Postseismic Relaxation Research Articles

Ductile Versus Brittle Tectonics in the Anatolian–Aegean–Balkan System

It is hypothesized that the present tectonic setting of the Anatolian, Aegean and Balkan regions has been deeply influenced by the different deformation styles of the inner and outer belts which constituted the Oligocene Tethyan system. Stressed by the Arabian indenter, this buoyant structure has undergone a westward escape and strong bending. The available evidence suggests that in the Plio–Pleistocene time frame, the inner metamorphic core mainly deformed without undergoing major fragmentations, whereas the orogenic belts which flanked that core (Pontides, Balkanides, Dinarides and Hellenides) behaved as mainly brittle structures, undergoing marked fractures and fragmentations. This view can plausibly explain the formation of the Eastern (Crete–Rhodes) and Western (Peloponnesus) Hellenic Arcs, the peculiar time-space features of the Cretan basins, the development of the Cyprus Arc, the North Aegean strike-slip fault system, the southward escapes of the Antalya and Peloponnesus wedges and the complex tectonic setting in the Balkan zone. These tectonic processes have mostly developed since the late Late Miocene, in response to the collision of the Tethyan belt with the Adriatic continental domain, which accelerated the southward bending of the Anatolian and Aegean sectors, at the expense of the Levantine and Ionian oceanic domains. The proposed interpretation may help us to understand the connection between the ongoing tectonic processes and the spatio-temporal distribution of major earthquakes, increasing the chances of estimating the long-term seismic hazard in the study area. In particular, it is suggested that seismic activity in the Serbo–Macedonian zone may be favored by the post-seismic relaxation that develops after seismic crises in the Epirus thrust front and inhibited/delayed by the activations of the North Anatolian fault system.

Spatiotemporal dominance of afterslip and viscoelastic relaxation revealed by four decades of post-1973 Luhuo earthquake observations

Measuring surface displacements driven by afterslip and viscoelastic relaxation following large earthquakes enables inferring frictional and rheological properties of Earth's outermost layers where hazardous earthquakes occur. However, the two concurrent mechanisms generate similar and intertwined surface deformations, complicating the extraction of physical information from the measurements. Here, we quantify the spatiotemporal dominance of co-evolving afterslip and viscoelastic relaxation following the 1973 Ms 7.6 Luhuo earthquake in eastern Tibet, using 42 years (1976‒2018) of fault-crossing short-baseline observations. The finite-fault slip of this M7+ earthquake was constructed from triangulation data measured in 1961‒1975. Based on the model incorporating two postseismic relaxation mechanisms and calibrated by the measurements over four decades, we show that, in the temporal domain, afterslip may produce geodetically measurable deformations (>1.5 mm/yr) in local near-field regions even 20 years after the mainshock, with spatially broader impact within ∼10 years. Viscoelastic relaxation may last for nearly six decades, imposing a broad influence for three decades. In the spatial domain, afterslip may produce deformations within ∼2 times the seismogenic depth (SD; 20 km) from the fault, but dominantly in the near-field of ∼1 times the SD. In contrast, viscoelastic relaxation may affect a much wider region reaching 200 km. While afterslip and viscoelastic relaxation collectively affected the region ∼2 times the SD from the fault in the early postseismic period, their resulting deformations gradually separated in space with time, with afterslip-induced deformation shrinking toward the fault, and surface deformation caused by viscoelastic relaxation concentrating in the mid-field, ∼2–3 times the SD from the fault. Their spatiotemporal partitioning helps better learn fault frictional properties and lithospheric rheology based on geodetic data acquired from different domains.

Spatiotemporal Variations and Postseismic Relaxation Process Around Mt. Fuji, Japan, During and After the 2011 Tohoku‐Oki Earthquake

AbstractTo monitor the volcanoes at a high spatiotemporal resolution, we introduce the singular value decomposition‐based Wiener filter and the three‐component waveforms in ambient noise velocity monitoring. The continuous ambient noise data from 63 stations around Mt. Fuji and Mt. Hakone, Japan, during the January‐September 2011 were analyzed to estimate the seismic velocity variations at a 1‐day temporal resolution, allowing us to distinguish the velocity drops caused by the 2011 Mw 9.0 Tohoku‐oki and the Mw 6.0 East Shizuoka earthquake. The velocity drop during the Tohoku‐oki earthquake was large in volcanic areas and was larger around Mt. Hakone than Mt. Fuji. This difference is possibly due to the existence of fluid‐ and gas‐rich zones at shallower depths and a higher crack density around Mt. Hakone. In addition, the velocity drop at Mt. Fuji during the Tohoku‐oki and the East Shizuoka earthquake was the same level, despite larger static stress changes beneath Mt. Fuji during the East Shizuoka earthquake. We interpret this inconsistency between the velocity drops and static stress changes to arise from incomplete recovery of the generated cracks during the Tohoku‐oki earthquake when the East Shizuoka earthquake occurred. This study also investigates the spatial variations in recovery speed and recovery amount, finding slow recovery speeds in the volcanic areas and fault areas, possibly due to larger crack densities in the crust. Furthermore, we observe the lowest velocity recovery amount in the volcanic areas, which is likely attributed to the maintained increase in pore pressure due to the volcanic gas bubbles.

GRACE and GRACE Follow‐On Gravity Observations of Intermediate‐Depth Earthquakes Contrasted With Those of Shallow Events

AbstractEarthquakes involve mass redistribution within the solid Earth and the ocean, and as a result, perturb the Earth's gravitational field. For most of the shallow (<60 km) earthquakes with Mw > 8.0, the GRACE satellite gravity measurements suggest considerable volumetric disturbance of rocks. At a spatial scale of hundreds of km, the effect of volumetric change exceeds gravity change by vertical deformation; for example, negative gravity anomalies associated with volumetric expansion are characteristic patterns after shallow thrust events. In this study, however, we report contrasting observations of gravity change from two intermediate‐depth (100–150 km) earthquakes of 2016 & 2017 Mw 8.0 (two combined) Papua New Guinea thrust faulting events and 2019 Mw 8.0 Peru normal faulting and highlight the importance of compressibility in earthquake deformation. The combined 2016/17 thrust events resulted in a positive gravity anomaly of 5–6 microGal around the epicenter, while the 2019 normal faulting produced a negative gravity anomaly of 3–4 microGal. Our modeling found that these gravity changes are manifestation of vertical deformation with limited volumetric change, distinct from gravity changes after the shallow earthquakes. The stronger resistance of rocks to volume change at intermediate‐depth results in largely incompressible deformation and thus in a gravity change dominated by vertical deformation. In addition, malleable rocks under high pressure and temperature at depth facilitated substantial afterslip and/or fast viscoelastic relaxation causing additional vertical deformation and gravity change equivalent to the coseismic change. For the Papua New Guinea events, this means that postseismic relaxation enhanced coseismic uplift and relative sea level decrease.

Velocity Changes Associated to the 2019 Ambon 6.5 Mw Earthquake Using Ambient Seismic Noise

Seismic velocity changes which occurred before, during and after large magnitude earthquake carry information about damage fault surrounding the area. Research focused on Ambon 6.5 Mw earthquake on September 26, 2019. We use ambient noise cross-correlation technique to compute empirical Green’s Function between station pairs from January to December 2019. We use doublet method to determine perturbation calculated in frequency domain by using Moving Windows Cross Spectrum in frequency between 0.1 to 1.0 Hz. Our results show no obvious significant velocity change before the mainshock. Velocity fluctuation average about ±0.1% caused by non-isotropic of noise source around the area. Sudden velocity decrease occurred and reached average value of -0.35% in 7 days after mainshock. After reaching its lowest value, velocity gradually increased about +0.4% in 14 days. Velocity fluctuations about ±0.2% also occurred for three months after the earthquake which may caused by the aftershocks around the earthquake epicentre. Following the large magnitude mainshock, velocity reduction may be induced by cracks opening and fractures due to deformation. While the velocity increase may caused by post-seismic relaxation due to elastic behaviour and crack healing of the fault throughout the surrounding region.

MCQsim: A Multicycle Earthquake Simulator

ABSTRACT Multicycle earthquake rupture simulators generate long sequences of earthquakes along predefined fault geometries for statistical analysis of earthquake recurrence and related rupture characteristics. Based on the physical approximations of long-term crustal deformation and the short-term rupture process, they can provide a deeper understanding of the inner workings of the “earthquake machine” as well as inform seismic hazard assessment by filling the observational gap between instrumental seismology and paleoseismology. With MCQsim, we introduce a multi-cycle rupture simulator that (a) can work with complex, nonplanar fault geometries and heterogenous models of fault strength, (b) includes different approximations of long-term stress accumulation, (c) incorporates elastic signal propagation velocity, (d) distinguishes between stable, conditionally stable, and unstable portions of the fault, and (e) includes postseismic relaxation and afterslip. The generated earthquake catalogs allow the identification of parameters that dominate system behavior while also providing probabilities of future ruptures based on their preceding behavior. Here, we focus on the technical aspects of how MCQsim, which is based on elastostatic dislocation theory and the boundary element method, approximates both the seismic cycle and the earthquake’s rupture process. We provide exemplary simulation outputs for verification and validation purposes.

Mosaicking Andean morphostructure and seismic cycle crustal deformation patterns using GNSS velocities and machine learning

We use unsupervised machine learning techniques to analyze continental-scale crustal motions in areas affected by the seismic cycle of large subduction earthquakes along the Chilean Trench. Specifically, we use the agglomerative clustering algorithm as an exploratory tool to investigate spatial patterns in GNSS regional velocities without the complexity of modeling a physical source. We present a continental-scale velocity field including all available GNSS data for two-time windows (pre-2014, 2018–2021) that represents two periods with different deformation patterns of the seismic cycle. We test two different pre-processing methodologies for the design of machine learning features from the GNSS-derived velocities. The first method uses the direction and magnitude of the secular rates as input features to the clustering algorithm. These results show a clustering spatially related to seismic cycle deformation, separating latitudinal segments with different velocities in the fore-arc and back-arc, as well as regions affected by postseismic relaxation. Thus, highlighting the effectiveness of this method for mapping first-order patterns of active deformation in a subduction zone, that are particularly related to variations on interplate coupling and postseismic transient deformation. In a more sophisticated approach, we use surface strain and rotational rates from GNSS velocities as features in the second methodology. Here, we develop a novel methodology to estimate strain and rotation rates accounting for the spatial heterogeneity of the GNSS-network. We determine the spatial scale at which these features are estimated by least squares inversions, by using a Bayesian model class selection method. The distribution of stations allows to identify heterogeneities in strain and rotation rates at spatial scales larger than 50 km, being particularly notorious the main features of regional deformation at scales > 100 km. Interestingly, the results show a spatial correlation between seismic segmentation in the fore-arc and geologic and structural domains in the arc and back-arc. Our results demonstrate the ability of the combination of inverse and machine learning methods to efficiently identify active deformation patterns and their relationship to the subduction seismic cycle and regional-scale geological structures. Furthermore, our analysis suggests that Andean geological structures influence the observed deformation field.

Can Plate Bending Explain the Observed Faster Landward Motion of Lateral Regions of the Subduction Zone After Major Megathrust Earthquakes?

AbstractGreater landward velocities were recorded after six megathrust earthquakes in subduction zone regions adjacent to the ruptured portion. Previous explanations invoked either increased slip deficit accumulation or plate bending during postseismic relaxation, with different implications for seismic hazard. We investigate whether bending can be expected to reproduce this observed enhanced landward motion (ELM). We use 3D quasi‐dynamic finite element models with periodic earthquakes. We find that afterslip downdip of the brittle megathrust exclusively produces enhanced trenchward surface motion in the overriding plate. Viscous relaxation produces ELM when a depth limit is imposed on afterslip. This landward motion results primarily from in‐plane elastic bending of the overriding plate due to trenchward viscous flow in the mantle wedge near the rupture. Modeled ELM is, however, incompatible with the observations, which are an order of magnitude greater and last longer after the earthquake. This conclusion does not significantly change when varying mantle viscosity, plate elasticity, maximum afterslip depth, earthquake size, megathrust locking outside of the rupture, or nature and location of relevant model boundaries. The observed ELM consequently appears to reflect faster slip deficit accumulation, implying a greater seismic hazard in lateral segments of the subduction zone.

Offshore Landward Motion Shortly After a Subduction Earthquake Implies Rapid Relocking of the Shallow Megathrust

AbstractGeodetic observations after large subduction earthquakes reflect multiple postseismic processes, including megathrust relocking. The timing of relocking and the observational constraints on it are unclear. Relocking was inferred to explain some observed landward motion that occurs within months. It was also considered unable to explain other, greater landward motion, including that off the coast of Japan beginning weeks after the 2011 Tohoku earthquake, attributed to postseismic relaxation. We use generic, 3D numerical models to show that relocking, particularly of the shallow interface, is needed for postseismic relaxation to produce landward motion on the tip of the overriding plate. We argue that this finding is consistent with previous simulations that implicitly relock the megathrust where afterslip is not included. We conclude that the Tohoku megathrust relocked within less than 2 months of the earthquake. This suggests that the shallow megathrust probably behaves as a true, unstably sliding asperity.

Interseismic Strain Accumulation between the Colorado Plateau and the Eastern California Shear Zone: Implications for the Seismic Hazard near Las Vegas, Nevada

ABSTRACTCrustal deformation in the central Basin and Range between the Colorado plateau and the Eastern California Shear Zone is active but slow, making it a challenge to assess how strain is distributed and crustal motion transferred. However, knowledge of strain rates is very important, particularly for addressing the seismic hazard for both the Las Vegas urban area and the site of the proposed Yucca Mountain nuclear waste repository, in southern Nevada. Global Positioning System (GPS) data provide important constraints, particularly now that the GPS network in the area has substantially expanded in recent years. However, because deformation is slow, it is important to mitigate any transient tectonic and nontectonic signals to obtain the most accurate long-term interseismic motion and robust estimation of strain rates. We use data from all GPS stations in the region including both long-running continuous and semicontinuous stations. We model and remove postseismic displacements at these stations using source parameters for 41 events, dating back to the 1700 Cascadia megathrust earthquake, which contribute significantly to the deformation field within the central Basin and Range. We also remove correlated noise from the time series with the common-mode component imaging technique. We find that removal of both the postseismic transients and common-mode noise substantially reduces the uncertainties and spatial variation in the velocities. We find east–west extension across the Las Vegas Valley of 0.5–0.6 mm/yr. The interseismic strain rate field, calculated with the final velocities, reveals higher strain rates through southern Nevada than in previous studies, with rates within Las Vegas Valley of 8.5±2.4×10−9 yr−1. Our results also confirm shear along the Pahranagat shear zone, but the estimated amplitude is strongly affected by postseismic relaxation.

Time-dependent decrease in fault strength in the 2011–2016 Ibaraki–Fukushima earthquake sequence

SUMMARY Two near-identical Mw 5.8 earthquakes in 2011 and 2016 ruptured the Mochiyama Fault in the Ibaraki–Fukushima region of Japan. The unusually short repeat time between the two earthquakes provides a rare opportunity to estimate the evolution of stress on a fault through an earthquake cycle, as the stress drop in the first earthquake provides a reference value from which we can infer variations through time in the stresses required to cause earthquake rupture. By combining observations of crustal deformation from GPS, InSAR and seismology with numerical models of stress transfer due to coseismic deformation and post-seismic relaxation, we demonstrate that the rupture area on the Mochiyama Fault could only have been reloaded by up to 50–80 per cent of the 2011 earthquake stress drop (3–10 MPa) between that event and the subsequent 2016 earthquake. Most of this reloading was caused by afterslip around the rupture area driven by stress changes from the 2011 Mochiyama and Tohoku-oki earthquakes. We therefore infer that the Mochiyama Fault became weaker in the intervening 6 yr, with at least a 1–5 MPa drop in the shear stresses needed to break the fault in earthquakes. The mechanism(s) that led to this weakening are unclear, but were associated with extensive aftershock seismicity that released a cumulative moment similar to the 2011 main shock. Temporal changes in fault strength may therefore play a role in modulating the timing of moderate-magnitude earthquakes.

Seismically active structures of the Main Himalayan Thrust revealed before, during and after the 2015 Mw 7.9 Gorkha earthquake in Nepal

SUMMARY The Mw 7.9 2015 April 25 Gorkha earthquake is the latest of a millenary-long series of large devastating Himalayan earthquakes. It is also the first time a large Himalayan earthquake and its aftershocks were recorded by a local network of seismic stations. In the 5 yr following the main shock, more than 31 000 aftershocks were located by this permanent network within the ruptured area, including 14 362 events with ML greater than 2.5, 7 events with ML > 6, including one large aftershock with Mw 7.2 on 2015 May 12. In 2020, 5 yr after the main shock, the seismicity rate along the ruptured fault segments was still about 5 times higher than the background seismicity before the Gorkha earthquake. Several bursts of earthquakes, sometimes organized in clusters, have been observed from a few days to several years after the main shock. Some of these clusters were located at the same place as the clusters that happened during the decades of interseismic stress build-up that preceded the large earthquake. They also happened in the vicinity of the high frequency seismic bursts that occurred during the main shock. These heterogeneities contribute to a persistent segmentation of the seismicity along strike, possibly controlled by geological structural complexities of the Main Himalayan Thrust fault. We suggest that these pre-2015 clusters revealed the seismo-geological segmentation that influences both the coseismic rupture and the post-seismic relaxation.

Pseudotachylyte-Mylonites Record of Transient Creep From Inter-Seismic Ductile to Co-Seismic Rupture

Transient creep during an earthquake cycle is very important to understand the rheology of fault and deformation mechanisms in the brittle–plastic transition zone. In this paper, we analyzed the microstructures of samples for mylonites, pseudotachylyte, and cataclasite under optical microscope, SEM, and EBSD, which were collected from the Red River fault in southwest of China, where we uncovered a series of ductile to brittle deformed rocks which recorded transient creep related to earthquakes. The results show that mylonites formed at the inter-seismic creep were overprinted by pseudotachylyte and cataclasite which were produced during co-seismic rupture, and cracks in cataclasite were healed during the post-seismic relaxation. Based on the analysis of the microstructures and deformation mechanism of fault rocks, we propose the oscillation deformation model to explain transient creep of the brittle–plastic transition zone during the seismic cycle in the Red River fault.

On the co-evolution of dislocations and grains in deforming rocks

Deformation at tectonic plate boundaries is governed by the mechanical properties of crustal and lithospheric rocks, which evolve through changes in the microstructure of their constituent minerals. Geological observations of microstructure can, therefore, provide estimates of the rock's deformation conditions through various piezometers (e.g., the relationship between stress and grain size or dislocation density). However, grain size and dislocation density do not evolve independently, and thus their associated piezometers are unlikely to be decoupled. We present a new theoretical model coupling the evolution of grain boundaries and dislocations in a deforming rock and make a novel prediction that the equilibrium dislocation density at a given stress (the dislocation piezometer) can be non-unique, with co-existing deformation states or piezometric branches. One stable piezometric branch exists at high stress and large grain-size, for which the steady state dislocation density is large and dictated by stress, in agreement with experimental observations and earlier theoretical models. However, another stable branch exists at low stress and small grain-size, in which case dislocation density is small, weakly stress-dependent, and is governed by the balance between dislocation sinks and sources at grain boundaries. The contrast in dislocation density between grains of different sizes induces grain boundary migration from smaller grains into larger grains. Dislocation-induced grain boundary forces thus act in the opposite direction to those dictated by surface energy, for which grain boundaries migrate from larger grains into smaller ones. When forces due to surface and dislocation energies are balanced, the system reaches a stable microstructural equilibrium, thereby halting grain growth and helping preserve plate boundary weakness. When the forces are not in balance, unstable and oscillatory behavior can ensue, with oscillation periods on the order of decades. Finally, the new theory provides a microphysical model for transient rheological behavior, which is particularly relevant to tectonic activity in which deformation is relatively rapid and where the steady state flow laws are a poor approximation, such as during postseismic relaxation or postglacial rebound.

Nearest Neighbor Method for Discriminating Aftershocks and Duplicates When Merging Earthquake Catalogs

Early aftershocks contain important information about the physics of earthquake occurrence and postseismic relaxation processes. However, the standard catalogs of early aftershocks are usually incomplete. Many events can be missed in the main shock coda, some of which are strong enough due to the extremely high noise level. Under these conditions, the process of event identification becomes largely stochastic. Due to different network configurations and record processing methods, different agencies may register/miss different events, thus merging catalogs can improve the completeness of the aftershock sequence. When merging catalogs, the problem of identifying duplicates (records related to the same seismic event) arises. The main difficulty is discriminating aftershocks and duplicates, since both are events close in space and time. The problem is analogous to the problem of discriminating aftershocks and independent events. The solution methods are usually similar too. In this paper, we apply the nearest neighbor method modified for our problem. This method has become widespread in recent years in the problem of identifying aftershocks, and a probabilistic metric in the space of network errors in determining the epicenters and times of seismic events. It is applied for automatic identification of duplicates when merging catalogs of aftershocks for the Tohoku earthquake. An analysis of the space-time structure of duplicates and aftershocks shows their significant difference, which makes it possible to successfully solve the problem. In a sample from the global Advanced National Seismic System (ANSS) catalog (M> 4), were found more than 700 events missed by the Japan Meteorological Agency (JMA) seismic network, which is one of the best in the world. Among the misses, there are several events with M> 6 in the first hours after the main shock. Duplicate identification reliability is >97%. The method can be used to improve the completeness of aftershock sequences. The reliable identification of duplicates allows, in addition, to study the correspondence of the magnitudes determined by different agencies. Therefore the present method is an effective tool for creating merged catalogs of earthquakes with a uniform magnitude.

Horizontal and Vertical Deformation Rates Linked to the Magallanes‐Fagnano Fault, Tierra Del Fuego: Reconciling Geological and Geodetic Observations by Modeling the Current Seismic Cycle

AbstractWe integrate geodetic, geological and seismological observations in Tierra del Fuego, into a consistent and quantitative analysis, to better understand the current crustal deformation associated to the Magallanes‐Fagnano Fault, that is, the transform boundary between the South American and Scotia plates at the southern tip of Patagonia. To obtain reliable geodetic estimates of the thickness of the seismogenic layer, we model the current seismic cycle from the great 1949 Mw ≃ 7.7 earthquake to the present, including the lasting effects of postseismic relaxation. The model parameters are constrained by Global Navigation Satellite System velocities obtained by reprocessing 24 years of observations in the island with up‐to‐date models and satellite products. We combine the observed deformation rates with long‐term geological estimates of the slip rate in this transform system during the Holocene. The modeling results point to a seismogenic layer thickness of 15 ± 3 km and to fault planes inclined 63° ± 4°, dipping to the South. Along the sections of the Magallanes‐Fagnano Fault in the island these results are consistent with a seismic moment deficit rate, per unit of length, of 3.2 ± 0.8 × 1012 N m a−1 km−1, and a cumulative seismic moment, to date, equivalent to an earthquake of magnitude Mw ≃ 7. The postseismic viscoelastic relaxation, probably related to viscous flow in the mantle, affects the entire region up to ∼200 km away from the Magallanes‐Fagnano Fault, and more than 60 years after the earthquake.

Velocity changes around the Kaikōura earthquake ruptures from ambient noise cross-correlations

SUMMARYSeismic velocity changes before and after large-magnitude earthquakes carry information about damage present in the surrounding region. This study presents temporal velocity changes detected prior to and following the 2016 November Mw 7.8 Kaikōura earthquake in Canterbury, New Zealand. We use continuous waveform data from 11 short-period seismometers within the Kaikōura region with an average interstation distance of 83 km. Nine-component day-long empirical Green’s functions were computed for frequencies between 0.1 and 0.9 Hz for continuous seismic records from 2012 January 1 to 2018 February 28, which also include the 2013 Cook Strait and Lake Grassmere earthquakes. Using the moving-window cross-spectral method, seismic velocity changes were calculated. Immediately following the 2016 Kaikōura earthquake, a decrease in seismic velocity averaged across all component pairs of approximately 0.2 per cent was observed. An increase in seismic velocity of approximately 0.1 per cent after the earthquake was visible over a 1.5 yr period averaged across all component pairs. A depth sensitivity analysis suggests that observed velocity changes were confined to the uppermost 5 km of the subsurface. We consider strong ground motions a likely candidate for the seismic velocity decrease, followed by post-seismic relaxation via crack healing of the faults that ruptured in the Kaikōura region. Fault-zone damage may also have contributed to observed decreases in the vicinity of ruptured faults.

Postseismic Relaxation Following the 2019 Ridgecrest, California, Earthquake Sequence

ABSTRACT The 2019 Ridgecrest, California, earthquake sequence involved predominantly right-lateral strike slip on a northwest–southeast-trending subvertical fault in the 6 July M 7.1 mainshock, preceded by left-lateral strike slip on a northeast–southwest-trending subvertical fault in the 4 July M 6.4 foreshock. To characterize the postseismic deformation, we assemble displacements measured by Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar. The geodetic measurements illuminate vigorous postseismic deformation for at least 21 months following the earthquake sequence. The postseismic transient deformation is particularly well constrained from survey-mode GPS (sGPS) in the epicentral region carried out during the weeks after the mainshock. We interpret these observations with mechanical models including afterslip and viscoelastic relaxation of the lower crust and mantle asthenosphere. During the first 21 months, up to several centimeters of horizontal motions are measured at continuous GPS and sGPS sites, with amplitude that diminishes slowly with distance from the mainshock rupture, suggestive of deeper afterslip or viscoelastic relaxation. We find that although afterslip involving right-lateral strike slip along the mainshock fault traces and their deeper extensions reach a few decimeters, most postseismic deformation is attributable to viscoelastic relaxation of the lower crust and mantle. Within the Basin and Range crust and mantle, we infer a transient lower crust viscosity several times that of the mantle asthenosphere. The transient mantle asthenosphere viscosity is ∼1.3×1017 Pa s, and the adjacent Central Valley transient mantle asthenosphere viscosity is ∼7×1017 Pa s, about five times higher and consistent with an asymmetry in postseismic horizontal motions across the mainshock surface rupture.

Decaying Post-Seismic Deformation Observed on the Korean Peninsula Following the 2011 Tohoku-Oki Earthquake.

We investigated decaying post-seismic deformation observed on the Korean Peninsula associated with the 2011 Mw 9.0 Tohoku-Oki earthquake using Global Navigation Satellite System (GNSS). The GNSS velocity vectors were estimated in five periods from 2005 to 2019. A co-seismic offset of the Korean Peninsula caused by the 2011 earthquake was inversely proportional to epicentral distances. According to the temporal variations of two components (magnitude and direction) of the GNSS velocity vector with the epicentral distance, the difference between the eastern and western regions for the two components becomes smaller over time. For approximately nine years after the 2011 event, the direction for the crustal movement in South Korea showed a recovery pattern returning to the pre-earthquake motion. In addition, the recovery patterns of the crustal movement were observed differently with the regional geologic structure (e.g., the crustal thickness) and each period. Our estimates of the decay in post-seismic deformation of the Korean Peninsula suggest that post-seismic relaxation will be complete within 5–20 years after the 2011 earthquake. The results suggest that the crustal movement on the Korean Peninsula is gradually recovering to its pre-earthquake motion.

Multiscale Post-Seismic Deformation Based on cGNSS Time Series Following the 2015 Lefkas (W. Greece) Mw6.5 Earthquake

In the present work, a multiscale post-seismic relaxation mechanism, based on the existence of a distribution in relaxation time, is presented. Assuming an Arrhenius dependence of the relaxation time with uniform distributed activation energy in a mesoscopic scale, a generic logarithmic-type relaxation in a macroscopic scale results. The model was applied in the case of the strong 2015 Lefkas Mw6.5 (W. Greece) earthquake, where continuous GNSS (cGNSS) time series were recorded in a station located in the near vicinity of the epicentral area. The application of the present approach to the Lefkas event fits the observed displacements implied by a distribution of relaxation times in the range τmin ≈ 3.5 days to τmax ≈ 350 days.