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The establishment of a unified earthquake catalog is a basic requirement for attaining a detailed seismological analysis and an improved seismic hazard assessment. Accordingly, the current research paper aims to present a reliable instrumental unified earthquake catalog for the northern part of Algeria. This unified catalog is compiled using regional empirical relationships derived from converting different magnitude scales to an advanced physical-based moment magnitude scale suggested in the recent literature and denoted by Mwg. The regression methodology used in this study is called the New General Orthogonal Regression approach (N.GOR), which was selected since it is the most trustworthy procedure for scaling the magnitudes to Mwg. To highlight the significance of using the N.GOR, the obtained outcomes of the scaling relations were compared with those issued from the conventional general orthogonal regression (GOR) method, which is deemed to be the most dependable methodology used worldwide for magnitude conversion problems. The unified earthquake catalog was assembled from 1954 to 2022. Reliably converting the original magnitudes into homogenized moment magnitude leads to a complete and consistent unified earthquake catalog. Indeed, the unified moment magnitude catalog presented in this paper could provide reliable data for studying earthquakes distribution and the assessment of seismic hazards in the north of Algeria. [ABSTRACT FROM AUTHOR]

Large-magnitude earthquakes in northern Chile and southern Peru occur every 108 years on average. It should be noted that over 143 years since the catastrophic earthquake of May 9, 1877, any similar events were completely absent. In 2007, a 7.7 Mw earthquake occurred near Tocopilla, and in 2014 a catastrophic M = 8.1 earthquake hit Pisagua. It is believed that only part of the energy accumulated over 143 years has been released during those events, while most of it is yet to be released. Thus, we can conclude that a serious tsunami hazard exists for all coastal cities of southern Peru and northern Chile. In this paper, on the basis of the available historical data and geodynamic studies, numerical simulation of the historical catastrophic earthquake and tsunami of May 9, 1877 is carried out assuming the blockwise earthquake source configurations. We implemented 23 simulation scenarios for different kinematic behavior patterns of such a source, sequentially updating the source fragmentation to reduce the misfit between the simulated and observed wave height data. Using the proposed methodology, for each scenario, the generation of a tsunami source is simulated and the computation of wave fields up to the 5-m isobath is carried out. The results obtained are compared with historical data. Analysis of the entire set of simulated earthquake scenarios makes it possible to choose a tsunamigenic earthquake scenario with the most adequate characteristics of tsunami waves in the coastal zone. [ABSTRACT FROM AUTHOR]

The tectonic setting of Lebanon in the eastern Mediterranean region is a restraining bend along the Dead Sea Transform Fault, which is the plate boundary between Arabia and Africa. Within the Lebanese Restraining Bend, the plate boundary splays into several fault branches that are mapped onshore Lebanon and known to have contributed to the evolution of the Lebanese structural framework. Different models have been proposed about the geological structures offshore Lebanon, and how those structures could relate to the onshore tectonics. Based on 2D seismic reflection data, a previously interpreted thrust fault system offshore Lebanon referred to as "Mount Lebanon thrust" was suggested to be responsible for the 9 July 551 M 7.2 earthquake, which was one of the most destructive in the history of the Levant. This thrust system has been accepted as the main structure behind the offshore seismic activity. The objective of this paper is to use current 3D seismic reflection data in order to interpret the main tectonic structures offshore Lebanon, and to differentiate features formed by tectonic activity from others caused by different mechanisms such as subsurface salt movement. Such information is very useful in understanding the tectonic framework of the region from the earthquake geology perspective. A bathymetric map of the Lebanese offshore area was generated and used to delineate the seafloor features. Major and minor faults were interpreted and used to identify and understand the behavior of potential tsunami-generating structures. After careful investigation of the offshore area using the available 3D seismic data, the authors were not able to confirm the existence of Mount Lebanon thrust. Instead, they propose that the Latakia ridge that lies between Lebanon and Cyprus is a major and prominent structure that can trigger high-magnitude tsunamigenic earthquakes. [ABSTRACT FROM AUTHOR]

The geological activity in Sichuan and Yunnan of China is intense, and large earthquakes such as the Tonghai earthquake in 1970 have caused huge casualties and economic losses. Therefore, it is significant to reduce the long-period earthquake damage of dams and high-rise buildings to reproduce the near-fault long-period ground motion characteristics of the Tonghai earthquake. In this paper, the finite difference method is used to predict the long-period characteristics of the Tonghai Ms 7.8 earthquake. Considering the uncertainty of the initial fracture point, number of asperities, and slip angle, 18 seismic scenarios are established. The predicted results are verified by comparing intensity, attenuation relation and pulse probability distribution. At the same time, based on the velocity pulse recognition method, the pulse probability distribution of the near-field is obtained. We selected six seismic scenarios with high probability from 18 seismic scenarios. Our results show that the spatial distribution and pulse probability distribution of near-field simulated ground motion is significantly affected by the initial rupture positions. After the characteristics of the largest asperity on the fault are determined, the remaining asperities have limited influence on the spatial distribution of simulated ground motion. This study expands the application scope of the finite difference method, which can simulate the seismic intensity characteristics of destructive earthquakes in areas with few seismic records. [ABSTRACT FROM AUTHOR]

Quickly determining earthquake size by extracting information after P-wave arrival is a critical issue for earthquake early warning systems. However, early warning parameters are often calculated using a fixed-length (3–5 s) P-wave time window for rapid magnitude estimation. In this paper, we directly investigate magnitude-scaling relationships within the available P-wave time window (APTW, defined as the window period starting from the trigger time of the P wave and ending at the arrival of the S wave) and explore a new real-time magnitude estimation approach (the APTW method). The results of an offline application demonstrate the good performance of the APTW method in terms of the stability of magnitude estimation for small to moderate earthquakes and improved estimation for large earthquakes. The APTW method can provide estimates earlier, and the estimated magnitudes are more stable. The proposed method of establishing magnitude relationships could provide an alternative choice for magnitude estimation in earthquake early warning systems. [ABSTRACT FROM AUTHOR]

In this paper, we explore the Western Pacific subduction impact on the geodynamics of the Asian continent. The data on migration of slow strain and earthquakes from the Nankai, Japan and Kuril-Kamchatka segments of the Western Pacific subduction zone deep into mainland Asia are analyzed. The calculations performed on five profiles, crossing the Kuril Islands, the Japanese Archipelago and Sakhalin Island toward the Asian continent, have revealed the transverse migration of earthquakes from the Japan–Kuril-Kamchatka subduction zone. The velocities of hypocenter migration of M ≥ 4.5 earthquakes from the Kuril-Kamchatka Trench via northern and central Sakhalin vary from 6 to 17 km/year, on average, at different depths. The profiles crossing the islands of Hokkaido and Sakhalin show the M ≥ 4.earthquake migration from the Kuril and Japan trenches at velocities of 8–27 km/year. [ABSTRACT FROM AUTHOR]

A modified k-nearest-neighbors method is introduced that provides an efficient nonlinear estimate of the intensity of a point process (field) based on locations of events. The method is applied to perform a detailed statistical analysis of the spatial structure of the seismic intensity field (in the context of this paper, the term "intensity" refers to the intensity of the seismic field, and should not be mistaken for the intensity of ground motion caused by an earthquake). The proposed method requires neither a preliminarily delineated area nor a normalization procedure for the estimates. In contrast to many interpolation methods, estimates based on the proposed method are statistically justified. The "uncertainty relation" between the spatial smoothing effect and random errors is established for the method in an explicit form. A procedure for choosing the number of nearest neighbors k* that controls the effective radius of smoothing is suggested. The proposed method is applied to analyze the seismic intensity field in two seismogenic regions surrounding the Kuril Islands and Japan during the period from 1904 to 2011. Spots of increased seismic activity in these regions are localized by the new method, and some quantitative statistical characteristics of these spots are determined and discussed. [ABSTRACT FROM AUTHOR]

Because of the scarcity of recorded seismic ground motions from Vrancea intermediate-depth source that can be used in designing a structure, engineers are bound to use scaled, artificial, or simulated accelerograms. Out of these options, the first two might incompletely account for the phenomena that may appear. Although complicated, simulated accelerograms provide one of the best options for defining the seismic demand in engineering design, but further research is needed to adapt the simulation methods to the source, path, and site-specific characteristics. As some parameters used in simulations were not addressed yet specifically for the Vrancea-intermediate seismic source, the specific path duration is investigated in this paper. Using a database with the recorded ground motions from March 4, 1977, August 30, 1986, May 30 and May 31, 1990, October 27, 2004 Vrancea earthquakes (the only five recorded earthquakes with moment magnitudes at least equal to 6), the path and magnitude dependent duration specific to Vrancea intermediate-depth seismic source to be used in the stochastic simulation is developed. [ABSTRACT FROM AUTHOR]

The goal of this paper is to describe cluster analyses of the focal mechanism solutions estimated from local and teleseismic measurements and stress inversions to support the recent and previously published studies in the investigated region, that is the Vrancea seismic zone. We have applied different established clustering methods—e.g. hierarchical density-based clustering for applications with noise (HDBSCAN) and agglomerative hierarchical analysis—to the geographical coordinates, focal depths and parameters of the focal mechanism solutions of the gathered seismic events. We have attempted to develop a fully automated algorithm for the classification of earthquakes and to support the further investigation of stress inversions. This algorithm does not call for the setting of hyper-parameters by the users, thus the contribution from any bias of the user can be reduced significantly and the time required to carry out the clustering can also be decreased. In most cases, the resulting stress tensors are in close agreement with those we found in the literature. [ABSTRACT FROM AUTHOR]

This paper presents the quantification of the role of structural parameters and impedance contrast in the insulation and meta-capacity of a city for Rayleigh waves at an earthquake engineering scale. The feasibility of developing heterogeneous and homogeneous meta-cities in a soft sediment deposit is investigated using the meta-behavior of structures and meta-blocks in the epicentral zone of shallow crustal earthquakes. The Rayleigh wave and horizontally propagating plane SH-wave responses of the city with different structural parameters and impedance contrast are simulated at the top of the structure as well as at the free field after crossing the city. It is concluded that the structures act as a meta-structure for the Rayleigh waves but not for the Love waves, and the meta-capacity of the city increases with the increase in the number and stiffness of structures and decrease in damping and impedance contrast. An increase in the width of bandgaps at different longitudinal modes of vibration of structures is obtained with a decrease in impedance contrast, particularly when it is less than 15. It is concluded that meta-blocks can be developed using appropriate ceramic material considering the half-space impedance to develop a desired bandgap for Rayleigh waves. Based on the obtained increase in the city's insulation capacity for Rayleigh waves with the increase in the number and width of structures and decrease in impedance contrast, it is recommended that engineers consider the urban layer as lying in the path of Rayleigh waves for the estimation of seismic hazard in the epicentral zone of shallow crustal earthquakes. [ABSTRACT FROM AUTHOR]

In December 2007, an Mw2.2 earthquake occurred in a gabbroic dike at 3.3 km depth in a deep gold mine in South Africa. The fore- and aftershock activity was analyzed in an effort to understand the preparation and generation processes of earthquakes; these findings have already been published. The present paper focuses on the stress state in the source region of the mainshock. A 90-m-long borehole across the mainshock fault was drilled ~ 1.5 years after the mainshock and logged by an optical televiewer. The fault can be identified by severe damage to the borehole wall at the point where the borehole intersected the aftershock cluster. Except for a 10-m section in the hanging wall of the fault, borehole cores were fully recovered. Borehole breakout (BO) and core disking (CD) were found to occur. Two stress measurement techniques [Deformation Rate Analysis (DRA) and Diametrical Core Deformation Analysis (DCDA)] were applied to the borehole cores. By combining their results with occurrence criteria for BO and CD, the principal stress state in the source region of the mainshock was determined. The principal directions in the hanging wall of the fault were nearly identical to the virgin stress state, while it was significantly disturbed in the footwall. The vertical stresses were 106 MPa and 40 MPa in the hanging wall and footwall, respectively. The significant difference in the vertical stress between the two sides of the fault can be explained by the stress redistribution associated with a nonuniform slip on a nonplanar fault. [ABSTRACT FROM AUTHOR]

Earthquake depth frequency distribution is often used as a constraining factor in assessing lithospheric strength. The accuracy of hypocenter locations is therefore a key factor in these types of analyses. This paper presents a summary of earthquake depth distributions in the Baikal rift system reported in various previous studies. Datasets on both background seismicity and aftershock sequences were analyzed by taking into account hypocenter depth uncertainties. The results show that the most seismically active part of the Earth's crust is in a depth range of 10–25 km. The lower cutoff depth varies for different datasets from 20 to 40 km. The latter value corresponds to the Kichera sequence and coincides with the Moho depth, but other datasets show that the lower 8–18 km of the crust is aseismic. The South Baikal basin is distinguished by the shallower cutoff depth (20–24 km) compared to the datasets outside of it. The peak depths of the earthquake depth frequency distributions vary from 8 to 23 km. Given the depth uncertainties, only the Kumora sequence appears to have double peaks at 9–10 km and 16–17 km. Variations in the lower limit of earthquake distribution and depth of the main peak may imply a spatial change in the depth of the brittle–ductile transition and consequently in the thermomechanical properties of the crust. [ABSTRACT FROM AUTHOR]

Site and path effects are always required to isolate the source term from a recorded seismograph for the estimation of source characteristics. In this paper, the site and path effects are assessed in terms of site amplification and anelastic attenuation (quality factor), respectively, to obtain an isolated source term for the estimation of earthquake source parameters in the Kinnaur Himalaya. These terms are then applied to calculate the source parameters of 75 local earthquakes in the range 1.5 ≤ moment magnitude (Mw) ≤ 3.6 recorded in the Kinnaur region. The scaling relations of earthquake source parameters are also established for this region. The site amplification curves and S-wave quality factor Qs(f) are determined and further utilized to correct the Fourier spectrum of earthquake records. The obtained source spectrum corrected for these two terms is compared with the theoretical source spectrum based on the Brune (J Geophys Res 75:4997–5009, 1970) source model. The root-mean-square error (RMSE) between the observed and theoretical spectrum is formulated by implementing iterative forward modeling. The obtained range of source parameters of 75 events reveals ranges of 2.73 × 1011–3.44 × 1014 N-m for seismic moment, 0.03–13 bar for stress drop, 0.3–0.9 km for source radius, and 5.64 × 1002–1.19 × 1008 J for radiated energy. [ABSTRACT FROM AUTHOR]

On February 28th, 1969, a large earthquake (M SB w sb = 7.8) strokes the Iberian Peninsula and northern Morocco, producing some casualties and important damage and economic losses. This earthquake is the last large earthquake that has occurred in this region, very near where the catastrophic 1755 Lisbon earthquake took place. The paper by Udías ([11]) is a review of large tsunamigenic earthquakes that occurred at SW of Iberia before the 1755 Lisbon earthquake. [Extracted from the article]

China suffers severe continental earthquakes. The 2008 Wenchuan earthquake (Ms 8.0) and the 2014 Ludian earthquake (Ms 6.5) are two representative examples from the modern era. The Wenchuan earthquake caused considerable economic losses, and the Ludian earthquake caused unexpected losses with only moderate magnitude. Based on disability-adjusted life years (DALYs), this paper first calculates aggregated disaster losses and the contributions of three factors: number of deaths or missing persons, number of people affected, and direct economic losses. Then, the losses per unit energy released or area affected are calculated for each earthquake to investigate which of the two earthquakes caused more damage. The paper finishes by investigating the implications and revealing three results. (1) The Wenchuan earthquake induced more damage than the Ludian earthquake, but the latter had relatively higher losses per unit energy than the former. (2) Injuries and affected people contributed most to the total losses for each earthquake, more than fatalities or direct damage. This outcome means that unnoticed aspects have the most severe disaster impacts. In addition, long-term impacts of earthquakes are more serious than usually acknowledged, and more attention needs to be paid to these. (3) The damage from moderate earthquakes can exceed expectations, meaning that disaster mitigation policies in all regions are necessary, regardless of the degree of anticipated hazard. [ABSTRACT FROM AUTHOR]

The present study reports measurements of radon in groundwater conducted in China between January and December 2013. During the measurement period, a great Ms = 6.6 earthquake occurred in Gansu Province, China. The paper analyses data derived from three nearby stations via monofractal and multifractal detrended fluctuations analysis and explores whether pre-earthquake fractal and long-memory trends exist in the time series. Several critical epochs with characteristic pre-seismic long-lasting fractal patterns are identified in segmented parts of the series. In the whole data series, the fluctuation function, scaling exponent and generalised Hurst exponent depend on the fractal scales, which indicates multifractality. The multifractal spectrum of the raw, shuffled and truncated series showed a significant increase in the degree of fluctuation that can be explained as a pre-earthquake indicator attributable to the Gansu earthquake due to its very large magnitude. [ABSTRACT FROM AUTHOR]

On May 21, 2021, an Ms 6.4 earthquake occurred in Yangbi County, Yunnan Province, China. The epicenter was located on a secondary blind fault on the southwestern boundary of the Sichuan-Yunnan Block, and this area has a complex tectonic setting and seismogenic background. To improve the understanding of the seismogenic mechanism of this earthquake, we conduct an in-depth analysis on the regional stress and strain characteristics based on geodetic data from the Global Navigation Satellite System (GNSS) and seismological data (focal mechanism solutions) and analyze the stress changes at the epicenter of this earthquake from the perspective of stress triggering. The results show that the epicenter area of the Yangbi earthquake was characterized by significant shear and extension deformation, with a dextral shear rate of 2.5 × 10−8/a and an extension rate of 3.5 × 10−8/a. The present tectonic stress of the study area is dominated by NNW-directed principal compressive stress and NEE-directed principal extension stress, and the Yangbi main shock occurred on the optimal release nodal plane of the tectonic stress field where the relative shear stress reached 0.984. Furthermore, the cumulative Coulomb stress changes caused by four strong historical earthquakes exceeded the stress trigger threshold of 0.1 bar. All of these results suggest that the Yangbi Ms 6.4 earthquake was the result of the maximum effective shear stress concentration on the blind fault on the southwest side of the Weixi-Qiaohou fault, which was mainly driven by the regional tectonic stress and the surrounding historical strong earthquakes. [ABSTRACT FROM AUTHOR]

The devastating tsunami after the 2018 Indonesian Sulawesi-Palu strike-slip earthquake was a surprise because strike-slip faulting was a known phenomenon of primarily lateral movement of land, while tsunamis were believed to be caused by vertical movements of seafloor or landslides. Here we demonstrated how the strike-slip faulting could have pushed waters from outside and inside the Palu Bay to form a powerful tsunami in the Palu Bay. We constructed three earthquake inversions from seismographs, satellite radar and optical imagery, and used an open-source ocean circulation model to replicate the tsunami. Our experiments revealed that: (1) the southward horizontal displacement of deeper-water slopes along the Makassar coast generated a long-wave tsunami of 40 km, propagating southward into the Palu Bay and consisting with the two distinguished tsunami-peaks in the Pantoloan tide-record, twice higher than the local resonance waves; (2) the two types of tsunamis in the Mamuju tide-record—the "early arrival" tsunami and the late larger tsunami—were originated from the outside and inside sources; and (3) the eyewitness account of the whirlpool circulation in the Palu Bay could be explained by the horizontal strike-slip forcing of the two involved tectonic plates. The east plate was largely responsible for pushing the long-wave tsunami southward that inundated the Palu City and resulted in the devastation. Our findings suggest that the tsunami's behavior of strike-slip earthquakes is more complex than previously thought and should be considered in future tsunami early warnings. [ABSTRACT FROM AUTHOR]

On January 24, 2020, a Mw 5.0 earthquake occurred in the El Aouana region, northeastern Algeria. This region is located at the western end of the Lesser Kabylian Block (LKB), a rigid body that was weakly deformed during the late Cenozoic tectonic phase, and it is characterized by a lower seismic activity than that in its bounding regions. The mainshock focal mechanism was estimated via both the P-wave first motion and waveform modeling methods. The earthquake was associated with the rupture of a NW–SE-oriented right-lateral strike-slip fault, as revealed by a 6 km long and 2 km wide aftershock cluster. The seismic moment estimated through waveform modeling was 3.6 × 1016 Nm, while spectral analysis yielded a value of 3.9 × 1016 Nm corresponding to a magnitude of Mw 5.0, a source radius of 1.6 km, and a stress drop of 4 MPa. The spatiotemporal evolution of the aftershock sequence, as modeled using a restricted epidemic-type aftershock sequence (RETAS) stochastic model, yielded a slope p = 1, indicating that the earthquake was generated by tectonic forces and that the aftershock sequence included many subsequences. The calculated stress tensor suggested N–S compression, rotated clockwise relative to NW–SE Eurasia–Africa convergence. Finally, the recent seismic activity (2012–2021) and geological observations in the area led to the suggestion of a new NW–SE right-lateral shear zone, namely, the Bejaia-Babors shear zone, which was incorporated into a seismotectonic growth model involving slip along inherited E–W structures. The pattern of stepover structures throughout this wide shear zone was enhanced during the recent seismic evolution. [ABSTRACT FROM AUTHOR]

Tsunamis cause significant damage and loss of life, particularly for the nearest communities, where the tsunami may arrive in minutes. These local communities often do not receive an informed or timely alert under traditional warning pathways. In response, numerous tsunami early warning (TEW) algorithms have been developed with the goal of providing informed tsunami source characterization for use in rapid, localized warning. An overlooked aspect of TEW is the means that this crucial information is disseminated. Current operations focus heavily on the time an alert is issued from a warning center, however, that alert passes through multiple groups and agencies before it is conveyed to affected communities. This distribution path can create further delays and contributes to inconsistencies in the message timeliness and content. In this study, we provide the framework and advocate for the use of a rapid dissemination tool, that we call WaveAlert, that would leverage preexisting advances in earthquake early warning systems to provide timely, clear, and consistent alerts to the public by use of the MyShake EEW phone app. This proposed tsunami dissemination tool would be able to provide consistent, public facing tsunami alerts over the duration of the hazard with the added benefit of low message latencies and high spatial resolution in who can be targeted for messages. We illustrate the need for rapid alerting strategies through a retrospective look at the alerting process during the 2022 Tonga tsunami and through a modeled potential near-field Cascadia timeline example affecting the west coast of the US. [ABSTRACT FROM AUTHOR]

We have examined the tsunami hazard in the Bering and Chukchi seas by determining whether trans-oceanic tsunamis penetrate into these regions through the Aleutian Islands and Bering Strait. Results are based on numerical modeling of eight major far-field earthquakes in the North Pacific Ocean and one near-field earthquake that occurred in the Bering Sea region: the 1946 Aleutian, 1952 Kamchatka, 1960 Chile, 1964 Alaska, 1965 Rat Islands, 1957 Andreanof Islands, 2011 Tohoku, 2012 Haida Gwaii, and 2017 Commander Islands earthquakes. It was found that the Bering Sea was most affected by the 1960 tsunami caused by the Chilean earthquake of magnitude Mw 9.5. According to our numerical simulations, it produced tsunami waves with amplitudes up to 294 cm in the sea. For the next strongest event, the 1952 Kamchatka tsunami, the wave amplitudes were up to 146 cm. The 1964 Great Alaska earthquake (Mw 9.2) did not produce an intense tsunami in the Bering Sea because the long and narrow southwestern extension of the Alaska Peninsula sheltered the region from incoming tsunami waves. Modeling further shows that, in separate bays, tsunami waves formed by strong distant earthquakes could reach 1.5–2 m. Results show that the typical attenuation coefficient for the straits of the Aleutian Islands is 0.75, on average, while the corresponding coefficient of the Bering Strait is ~ 0.29. Based on these estimates, we conclude that tsunami penetration into the Arctic Ocean from remote sources in the Pacific is unlikely. Even for such powerful events as the 1960 Chilean tsunami, the tsunami wave amplitudes in the Chukchi Sea would not exceed a few centimeters. For the Bering Sea, the amplitudes from distant earthquakes can be significant. [ABSTRACT FROM AUTHOR]

Logit analysis is widely used for binary data classification in geoscience. In this study, logistic regression was used as a tool for deriving the binary classifier of tsunamigenic and non-tsunamigenic earthquakes for near-source early warning. The catalogue of submarine earthquakes and tsunami database were merged into one seismic database, in which an additional binary variable associated with the tsunamigenic class (true or false) was assigned to each seismic event. The training dataset consisted of 712 M6.0+ submarine earthquakes, including 80 tsunamigenic and 632 non-tsunamigenic events that occurred in the northwestern part of the Pacific Ocean from 1960 to 2020. The target area has already experienced significant and catastrophic tsunamis. The best performance metrics were archived with the predictors given by the earthquake magnitude, logarithm of the source depth and the seafloor depth in the epicenter location. The current analysis clearly showed that the data-driven logit model significantly improved the performance metrics of the threshold magnitude criteria that are widely used by tsunami warning agencies. Authors suggested that logit-based binary classifier led to improve the tsunami alert efficiency in the Northwestern Pacific Ocean. [ABSTRACT FROM AUTHOR]

The 2012 Mw7.8 Haida Gwaii earthquake triggered a tsunami that highlighted the importance of tsunami hazard assessments along Canada's Pacific coast. Stochastic source modelling offers a valuable method to assess future tsunami hazard and has not been previously performed for this region. The stochastic source models characterize the variability of earthquake ruptures by accounting for uncertain fault geometry and slip heterogeneity, allowing the consideration a wide range of possible tsunami scenarios. The model predictions are constrained by observational data and past source inversion studies. A total of 1500 stochastic tsunami models are generated using the stochastic source modelling method to assess tsunami hazard via Monte Carlo tsunami simulations of the target region and conduct sensitivity analyses of tsunami height variability. 500 stochastic models are synthesized by reflecting the key features of the observed tsunami wave profiles as well as ground deformations during the 2012 event, whereas 1,000 stochastic models are generated to represent future potential rupture scenarios of Mw7.7 to Mw8.1 in the Haida Gwaii region. The comparison of the offshore tsunami data based on the 2012 observations and the stochastic tsunami simulations matched reasonably well, while the comparison of the run-up measurements with the simulation results showed that the latter is generally smaller than the former. To improve the tsunami simulation, accurate bathymetry and elevation data are necessary. Since the prospective evaluations of regional tsunami hazards using the stochastic source models capture both moderate and extreme tsunami hazard scenarios, the developed models can be used to perform future probabilistic tsunami hazard analysis in this region to promote better-informed risk management decisions. [ABSTRACT FROM AUTHOR]

Research on earthquake occurrences using a statistical methodology based on point processes has mainly focused on the data featuring earthquake catalogs which ignores the effect of environmental variables. In this paper, we introduce inhomogeneous versions of the Cox process models which are able to quantify the effect of geological factors such as subduction zone, fault, and volcano on the major earthquake distribution in Sulawesi and Maluku, Indonesia. Specifically, we compare Thomas, Cauchy, variance-gamma, and log-Gaussian Cox models and consider parametric intensity and pair correlation functions to enhance model interpretability. We perform model selection using the Akaike information criterion (AIC) and envelopes test. We conclude that the nearest distances to the subduction zone and volcano have a significant impact on the risk of earthquake occurrence in Sulawesi and Maluku. Furthermore, the Cauchy and variance-gamma cluster models fit well the major earthquake distribution in Sulawesi and Maluku. [ABSTRACT FROM AUTHOR]

Monitoring precursory decline in groundwater radon at the Antung hot spring is a useful means of forecasting the magnitude and precursor time of local disastrous earthquakes. With the help of a case study in southeastern Taiwan, this paper demonstrates the effect of tectonic setting in the subduction zone on the correlation between radon decline, precursory time and earthquake magnitude. Given a radon-monitoring site located near the plate boundary in the tectonic setting of advanced arc-continental collision, the observed radon decline and precursory time prior to the earthquakes in the tectonic setting of initial arc-continental collision are smaller than those observed prior to the earthquakes occurring on the plate boundary in the tectonic setting of advanced arc-continental collision. In the advanced arc-continental collision state, the coupling between the plates is strong and the stress transfer is efficient, whereas in the incipient collision state, the coupling and stress transfer are not as good. It also takes additional time lag and attenuation for the stress transfer from one tectonic setting to the other. This paper presents the difference in the precursory behavior of groundwater radon between earthquakes which occurred in two different tectonic settings: advanced and initial arc-continental collision. [ABSTRACT FROM AUTHOR]

Nowcasting refers to the use of proxy data to estimate the current dynamic state of driven complex systems such as earthquakes, neural networks, or the financial markets. In previous papers, methods to nowcast earthquakes have been presented based on the natural time count of small earthquakes after the last large earthquake in a defined, seismically active geographic region. In this method, a large geographic region is identified in which a local region of interest is embedded. The primary assumption in the method is that the frequency-magnitude statistics of the local region are the same as the frequency-magnitude relation of the large region. The nowcasting technique relies on seismic catalogs that are complete in the sense that all events whose magnitude is larger than a completeness threshold have been detected. For this reason, the previous papers have been limited to nowcasting large earthquakes with magnitudes of approximately M7.5. In this article, we extend the nowcasting method to great global earthquakes of magnitudes as large as M9 by defining the surrounding large region as the entire earth. We then analyze the current hazard of a number of selected "local" regions and rank the regions in terms of current risk of great earthquakes. These great events also present significant hazards for generating mega-tsunamis as well as local damage due to intense shaking. We also perform a sensitivity analysis to establish the reliability of the nowcasts. One of our main results is that the eastern Aleutian Islands, site of the M8.6 earthquake of 1 April 1946, is currently the region most at current risk of a great earthquake larger than M8.0. We finish by presenting comments on the applicability of our methods for anticipating the occurrence of great destructive earthquakes and tsunamis. [ABSTRACT FROM AUTHOR]

According to theory, earthquakes may produce disturbances in surrounding areas and affect the major earthquake occurrence rate of surrounding faults. The MS 6.6 Jinghe Earthquake on 9th August, 2017 occurred in the Tienshan Seismic Belt where there is much tectonic activity and many strong earthquakes occur. The impact of this earthquake on major earthquake activity of surrounding areas is worth studying. This paper attempts to combine two models: the Brownian passage-time (BPT) model, which describes the quasi-periodic recurrence of major earthquakes, and the Coulomb Failure Stress model, which describes rock failure on faults. The amount of change in the rate of major earthquake occurrence caused by a nearby earthquake was calculated, and changes in major earthquake activity in the surrounding areas after the Jinghe Earthquake were re-estimated. There are few differences in major earthquake occurrence rate change by stress disturbances calculated with the BPT model or the Coulomb Failure Stress model. Calculation results are consistent with actual earthquake cases. The 2017 Jinghe Earthquake increased major earthquake activity on the Boluokenu Fault and Yili Basin North Fault, and the expected major earthquake recurrence times of the two faults were advanced by 11 years and 59 years, respectively. Conclusions of this paper provide a reference for time-dependent seismic hazard assessments in related areas. [ABSTRACT FROM AUTHOR]

Tectonic movement is closely related to earthquake preparation. Across-fault sites and InSAR data are applied to reveal crust deformation from different precision, different temporal scale, and different spatial scale. In this paper, Xianshuihe fault zone is taken as an example; dislocation model in near-fault field is derived from screw dislocation model proposed by Savage and Burford (1973), which can reveal the locking information of the across-fault site using baseline data. Locking depth of each across-fault site on Xianshuihe fault zone is calculated using baseline data, which indicates that the locking depth of Laoqianning across-fault site is about 18 km and the locking depth of other sites is shallow. Time series analysis of SBAS-InSAR is used to study interseismic tectonic deformation. Deformation field from InSAR reveals the left-lateral slip property of Xianshuihe fault zone. The average locking depth is about 2-4 km calculated by the pixels' slip rate around in near-fault field, which indicates that locking depth is shallow and consistent using InSAR data as well as across-fault data. Screw model is used to obtain the best-fitting result of fault locking depth and slip rate. Layered dislocation model is put forward to explain shallow locking depth of fault zone. According to the theory of fault-slip deficit and seismic moment equilibrium, accumulative seismic moments caused by fault slip and the release amount of MS ≥ 5.0 earthquakes are calculated to estimate seismic risk of study area. All results indicate that Qianning segment of Xianshuihe fault zone is the future danger area with strong earthquakes. [ABSTRACT FROM AUTHOR]

Investigating the extension of aftershocks and the rupture mechanism can effectively constrain the geometric structure of a seismogenic fault and provide a scientific explanation for the earthquake rupture model and the seismogenic structure. On January 8, 2022, an MS6.9 earthquake occurred in Menyuan County, Qinghai Province. The epicenter was located at the intersection of the Tuolaishan and Lenglongling faults in the Qilian-Haiyuan fault zone. The precise relocation of the earthquake sequence shows that the mainshock appeared to be an asymmetric bilateral rupture. The fault plane had a steep dip angle and was inclined toward the SW. The Menyuan earthquake sequence was characterized by two obvious belts with NWW and SSE orientations, exhibiting clear segmentation and non-uniform spatial characteristics. The focal mechanism solutions of the mainshock and partially mediated aftershocks with MS ≥ 4.0 indicate that the source type of the four early aftershocks was consistent with that of the mainshock, predominantly strike-slip faulting, while the two large later aftershocks located at the eastern tip of the aftershock belt were mainly thrust-type. This reveals that the terminus of the mainshock rupture was arrested by the bifurcation fault of the Lenglongling fault. The release of energy from recent accumulated compressional tectonic stress led to the variation in the types of earthquakes. The regional stress inversion indicates that the focal region is undergoing compression in the NE direction, consistent with the northeast convergence of the characteristics of the stress field of the northeast margin of the Qinghai-Tibet Plateau. Integrated with the regional geotectonic environment, we infer that the seismogenic fault of the mainshock was associated with the Tuolaishan fault. The earthquake rupture propagated simultaneously toward the connected Tuolaishan and Lenglongling faults, generating a typical strike-slip cascading rupture event, and the energy release of the mainshock was concentrated at the intersecting fault. The Menyuan earthquake revealed the complexity and heterogeneity of the fracture at the intersection of the faults under a strike-slip regime. In addition, it also revealed that the occurrence of large-scale strike-slip earthquakes in the Qilian-Haiyuan fault zone accommodates the compressive force from the northeastern margin of the Qinghai-Tibet Plateau. [ABSTRACT FROM AUTHOR]

We performed a study to test the performances of the Hybrid Heuristic Search (HHS) algorithm (Nagashima et al. 2014) using earthquake recordings at 23 instrumented sites in Switzerland. The HHS algorithm is based on the diffuse field theory and estimates the P- and S-wave velocity profiles by inverting horizontal-to vertical spectral ratio (H/V) curves of earthquake recordings. In this study we inverted the H/V curves computed using the arrivals of local and regional earthquakes with a maximum local magnitude of 4.6. We compared the results of the inversion technique to the information available for each investigated site (e.g., site characterization analysis). Our results emphasize the importance of site characterization data for the definition of the parameter space in the near surface, and the potential to extend the investigated depth to much higher depths than the standard site characterization measurements. The additional constraints in the inversion are important to drive the inversion towards a unique solution. Our analysis confirms the potential of the HHS algorithm to invert the full earthquake H/V curve for subsurface investigation and its applicability to areas with low magnitude earthquakes. In addition, the HHS algorithm can be a useful technique to investigate deeper structures and extend the velocity profile to depths that are not resolved by classical site characterization techniques. [ABSTRACT FROM AUTHOR]

We analyze high-resolution borehole strainmeter signals to constrain the fault plane solutions and locations of the 2009 Fengpin-Hualien earthquakes, in eastern Taiwan. The December M w 6.6 earthquake has ruptured a narrow section of the Collision Seismic Zone (within depth of 34 to 44 km), an active structure located offshore northeast Taiwan. The October M w 6.3 event has ruptured the deep section of the Longitudinal Valley fault (25–32 km depth) located between Fengpin and Fanglin. Coulomb stress changes induced by the October event in the hypocenter region of the December event are low (< +30 kPa), which suggests that the latter event was not induced by static stress transfer. The December event is characterized by an abundant seismicity for about 2.5 days following the mainshock. The temporal evolution between the aftershock rate and geodetic strain suggests that seismicity could have been controlled by a short-lived frictional afterslip. Besides, we infer that significant positive static Coulomb stress changes (> +100 kPa) induced by this sequence at shallow to moderate depths in the Longitudinal Valley (0–12 km) may have influenced the occurrence of large mainshocks in eastern Taiwan during the past decade. Conversely, limited stress (< +30 kPa) is transferred to the shallow, locked section of the southernmost Ryukyu trench. This study emphasizes the importance of borehole strain measurements in active regions to enhance detection of seismic and aseismic events unresolved by surface geodesy. [ABSTRACT FROM AUTHOR]

Grouping of earthquakes into distinct clusters is applied to improve mechanism identification and pattern recognition for active seismicity in a region. One of the important issues concerning earthquake data clustering is determining the optimum number of clusters (ONC) at the early stages of algorithms. In this paper a robust method based on K-nearest neighbor search (KNNS) is presented to achieve three goals: improving output accuracy, improving output stability, and adding the ability to weight the features used in ONC determination. By introducing a new formula, the proposed method utilizes the error calculated for clustered data based on the similarity between the members in each cluster. An outlier attenuation algorithm is also used to improve the performance of the method. Both the Krzanowski–Lai Index (KLI) and the silhouette coefficient (SC), as two conventional methods, were used to compare the results and evaluate the performance. Experiments on synthetic data sets verified the effectiveness of the method, with considerable differences found. The clustering of a real earthquake catalogue related to the seismogenic province of Zagros in Persia using our proposed methodology suggests using 13-cluster analysis for clustering based on the spatiotemporal features with the same weights, and seven-cluster analysis for a case where priority is given only to the spatial parameters of the epicenters. Under the same circumstances, the KLI and SC methods suggest three and 18 clusters, respectively. The results of the experiments on synthetic data sets indicate that the proposed method is quantitatively more stable and more accurate than the other two methods. [ABSTRACT FROM AUTHOR]

Seismic risk analyses aim at establishing a relation that links the earthquake activity rate to the magnitude, using earthquake catalogs. The most widely used relation is the log-linear relation proposed by Gutenberg and Richter (Science 83:183–185, 1936) and Gutenberg and Richter (Bull Seismol Soc Am 46(3):105–145, 1945): log E N m = a - b m , where E N m is the mean number of earthquakes which magnitude is greater than m, with modification at larger magnitudes by Cosentino et al. (Bull Seismol Soc Am 67:1615–1623, 1977), Kijko and Sellevoll (Bull Seismol Soc Am 79(3):644–654, 1989), Page (Bull Seismol Soc Am 58:1131–1168, 1968), Pisarenko and Sornette (Pure Appl Geophys 160:2343–2364, 2003) and Weichert (Bull Seismol Soc Am 70(4):1337–1346, 1980). That relation leads to an Exponential distribution for the magnitudes, that we assume to be truncated to a maximum magnitude m max , a priori fixed under geophysical considerations. The objective of this paper is to simultaneously estimate the parameters (a, b) from data which main features are: (a) data are observed on unequal observation periods; (b) magnitudes are imprecisely known. Within the assumption that earthquakes are modeled by a continuous Poisson point process, we propose an estimator of (a, b) that maximizes the likelihood of that Poisson point process which has been discretized on classes of magnitudes. The asymptotic distribution of the estimator is a Normal distribution of dimension 2. That distribution is used to propagate the estimation uncertainties on (a, b) until the recurrence model. Uncertainty analyses enable to draw the 5 % and 95 % quantile curves around the estimated recurrence model. [ABSTRACT FROM AUTHOR]

In the present paper, a new method referred to as the Poisson probability map (PMAP) method is presented for identifying and visualizing seismic quiescence. With the PMAP, the P-value is defined as the probability that consecutive earthquakes occur according to a homogeneous Poisson process: the smaller the P-value, the less frequently the longer time interval is observed, i.e. the more significant the seismic quiescence. The PMAP method was applied to the sequence which preceded the Kurile Islands earthquake that occurred on 15 November 2006 [Mw = 8.3 and the centroid = (154.33 °E, 46.71 °N)]. The seismic quiescence is identified by a small P-value of 9.0 × 10–5 that was found to start in 1990.1, which lasted for 15.4 years and ended in 2005.5 within a circular area centered at (153.8 °E, 47.1 °N) and with a radius of 26 km. This seismic quiescence has not previously been recognized using any other method. [ABSTRACT FROM AUTHOR]

We review properties and processes of earthquake rupture zones based on field studies, laboratory observations, theoretical models and simulations, with the goal of assessing the possible dominance of different processes in different parts of the rupture and validity of commonly used models. Rupture zones may be divided into front, intermediate, and tail regions that interact to different extents. The rupture front is dominated by fracturing and granulation processes and strong dilatation, producing faulting products that are reworked by subsequent sliding behind. The intermediate region sustains primarily frictional sliding with relatively high slip rates that produce appreciable stress transfer to the propagating front. The tail region further behind is characterized by low slip rates that effectively do not influence the propagating front, although it (and the intermediate region) can spawn small offspring rupture fronts. Wave-mediated stress transfer can also trigger failures ahead of the rupture front. Earthquake ruptures are often spatially discontinuous and intermittent with a hierarchy of asperity and segment sizes that radiate waves with different tensorial compositions and frequency bands. While different deformation processes dominating parts of the rupture zones can be treated effectively with existing constitutive relations, a more appropriate analysis of earthquake processes would require a model that combines aspects of fracture, damage-breakage, and frictional frameworks. [ABSTRACT FROM AUTHOR]

We investigate the intermediate-term earthquake forecasting of the central China North–South Seismic Belt for the last quarter-century using pattern informatics (PI) of seismicity. While the number of earthquakes above the cut-off magnitude is typically used in the original PI analysis, we use the mean magnitude of earthquakes as an alternative index of the 'pattern'. This alternative index can be understood in physics considering that the mean magnitude of earthquakes, related to the b value in the sense of the maximum-likelihood estimation, is associated with the stress level. While the original PI considers the number of earthquakes with different magnitudes above the cut-off magnitude, the PI of micro-seismicity considering earthquake magnitude, noted as PIm, assigns more weight to the larger events. Compared with the original PI, the PIm, with a different distribution of 'hotspots', does not demonstrate better forecast performance, implying that the simple number counting approach of PI effectively grasps the key physics of earthquake dynamics. Meanwhile, the combination of the original PI and the PIm, noted as PIJ, significantly reduces the number of hotspots and thus can reduce the false alarms. [ABSTRACT FROM AUTHOR]

Induced seismicity is an increase in seismic activity caused by human engineering. Examples of such activity include mineral exploration, large water reservoir construction, and exploitation of underground oil and gas storage. The authors studied the seismicity in the Uglegorsky district of the Sakhalin region, where the Solntsevskoye brown coal field is located, which is the most promising on the island. Its area is over 100 km2, and productive strata of the Verkhneduiskaya formation with a thickness of up to 600 m contain 12 coal seams, eight of which are working. Active mining of brown coal is carried out at the Solntsevsky coal mine, and blasting operations are performed on a large scale, which therefore does not exclude the relation between seismic processes and anthropogenic seismicity. Earthquake recurrence curves for two decades beginning from 2000 to the present were constructed in this work to compare the characteristics of the seismic regime in the study area. The difference in the slope angle of the recurrence graph during the period of 2011–2020 (the period of the most active development of the Solntsevsky coal mine) relative to the previous decade is quite significant. Maps of the spatiotemporal distribution of seismic event epicenters in the vicinity of Solntsevsky coal mine were constructed, and contraction of zones of seismic event concentration to the mining areas, first of all to the Solntsevsky coal mine, was found. Such a combination allows us to note an increase in seismicity in the region during the past few years and a change in its character from natural to mixed natural and technogenic. The focal mechanisms of the largest earthquakes occurring in the Uglegorsky district were constructed in order to confirm the change in the nature of the seismicity and reasons for earthquake occurrence in the study area. The mechanisms of seismic events of 2020 were classified as strike-slip faults, which is not characteristic of most earthquakes in the territory of Sakhalin Island. The authors made an attempt to determine the regularities of the parameters of the produced blasts and earthquakes through dynamic parameters of the seismic event foci by studying the frequency content of earthquakes and blasts in order to determine a corner frequency from the focal velocity spectrum. [ABSTRACT FROM AUTHOR]

At the high pressures and temperatures typical of the Earth's interior, the brittle failure in accordance with the generally accepted Reid elastic rebound model cannot occur at depths greater than a few dozen kilometers. Nevertheless, earthquakes do occur at depths to about 700 km depth and some more. The paradox is explained by the presence of the deep fluid that decreases the effective friction in rocks and/or by metamorphic processes occurring in down-going slabs. These models imply an essential difference between characteristics of earthquakes occurring at different depth. However, no appreciable differences have so far been found between the source characteristics of the shallow, intermediate-depth, and the deep earthquakes. A few such differences are found and discussed below. Firstly, a few examples are presented demonstrating that the stress and tectonic deformations are not sufficient for the occurrence of earthquakes deeper than 30–50 km. An additional factor associated with metamorphic transformations appears to be required for the occurrence of deeper earthquakes. It is shown also that the typical values of a few source parameters of earthquakes (the normalized duration of seismic process, apparent stress, etc.) vary substantially over depth. What is more, these variations are consistent with suggested changes in mechanisms of earthquakes generation with the depth. The depth boundaries between different models of earthquakes origin are marked by increased median values of the apparent stresses and durations of seismic radiation process. "Normal" earthquakes can occur only in the upper 50 km of the lithosphere. The intermediate-depth earthquakes appear to be connected mainly with the dehydration embrittlement; the deep earthquakes appear to be connected with the solid-state (phase) transformations. [ABSTRACT FROM AUTHOR]

We document magnetic disturbances that occurred during the Tohoku-Oki tsunami of 11 March 2011 using empirical mode decomposition (EMD) in a dataset derived from a network of ground-based magnetometers (INTERMAGNET and GIS). The disturbances, obtained by filtering the magnetic field data using the first intrinsic mode function (IMF1) of EMD, propagate ahead of the tsunami at a speed in the range of 600–1.6 km/s. They also appear 3 min (near-field) to 2 h (far-field) earlier than the tsunami arrival at the magnetic observatories. We refer to these disturbances as ahead-of-tsunami magnetic disturbances (ATMDs). A comparison with seismometer data shows them arriving ∼ 10 min after the arrival of Rayleigh waves. Their association with both seismogenic and tsunamigenic processes is discussed, and it is argued that the tsunamigenic process can well explain the magnetic disturbance propagation characteristics at the far-field. At near-field, the ATMDs are the coseismic magnetic signatures mainly driven by surface Rayleigh waves. Monitoring these ATMDs can be extremely useful for the early warning of the tsunami. [ABSTRACT FROM AUTHOR]

A strong earthquake with a magnitude Mw = 7.0 occurred on February 13, 2020, at a depth of 142 km in the vicinity of the Southern Kuril Islands. Over recent decades, there has been an anomaly near the focal zone in the earthquake distribution over the layers of the double seismic focal zone in accordance with the seismic dislocation kinematic type compared to the other areas of the intermediate depths of the southern part of the Kuril arc. Inversion of the earthquake focal mechanisms of intermediate depth was performed for the southern part of the Kuril Islands along the upper and lower layers of the seismic focal zone. In the upper layer, compression prevails along the slab; the axis of maximum compression σ3 is mainly parallel to the slab and is oriented at an angle of 140–160° (320–340°) to the direction of its dip, i.e., subparallel to the Pacific Plate motion vector in the mantle in this area. The stress state of extension along the slab prevails in the lower layer; the axis of minimum compression σ1 is parallel to the slab and is mainly rotated by 20° (200°) clockwise from the direction of its dip. This orientation of the principal stress axes could indicate that, in addition to the slab unbending, their formation is affected by the mantle resistance to its movement. In the upper and lower layers, we marked the areas where the shear stress prevails that correspond to the kinematic type of transform faults, which are presumably seismically active on the descending plate. The most complex pattern of the stress state of the medium was obtained near the focus of the earthquake that occurred on February 13, 2020. Here, in the upper layer, an area with the stress state of extension and orientation of the principal axes corresponds more to the lower layer. Seismic quiescence is observed in the lower layer, near the focus of this earthquake within the area 60 km wide along the island arc and at the entire interval of intermediate depths. [ABSTRACT FROM AUTHOR]

In this paper, we report the co-seismic temperature response of the Ms6.3 Kangding earthquake in Sichuan, China, which occurred within a bedrock temperature monitoring station network constructed in the east of the Qinghai-Tibet Plateau. Results indicate that two kinds of co-seismic responses of bedrock temperature, exponential and step-rise ones, were recorded at different stations. As for the former, the amplitude of co-seismic response is about 5.6-11.5 mK (the average value is 7.33 mK), while the latter is about 0.31-0.98 mK (the average value is 0.52 mK). These two kinds of responses would be indicative of the two different mechanisms for change in temperature. On the one hand, the exponential responses represent the effects of fluid advection along the borehole wall, which are associated with a finite quantity of heat produced by earthquake-driven transient fluid flow. The step-rise responses, on the other hand, represent the direct response of temperature to change in the crustal stress accompanying with earthquake. We still tested relationship between stress variation and temperature response on the rock samples taken from the boreholes, and the average value is 0.77 mK/MPa. Upon thus, these step-rise temperature responses imply that co-seismic change in mean stress is about 0.40 MPa. We simplify effects of fluid by presuming a one-dimensional model where the flow is either up or down based on a pressure change due to compression or tension. The stress state, compression or tension, deduced by the direction of the fluid flow is qualitatively accordant with the direct response of bedrock temperature to stress change. In summary, characteristic changes in bedrock temperature occurred coincidentally with the earthquake at different stations, agreeing generally with regional stress adjustments accompanying the earthquake. Changes in bedrock temperature do seem to offer a useful means of monitoring dynamic changes in shallow crustal stress. [ABSTRACT FROM AUTHOR]

Earthquakes with submarine foci are generally located with high uncertainties, and their focal mechanisms are poorly resolved, due to the confinement of the monitoring network onshore and the consequent poor azimuthal coverage. The use of amphibious networks, combining ocean-bottom seismometers (OBSs) and land stations, helps to reduce the epicentral distance and the azimuth gap, thus better constraining the hypocentral locations and decreasing the focal mechanism uncertainties. A second important factor in improving the location accuracy for offshore seismicity is the use of a suitable velocity model. The objective of this paper is to study how the combination of an amphibious network with 3D crustal models can improve offshore earthquake hypocenter locations and focal mechanisms. The study area is SW Iberia near Cape St. Vincent, which generated some of the most striking earthquakes and tsunamis in Europe in past centuries, such as the 1755 Lisbon earthquake (Imax = X). We deployed an array of six broadband OBSs 200 km offshore Cape St. Vincent to study the seismicity of the region for a period of 8 months. During this period, we detected 52 earthquakes, the largest with magnitude M (mbLg) ≈ 5. Thirty-eight earthquakes are relocated using land stations in Iberia and North Africa and the OBS array with different velocity models. Focal mechanisms and moment tensors are computed for a data set of seven earthquakes based on first-motion polarities, body-wave amplitude spectra and waveform cross-correlation. We show that if we include offshore OBS data and consider accurate 3D velocity models to locate earthquakes, the focal parameter uncertainties decrease substantially, thus improving the depth constraint. We also show that the locations and focal mechanisms obtained using the amphibious network agree with the regional stress pattern in the SW Iberia region. [ABSTRACT FROM AUTHOR]

During May 12–14, 2018, the International Conference for the Decade Memory of the Wenchuan Earthquake, in connection to the 4th International Conference on Continental Earthquakes (4th ICCE) and the 12th General Assembly of the Asian Seismological Commission (ASC), was held in Chengdu, China. There have been several outcomes of the Conference (for example, the book published jointly by Higher Education Press and Springer) as well as the present special issue. Among the 58 submissions, 34 have been accepted after peer review. Some of the papers came from the presentations during the Conference, but with additional submissions, the scope of this special issue is far beyond that of the proceedings of the meeting. [ABSTRACT FROM AUTHOR]

The MS8.0 earthquake that occurred in Wenchuan, Sichuan in 2008 provides an important case for the study of operational earthquake forecasting and short-term aftershock forecasting of major disaster-inducing earthquakes in China. This paper focuses on the comparative study of the applicability of the epidemic-type aftershock sequence (ETAS) model, the Reasenberg–Jones (R–J) model and the Omi–R–J model, which are widely adopted internationally for short-term aftershock forecasting and seismic hazard mitigation strategy research. We compare the stability of model parameters and aftershock occurrence rate forecasting, and evaluate the effectiveness of forecasting using the N-test and T-test with multiple time windows. The results show that the sequence parameters of the ETAS model, the R–J model and the Omi–R–J model tend to stabilize after 15.50, 15.50 and 6.00 days following the earthquake respectively, and the attenuation of the Wenchuan MS8.0 earthquake is rather normal. Compared to the ETAS model and the R–J model, the Omi–R–J model obtain steadier model parameters in a shorter time with significantly smaller parameters pORJ, cORJ, bORJ and standard deviations. Among the three models, the overall aftershock occurrence rate forecasted by the R–J model is the highest, followed by the Omi–R–J model, while that of the ETAS model is the lowest. N-test results show overall forecasting effectiveness of 93.8, 80.7 and 97.7% for the ETAS, R–J and Omi–R–J models, respectively, with the ETAS and Omi–R–J models superior to the R–J model, and the Omi–R–J model slightly better than the ETAS model. The overall "information gain per earthquake" calculation results show that the ETAS model is superior to the Omi–R–J and R–J models, while the Omi–R–J model is better than the R–J model; thus the combined use of the ETAS and Omi–R–J models by focusing on their respective strengths might ensure optimal performance. These "maneuverable" forecasting approaches to short-term aftershock model forecasting will play a vital role in efficient post-disaster relief, emergency management decision-making and post-disaster reconstruction. [ABSTRACT FROM AUTHOR]

A systematic inspection of the magnetic field and electron density, recorded by Swarm three-satellite constellation over the seismic region hit by the 2016–2017 Amatrice-Norcia (Central Italy) seismic sequence, has allowed us to identify some possible precursory anomalies, when disturbed periods of the geomagnetic conditions are properly taken into account and/or avoided. This paper aims at studying and interpreting the electromagnetic phenomena occurred before and during the 2016–2017 Amatrice-Norcia (Central Italy) seismic sequence, in order to look for any possible evidence of precursory anomalies. Results show magnetic field and electron density anomalies of four tracks that precede the major earthquakes of the seismic sequence. After an inspection of the geomagnetic conditions, a Swarm Charlie track, acquired on 20/08/2016 that precedes by 3.2 days the beginning of the whole seismic sequence, remains unexplainable with the normal geomagnetic disturbance phenomena of the Earth's magnetic field. Furthermore, we carry out a blind study of possible relationship between abnormal magnetic field signals detected by Swarm satellites during geomagnetic quiet conditions and major seismic events from about 4 months before the start of the seismic sequence until about the first 8 months from the seismic sequence (i.e. a total of one year of analysed data). We find a very interesting increase of such anomalies starting about 40 days before the beginning of the seismic sequence. It coincides and follows surface and atmospheric alterations, resulting in a temporal sequence of anomalies from Earth's surface up to ionosphere, supporting the possibility of lithosphere–atmosphere–ionosphere coupling models. [ABSTRACT FROM AUTHOR]

In this paper we investigated the feasibility of using the higher-frequency waveforms to characterize the seismicity in the Koyna–Warna region, globally well-known for its reservoir-triggered earthquakes. The local seismicity and source parameters have been extensively explored by many researchers using various types of data, e.g. analog data, broadband seismological data, and deep seismic sounding data. However, here we have utilized the seismological data recorded by 97 temporary 4.5-Hz geophones to locate the local seismicity in the region. We first investigated an appropriate 1-D P-wave velocity model through the inversion of P- and S-times. In order to compensate for the thick and laterally heterogeneous basalt, we incorporated the station corrections in the 1-D velocity inversion process. The station correction varies from + 0.09 to − 0.20 s for the P-waves, with clear demarcations of positive and negative station corrections corresponding to the northern and southern regions, respectively. The patterns of positive and negative station corrections suggested that the northern part is characterized by lower velocity, while slighter higher velocities present in the southern part of the study area are relative to the estimated 1-D velocity model. The earthquake locations are further improved by using the double-difference (hypoDD) approach, which enabled us to accurately map shallow subsurface seismogenic faults in the region. The data comprises 10,062 P-wave and 9049 S-wave catalog differential times from 348 earthquakes (1.0 ≤ ML ≤ 4.5). The precisely relocated earthquakes (ERH < 0.05 km ERZ < 0.5 km and rms < 0.02 s) form the three clusters of earthquakes in the NW–SE direction along the lineaments close to Warna Reservoir. The observed seismicity trends on the surface could be due to the vertical manifestations of faults or fractures. Their alignments are confirmed by the analysis of focal mechanism which exhibits a predominant normal type of faults with one nodal plane consistent with the main alignment of the seismic events. [ABSTRACT FROM AUTHOR]

China suffers severe earthquake disasters along the Pacific Rim. Rapid urbanization, population growth and economic production have dramatically changed China both spatially and temporally. Exposure has increased rapidly compared with hazards and vulnerability, and exposure has increased earthquake risks more than hazards and vulnerability. Based on county-level data, this paper emphasizes the rising exposure of high earthquake risk regions (HERRs) by analyzing the concentration of population and GDP in HERRs. The results reveal three findings. (1) Population and GDP transferred to HERRs from 2000 to 2010. The vast majority of the population growth (88%) and GDP growth (63.8%) occurred in HERRs, and 16.4% of the population growth and 16.4% of the GDP growth occurred in counties where an earthquake of M 7.5 + may take place. (2) The shift of population and GDP growth in East China was more dramatic than that in West, Central and Northeast China. Overall, 56.8% of the population growth and 38.0% of the GDP growth occurred in HERRs in East China. (3) The Theil index showed that the internal population distribution for each region was also distinct. East China had the largest population growth in HERRs but had the most separation among the regions in China. In both 2010 and 2000, East China had the largest internal Theil index. The population and GDP were concentrated in the places with high earthquake risks, especially in the developed areas of East China. And East China had the most internal inequality. Increasing earthquake risks are a challenges for the country's social and economic development. [ABSTRACT FROM AUTHOR]

This article characterizes the regions of earthquake slip that are responsible for strong motion generation (SMG), using extreme value theory (EVT). A three-step iterative procedure involving Zipf plot, mean excess function plot and moment ratio plot is used to define the threshold value for a slip model. This threshold slip demarcates the region of SMG in the slip model. The iterative procedure ensures that slip values above the threshold slip follow generalized Pareto distribution (GPD). The regions where slip is greater than the corresponding threshold slip are defined as regions of SMG. A total of 159 slip samples filtered from the SRCMOD catalogue are analyzed in this paper. In order to identify the region of SMG, the first step is to obtain effective slip dimensions. A novel method based on strong motion duration (SMD) is proposed to obtain the effective dimensions of slip model. In the next step, the iterative procedure is employed to obtain threshold slip and the regions of SMG for all the slip samples. The corresponding area of SMG is estimated from the number of sub-faults in the region of SMG and the sub-fault dimensions. Regression analysis is carried out to establish source scaling relationships for the rupture parameters with respect to seismic moment. The considered rupture parameters are effective length, effective width, effective area, effective mean slip, threshold slip and area of SMG of the slip model. On a logarithmic scale, these rupture parameters are observed to follow a linearly increasing trend with the seismic moment. The proposed equations can be used to estimate the rupture parameters for future earthquake events. [ABSTRACT FROM AUTHOR]

An Mw 5.7 earthquake occurred in Xegar, Tingri Basin, Tibetan Plateau, on 20 March 2020. Determining the precise focal mechanism solution is helpful for understanding the seismogenic mechanism and the geodynamic process of this earthquake. Here, we used Sentinel-1A data to obtain line-of-sight coseismic deformation. Fault geometric parameters and coseismic slip distribution can be estimated with the Bayesian method and the steepest descent method, respectively. The inversions show that the strike of the seismogenic fault is ~ 330.0°, with a dip angle of ~ 62.7°. The main rupture zone covers an area of ~ 5 × 5 km2. Only one slip asperity appears at a depth of 1.9–5.0 km, the maximum slip on the fault is 0.98 m at a depth of 3.26 km, with a centroid location of 87.40° E, 28.66° N, and the mean rake angle is ~ −104.6°. The results reveal that this earthquake is dominated by normal faulting, and the derived seismic moment is ~ 3.23 × 1017 Nm (Mw 5.6). Furthermore, it is found that the 2015 Mw 7.9 Gorkha earthquake played a key role in triggering the 2020 Mw 5.7 Xegar earthquake based on calculation of the coseismic and postseismic Coulomb failure stress with different viscosities and depths. [ABSTRACT FROM AUTHOR]