Maximum Slip Research Articles

Comprehensive Investigation of Capabilities of the Left-Looking InSAR Observations in Coseismic Surface Deformation Mapping and Faulting Model Estimation Using Multi-Pass Measurements: An Example of the 2016 Kumamoto, Japan Earthquake

We here present an example of the 2016 Kumamoto earthquake with its coseismic surface deformation mapped by the ALOS-2 satellite both in the right- and left-looking observation modes. It provides the opportunity to reveal the coseismic surface deformation and to explore the performance of the unusual left-looking data in faulting model inversion. Firstly, three tracks (ascending and descending right-looking and descending left-looking) of ALOS PALSAR-2 images are used to extract the surface deformation fields. It suggests that the displacements measured by the descending left-looking InSAR coincide well with the ascending right-looking track observations. Then, the location and strike angle of the fault are determined from the SAR pixel offset-tracking technique. A complicated four-segment fault geometry is inferred for explaining the coseismic faulting of the Kumamoto earthquake due to the interpretation of derived deformation fields. Quantitative comparisons between models constrained by the right-looking only data and by joint right- and left-looking data suggest that left-looking InSAR could provide comparable constraints for geodetic modelling to right-looking InSAR. Furthermore, the slip model suggests that the series of events are dominated by the dextral strike-slip with some normal fault motions. The fault rupture initiates on the Hinagu fault segment and propagates from southwest to northeast along the Hinagu fault, then transforms to Futagawa fault with a slip maximum of 4.96 m, and finally ends up at ~7 km NW of the Aso caldera, with a rupture length of ~55 km. The talent of left-looking InSAR in surface deformation detection and coseismic faulting inversion indicates that left-looking InSAR can be effectively utilized in the investigation of the geologic hazards in the future, same as right-looking InSAR.

Normal Faulting Movement During the 2020 Mw 6.4 Yutian Earthquake: A Shallow Rupture in NW Tibet Revealed by Geodetic Measurements

The ENE striking Longmu Co fault and the North Altyn Tagh left-lateral slip fault have led to the complex regional structure in the northwestern Tibetan Plateau, resulting in a series of normal faulting and strike slip faulting earthquakes. Using both the ascending and descending Sentinel-1A/B radar images, we depict the coseismic deformation caused by the 2020 Yutian Mw 6.4 earthquake with a peak subsidence of ~ 20 cm. We determine the seismogenic fault geometry by applying the Bayesian approach with a Markov Chain Monte Carlo sampling method, which can better characterize the posterior probability density functions of the source model parameters. The estimation results reveal that the earthquake is a normal faulting event with a moderate strike slip component. Based on the optimal fault geometry model, we extend the fault plane and invert for the distributed coseismic slip model. The optimal slip model shows that the coseismic slip is mainly concentrated at shallow depths of 3–10 km with a maximum slip of ~ 1.0 m. Our preferred geodetic coseismic model exhibits no surface rupture, which may likely be due to the shallow slip deficit in the uppermost crust. We calculate the combined loading effect of the Coulomb failure stress changes induced by the coseismic dislocations and postseismic viscoelastic relaxation of the 2008 Mw 7.1, 2012 Mw 6.4 and 2014 Mw 6.9 Yutian events. Our study demonstrates that the three preceding major Yutian shocks were insufficient to trigger the 2020 Yutian earthquake, which we consider perhaps reflects the natural release of elastic strain accumulated mainly through localized tectonic movement. We attribute the 2020 Yutian event to the release of extensional stress in a stepover zone controlled by the Longmu Co and the North Altyn Tagh sinistral strike slip fault systems. The seismic risk in the southwest end of the North Altyn Tagh fault has been elevated by the Yutian earthquake sequences, which require future attention.

Load-slip relationship of multi-type shear connectors based on push-out test

The push-out test used in this study to present the performance of continuous perfobond connectors. A total of seven specimens that are composed of lightweight concrete-filled steel tubes with two reinforced blocks were tested. The main investigated parameters were the concrete compressive strength of the blocks, specification of perfobond shear connector, and the type of shear connector. Specimens were inspected under a concentrated load applied on the steel tube filled with lightweight concrete while the slip is measured by two LVDTs. The results show that the ultimate shear strength and stiffness are increasing with increment concrete compressive strength. The T-type and mixed-type show increase in the ultimate shear strength compared with perfobond type, while a maximum slip of specimens decreasing with increasing the compressive strength of the concrete.

An Improved Quadtree Sampling Method for InSAR Seismic Deformation Inversion

With the development of interferometric synthetic aperture radar (InSAR), the seismic deformation observation density increases sharply. Data down-sampling can effectively reduce the observation density and the computational cost for subsequent researches. Considering the saliency of the deformation field, we introduce a saliency-based quadtree algorithm for down-sampling (SQS). Three simulation experiments show that SQS can effectively distinguish the near-field and far-field deformation, as well as reduce the amount of observation, while keeping the detailed information of the main deformation near the fault. SQS can avoid the interference of far-field local deformation better than the traditional quadtree sampling algorithm (QS), thus obtaining better inversion results. We took the Dingri earthquake on 20 March 2020 as a case study to verify the advantages of SQS in dealing with real earthquake deformation. We obtained the co-seismic deformation from the ascending and descending Sentinel-1 for the Dingri earthquake, using QS and SQS for sampling and inversion separately. The results show the advantages of SQS in data volume reduction, observation distribution, anti-interference of local deformation, and inversion accuracy. Our preferred solution based on SQS shows that the Dingri earthquake was caused by a normal fault slip. The main slip area is 2–5.5 km deep with a maximum slip of 0.68 m. The estimated geodetic moment is 3.14 × 1017 Nm, corresponding to a magnitude of Mw5.63.

Quantifying Normal Fault Evolution from River Profile Analysis in the Northern Basin and Range Province, Southwest Montana, USA

Abstract Over the past few decades, tectonic geomorphology has been widely implemented to constrain spatial and temporal patterns of fault slip, especially where existing geologic or geodetic data are poor. We apply this practice along the eastern margin of Bull Mountain, Southwest Montana, where 15 transient channels are eroding into the flat, upstream relict landscape in response to an ongoing period of increased base level fall along the Western North Boulder fault. We aim to improve constraints on the spatial and temporal slip rates across the Western North Boulder fault zone by applying channel morphometrics, cosmogenic erosion rates, bedrock characteristics, and calibrated reproductions of the modern river profiles using a 1-dimensional stream power incision model that undergoes a change in the rate of base level fall. We perform over 104 base level fall simulations to explore a wide range of fault slip dynamics and stream power parameters. Our best fit simulations suggest that the Western North Boulder fault started as individual fault segments along the middle to southern regions of Bull Mountain that nucleated around 6.2 to 2.5 Ma, respectively. This was followed by the nucleation of fault segments in the northern region around 1.5 to 0.4 Ma. We recreate the evolution of the Western North Boulder fault to show that through time, these individual segments propagate at the fault tips and link together to span over 40 km, with a maximum slip of 462 m in the central portion of the fault. Fault slip rates range from 0.02 to 0.45 mm/yr along strike and are consistent with estimates for other active faults in the region. We find that the timing of fault initiation coincides well with the migration of the Yellowstone hotspot across the nearby Idaho-Montana border and thus attribute the initiation of extension to the crustal bulge from the migrating hotspot. Overall, we provide the first quantitative constraints on fault initiation and evolution of the Western North Boulder fault, perhaps the farthest north basin in the Northern Basin and Range province that such constraints exist. We show that river profiles are powerful tools for documenting the spatial and temporal patterns of normal fault evolution, especially where other geologic/geodetic methods are limited, proving to be a vital tool for accurate tectonic hazard assessments.

Seismic source characteristics of the intermediate-depth and intraslab 2019 northern Peru earthquake (Mw 8.0)

On May 26, 2019, an unusual depth-intermediate focus earthquake, in the interior of the Nazca plate, shook the northern Peruvian jungle and caused the death of two victims. This earthquake was felt in almost all of Peru as well as Brazil, Ecuador and Colombia. The focal mechanism calculated is of the normal fault type (strike = 353∘, dip = 57∘, rake = − 99∘) for a focal depth of 114 km. The slip distribution was obtained through an inversion process of teleseismic waveforms. The maximum slip was 6.3 m, located around the central side of the fault plane. The moment magnitude calculated was Mw 8.0; the spectral analysis indicated a similar magnitude of Mw 8.0. The source time function indicates a multiple rupture earthquake, with the presence of several pulses of seismic energy release, with a total duration around 80 s. The rupture directivity was from the south to the north; we have assumed a propagation rupture velocity of 2.8 km/s. The modeled fault plane geometry has dimensions of 208×105 km2. The predicted vertical coseismic deformation field shows a pattern of mostly subsidence.

Heterogeneity of Aftershock Productivity Along the Mainshock Ruptures and Its Advantage in Improving Short‐Term Aftershock Forecast

AbstractThis study introduces an improved hypocentral version of the space‐time Epidemic‐Type Aftershock Sequence (ETAS) model, entitled as the 3D‐finite source (3D‐FS) ETAS model, and applies it to the analysis of the Southern California earthquake catalog. By stochastic reconstruction, we are able to reconstruct the patterns of aftershock productivity density along the mainshock ruptures. Detailed analysis of the productivity patterns reveals that: (1) Directly triggered aftershocks make up 21% to 41% of all earthquakes within the mainshock rupture areas, and show significant spatial heterogeneity and temporal migrations; (2) Major aftershocks tend to locate in low productivity areas, at the edges of clusters formed by small aftershocks; (3) Large slip areas are depleted of aftershocks, over 60% of all productivity distributes in areas with slip less than 0.3 times of the maximum slip, and the trajectory of the productivity pattern on the fault plane demonstrates apparent compensation to coseismic slip. We relate the difference in triggering abilities of four mainshocks to the heat flow in corresponding regions. Simulation results suggest that the 3D‐FS ETAS model has apparent advantages of improving the performance of short‐term aftershock forecast. Moreover, the later aftershocks are more correlated with the locations of subsequent events than earlier aftershocks, suggesting that the migration of aftershocks is important for mitigating aftershock hazard.

Source modeling and spectral analysis of the Crete tsunami of 2nd May 2020 along the Hellenic Subduction Zone, offshore Greece

Tsunami hazard in the Eastern Mediterranean Basin (EMB) has attracted attention following three tsunamis in this basin since 2017 namely the July 2017 and October 2020 Turkey/Greece and the May 2020 offshore Crete Island (Greece) tsunamis. Unique behavior is seen from tsunamis in the EMB due to its comparatively small size and confined nature which causes several wave reflections and oscillations. Here, we studied the May 2020 event using sea level data and by applying spectral analysis, tsunami source inversion, and numerical modeling. The maximum tsunami zero-to-crest amplitudes were measured 15.2 cm and 6.5 cm at two near-field tide gauge stations installed in Ierapetra and Kasos ports (Greece), respectively. The dominant tsunami period band was 3.8–4.7 min. We developed a heterogeneous fault model having a maximum slip of 0.64 m and an average slip of 0.28 m. This model gives a seismic moment of 1.13 × 1019 Nm; equivalent to Mw 6.67. We observed three distinct wave trains on the wave record of the Ierapetra tide gauge: the first and the second wave trains carry waves with periods close to the source period of the tsunami, while the third train is made of a significantly-different period of 5–10 min.

Fracture mechanics of rate-and-state faults and fluid injection induced slip.

Propagation of a slip transient on a fault with rate- and state-dependent friction resembles a fracture whose near tip region is characterized by large departure of the slip velocity and fault strength from the steady-state sliding. We develop a near tip solution to describe this unsteady dynamics, and obtain the fracture energy Gc, dissipated in overcoming strength-excursion away from steady state, as a function of the rupture velocity vr. This opens a possibility to model slip transients on rate-and-state faults as singular cracks characterized by approximately steady-state frictional resistance in the fracture bulk, and by a stress singularity with the intensity defined in terms of Gc(vr) at the crack tip. In pursuing this route, we develop and use an analytical equation of motion to study 1-D slip driven by a combination of uniform background stress and a localized perturbation of the fault strength with the net Coulomb force ΔT. In the context of fluid injection, ΔT is a proxy for the injection volume Vinj. We then show that, for ongoing fluid injection, the propagation speed of a transient induced on a frictionally stable fault is bounded by a large-time limiting value proportional to the injection rate dVinj/dt, while, for stopped injection, the maximum slip run-out distance is proportional to [Formula: see text]. This article is part of the theme issue 'Fracture dynamics of solid materials: from particles to the globe'.

Bond behavior between BFRP bar and basalt fiber reinforced seawater sea-sand recycled aggregate concrete

In order to meet the demand of construction waste treatment and using local materials, the use of recycled coarse aggregate (RCA), sea-sand and seawater is of great significance to coastal areas and sea-related engineering construction. Basalt fiber (BF) reinforced seawater sea-sand recycled aggregate concrete (BFSSRAC) with good workability was prepared in this paper. The mechanical properties of BFSSRAC and bond behavior between BFRP bar and BFSSRAC were investigated considering the effects of BF content and concrete strength through a series of tests and microstructure analysis. The results showed that the combination use of BF, seawater, sea-sand and RCA could reduce the adverse effects of RCA on the mechanical property of concrete. SEM analysis showed that the addition of BF could effectively improve the pore structure of concrete matrix, especially the interfacial transition zone (ITZ). The usage of RCA reduced the bond strength between BFRP bar and RAC, while the usage of seawater and sea-sand had little effect on the bond-slip constitutive relation. After adding BF, the ultimate bond strength and maximum slip of BFSSRAC were larger than those of concrete without fibers and increased with the increasing BF content. Based on the experimentally obtained τ-s curves, the bond-slip constitutive relation model between BFRP bar and BFSSRAC was proposed, and the model was verified by deviation analysis between calculated and measured values. The findings of this study can provide new and meaningful reference for the application of BFSSRAC structures reinforced with BFRP bars.

Present-day orogenic processes in the western Kalpin nappe explored by interseismic GNSS measurements and coseismic InSAR observations of the 2020Mw 6.1 Kalpin event

SUMMARYAs the largest and most active intracontinental orogenic belt on Earth, the Tien Shan (TS) is a natural laboratory for understanding the Cenozoic orogenic processes driven by the India–Asia collision. On 19 January 2020, a Mw 6.1 event stuck the Kalpin region, where the southern frontal TS interacts with the Tarim basin. To probe the local ongoing orogenic processes and potential seismic hazard in the Kalpin region, both interseismic and instantaneous deformation derived from geodetic observations are used in this study. With the constraint of interseismic global navigation satellite system (GNSS) velocities, we estimate the décollement plane parameters of the western Kalpin nappe based on a 2-D dislocation model, and the results suggest that the décollement plane is nearly subhorizontal with a dip of ∼3° at a depth of 24 km. Then, we collect both Sentinel-1 and ALOS-2 satellite images to capture the coseismic displacements caused by the 2020 Kalpin event, and the interferometric synthetic aperture radar (InSAR) images show a maximum displacement of 7 cm in the line of sight near the epicentral region. With these coseismic displacement measurements, we invert the source parameters of this event using a finite-fault model. We determine the optimal source mechanism in which the fault geometry is dominated by thrust faulting with an E–W strike of 275° and a northward dip of 11.2°, and the main rupture slip is concentrated within an area 28.0 km in length and 10.3 km in width, with a maximum slip of 0.3 m at a depth of 6–8 km. The total released moment of our preferred distributed slip model yields a geodetic moment of 1.59 × 1018 N$ \cdot $m, equivalent to Mw 6.1. The contrast of the décollement plane depth from interseismic GNSS and the rupture depth from coseismic InSAR suggests that a compression still exists in the Kalpin nappe forefront, which is prone to frequent moderate events and may be at risk of a much more dangerous earthquake.

Slip distribution and rupture history of the August 11, 2012, double earthquakes in Ahar – Varzaghan, Iran, using joint inversion of teleseismic broadband and local strong motion data

We use combined teleseismic and strong motion data sets to investigate finite-fault slip models for a double of earthquakes that occurred on August 11, 2012, in northwestern Iran near the cities of Ahar and Varzaghan. The data include teleseismic P-waveforms retrieved from broadband seismic stations located between 30°–94° from the earthquakes and local strong motion data recorded by the Iran Strong Motion Network, installed and operated by the Building and Housing Research Centre. We first invert teleseismic P-waveforms and local strong motion data separately. For the first event (12:23 UTC), the teleseismic broadband inversion yields a somewhat deeper and simpler distribution of slip than the local strong motion inversion. The strong motion inversion results in a more complex distribution because of higher frequency content but can also be influenced by complexities in the propagation path. For the second event (12:34 UTC), the slip distribution from strong motion data is more similar to the teleseismic result and shows a simple slip area with a small relative movement to the west. To resolve the differences between the results of these two data sets and obtain a better constrained slip model, we perform a joint inversion of teleseismic broadband and local strong motion data.The joint inversion for the first event shows two asperities with a maximum slip of 3.9 m up- dip from the hypocenter and extending to the west between depths of 1 and 5 km. A second narrower high-slip area is seen just above the hypocenter from 6 to 10 km depth. The total moment for this earthquake is calculated to be Mo = 3.8 × 1025 dyn-cm (3.8 × 1018 N.m) (Mw 6.4). For the second event, the results of the joint inversion show a simple slip distribution that is mainly confined in a single patch around the hypocenter with a depth range from about 10 to 13 km and maximum slip of 1.9 m. We compute a total seismic moment of Mo = 1.6 × 1025 dyn-cm (1.6 × 1018 N.m) (Mw 6.1) for the second event. The largest stress drops for the first event occur above the hypocenter with an average stress drop over the rupture area of 120 bar (12 Mpa). For the second event, the maximum stress drop occurs at the reported focal depth with an average stress drop over the rupture area of 80 bar (8 Mpa).

Subducting oceanic basement roughness impacts on upper-plate tectonic structure and a backstop splay fault zone activated in the southern Kodiak aftershock region of the Mw 9.2, 1964 megathrust rupture, Alaska

AbstractIn 1964, the Alaska margin ruptured in a giant Mw 9.2 megathrust earthquake, the second largest during worldwide instrumental recording. The coseismic slip and aftershock region offshore Kodiak Island was surveyed in 1977–1981 to understand the region’s tectonics. We re-processed multichannel seismic (MCS) field data using current standard Kirchhoff depth migration and/or MCS traveltime tomography. Additional surveys in 1994 added P-wave velocity structure from wide-angle seismic lines and multibeam bathymetry. Published regional gravity, backscatter, and earthquake compilations also became available at this time.Beneath the trench, rough oceanic crust is covered by ∼3–5-km-thick sediment. Sediment on the subducting plate modulates the plate interface relief. The imbricate thrust faults of the accreted prism have a complex P-wave velocity structure. Landward, an accelerated increase in P-wave velocities is marked by a backstop splay fault zone (BSFZ) that marks a transition from the prism to the higher rigidity rock beneath the middle and upper slope. Structures associated with this feature may indicate fluid flow. Farther upslope, another fault extends >100 km along strike across the middle slope. Erosion from subducting seamounts leaves embayments in the frontal prism.Plate interface roughness varies along the subduction zone. Beneath the lower and middle slope, 2.5D plate interface images show modest relief, whereas the oceanic basement image is rougher. The 1964 earthquake slip maximum coincides with the leading and/or landward flank of a subducting seamount and the BSFZ. The BSFZ is a potentially active structure and should be considered in tsunami hazard assessments.

Push-out tests on novel enhanced C-channels in steel-concrete-steel sandwich structures using ultra-high performance concrete

This paper firstly developed steel-UHPC-steel sandwich structures (SUSSSs) with new enhanced C-channels (ECs). Shear behaviours of ECs embedded in ultra-high performance concrete (UHPC) were firstly investigated by a testing program with 14 specimens. The test results primarily reported the failure mode, shear strength, and shear-slip behaviours of ECs in UHPC. The ECs in UHPC exhibited a successive shear fracture of external bolts and C-channels, and a ductile shear-slip behaviour with a maximum slip capacity exceeding 6 mm. These test results also revealed the effects of length, height, and layout of the C-channel, and concrete core strength on shear behaviour of ECs. Theoretical models were also developed to describe the load-slip behaviour and predict ultimate shear resistances of novel EC connectors in UHPC. The validations against reported tests exhibited that the modified code equations greatly overestimated ultimate shear resistances of ECs, and the developed theoretical models predicted well the shear-slip behaviour and ultimate shear resistances of ECs in UHPC. The average prediction discrepancy for the developed theoretical model is only 5% with a small standard deviation of 0.06 for the 12 reported tests.

Seismic source characteristics of the 2016 Pedernales-Ecuador earthquake (Mw 7.8)

The 2016 Pedernales earthquake (Mw 7.8), is part of the sequence of large earthquakes in Ecuador and Colombia: 1906 (~8.8 Mw), 1942 (Mw 7.8), 1958 (Mw 7.7), 1979 (Mw 8.2), all these events with a focal mechanism of the thrust fault type. In this research, we have obtained the seismic source distribution (indicating the presence of 2 main asperities), from inversion of teleseismic signals, as well as the parameters of the seismic rupture process: focal mechanism that indicates a type of inverse fault (strike = 25∘, dip = 12∘, rake = 118∘) for a focal depth of 23 km and the source time function indicating a composite event, the duration of the rupture process was around 70 s, assuming an average rupture velocity of 3.0 km/s. The seismic moment was calculated at 5.77×1020 Nm, equivalent to a magnitude of Mw 7.8. The smallest asperity is located around the epicentre and the main asperity is located around the coastline to the south-west from the epicentre. The maximum dislocation or slip was 3.75 m. The coseismic vertical displacement is not conductive to the generation of a large tsunami, as it is around 1 m only. The zone of Pedernales have a Coulomb stress transfer of 22 bar, therefore, this area is loaded with stresses and is a potentially seismic zone for an upcoming earthquake.

Mechanical behavior of perfobond connector group in steel–concrete joint of hybrid bridge

The perfobond connector is the most widely used connector in the steel–concrete joints of long-span hybrid bridges, and the behavior of the perfobond connector group directly determines the reliability of the connection and the smoothness of force transmission of the joint. However, the existing model test can only reveal some of the connector group’s information in the joint, which leads to the force transfer mechanism of the joint group remaining unclear. To solve these problems, establishing the numerical and theoretical models is necessary. Numerical analysis shows that the stress of the connector on the steel beam loading side is far greater than that on the concrete loading side at the initial stage of loading, and the connector has enough slip capacity to ensure the uniform distribution of interface shear when it reaches the limit state. The reduction of connector spacing weakens the effective support on the concrete dowel and then reduces the shear capacity. In terms of theoretical analysis, four factors determine the interface slip distribution: the length of the steel–concrete joint, the interface stiffness, the axial stiffness of concrete members and the axial stiffness of steel members. The increase of the interface stiffness and the steel–concrete joint length can decrease the maximum slip and reduce the effective utilization ratio of the connector group. When the axial stiffness of concrete or steel members tends to be the same, the effective utilization ratio of the whole group of connectors can improve. Finally, the simplified calculation formula of the maximum slip Smax, the effective utilization ratio μj of the connector group and the load ratio ξj of the connector are put forward.

Strong and Tough Bioinspired Additive-Manufactured Dual-Phase Mechanical Metamaterial Composites

Nature's materials are generally hybrid composites with superior mechanical properties achieved through delicate architectural designs. Inspired by the precipitation hardening mechanisms observed in biological materials as well as engineering alloys, we develop here dual-phase mechanical metamaterial composites by employing architected lattice materials as the constituent matrix and reinforcement phases. The composite metamaterials made from austenitic stainless steel are simply fabricated using selected laser melting based additive manufacturing. Using quasi-static compression tests and simulation studies, we find that strength and toughness can be simultaneously enhanced with the addition of reinforcement phase grains. Effects of reinforcement phase patterning and connectivity are examined. By fully utilizing the energy dissipation from phase-boundary slip, an optimized dual-phase metamaterial is designed with the maximum slip area, where every truss unit in the matrix phase is completely surrounded by reinforcement phase lattices; this material exhibits a specific energy absorption capability that is ~2.5 times that of the constituent matrix phase lattices. The design rationale for dissipative dual-phase metamaterials is analyzed and summarized with a focus on phase pattering. The present digital multi-phase mechanical metamaterials can emulate almost any of nature's architectures and toughening mechanisms, offering a novel pathway to manipulate mechanical properties through arbitrary phase-material selection and patterning. We believe that this could markedly expand the design space for the development of future materials.

Spatial\u2013temporal properties of afterslip associated with the 2015 Mw 8.3 Illapel earthquake, Chile

In order to characterize the spatial–temporal properties of postseismic slip motions associated with the 2015 Illapel earthquake, the daily position time series of 13 GNSS sites situated at the near-field region are utilized. Firstly, a scheme of postseismic signal extraction and modeling is introduced, which can effectively extract the postseismic signal with consideration of background tectonic movement. Based on the extracted postseismic signal, the spatial–temporal distribution of afterslip is inverted under the layered medium model. Compared with coseismic slip distribution, the afterslip is extended to both deep and two sides, and two peak slip patches are formed on the north and south sides. The afterslip is mainly cumulated at the depth of 10–50 km, and the maximum slip reaches 1.46 m, which is situated at latitude of − 30.50°, longitude of − 71.78°, and depth of 18.94 m. Moreover, the postseismic slip during the time period of 0–30 days after this earthquake is the largest, and the maximum of fault slip and corresponding slip rate reaches 0.62 m and 20.6 mm/day. Whereas, the maximum of fault slip rate during the time period of 180–365 days is only around 1 mm/day. The spatial–temporal evolution of postseismic slip motions suggests that large postseismic slip mainly occurs in the early stage after this earthquake, and the fault tend to be stable as time goes on. Meanwhile, the Coulomb stress change demonstrate that the postseismic slip motions after the Illapel earthquake may be triggered by the stress increase in the deep region induced by coseismic rupture.

Rupture Process of the Mainshock of the 2019 Ridgecrest Earthquake Sequence from Waveform Inversion with Empirical Green’s Functions

ABSTRACTThe 2019 Mw 7.1 mainshock of the Ridgecrest earthquake sequence, which was the first event exceeding Mw 7.0 in California since the 1999 Hector Mine earthquake, caused near-fault ground motions exceeding 0.5g and 70 cm/s. In this study, the rupture process and the generation mechanism of strong ground motions of the mainshock were investigated through waveform inversions of strong-motion data in the frequency range of 0.2–2.0 Hz using empirical Green’s functions (EGFs). The results suggest that the mainshock involved two large slip regions: the primary one with a maximum slip of approximately 4.4 m was centered ∼3 km northwest of the hypocenter, which was slightly shallower than the hypocenter, and the secondary one was centered ∼25 km southeast of the hypocenter. Outside these regions, the slip was rather small and restricted to deeper parts of the fault. A relatively small rupture velocity of 2.1 km/s was identified. The robustness of the slip model was examined by conducting additional inversion analyses with different combinations of EGF events and near-fault stations. In addition, using the preferred slip model, we synthesized strong motions at stations that were not used in the inversion analyses. The synthetic waveforms captured the timing of the main phases of observed waveforms, indicating the validity of the major spatiotemporal characteristics of the slip model. Our large slip regions are also generally visible in the models proposed by other researchers based on different datasets and focusing on lower frequency ranges (generally lower than 0.5 Hz). In particular, two large slip regions in our model are very consistent with two of the four subevents identified by Ross et al. (2019), which may indicate that part of the large slip regions that generated low-frequency ground motions also generated high-frequency ground motions up to 2.0 Hz during the Ridgecrest mainshock.

Co-seismic deformation and slip distribution of 5 April 2017 Mashhad, Iran earthquake using InSAR sentinel-1A image: implication to source characterization and future seismogenesis

We analyzed interferometric synthetic aperture radar sentinel-1A-based observations to characterize the source of 5 April 2017 Mashhad, Iran mainshock (Mw 6.1), for the first time to understand the seismogenic potential of the source area using the estimates of co-seismic displacement and slip distribution on applying the steepest descent method (SDM). SAR pixel offsets (SPO) provided a deep insight into the co-seismic surface deformation of the entire source zone. Based on iterations of a total of 451 models, our analysis of sentinel-1A data from ascending and descending tracks revealed surface deformation occurred in an area of 40 × 30 km with a maximum co-seismic uplift of 10 cm. We estimated geodetic moment of 1.9 × 1020 Nm corresponding to the magnitude (Mw 6.0) of the Mashhad mainshock for which the rupture has a planner geometry with uniform slip dislocation in an elastic half-space with slip of 0.35 ± 0.1 m; strike of N313°E having dip of 48° of the thrust fault associated with oblique motion. The rupture length of 45 ± 3 km along-strike and 30 ± 3 km down dip has been estimated. The best-fit fault model geometry derived from SDM suggests that rupture occurred in the vicinity of the Kashafrud thrust fault, located west to the main Kopeh-Dagh Fault with its strike of 315°E. It is observed that a maximum slip of 0.35 m occurred at a depth of 8 km that extended to 10 km in the crust, which is found to be in unison to the Coulomb stress model that showed low-stressed zone is associated with the majority of events of lower magnitude (M ≤ 4.5) in NE–SW to the mainshock, whilst the EW zone to the mainshock found relatively highly stressed as a probable source for generating relatively higher magnitude earthquakes (M > 4.5) in the future. We infer that the estimates of co-seismic source attributes are essentially important for understanding the nature and extent of earthquake risks for Mashhad, Iran earthquake source area and of the areas of analogous geotectonic settings, elsewhere in the world.