Interplate Earthquakes Research Articles

Not trench-parallel but trench-normal source fault of the 1994 Hokkaido Toho-oki earthquake as revealed by the aftershock relocation using HypoDD

In the southwestern area of the Kurile Islands subduction zone, three large earthquakes including the Mw8.2 1969 and the Mw8.3 1994 Hokkaido Toho-oki earthquakes and the Mw7.8 1975 tsunami earthquake occurred in almost the same location. The three main shocks and their aftershocks were re-determined simultaneously using a double-difference earthquake location method, HypoDD. As a result, the 1969 and the 1975 main shocks and their aftershocks were concentrated near the upper surface of the subducting Pacific plate, indicating that both events were plate boundary megathrust earthquakes as shown by previous studies. On the other hand, the 1994 main shock and its aftershocks were distributed within the Pacific plate indicating that the 1994 event was an intraslab event as reported by previous studies. The result that interplate earthquakes and intraslab earthquakes could be appropriately distinguished is strong evidence that the accuracy of hypocenter determination is sufficiently high. The centroid moment tensor solution of the 1994 main shock has two nodal planes which are parallel or normal to the Kurile Trench. According to the high-resolution hypocenter distribution obtained in the present study, the aftershocks appear to locate on the nodal plane normal to the trench, suggesting that the source fault ruptured by the 1994 main shock was not parallel but normal to the Kurile Trench. Moreover, we found that the Coulomb failure stress change (ΔCFS) produced by the trench-normal fault plane is more likely to trigger the largest aftershock than ΔCFS produced by the trench-parallel fault plane.Graphical abstract

A methodology for appropriate withdrawal of tsunami warnings based on numerical simulations

This study performed large-scale numerical simulations for predicting the attenuation of tsunamis caused by the 2011 Tohoku, Japan, and 2010 Maule, Chile earthquakes, recorded at Japanese tide gauges. Tsunami amplitude waveforms were generated by computing the moving root-mean-square of the data for quantitative analyses. Sensitivity analysis showed that tsunami nonlinearity and computational grid intervals near the tide gauges significantly impact the prediction of tsunami attenuation. The predicted withdrawal times of tsunami warnings agreed with the observations; however, time discrepancies were observed for advisory withdrawals at several stations. Using the proposed method, we predicted the warning period of a great interplate earthquake in the Nankai Trough to be approximately one day. These findings can provide critical information for disaster prevention because the withdrawal of warnings is directly related to permission to enter coastal areas affected by the tsunami, whereas unnecessarily long warnings hinder rescue operations.

Quantifying the expansion rates of aftershock zones for magnitude-7 class earthquakes around the Japanese archipelago

Earthquakes (mainshocks) trigger sequences of aftershocks, the frequency of which diminishes following a power-law decay, while the spatial domain of these aftershocks extends logarithmically over time. The delineation of the aftershock zone can be modulated by variables beyond the magnitude of the mainshock, encompassing the location of the fault (whether the fault is at a plate boundary), the depth at which the event occurs, and the prevailing local stress conditions. Here, we evaluate the expansion rate of aftershock zones by analyzing earthquakes of magnitude-7 class in the vicinity of the Japanese archipelago. Prior studies have offered approximate assessments of expansion rates; however, our approach involves the utilization of a straightforward algorithm for the automated estimation of this metric, facilitating the compilation of a catalog. Across the dataset, no pronounced correlations were discerned between the expansion rate and other examined parameters. Yet, an inverse relationship is identified between the expansion rate and the b value of aftershocks for mainshocks occurring at plate boundaries. This observation suggests that the expansion rate of aftershock zones predominantly mirrors the stress field following the mainshock. Such a pattern is not detected in mainshocks occurring within the plate's interior. While the expansion rate of aftershock zones is likely influenced by various factors, aftershock zones may expand more rapidly with higher differential stress in areas surrounding hypocenters of major interplate earthquakes of magnitude 8 or 9.

Prediction of Fourier Amplitude Spectrum of Ground Motion in Mexico City from Subduction Thrust Earthquakes

Mexico City, one of the largest cities in the world, experiences frequent, devastating interplate earthquakes that originate on the subduction thrust along the Pacific coast of Mexico more than 250 km away. A notorious example is the 19 September 1985 Michoacán earthquake (Mw 8.0) which killed ~ 10,000 persons and caused wide-spread destruction in the city. The main cause of seismic damage in Mexico City is its subsoil that amplifies the ground motion. In view of the seismic hazard faced by the city, a reliable estimation of ground motion during future earthquakes is of vital importance. We present a new ground motion prediction equation (GMPE) for the Fourier amplitude acceleration spectrum (FAS) from subduction thrust earthquakes along the Pacific coast of Mexico at the reference station, CU, located in the firm zone of Mexico City. The GMPE is derived via Bayesian regression analysis and is based on an enlarged set of recordings (1965-2020; 40 earthquakes; 250 ≤ R≤ 500 km; 5 ≤ Mw ≤ 8.0). An important feature of the new GMPE is that it includes a term to model different attenuation along different ray paths. The inclusion of this term leads to a reduction in the aleatory variability of the GMPE, particularly at high frequencies. Since spectral amplification of seismic waves with respect to CU is known at many sites in the city, the FAS at these sites can be computed if it is known at CU. The new GMPE has been incorporated in a fully Fourier-based probabilistic seismic hazard analysis of Mexico City.

Study of the rupture processes of the 1989 (Mw 6.9) and 2021 (Mw 7.0) Guerrero earthquakes using teleseismic records: Sismotectonic implications

Slip models of the April 25, 1989 (Mw 6.9) earthquake and the Acapulco earthquake of September 8, 2021 (Mw 7.0) were determined to analyze its rupture zone and understand the relationship of interplate earthquakes in the Acapulco–San Marcos subduction zone. For both events, velocity P and SH waves recorded at teleseismic distances were modeled. For the 1989 San Marcos event, two slip zones were determined with a maximum slip of 121cm, a stress drop of 8bars and a rupture duration of approximately 13s. This event showed a complex rupture. For the 2021 Acapulco event, a zone of maximum slip of 267cm, a stress drop of 5bars, and a rupture duration of approximately 24s were determined. Analysis of rupture zones suggests that complex events occur in the Acapulco–San Marcos subduction zone. Therefore, these results have important implications in seismic potential studies since the source parameters are critical.

Mechanically Coupled Areas on the Plate Interface in the Kanto Region, Central Japan, Generating Great Earthquakes and Slow-Slip Events

ABSTRACT We detected the mechanically coupled areas, or high stress rate patches, on the plate interface in the Kanto region, central Japan, by analyzing the Global Navigation Satellite Systems data. The estimated patches correspond well with the focal areas of past great earthquakes and slow-slip events (Mw∼6.5) occurring every ∼5 yr. Using one of the estimated patches, we created a model of a slow-slip event as a stress release with a recurrence interval of 5 yr. This synthetic can reproduce observed features of the slow-slip events such as the slip distribution and the magnitude. We use the strain-energy magnitude Mw0 defined by the minimum strain-energy release to quantify the magnitude. This is useful to compare slow-slip events with ordinary earthquakes in terms of the strain energy release, whereas the moment magnitude does not represent the difference of the energy release in this case. The strain-energy magnitude of the slow-slip event was Mw0 4.9, which was considerably smaller than the moment magnitude, because the smaller stress drop of the slow-slip event results in a smaller strain-energy release. Furthermore, by assuming that stress has accumulated at the other patches corresponding to the source region of past earthquakes since the occurrence of the last earthquakes, we obtain a model of the stress accumulation in 2023. We then create various rupture scenarios of great earthquakes as combinations of ruptures of the different patches. When two or three of the patches release the accumulated stress simultaneously, an interplate earthquake with Mw≥7.8 can occur in the Kanto region.

Estimation of the Hurst exponents of irregularly sampled subsurface fault geometries by the lifting scheme

SUMMARY Seismic fault surfaces have complex geometries over a broad-scale range. The Hurst exponent (H) is an important parameter characterizing the complexity of fault geometries. This exponent of seismic faults has only been estimated at outcrops and from surface traces of large earthquakes. This is because geometry data of subsurface faults usually have large uncertainties and many gaps. This study examined the applicability of the lifting scheme, which is one of the wavelet transform methods, to estimate H of irregularly sampled geometry of subsurface faults. We analysed the surface geometry of the subducting oceanic plate at the Nankai trough, Japan, which is part of the fault plane of interplate earthquakes. The geometries of the subducting plate were estimated along six survey lines by integrating seismic refraction and reflection surveys. Two-way traveltimes of reflected waves from the plate surface, which were measured from the reflection survey data, were converted to depth using the velocity structure estimated by refraction survey. The intervals between sampled points were irregular because the reflected waves were obscured or invisible in some places. The Hurst exponents were estimated from the scale dependence of the wavelet coefficients that were derived by the lifting scheme without interpolation. Analyses of the synthetic data simulating the irregularly sampled plate geometries indicated that the lifting scheme yields stable but largely biased estimates of H, especially for small H (<0.5). We introduced the empirical bias correction to achieve an unbiased estimation of the exponent. The analysis of plate geometry at the Nankai trough was conducted at narrow-scale ranges with consideration for the accuracy of velocity structures. We may conclude that H of five survey lines across the trough axis is > 0.8, and that of a line along trough is > 0.7. These estimated exponents had large uncertainties due to analyses at narrow-scale ranges, but were close to the estimates in studies of the surface traces of large earthquakes.

Recurrence intervals for M > 7 Miyagi-ken-Oki earthquakes during an M ~ 9 earthquake cycle

The 2011 Tohoku-Oki great earthquake increased the difficulty of evaluating the long-term probability of seismic activity along the Japan Trench because of the unknown impact of the unprecedentedly large slip. In this study, the long-term activity of “Miyagi-ken-Oki earthquakes”, an M > 7 earthquake sequence off Miyagi Prefecture, located at the edge of the source area of the Tohoku-Oki earthquake was simulated. We conducted numerical simulations of earthquake generation cycles based on the rate- and state-dependent friction law representing the stress accumulation and release process on the plate interface. We also validated the results based on analyses of observational data, including time intervals and afterslip distributions for the previous Miyagi-ken-Oki earthquakes. The simulation results were then compared with repeating-earthquake-derived interplate slip observations over 30 years. The results showed that the spatial and temporal characteristics of aseismic slips on the plate interface near the M > 7 Miyagi-ken-Oki source were qualitatively reproduced. The time interval between the M ~ 9 earthquake and the first M > 7 earthquake is shorter than the average recurrence interval of M > 7 earthquakes during the latter stage of each M ~ 9 earthquake cycle. In contrast, the interval between the first and the second M > 7 earthquakes is the longest in each M ~ 9 earthquake cycle. The time intervals of the M > 7 earthquakes fluctuated in the early stage compared to those in the latter stage of the M ~ 9 earthquake cycle. These characteristics were associated with the extent of the locked and afterslip areas in and around the source. Hence, monitoring the spatio-temporal distribution of the slip rate in and around the source area during the preparation process of earthquakes occurring in the downdip area off Miyagi Prefecture is very important to assess whether the next M > 7 earthquake is approaching. Furthermore, earthquake cycle simulations combined with earthquake and slow slip monitoring can provide meaningful insights for long-term assessments of great interplate earthquakes.

Geologic and Geophysical Evidence of Repeated Normal Faulting on the Itozawa Fault—the Source of the Mw 6.6 Iwaki Earthquake Triggered by the Mw 9.0 Tohoku-Oki Megathrust Earthquake, Northeast Japan

ABSTRACT The 2011 Mw 9.0 Tohoku-Oki earthquake ruptured the 500 km long and 200 km wide convergent plate margin between the North American and Pacific plates, and changed the crustal stress field and triggered widespread seismic activity in northeast Japan. In particular, many crustal earthquakes struck the southern Fukushima area. The largest inland normal-faulting earthquake with Mw 6.6 occurred on the northwest-trending Yunodake fault and the north-northwest-trending Itozawa fault on 11 April 2011. The coseismic surface ruptures appeared along the previously identified active and presumed active faults. To investigate the near-surface structure of the Itozawa fault, we conducted ground-penetrating radar (GPR) profiling across the fault, and two drilling surveys on its hanging-wall and footwall blocks. The GPR survey line about 50 m long was located near the previous trenching site. The GPR two-way travel-time sections were collected by common-offset modes by 50 and 100 MHz GPR systems. We acquired common midpoint ensembles on both sides of the surface rupture for depth conversion of the time sections. The resulting GPR sections show detailed subsurface structures to a depth of about 7 m. We identified event horizons for the 2011, penultimate, antepenultimate, and preantepenultimate earthquakes, indicating that the Itozawa fault has ruptured repeatedly as a normal fault. The vertical displacements during these earthquake events, obtained from the GPR sections, were about 0.8, 0.3, 0.4, and 0.7 m, respectively. These observations suggest that the coseismic surface offset amount on the Itozawa fault varies greatly. The recurrence intervals of surface-rupturing earthquakes on the Itozawa fault, calculated from the previous research and this study, are much longer than those of giant interplate earthquakes along the Japan trench, such as the 2011 Tohoku-Oki earthquake. We conclude that not every giant earthquake along the Japan trench triggers a rupture of the Itozawa fault.

Earthquake Damage Assessment of Buried Pipeline Networks in the Lima Metropolitan Area

Lifelines such as drinking water and sewage systems provide the means and conveyance for daily critical services, and they are essential systems for recovery operations after a damaging earthquake. Therefore, earthquake damage to lifeline components needs to be reliably assessed for a possible future seismic scenario. This study presents an earthquake damage assessment of buried pipeline networks in the Lima Metropolitan Area (LMA). It includes seismic hazard analysis, a review of pipeline network datasets, and the selection of empirical fragility functions. Deterministic seismic hazard analysis was performed for an inter-plate earthquake scenario using ground motion prediction equations and site conditions to compute the distribution of the peak ground velocity (PGV). Recommendations are offered for an adequate selection of fragility functions developed in other regions, and a logic tree of fragility functions is proposed to be used in pipelines of LMA according to the data of pipeline damage after the 2007 Pisco Earthquake. Finally, the pipeline repair ratios and the total number of repairs are estimated for the earthquake scenario, and the results are geographically presented for each pipeline network.

Subduction hydrothermal regime and seismotectonic variation along Kermadec–Tonga megathrusts

The calamitous phreatomagmatic eruption of the Hunga Tonga–Hunga Ha'apai (HTHH) volcano in January 2022 has been reported to have had a volcanic explosivity index (VEI) of 5-6 and a column height of 20 km and to have caused powerful tsunamis across the Pacific Ocean. Along the subvolcanic heterogeneous and tightly coupled megathrusts, the hydrothermal structure is considered essential to the deep arc volcanism and catalysis of focused interplate earthquakes. However, at present, such heterogeneity in slab hydrothermal regimes and the linkage of slab metamorphism to seismotectonics and magmatism remain poorly understood. Here, based on the recurrence of subduction earthquakes and the obtained surface heat flow estimates as constraints, we constructed a 3D thermodynamic model to study this subduction complex and the seismotectonic variation beneath Kermadec–Tonga. The subduction of the Pacific plate beneath the Kermadec–Tonga microplate is associated with a cold thermal transition from 300 °C to >900 °C on forearc interface. The distribution of seismicity is related to the dehydration of subducted water-bearing minerals which promotes the occurrence of both fast and slow subduction earthquakes. Slab metamorphism released large amounts of fluids, especially those from intraslab harzburgitization, which are key to influencing mantle melting and arc volcanism in Kermadec-Tonga.

Unveiling the Geological Divide: Contrasts Between Intraplate and Plate Boundary Earthquakes.

This review compares the distinctions between interplate earthquakes, primarily situated at plate boundaries, and intraplate earthquakes occurring within the interior of tectonic plates. While plate tectonics effectively explains the mechanisms of interplate seismicity, intraplate earthquakes, notably observed in northeast Asia, pose a more intricate puzzle with no overarching model. This study tackles the fundamental question of whether these two seismic categories exhibit similarities, exploring their respective origins and variations. Despite the challenges associated with understanding intraplate earthquakes due to their infrequency compared to plate-boundary seismic events, this comparative analysis seeks to provide insights into the shared and unique aspects of these significant geological occurrences.

The Mw 6.0–6.8 Quasi‐Repeating Earthquakes Off Miyagi, Japan, With Variable Moment Release Patterns Due To a Hidden Adjacent Slip Patch

AbstractAfter the 2011 M9 Tohoku earthquake, several interplate earthquakes of Mw 6 occurred in close proximity in the area off the northern Miyagi prefecture in Japan, including the 2015 Mw 6.8 earthquake. To advance our understanding of the similarity and diversity in earthquake ruptures at the same asperity, this study compares the source processes of six Mw 6.0–6.8 earthquakes in this region. The results of hypocenter relocation and source process inversion show that these Mw 6.0–6.8 events repeatedly ruptured the same patch west of their hypocenters; the rupture always began in this patch and tended to propagate westward. Moment releases for this patch are similar, approximately Nm (Mw 6.3) before and after the Tohoku earthquake. However, the Mw 6.0 event that occurred 20 days after the Tohoku earthquake had only half this moment release, likely reflecting less than usual healing of frictional strength. Moreover, the fourth earthquake after the Tohoku earthquake, which is the 2015 Mw 6.8 earthquake, released a much larger moment of Nm (Mw 6.5) in the same patch and caused a subsequent large slip in a second patch to the east. The presence of the eastern hidden patch likely affected the slip amount of the western patch that typically causes Mw 6.3 events. An examination of MJMA 2.0–5.3 repeating earthquake activity suggests that this eastern patch also affected the preparation period for the 2015 event. The present observation suggests that even locations with well‐predetermined rupture areas and slips can produce much larger slips than usual under the presence of adjacent hidden slip patches.

Estimation of source, path, and site factors of S waves recorded at the S-net sites in the Japan Trench area using the spectral inversion technique

S-net is a large-scale ocean bottom (OB) network in the Japan Trench area, consisting of inline-type 150 observatories equipped with seismometers and pressure gauges. Among them, 41 observatories have been buried about one meter beneath the seafloors in the shallow water regions (water depth <1500 m). We analyzed the strong-motion data recorded at the S-net sites from earthquakes with magnitudes 3.5 < Mw ≤ 7 and focal depths < 70 km to understand the site amplification effect on the recorded motions. We used the spectral inversion technique and obtained some fundamental properties of the earthquake source spectra, path attenuation, and site factors from the horizontal-component S-wave portions of the recordings. We obtained that the source spectra followed the {omega }^{-2} source model generally well, and the estimated magnitudes were mostly within ± 0.3 magnitude units of the catalog magnitudes. The stress drops increased systematically with the focal depths, and the values for the shallow earthquakes in the Pacific Plate were higher than those for the interplate earthquakes with comparable focal depths. The path-averaged quality factors were generally frequency-dependent and were somewhat larger than those in the past studies. The peak site frequencies ranged between about 0.2 and 10 Hz, while the peak amplification factors ranged between 10 and 50. Even though the peak frequencies and amplification factors differed from site to site, the peak frequencies were mostly higher than 2 Hz at the outer trench stations, while they were lower than 2 Hz at many inner trench stations. The amplification factors at a few OB sites in the shallow water regions were comparable with the theoretical ones computed from the 1-D subsea model. The amplification factors at intermediate frequencies (~ 0.3 to 2 Hz) generally increased with P-wave travel time in the sediments estimated from the multi-channel seismic survey. At about 20% of the sites (mainly at the unburied stations), spurious site spectra were recognized at frequencies higher than about 4 Hz. If the site spectra between 4 and 10 Hz are required, using only the X-component records is recommended.Graphical

Rupture direction and short‐period spectral level of moderate‐sized intraslab and interplate earthquakes off the coast of Fukushima prefecture, Japan

AbstractRupture directions and directivity effects of moderate‐sized intraslab and interplate earthquakes off the coast of Fukushima Prefecture were estimated using seafloor S‐net records and onshore K‐NET and KiK‐net records. The azimuthal dependance of the source spectral ratio of each earthquake relative to a reference earthquake observed at the same station was modeled based on a ω−2 model assuming unilateral rupture. The azimuthal dependance of the source spectral ratio observed for most earthquakes reached a maximum at 2–3 Hz and weakened at higher and lower frequencies. The rupture directivity effect contributes to stations located up to the maximum hypocentral distance of 150 km used in this study. All four down‐dip compressional intraslab earthquakes were estimated to rupture toward the up‐dip, i.e., the land. In addition, the six interplate earthquakes were estimated to rupture either up‐dip or in the opposite direction of the strikes, i.e., between south and southeast directions. Only one interplate earthquake was estimated to rupture down‐dip, i.e., toward the land. In the case of intraslab earthquakes, the regions of large radiation patterns and large rupture directivity overlap in the onshore coastal area. Therefore, when only onshore records were used, the estimates of short‐period spectral levels were biased.

Numerical estimation of a tsunami source at the flexural area of Kuril and Japan Trenches in the fifteenth to seventeenth century based on paleotsunami deposit distributions in northern Japan

Paleotsunami deposit investigations and numerical tsunami computations have been performed to elucidate the source and size of large tsunamis along the Kuril to Japan Trenches, particularly for unusual tsunamis that occurred in the seventeenth century, the 1611 CE Keicho tsunami (M 8.1) along the Japan Trench and seventeenth-century tsunami (> Mw 8.8) along the Kuril Trench, which caused serious damages on the coastal residents and environments. Moreover, several paleotsunami deposits dating from the thirteenth to eighteenth centuries have been reported along the area between the Kuril and Japan subduction zones, but their sources have not been clarified. In this study, we estimated the tsunami sources from numerical simulations using the distribution of fifteenth- to seventeenth-century tsunami deposits at Sekinehama along the coast of the Shimokita Peninsula. Based on numerical simulations with previously proposed fault models, the tsunami deposits showing similar ages at Sekinehama and another site on the coast of Shimokita Peninsula, which are within 50 km apart, could not be explained except with the huge earthquake models (> Mw 9.1), whose rupture zones extend to not only the Kuril or Japan Trenches but also their flexural area. Thus, we modified or newly proposed twelve fault models located in the flexural area between the two trenches to explain tsunami deposits possibly around the seventeenth century at the above-mentioned two sites on the coast of Shimokita Peninsula. Simulations using these models elucidated that the rupture in the shallow or deep plate boundaries with > 14–32 m slip (> Mw 8.55–8.76) is necessary. If the tsunami deposits around the seventeenth century along the Iburi–Hidaka coast in Hokkaido and those at the two sites mentioned above might be left by an identical event, an interplate earthquake with > 18–40 m slip (> Mw 8.62–9.2) in the flexural area is needed. Moreover, this interplate earthquake might have occurred in the deep plate boundary than in the shallower plate boundary based on slip deficit and slow earthquake distribution data. Our results offer significant insights into a large earthquake (> M 8) along the Kuril and Japan Trenches in the fifteenth to seventeenth century.

Effects of the Tsunami Generated by the 1662 Hyuga-Nada Earthquake off Miyazaki Prefecture, Japan

The Hyuga-nada region is located in the southwestern part of the Nankai Trough in the Pacific Ocean, where M7 class interplate earthquakes have been repeatedly occurring because of the subduction of the Philippine Sea Plate. The largest earthquake recorded in history for the Hyuga-nada region was the Hyuga-nada earthquake of 1662, which occurred off Miyazaki Prefecture in the southeastern area of Kyushu region, Japan, generating a tsunami. The region is also an area where slow earthquakes are active at the shallow part of the plate boundary. It is confirmed by the 2011 Tohoku earthquake that the active area of shallow slow earthquakes also became a tsunami source area. We hypothesize that the unusually large tsunami of 1662 was caused by the coseismic slipping of the active source area of shallow slow earthquakes. We constructed the fault model of the 1662 Hyuga-nada earthquake based on recent geophysical observations. A numerical simulation of the tsunami was carried out using the fault model constructed based on hypotheses. The total seismic moment was 9.8 × 1020 Nm (Mw 7.9) estimated from the fault model. The estimated fault model can explain the distribution of the likely tsunami event deposits which were recognized in this study and also have been reported by other studies and tsunami heights in historical records. Hypotheses based on geophysical findings have been found to be correct.

Tectonic tremors immediately after the 2011 Tohoku-Oki earthquake detected by near-trench seafloor seismic observations

Temporal seismic observations from pop-up type ocean-bottom seismometers were used to detect tectonic tremors immediately following the 2011 Tohoku-Oki earthquake in the northern periphery of the aftershock area. Near-field observations clearly distinguished tremors from regular earthquakes based on their spectral shape in the frequency band of 1–4 Hz. In addition to tremors accompanied by very low-frequency earthquakes (VLFEs), we detected 130 tremors without known VLFE activity during April–October 2011. The newly detected tremors were in the vicinity of a sequence of small repeating earthquakes, indicating a mixed distribution of tremors and regular interplate earthquakes in the region. Tremor activity was high immediately after the deployment of seismometers and gradually decreased. In addition, the tremor activity fluctuated with two activations with an interval of approximately 90 days, similar to the intervals between tremor bursts after 2016. The results of the study suggest that the observed tremors occurred under the influence of aseismic slip caused by the decaying afterslip of the preceding Tohoku-Oki and Mw 7.4 interplate earthquakes and episodic accelerations with a quasi-periodicity unique to the area.

Spatio‐temporal variation in b‐value prior to the 26 November 2021 Mizoram earthquake of northeast India

In this study, the variation in frequency magnitude distribution factor (b‐value) before the main shock has been assessed for the 26 November 2021 Mizoram earthquake (Mw 6.1) in northeast India. The study area covering the Indo‐Burma Ranges is divided into square grids of equal dimension and spatial mapping of the b‐value for each square grid has been carried out. The maximum likelihood method is implemented for the estimation of the b‐value of each grid. The epicentral location of the 26 November 2021 Mizoram earthquake (Mw 6.1) is found to be in the intermediate b‐value square grid. The temporal variation in the b‐value indicates a considerable decline in the b‐value before the occurrence of the main event. The depth‐wise variation in the b‐value suggests an antithetical relationship between the b‐value and crustal stress. The spatio‐temporal and depth‐wise fluctuation in the b‐value results have been strengthened by employing a non‐parametric Kolmogorov–Smirnov (K–S) test. Based on our study, we infer that interplate earthquakes occur mostly in the upper crust of the study region.

Spatio-temporal analysis of b-value prior to 28 April 2021 Assam Earthquake and implications thereof

Here, we report a Spatio-temporal analysis of the frequency magnitude distribution of earthquakes (b-value) before the 28th April 2021 (Mw 6.4) earthquake event observed in northeast India. To esti- mate the average b-value for the study region, a data set of 750 earthquake events with magnitude Mw ≥ 3.9 is extracted from the homogenous part of the earthquake catalog (1950-2021) documented by the United States Geological Survey (USGS) and International seismological center (ISC) in the region. For spatial analysis of the disparities in b-value, the whole study region is subdivided into 16 square grids of dimension 1o×1o and the b-value is calculated for each subsection. In congruence with other studies, this work yields b-values ranging from 0.66 to 1.25. After the calculation of the b-value for each grid, it is observed that the grid with the epicentral location of the 28th April 2021 (6.4) earthquake has a low b-value. Accordingly, the spatial correlation and aberrant pattern between b-value and focal depth have been comprehensively explored. It is observed that the b-value sig- nificantly dips within a depth range of ~15-35 km which implicates high-stress accumulation and crustal homogeneity. The depth-wise variation in b-value infers the antithetical relationship between b-value and crustal stress. Mostly interplate earthquakes are observed in the study region; thereby hinting at intense seismicity at the upper crust.