event or the Mw 7.5 2018 Palu, Indonesia, supershear earthquake. Despite the simplicity of the model, a large number of observational features ranging from different rupture-front isochrones and slip distributions to directional waveform effects or high slip patches are easy to model. The temporal evolution of slip rate and rise time are derived from the incremental growth of the rupture and the stress drop without imposing other constraints. The new model is fast and implemented in the open-source Python seismology toolbox Pyrocko, ready to study the physics of rupture and to be used in finite source inversions.

The effects of sediments on supershear rupture

The effects of sediments on supershear rupture have been discussed. Two sediment models with different media properties have opposite effects on supershear rupture. The rupture accelerates into supershear more easily if the sediment is thin in the model with a relatively low wave velocity. In contrast, the supershear rupture transition occurs more easily if the sediment is thick in the model with a relatively high wave velocity. Supershear is first triggered at the lower boundary of the sediment if the initial stress is sufficiently high. Otherwise, supershear is prevented or triggered at the free surface if the initial stress is relatively low. A deep hypocenter favors supershear rupture. The very long supershear transition distance in some cases may explain the extremely long supershear transition distance in the 2001 Kunlunshan earthquake. We also investigate the effects of the depth-dependent stress on the supershear rupture and find that similar results are concluded. Besides, a daughter crack is triggered but propagates in a subshear regime in some cases. A multi-layered sediment model makes it easier to trigger supershear rupture in relative high-velocity sediment but harder in relatively low-velocity sediment. Our results provide a possible explanation for the rarity of supershear earthquakes in nature.