Tohoku-Oki Earthquake Research Articles

Evaluation of recent tectonic movement in northeast Japan by using long-term GNSS and tide gauge measurements

The study investigates tectonic movements in northeast Japan by using long-term (2000–2022) global navigation satellite system (GNSS) and tide gauge measurements. The effect of the 2011 Tohoku-Oki earthquake including the other eight seismic events that occurred within this period is also discussed using GNSS time-series. The result showed the break in GNSS-time series because of occurred earthquakes and pointed out tectonic movements significantly. The GNSS sites located in the central and southern parts of northeast Japan showed that the velocity vectors have strong internal variation and suggest the existence source of alternative deformation because of geological terranes within the region. A least square approach was used, and the trend of sea-level measurements was fitted with the straight line. The obtained results from tide gauge measurements showed a rising trend at almost every site and indicated lithospheric uprising movement because of tectonic activities. This is possible because of the ongoing subduction of the Pacific and Philippine Sea plates beneath the Eurasian and North American plates.

Anisotropic structure at shallow depths across the Japan Trench

Anisotropic structures within the crust are frequently perceived to originate from stress-induced cracks, which have been mainly estimated on land through different wave speeds of orthogonally polarized S waves propagating in the anisotropic media. However, such estimations of crustal anisotropic structures in ocean areas, particularly for subduction zones around trenches, have not been investigated in detail due to the lack of long-term ocean bottom observations. In this study, we used ocean bottom seismometers of a permanent network deployed across the Japan Trench and the southern part of the Kuril Trench and applied the shear-wave splitting analysis to P-to-s converted waves extracted by receiver function analyses using teleseismic events. We estimated the anisotropic structures in marine sediments and oceanic crust for the incoming Pacific Plate and marine sediments for the overriding North American Plate. The obtained fast polarization directions for the incoming plate are mainly oriented to be parallel to the trench axis for the marine sediment and oceanic crust, which are formed by normal faults and cracks due to the upward plate bending in the outer-rise region, whereas results for marine sediments at the northern part of the Japan Trench are obliquely aligned to the trench axis. The oblique direction is consistent with the magnetic lineations of the incoming plate, indicating that ancient faults within the plate, which were formed in the shallow part of the crust during the creation of the oceanic plate at the ridge, are reactivated by the plate flexure. For the overriding plate, the fast polarization directions in the northern and southern parts of the study area are nearly normal to the trench axis. The central part shows two distinct features: the fast polarization directions parallel to the trench axis and small degrees of anisotropy. These patterns may reflect crack alignments associated with the lateral variation in postseismic crustal deformation after the 2011 Tohoku-Oki earthquake. Our results suggest substantial lateral variations in the stress field at the tip of the overriding plate along the strike direction.Graphical

The Elusive Role of Aseismic Slip Along a Seaward Dipping Normal Fault in the Indirect Triggering of a Normal Faulting Earthquake Sequence in Northeast Japan Following the 2011 Tohoku‐Oki Megathrust

AbstractAlthough blind normal faults are common in subduction environments, their rheology, kinematics and interaction with the upper crust are poorly constrained. A month‐long shallow normal faulting sequence in the Ibaraki‐Fukushima prefectural border (IFPB), northeast Japan, which followed the Mw9.0 Tohoku‐Oki earthquake (TOE) and culminated in the Mw6.7 Iwaki earthquake, provides a window into megathrust‐to‐normal fault interaction. Stress change calculations indicate that direct triggering by the TOE co‐ and post‐seismic slip does not provide a plausible explanation for the IFPB earthquake sequence. In quest for an alternative triggering mechanism, we analyzed post‐TOE GNSS data from eastern IFPB. A key step in this analysis is the removal of the large‐scale post‐TOE displacement field, after which a distinct highly‐localized strain along the coastline becomes apparent. The accumulation of this strain was mostly aseismic, and migrated with time prior to the Iwaki earthquake in a manner that correlates well with post‐TOE local seismicity. We attribute the pre‐Iwaki earthquake strain accumulation to aseismic slip along low‐angle seaward dipping blind normal fault, activated by the TOE. Stresses transferred by this slip episode accelerated the failure along the IFPB shallow normal faults. This indirect triggering of the Iwaki earthquake sequence by the TOE highlights the complexity of stress transfers in subduction environments.

Deep crustal fluids and their relation to cutoff depths of crustal earthquakes in the North Ibaraki area of northeastern Japan inferred from reflected S-waves

Crustal fluids play an essential role in the activity of crustal earthquakes. The north Ibaraki area in northeastern Japan has shown intense crustal seismicity after the 2011 Tohoku-oki earthquake. In this area, it is discussed that crustal fluids are supplied from the deep part of the Earth's crust and contribute to the genesis of these crustal earthquakes. To investigate the distribution of crustal fluids in this area, we focused on reflected S-waves, which are highly sensitive to the presence of crustal fluids. We developed an approach based on the Markov chain Monte Carlo method to precisely and quantitatively estimate the location of the crustal reflector and its geometry. The obtained results showed that the crustal reflector was located at depths of 15–25 km and dipped shallowly to the northwest. The crustal reflector was positioned above the region characterized by low seismic wave velocity and electrical resistivity anomalies in the lower crust, suggesting that the crustal reflector is the uppermost boundary of a fluid-rich zone. The distribution of the fluid-rich zone closely corresponded to the cutoff depths of crustal earthquakes. This fluid-rich zone was likely the source of the fluid that enhanced seismicity in the shallow part of the crust in the target area. In contrast, the fluid-rich zone itself may have suppressed the genesis of crustal earthquakes. We hypothesized that hydrothermal fluids might affect the shallowing of these cutoff depths. If the hydrothermal fluid was contained in the fluid-rich zone, it could induce a shallow brittle-ductile transition by increasing the temperature of the surrounding rocks.

Detectability of low-viscosity zone along lithosphere–asthenosphere boundary beneath the Nankai Trough, Japan, based on high-fidelity viscoelastic simulation

After the 2011 Tohoku-oki earthquake, a seafloor observation system observed rapid landward deformation. These seafloor observation data are potentially explicable via modeling of a low-viscosity zone under the subducting plate, called the lithosphere–asthenosphere boundary (LAB). However, the effect of a low-viscosity LAB has not been empirically examined in past earthquakes because of the absence of seafloor observation data, which are expected to have high sensitivity in the detection of a low-viscosity LAB. The Nankai Trough, southwest Japan, is one location where seafloor geodetic stations are in operation that could detect a low-viscosity LAB after a future earthquake. Therefore, we quantitatively model how the low-viscosity of the LAB is expected to affect postseismic crustal deformation following an anticipated future large earthquake under the Nankai Trough. Calculating viscoelastic deformation using a model that takes into account the realistic crustal structure in the southwestern Japanese Archipelago, we examine the effects of LAB viscosity on surface deformation patterns. The results show for very low LAB viscosities (2.5×1017\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$2.5 \ imes 10^{17}$$\\end{document} Pa s), rapid landward displacement as large as 38 cm/yr occurs in the offshore area, with uplift and subsidence patterns highly dependent on the viscosity structure. Especially in the first year after the earthquake, the difference in deformation between the cases with and without a low-viscosity LAB was much larger than the observational error level of the current seafloor observation system. For such low LAB viscosities, the probability of detection is therefore high. On the other hand, for moderately low LAB viscosities (2.5×1018\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$2.5 \ imes 10^{18}$$\\end{document} Pa s), the deformation patterns in the cases with and without a low-viscosity LAB are similar. Indeed, the difference in displacement lies within the observational error level of the current observational network, so detection is expected to be difficult for a LAB of only moderately low-viscosity. However, such a LAB may be detected by improving observational accuracy in the Global Navigation Satellite System–acoustic observation technique by performing more frequent measurements. Additional detection potential lies in expected future accuracy improvements in vertical displacement estimates at the DONET stations.Graphical

Volcanic arc rigidity variations illuminated by coseismic deformation of the 2011 Tohoku-oki M9.

Rock strength has long been linked to lithospheric deformation and seismicity. However, independent constraints on the related elastic heterogeneity are missing, yet could provide key information for solid Earth dynamics. Using coseismic Global Navigation Satellite Systems (GNSS) data for the 2011 M9 Tohoku-oki earthquake in Japan, we apply an inverse method to infer elastic structure and fault slip simultaneously. We find compliant material beneath the volcanic arc and in the mantle wedge within the partial melt generation zone inferred to lie above ~100 km slab depth. We also identify low-rigidity material closer to the trench matching seismicity patterns, likely associated with accretionary wedge structure. Along with traditional seismic and electromagnetic methods, our approach opens up avenues for multiphysics inversions. Those have the potential to advance earthquake and volcano science, and in particular once expanded to InSAR type constraints, may lead to a better understanding of transient lithospheric deformation across scales.

Detection of atmospheric radon concentration anomalies and their potential for earthquake prediction using Random Forest analysis

Various anomalies occurring before earthquakes are currently being studied to predict seismic events, with one of them being the radioactive element radon (222Rn). Radon concentrations in the soil, water, and atmosphere fluctuate in response to crustal movement. Recent research has statistically detected anomalies by analyzing the fluctuations in radon concentrations before earthquakes and conducting quantitative evaluations of radon. However, the method used to determine the parameters in the analysis was problematic. Therefore, in this study, we compared observed atmospheric radon concentration data with predicted values based on typical annual patterns using Random Forest analysis. We conducted a more objective analysis by employing this method and statistically determining anomalies using thresholds. This analysis was conducted using atmospheric radon concentration observation data obtained at Kobe Pharmaceutical University (KPU) before the 1995 Kobe Earthquake, and ionization currents emitted when radon decays were obtained at Fukushima Medical University (FMU) before the 2011 Tohoku-oki Earthquake. Consequently, before the major earthquakes occurred at both locations, the difference between the predicted and observed values exceeded the standard deviation by a factor of three. These results indicate the potential of Random Forest analysis to identify anomalies in atmospheric radon concentrations before earthquakes occur.

Dynamic Triggering of Earthquakes in Northeast Japan before and after the 2011 M 9.0 Tohoku-Oki Earthquake

ABSTRACT Previous studies documented a relative scarcity of remote dynamic triggering of earthquakes in Japan and suggested that it could be related to Japan’s predominantly compressive tectonic regime or the more frequent occurrence of large earthquakes in Japan. For example, remote triggering in California, characterized by extensional tectonics, occurs at levels of stress change as small as 0.1 kPa, whereas in Japan, transient stresses ≥30 kPa are required. However, the dynamic triggering threshold in Japan, following the 2016 Mw 7.0 Kumamoto earthquake, has been found to be of just a few kilopascals, significantly smaller than reported previously. It was proposed that a decrease in the triggering threshold may have taken place in Japan, in particular at volcanic and geothermal areas, after the 2011 Mw 9.0 Tohoku-Oki earthquake. In this study, we analyze the possible change in dynamic triggering conditions in five areas in northeast Japan, where swarm earthquakes have occurred immediately after the Tohoku-Oki earthquake. The triggering thresholds in these five areas have been estimated based on the analysis of waveform recordings of 49 teleseismic earthquakes that occurred between 2004 and 2020. A decrease of the triggering threshold (or triggering ability) is apparent in all but one region. However, a statistical significance Kolmogorov–Smirnov test could not reject, at a 5% level, the null hypothesis stating that “the distribution of dynamic stress changes for triggering earthquakes that occurred before and after the Tohoku-Oki event is the same.” We interpret the changes in the triggering threshold to be related to the pore pressure increase (and thus a fault strength decrease) in the crust following the Tohoku-Oki earthquake. Our results also indicate that dynamic triggering in Japan is more common than reported previously.

Stress field in northeastern Japan and its relationship with faults of recent earthquakes

Inversion tectonics, in which old normal faults act as reverse faults in current stress fields, are frequently observed in northeastern Japan (Tohoku District); however, the conditions that control these fault activities remain unclear. To improve the identification of faults that are more favorable to slip under current stress conditions, the regional fault mechanisms in the Tohoku District must be better understood. The stress field in the Tohoku District and the likelihood of fault activities were thus estimated using slip tendency (ST) analysis. The results show that in the eastern margin of the Japan Sea (EMJS), the reverse fault type is dominant in the stress field. The maximum horizontal direction changes clockwise from E–W to NW–SE from the northern to the southern regions and counterclockwise from NW–SE to E–W from the Japan Sea to the inland area. In the Tohoku inland area, the estimated direction of the maximum horizontal axis changed after the 2011 Tohoku-Oki earthquake, from E–W to WNW–ESE. ST values were calculated for seven events in the EMJS area. To avoid the influence of the Tohoku-Oki earthquake, only stress field data prior to the 2011 Tohoku-Oki earthquake were used to determine ST values for four of the events in the Tohoku inland area. The results showed eastward-dipping fault planes with low dip angles (approximately 30°–45°) and large ST values (approximately > 0.7). The large ST values indicate that the stress field fault is favorable to slip and the results were consistent with the actual fault plane in the EMJS area. However, in the Tohoku inland area and the southern part of fault model of the 1993 Hokkaido Nansei-Oki earthquake, fault planes with large ST values were found to be inconsistent with the slipped fault plane, thus indicating that slipping was unfavorable. The regional differences are consistent with the volcano distribution and thus, the fluid supply from volcanic activity may have helped the fault slip under difficult stress conditions.Graphical

A weak subducting slab at intermediate depths below northeast Japan.

Knowledge of the state of stress in subducting slabs is essential for understanding their mechanical behavior and the physical processes that generate earthquakes. Here, we develop a framework which uses a high-resolution focal mechanism catalog to determine the change in the position of the neutral plane before and after the M9 Tohoku-oki earthquake to determine that the deviatoric stress within the slab at intermediate depths must be very low (∼1 MPa). We show that by combining the static stress calculated from coseismic slip distributions with the stress orientations before and after the mainshock, we can determine the full deviatoric stress tensor within the subducting slab at intermediate depths. These results preclude earthquake source mechanisms that require large background driving stresses, favoring a mechanically weak subducting slab, thus providing quantitative constraints on the physical processes that generate intermediate-depth earthquakes.

Mesh size effect on finite source inversion with 3-D finite-element modelling

SUMMARY Three-dimensional finite-element models, which can handle the stress perturbations caused by subsurface mechanical heterogeneities and fault interactions, have been combined with the finite source inversion to estimate the coseismic slip distribution over the fault plane. However, the mesh grid for discretizing the governing equations in the finite-element model significantly affects the numerical accuracy. In this study, we performed kinematic finite source inversion with idealized (regular observation point array; M1A–M1D) and regional (GEONET, GPS Earthquake Observation Network System stations in Japan; M2A–M2H) models with different mesh sizes to quantitatively analyse the effect of the mesh grid size around the fault plane on the inverted fault slip distribution. Synthetic observation data vectors obtained from the finest models (M1A and M2A) are compared with those from the coarser models (M1B–M1D and M2B–M2H), which were adopted to construct Green's function matrix. We found that the coarser mesh models derived a smaller surface displacement, leading to a decrease in the norm of Green's function matrix, which in turn influences the fault slip magnitude from the finite source inversion. Finally, we performed the source inversion for the fault slip distribution of the 2011 Mw 9.0 Tohoku–Oki earthquake using the coseismic surface displacements recorded at the GEONET and seafloor stations and finite-element modelling. By reducing the mesh size on the fault, we confirmed that the estimated magnitude of fault slip converged to approximately 80 m, which is consistent with the range of fault slip amounts from previous studies based on the Okada model. At least 0.88 million total domain elements and a 6.7 km2 mesh size on the fault plane with an area of 240 × 720 km2 are required for the convergence of the fault slip. Furthermore, we found that the location of the maximum fault slip is less sensitive to the mesh size, implying that source inversion based on a coarse mesh model (i.e. less than 0.5 million elements and > ∼60 km2 mesh size) can quickly provide the rough fault slip distribution.

Change of deep subduction seismicity after a large megathrust earthquake

Subduction zones are home to the world’s largest and deepest earthquakes. Recently, large-scale interactions between shallow (0-60 km) and intermediate (80-150 km) seismicity have been evidenced during the interseismic period but also before and after megathrust earthquakes along with large-scale changes in surface motion. Large-scale deformation transients following major earthquakes have also been observed possibly due to a post-seismic change in slab pull or to a bending/unbending of the plates, which suggests the existence of interactions between the deep and shallow parts of the slab. In this study, we analyze the spatio-temporal variations of the declustered seismicity in Japan from 2000 to 2011/3/11 and from 2011/3/11 to 2013/3/11. We observe that the background rate of the intermediate to deep (150-450 km) seismicity underwent a deceleration of 55% south of the rupture zone and an acceleration of 30% north of it after the Tohoku-oki earthquake, consistent with the GPS surface displacements. This shows how a megathrust earthquake can affect the stress state of the slab over a 2500 km lateral range and a large depth range, demonstrating that earthquakes interact at a much greater scale than the surrounding rupture zone usually considered.

Detection of ULF Geomagnetic Anomalies Prior to the Tohoku-Oki Earthquake by the Multi-reference Station Method

Detection of ULF Geomagnetic Anomalies Prior to the Tohoku-Oki Earthquake by the Multi-reference Station Method

The submarine fault scarp of the 2011 Tohoku-oki Earthquake in the Japan Trench

Rupture of the plate boundary fault during the 2011 Tohoku-oki earthquake with moment magnitude 9.0 is thought to have propagated to the trench. Surface exposure of the fault has not yet been confirmed because of great depths that are challenging to access and study. Using a manned submersible in the Japan Trench, we explored and visually assessed the seafloor near the epicenter. On the eastern slope of a thrust ridge 59 m high, which appeared after the earthquake, we found a 26 m high subvertical cliff regarded as the fault scarp. Cross-section analysis suggests an 80–120 m slip on the fault when assumed to dip at 45–30°, to build up the observed relief. The estimated larger displacement in the trench than in more proximal parts can be attributed to local enhancement of the slip by extension of the wedge above a subducting graben on the Pacific plate.

Estimating geodynamic model parameters from geodetic observations using a particle method

SUMMARY Bayesian-based data assimilation methods integrate observational data into geophysical forward models to obtain the temporal evolution of an improved state vector, including its uncertainties. We explore the potential of a variant, a particle method, to estimate mechanical parameters of the overriding plate during the interseismic period. Here we assimilate vertical surface displacements into an elementary flexural model to estimate the elastic thickness of the overriding plate, and the locations and magnitudes of line loads acting on the overriding plate to produce flexure. Assimilation of synthetic observations sampled from a different forward model than is used in the particle method, reveal that synthetic seafloor data within 150 km from the trench are required to properly constrain parameters for long wavelength solutions of the upper plate (i.e. wavelength ∼500 km). Assimilation of synthetic observations sampled from the same flexural model used in the particle method shows remarkable convergence towards the true parameters with synthetic on-land data only for short to intermediate wavelength solutions (i.e. wavelengths between ∼100 and 300 km). In real-data assimilation experiments we assign representation errors due to discrepancies between our incorrect or incomplete physical model and the data. When assimilating continental data prior to the 2011 Mw Tohoku-Oki earthquake (1997–2000), an unrealistically low effective elastic plate thickness for Tohoku of ∼5–7 km is estimated. Our synthetic experiments suggest that improvements to the physical forward model, such as the inclusion of a slab, a megathrust interface and viscoelasticity of the mantle, including accurate seafloor data, and additional geodetic observations, may refine our estimates of the effective elastic plate thickness. Overall, we demonstrate the potential of using the particle method to constrain geodynamic parameters by providing constraints on parameters and corresponding uncertainty values. Using the particle method, we provide insights into the data network sensitivity and identify parameter trade-offs.

Acceleration spikes in strong earthquake ground motion induced by dilatancy cyclic mobility mechanism

Strong motion records with acceleration spikes have occurred frequently in recent major earthquakes, acceleration spikes contribute to the peak ground accelerations (PGAs) and the amplitudes are significantly higher than the predicted values based on present attenuation relation. The generation mechanism of acceleration spikes is revealed based on long-term monitoring after the 2008 Mw 8.0 Wenchuan earthquake. The horizontal accelerogram with obvious spikes from the Mw 9.0 Tohoku-Oki earthquake is reproduced by using numerical simulation, and it’s found that the dilatancy cyclic mobility (DCM) mechanism can explain the generation of acceleration spikes. The support vector machine (SVM) is used to efficiently identify DCM sites. We found that the spatial distribution of DCM sites is parallel to the earthquake source fault and the corresponding PGAs of ground motions recorded in DCM sites differ significantly from those recorded in non-dilatancy cyclic mobility (NDCM) sites.

Wedge plasticity and a minimalist dynamic rupture model for the 2011 MW 9.1 Tohoku-Oki earthquake and tsunami

One crucial yet unanswered question about the 2011 MW 9.1 Tohoku-Oki earthquake and tsunami is what generated the largest tsunami (up to 40 m) along the Sanriku coast north of 39°N without large slip near the trench. A minimalist dynamic rupture model with wedge plasticity is presented to address this issue. The model incorporates the important variation of sediment thickness along the Japan Trench into the Japan Integrated Velocity Structure Model (JIVSM). By revising a heterogeneous stress drop model, the dynamic rupture model with a standard rate-and-state friction law can explain the GPS, tsunami, and differential bathymetry data (within data uncertainties) with minimum tuning. The rupture is driven by a large patch of stress drop up to ∼10 MPa near the hypocenter with significantly smaller stress drop (< 3 MPa) in the upper ∼10 km. The largest shallow slip reaches 75.67 m close to the trench north of hypocenter, which is caused by the large fault width, free surface, shallowly dipping fault geometry, and northwardly increasing sediment thickness, dominated by elastic off-fault response. North of large shallow slip zone, however, inelastic deformation of thick wedge sediments significantly controls the rupture propagation along trench, giving rise to slow rupture velocity (∼850 m/s), diminishing shallow slip, and efficient seafloor uplift. The short-wavelength inelastic uplift produces impulsive tsunami consistent with the observations off the Sanriku coast in terms of timing, amplitude, and pulse width. Wedge plasticity and variation of sediment thickness along the Japan Trench thus provides a self-consistent interpretation to the along-strike variation of near-trench slip and anomalous tsunami generation in the northern Japan Trench in this earthquake.

Early Source Characterization of Large Earthquakes Using W Phase and Prompt Elastogravity Signals

Abstract In the minutes following a large earthquake, robust characterization of the seismic rupture can be obtained from full wavefield records at local distances or from early signals recorded by regional broadband seismometers. We focus here on the latter configuration, and evaluate the individual and joint performances of the early low-frequency elastic phases (W phase) and the recently discovered prompt elastogravity signals (PEGS). The 2011 Mw 9.1 Tohoku–Oki earthquake is a natural target for this evaluation, because the high quality of global and regional networks enabled to gather the best PEGS data set so far. We first confirm that the well-established W-phase method, using records from global seismological networks, is able to provide a reliable centroid moment tensor solution 22 min after the earthquake origin time. Using regional stations, an accurate W-phase solution can be obtained more rapidly, down to 10 min after origin time. On the other hand, a PEGS-based source inversion can provide even earlier, starting 3 min after origin time, a lower bound of the seismic moment (Mw 8.6) and constraints on the focal mechanism type. However, relying solely on PEGS introduces uncertainties caused by the hindering seismic noise and trade-offs between source parameters that limit the accuracy of source determination. We show that incorporating even a few early W phase signals to the PEGS data set reduces these uncertainties. Using more complete W phase and PEGS data sets available 5 min after origin time enables to converge towards a result close to the Global Centroid Moment Tensor solution.

Post-seismic gravity change modelling based on non-linear power-law upper mantle rheology

SUMMARY Post-seismic gravity change modelling is commonly based on earth model with Newtonian linear rheology. Here, we present a novel way of modelling post-seismic gravity change by using a non-linear power-law rheology earth model. The method is constructed based on the framework of spectral finite element method (SFEM). SFEM has been proven practical for the purpose of modelling gravity change occurring during megathrust earthquakes. Our method implements the strain rate expression of non-linear power-law rheology into the mathematical framework of SFEM. Using our method, simulations of geoid change caused by synthetic point source earthquakes were made. The results revealed the potential of using non-linear power-law rheology for the explanation of rapid gravity changes in the beginning of the post-seismic epoch. Further on, we computed the post-seismic gravity change of the 2011 M9.0 Tohoku-Oki earthquake based on Maxwell non-linear power-law rheology in the upper mantle. It demonstrated the potential of power-law upper mantle flow as a possible candidate for the explanation of post-seismic gravity change after the earthquake occurs. However, a Maxwell non-linear power-law rheology alone is insufficient for the explanation of the post-seismic gravity change occurred.

Sensitivity of slip distribution on tsunami trace heights and geological evidences: a case study of the 2011 Tohoku-Oki earthquake

We examined whether it is possible to estimate the tsunami source model of the 2011 Tohoku-Oki earthquake from a comparison of numerical simulations of tsunami propagation and sediment transport, the measured trace heights, and the sediment thickness of tsunami deposits. Twelve models with different subfault numbers were prepared based on a reference model inferred from tsunami waveform inversion. The reference model with 55 subfaults considering rupture propagation and the model with instantaneous slip successfully reproduced both the tsunami trace heights and sediment thickness distribution of tsunami deposits in the Idagawa Lowland and Sendai Plain. Other models with the same moment magnitude but fewer subfaults could not reproduce the observed trace heights, and the reproducibility of sediment thickness distribution strongly depended on the slip distribution. Models with increased slip amounts and moment magnitude could reproduce the trace heights; however, the simulated sediment thickness was underestimated for the Idagawa Lowland while overestimated for the Sendai Plain. Our results indicate that the combination of trace heights, sediment thickness of tsunami deposits, and numerical simulations of tsunami propagation and sediment transport can be used to estimate historical earthquakes and tsunamis. Efforts should be made to increase the number of subfaults for the study of historical events, although the obtained solutions may not be unique because of fewer trace heights or tsunami deposit data.