Earthquake Generation Research Articles

Seismic structure and its implication on the hydrothermal system beneath Mt. Ontake, central Japan

Mt. Ontake is an active volcano in central Japan where phreatic eruption activity is prominent. A shallow subsurface structure in the summit region of this volcano has been scarcely studied despite its importance. Our study examines the structure from three seismic analyses: a typical P-wave velocity from the semblance of the vertical propagation of intermediate-depth earthquakes (IDEs), a layered velocity model from P- and S-wave arrival times of shallow volcanic earthquakes, and a pseudoreflection profile from the autocorrelation functions of the IDEs. Our results consistently indicate the presence of three layers, which are interpreted as younger (> 0.1 Ma) and older (0.39–0.78 Ma) eruptive deposits and the basement. A comparison of the structure with hypocentres and deformation sources suggests fluid migration controlled by the structure.Graphical

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.

Daily to Centennial Behavior of Aseismic Slip Along the Central Section of the North Anatolian Fault

AbstractSlow, aseismic slip plays a crucial role in the initiation, propagation, and arrest of large earthquakes along active faults. In addition, aseismic slip controls the budget of elastic strain in the crust, hence the amount of energy available for upcoming earthquakes. The conditions for slow slip include specific material properties of the fault zone, pore fluid pressure, and geometrical complexities of the fault plane. Fine scale descriptions of aseismic slip at the surface and at depth are key to determine the factors controlling the occurrence of slow, aseismic versus rapid, seismic fault slip. We focus on the spatial and temporal distribution of aseismic slip along the North Anatolian Fault, the plate boundary accommodating the 2 cm/yr of relative motion between Anatolia and Eurasia. Along the eastern termination of the rupture trace of the 1944 M7.3 Bolu‐Gerede earthquake lies a segment that slips aseismically since at least the 1950s. We use Sentinel 1 time series of displacement and GNSS data to provide a spatio‐temporal description of the kinematics of fault slip. We show that aseismic slip observed at the surface is coincident with a shallow locking depth and that slow slip events with a return period of 2.5 years are restricted to a specific section of the fault. In the light of historical measurements, we discuss potential rheological implications of our results and propose a simple alternative model to explain the local occurrence of shallow aseismic slip at this location.

Triggering mechanism and brittle-ductile dynamics of active faults in the south-central Saurashtra horst, Gujarat, western India: A geospatial, geological, and geophysical approach

The seismically active Saurashtra horst is located within the intraplate volcanic continental margin of western India. The region is prone to moderate and low-magnitude earthquakes within the depth range of ∼ 3 to ∼ 24 km. We observed that the earthquakes in this region are associated with seismically active brittle and ductile crustal layers. To understand the dynamics of the earthquake generation process, we applied an integrated geological and geomorphological approach, supplemented by subsurface geophysical (magnetotelluric) studies. Additionally, the active surface deformation has been measured using the PSInSAR and GLA techniques. Based on the stream offset and geomorphic landform development pattern several NW-SE and NE-SW oriented strike-slip faults have been identified. The PSI-derived displacement analysis reveals that the area is deforming at the rate of ± 5 mm/yr. Furthermore, subsurface crustal heterogeneity with increasing depth has been identified using the magnetotelluric technique, which is reflected in the form of basaltic lava flows, plutonic emplacement within the granitic basement, and the presence of semi-crystallized magmatic bodies below the brittle-ductile level. Additionally, we proposed a model to depict the plutonic emplacement within the highly fractured/faulted granitic basement and their relationship to the earthquake generation process. Our model shows that crustal heterogeneity and the migration of hydrothermal fluid from the semi-crystallized magmatic body along the active fault cause earthquake nucleation processes within the brittle and ductile layers. We concluded that the upwelling magmatic fluid above the brittle-ductile transition (BDT) acted as a lubricant for the nucleation and triggering of the earthquake along the active faults. Similarly, the fractured ductile crust is weakened by fluid migration, which causes high fluid pressure in the ductile crust thereby decreasing the confining pressure and endorsing the velocity weakening in the aseismic layer, responsible for the shear instability that causes deep crustal earthquakes. More specifically, the lithological heterogeneity at brittle and ductile regimes is an important factor for the earthquake nucleation process in this part of the Indian plate.

Re‐Examining Temporal Variations in Intermediate‐Depth Seismicity

AbstractChanges in the frequency of intermediate‐depth (60–300 km) earthquakes in response to static stress transfer can provide insights into the mechanisms of earthquake generation within subducting slabs. In this study, we use the most up‐to‐date global and regional earthquake catalogs to show that both the aftershock productivity of large earthquakes, and the changes in the frequency of intermediate‐depth earthquakes around the timing of major megathrust slip, support the view that faults within the slab are relatively insensitive to static stress transfer on the order of earthquake stress drops. We interpret these results to suggest the population of faults within the slab are much further from their failure stress than is typical for shallow fault systems. We also find that aftershock productivity varies within slabs over small spatial scales, indicating that the mechanism that enables faults to rupture at intermediate depths is likely to be spatially heterogeneous over length‐scales of a few tens of kilometres. We suggest dehydration‐related weakening mechanisms can best account for this heterogeneity.

Carbonates and intermediate-depth seismicity: Stable and unstable shear in altered subducting plates and overlying mantle

A model for intermediate-depth earthquakes of subduction zones is evaluated based on shear localization, shear heating, and runaway creep within thin carbonate layers in an altered downgoing oceanic plate and the overlying mantle wedge. Thermal shear instabilities in carbonate lenses add to potential mechanisms for intermediate-depth seismicity, which are based on serpentine dehydration and embrittlement of altered slabs or viscous shear instabilities in narrow fine-grained olivine shear zones. Peridotites in subducting plates and the overlying mantle wedge may be altered by reactions with CO2-bearing fluids sourced from seawater or the deep mantle, to form carbonate minerals, in addition to hydrous silicates. Effective viscosities of magnesian carbonates are higher than those for antigorite serpentine and they are markedly lower than those for H2O-saturated olivine. However, magnesian carbonates may extend to greater mantle depths than hydrous silicates at temperatures and pressures of subduction zones. Strain rates within altered downgoing mantle peridotites may be localized within carbonated layers following slab dehydration. A simple model of shear heating and temperature-sensitive creep of carbonate horizons, based on experimentally determined creep laws, predicts conditions of stable and unstable shear with strain rates up to 10/s, comparable to seismic velocities of frictional fault surfaces. Applied to intermediate-depth earthquakes of the Tonga subduction zone and the double Wadati-Benioff zone of NE Japan, this mechanism provides an alternative to the generation of earthquakes by dehydration embrittlement, beyond the stability of antigorite serpentine in subduction zones.

Dynamic Response Analysis of Roadway Surrounding Rock Induced by Dynamic Load under the Action of Hard and Thick Rock Stratum

In the process of coal seam mining, there are often hard thick key layers in the overlying strata. Due to the high strength and good integrity of the hard thick key layer, after the hard thick key layer is broken, the overlying strata will collapse and lose stability in a large area, which is very easy to induce dynamic disasters such as rock burst, mine earthquake, coal wall caving, and roof slab caving. Aiming at the hard and thick key layer overlying the working face, the dynamic response of the mine under the strong mine earthquake induced by the breaking of the main key layer of high-level magmatic rock is numerically simulated and analyzed by using FLAC2D numerical simulation software, and the variation laws of the stress field, displacement field, and velocity field of the coal seam roadway under different boundary conditions and different focal heights are studied. The research shows that the roof of solid coal roadway is prone to vibration in a small range, and the displacement increases and decreases with the disturbance. The displacement of the floor and two sides of the solid coal roadway and the top floor and two sides of the roadway along the goaf continues to increase in the initial stage of the disturbance, and the displacement will remain stable with the continuation of the disturbance. The displacement of both sides and roof and floor of gob roadway can reach stability in the later stage of disturbance, and with the increase of the number of adjacent goaf, the longer it takes for the displacement of surrounding rock to reach stability. When the focal height is lower than 90 m, the variation of surrounding rock response increases sharply with the decrease of focal height. When a strong earthquake occurs in the low rock stratum, the impact damage of roadway surrounding rock is almost inevitable. The influence degree of strong earthquake on the stability of roadway surrounding rock is arranged as follows: gob-side roadway (mined out on one side) > solid coal roadway (mined out on both sides) > solid coal roadway (mined out on one side). The evolution process also shows that the working face boundary conditions have an important influence on the energy propagation of mine earthquake. With the increase of the number of adjacent goafs, the faster the energy attenuation rate of mine earthquake propagation is. The research results have important reference significance for the safe mining of working face under similar geological conditions.

Do upper-plate material properties or fault frictional properties play more important roles in tsunami earthquake characteristics?

Tsunami earthquakes are a type of shallow subduction zone events that rupture slowly (<1.5 km/s) with exceptionally long duration and depleted high frequency radiation, resulting in a large discrepancy of Mw and Ms magnitudes and abnormally large tsunami along coastal areas. Heterogeneous fault frictional properties at shallow depth have been thought to dominate tsunami earthquake generation. Some recent studies propose heterogeneous upper-plate material properties determine rupture behavior of megathrust earthquakes, including characteristics of tsunami earthquakes. In this study, we use a recently developed 3D dynamic earthquake simulator to explore tsunami earthquake generation and systematically examine roles of upper-plate material properties and fault frictional properties in tsunami earthquake characteristics in a physics-based framework. For heterogeneous fault friction, we consider isolated asperities with strongly velocity-weakening properties embedded in a conditionally stable zone with weakly velocity-weakening properties. For heterogeneous upper-plate properties, we consider a generic depth profile of seismic velocity and rigidity constrained from seismic surveys. We design a set of models to explore their effects on tsunami earthquake generation and characteristics. We find that the conditionally stable zone can significantly slow down rupture speeds of earthquakes that nucleate on asperities to be <1.5 km/s over a large depth range (1–20 km), while heterogeneous upper-plate properties can only reduce rupture speeds to be ∼1.5–2.0 km/s over a narrow depth range (1-3 km). Nevertheless, heterogeneous upper-plate properties promote cascading rupture over multiple isolated asperities on the shallow subduction plane, contributing to large tsunami earthquake generation. We also find that fault frictional properties play much more important roles than upper-plate material properties in long normalized duration, high-frequency depletion and low moment-scaled radiated energy in tsunami earthquakes. In addition, the effective normal stress on the subduction plane, which affects fault frictional strength, also influences the characteristics of tsunami earthquakes, including duration, stress drop and moment-scaled radiated energy.

Earthquake Hazard Analysis in Ciletuh Pelabuhan Ratu Geopark Area, West Java

Indonesia is situated between Asia and Australia, as well as the Indian and Pacific Oceans. This circumstance places Indonesia in a geologically complicated pattern, which increases the intensity of earthquakes, tsunamis, volcanoes, and landslides. UNESCO has classified Pelabuhanratu Ciletuh Geopark as a world heritage area. This has resulted in an explosion in tourist and development activity. Even though this tourism destination has the potential for high seismicity, there has been no analysis of the seismic hazard in this region. This study aims to map the earthquake hazard in the Ciletuh Pelabuhan Ratu Geopark Area. This will be important for determining vital assets' placement and development planning. This study employs remote sensing studies and geographic information systems to examine and classify earthquake-prone locations. We evaluated data from DEM, RBI, and soil-type maps. This method evaluates each earthquake hazard metric using the Analytical Hierarchy Process (AHP). The large research area has a slope between 0-300. Few locations have steep slopes. In the Districts of Cisolok, Cikakak, and portions of Pelabuhan Ratu, the characteristics of the huge hard rock are derived from the Quaternary volcanic deposits of Mount Endut. As a result, this region possesses a solid rock structure that can absorb an earthquake propagation wave. This dynamic process of geomorphological creation can also demonstrate that the region surrounding the lineage will feel the effects of a future earthquake. The results show that Ciemas, Simpenan, parts of Cisolok, and Pelabuhanratu districts are included in the moderate to high category of earthquake threats.

Evaluation of Anti-Burst Performance in Mining Roadway Support System

The hazardous effect of a mine earthquake on a roadway is not only related to its energy scale but also to its distance from the roadway. In this study, a signal attenuation model and a disaster-causing model were established to evaluate the mine earthquake effects based on peak particle velocity (PPV) data recorded for 37221-1 upper roadway of the Dongxia Coal Mine, China. The characteristic of dynamic loads due to mine earthquake propagation to roadway surfaces was researched, and critical PPV values were identified using FLAC3D numerical simulation, which can be used to evaluate the roadway anti-burst performance under the existing support system. The results show that the support system is able to resist a mine earthquake with energy below 2.33 × 103 J; however, considering the energy accumulation volume of surrounding rocks and the range of source fracture, the maximum resistible mine earthquake energy can be up to 7.09 × 106 J when the roadway is 50 m away from the source. The validity and applicability of the disaster-causing models was verified by two rockburst cases that occurred during the excavation of the working face.

Nowcasting earthquakes in Iran: A quantitative analysis of earthquake hazards through natural times

Despite the extremely complex earthquake generation mechanism in a driven nonlinear threshold system, simple empirical scaling laws apply to the collective behavior of earthquakes in a network of geological faults. The physical interpretation of these properties and associated relations exhibit a wealth of information to study earthquake processes and consequent earthquake hazards in a defined area. In this article, an area-source-based earthquake nowcasting method is implemented to statistically determine the current state of earthquake hazards at several cities of Iran through the formulation of a discrete time-series of cumulative interevent counts, known as natural times. Use of natural times to mark the evolution of the seismic process, rather than the passage of clock time, essentially provides spatiotemporal and clustering invariable properties. Statistical inference of natural time counts corresponding to M ≥ 6.0 events results in an earthquake potential score (%) as Ahvaz (94), Hamadan (50), Isfahan (98), Karaj (76), Kerman (78), Kermanshah (73), Mashhad (40), Qom (92), Rasht (21), Shiraz (98), Tabriz (45), Tehran (78), Urmia (51), and Zahedan (53). These nowcast values provide a snapshot of the current progression of a city in its earthquake cycle of large-sized events and thus facilitate better decision-making, city-planning, and earthquake awareness, leading to an efficient seismic risk reduction strategy in the densely populated study region.

INTEGRATED MONITORING OF HAZARDOUS GEOLOGICAL PROCESSES IN PRIBAIKALYE: PILOT NETWORK AND FIRST RESULTS

The article deals with the first results of integrated geohazard monitoring conducted by the Institute of the Earth’s Crust SB RAS on the territory of Pribaikalye in 2020. The pilot network consists of three sites: "Buguldeika", "Priolkhonye" and "Listvyanka", each of which is equipped with high-precision digital devices including a broadband seismic station, a GPS receiver, deformometers, a sensor of soil radon emanations, and an observation station for the Earth’s electromagnetic environment. This equipment is designed to acquire quantitative information on rock deformation, recent movements and geophysical field variations for solving theoretical and applied problems of geodynamics and seismology, including development of earthquake prediction methods. In the vicinity of the sites, there have been made the hydroisotopic measurements as well as observations over the character of some of exogenous processes. Based on the comprehensive analysis of the seismological, tectonic, deformation and emanation data, acquired also through monitoring, there has been obtained the preliminary characteristics of the Kudarinsky earthquake (December 9, 2020, M W =5.6) that was followed by intensity 5 aftershocks in large cities of the southeastern East Siberia – Irkutsk, Shelekhov, Angarsk, Usolye-Sibirskoe and others. It has been found that the seismic event manifested itself almost in all the fields monitored. This implies the network efficiency for a purposeful study of the precursors of large earthquakes which can initiate the development of other hazardous geological processes in Pribaikalye. The deformation monitoring data show some general patterns of earthquake source evolution which corresponds to the fundamental principles of physical mesomechanics. This opens the prospects for diagnostics of the final phase of earthquake generation in the context of meta-instable state of deformation process and rock mass disintegration.

Tsunami Numerical Modelling in Ujung Kulon National Park with the Earthquake Magnitude Scenario of 6.5 and 6.9 Mw

Tsunami modelling was conducted to simulate tsunami events based on earthquake data previously occurring around Ujung Kulon National Park (TNUK), Banten Province using TUNAMI-F1 and TUNAMI-N3. TUNAMI-F1 is a modelling method to get the wave height and tsunami arrival time with a grid spacing of 0.00167°. While the TUNAMI-N3 is a model that applies the theory of wave propagation linearly in a grid variation to get a high output of tsunami inundation. The tsunami source data was used from historical earthquake generation on January 14, 2022 (6.5 Mw) and August 2, 2019 (6,9 Mw). The reverse fault type scenario from Wells & Coppersmith (1994) is used to calculate the scaling law with length, width, and the focal mechanism value obtained from the United States Geological Survey (USGS) to generate initial waves based on the multideform model. The TUNAMI-N3 model applies a nested model with variations in grid spacing and the most detailed value is 68,556 m. The tsunami wave height based on the magnitude 6.5 Mw scenario is around 0.001 m to 0.05 m with the highest wave height found on the southern coast of TNUK and the southern coastal area of Peucang Island (0.03-0.05m). The inundation produced is only visible in the E1 domain reaching 96-192 m in areas A, B, C. Then, the maximum inundation area is the south coast of TNUK (0.01-0.03m). Based on the second scenario, the height of the tsunami waves generated is in the range of 0.001 − 0.3 m. The maximum tsunami height was in the Rancecet Beach area, south of Tinjil Island, the south, east, and north coasts of TNUK (0.03-0.3m). The run-up distance resulting from scenario 1 is less than 1 km. Based on TUNAMI-F1 model results, wave height maximum caused by 6.5 Mw magnitude is higher than 6.9 Mw.

Seismic Anisotropy Tomography and Mantle Dynamics of Central‐Eastern USA

AbstractWe present high‐resolution 3‐D tomographic models of isotropic P‐wave velocity (Vp), azimuthal anisotropy, and radial anisotropy down to 1,300 km depth beneath the central and eastern United States (CEUS), which are obtained by inverting a great number of local and teleseismic data and making a whole‐mantle correction to teleseismic travel‐time data recorded by the USArray. Trade‐offs between azimuthal and radial anisotropies occur due to the correlation between the azimuthal and incidence angles of seismic rays, but the use of uniform and crisscrossing rays can reduce the trade‐off effect. Our tomographic images reveal the North American Craton with layered anisotropy and strong anisotropies in the lower mantle related to the deeply subducted Farallon slab and passage of the Bermuda hotspot. In the upper mantle, low‐velocity anomalies with significant seismic anisotropy are revealed beneath three intraplate seismic zones in New Madrid, East Tennessee, and South Carolina, suggesting that hot and wet mantle upwelling occurs under the seismic zones, and the related fluids affect the earthquake generation. The most important dynamic processes in the mantle beneath the CEUS are the deep subduction of the Farallon plate and the passage of the Bermuda hotspot, which have caused strong structural heterogeneities, seismic anisotropy, as well as thermal anomalies and fluids that contributed to the formation of intraplate seismic zones in the CEUS.

Geometric Control on Seismic Rupture and Earthquake Sequence Along the Yingxiu‐Beichuan Fault With Implications for the 2008 Wenchuan Earthquake

AbstractThe 2008 Mw 7.9 Wenchuan earthquake is the most disastrous seismic event in China since 1976. Both field and seismological investigations suggest a multi‐stage coseismic rupture with most damage associated with the Yingxiu‐Beichuan Fault (YBF) of spatially variable fault strike and dip angles. To investigate the effect of fault geometric complexity on coseismic rupture and paleoseismic pattern on the YBF, we perform earthquake sequence modeling on 3D fault geometry in the framework of rate‐and‐state friction law. Our model produces a long‐term earthquake sequence with quasi‐regular recurrences of whole‐fault ruptures and segmented ruptures. The along‐strike rupture segmentation, earthquake propagation speed, and slip rate are mainly controlled by along‐strike variations of fault dip and strike angles. Particularly, the YBF can be divided into two segments: the southern segment featured by large events (Mw &gt; 8.0) and the northern segment with smaller events (Mw &lt; 7.5), with recurrence intervals primarily determined by the tectonic loading rate. In a whole‐rupture event, the rupture speed mainly correlates with the fault dip angle; a smaller dip angle results in a wider seismogenic zone and higher rupture speeds, whereas the small‐scale variation in rupture speed is regulated by the fault strike angle. The effect of strike variation on the total coseismic slip amount is more pronounced due to the large strike angle gradient along the YBF. After varying the slip vector to reflect the northward transition from the dominant thrust‐slip to strike‐slip faulting during the Wenchuan coseismic rupture, we obtain reasonably good agreement of model simulated coseismic surface displacements with GPS observations.

Urban water supply systems’ resilience under earthquake scenario

Threats to water supply systems have increased in number and intensity. Natural disasters such as earthquakes have caused different types of damage to water distribution networks (WDN), particularly for those with aged infrastructure. This paper investigates the resilience of an existing water distribution network under seismic hazard. An earthquake generation model coupled with a probabilistic flow-based pressure driven demand hydraulic model is investigated and applied to an existing WDN. A total of 27 earthquake scenarios and 2 repair strategies were simulated. The analysis examined hydraulic resilience metrics such as pressure, leak demand, water serviceability, and population impacted. The results show that nodal pressure drops below nominal pressure and reaches zero in some earthquake scenarios. Leak demand could reach to more than 10 m3/s within hours following an earthquake. Water serviceability drops to a low of 40% and population impacted reaches up to 90% for a 6.5 M earthquake, for example. This study highlights and quantifies vulnerabilities within the simulated WDN. The tools outlined here illustrate an approach that can: (1) ultimately help to better inform utility water safety plans, and (2) prepare proactive strategies to mitigate/repair before a hazard of this nature occurs.

Nanoscale Damage Production by Dynamic Tensile Rupture in α‐Quartz

AbstractThe creation of new fractures during an earthquake produces rock damage and contributes to the dissipation of strain energy. During dynamic rupture propagation, tensile microfractures can form in the earthquake process zone and in the domains around a fault that host large transient tensile stress. These microfractures can produce rock fragments with a wide range of sizes. Using molecular dynamics simulations, we model tensile rupture propagation in α‐quartz under conditions of stress that occur during earthquake propagation. Our results show that for rupture speeds below 15% of the Rayleigh wave speed, fractures propagate linearly. At higher speeds, fracture propagation undergoes path instabilities with crack oscillations and microbranching leading to the formation of nanoscale roughness and fragments. This nanoscale damage can form in and around the earthquake process zone before any significant slip has occurred on the fault. The produced nanoparticles may control further energy dissipation during frictional slip.

Anatomy of large earthquakes in North China

• Most of the large events occurred in high-V or close to the boundary between low-V and high-V zones. • Prominent low-V and high-σ anomalies in the middle and lower crust reflect crustal fluids . • Crustal fluids may originate from hot and wet upwelling flows in the big mantle wedge. • Crustal fluids affected the nucleation and generation of intracontinental large earthquakes. We present detailed 3-D isotropic P and S wave velocity (Vp, Vs) images and Vp azimuthal anisotropy in source areas of 26 large crustal earthquakes (M5.6–7.8) that occurred in North China during 1966 to 2021. Most of the large earthquakes are located in high-velocity (high-V) zones or close to the boundary between low-velocity (low-V) and high-V anomalies. Prominent low-V and high Poisson’s ratio (high-σ) anomalies are revealed in the middle and lower crust under almost all the source areas, which may reflect crustal fluids. The low-V and high-σ anomalies may cause local thinning and weakening of the brittle seismogenic layer above them. The crustal fluids may originate from hot and wet upwelling flows in the big mantle wedge (BMW) beneath East Asia due to corner flows in the BMW and deep dehydration reactions of the stagnant Pacific slab in the mantle transition zone.

STUDY, FORECAST AND CONTROLLED SEISMIC HAZARD REDUCTION IN THE IDENTIFIED SEGMENTS OF THE MAIN FAULTS BY CYCLIC INJECTION OF FLUID THROUGH DEEP MULTI-BRANCH DIRECTIONALLY INCLINED WELLS

The methods developed by the world community to date to withstand strong natural and induced destructive earthquakes do not effectively reduce material losses and the number of victims. The authors propose for discussion an integrated approach to solving the problem of ensuring seismic safety, based on the use of new important information about the geological conditions for earthquake generation. This involved the use of results of numerical and physical modeling, as well as physical full-scale experiments in the natural fault areas. The paper analyzes the petrophysical conditions of deep-seated frictional processes in coseismic faults, revealed through detailed studies of the fragments of paleoearthquake centers that became accessible after their exhumation from seismic-focal depths of the Earth’s crust. The collected information allowed the authors to clarify with a high degree of certainty the origin and occurrence of seismic motions. This paper presents briefly the results of the medium-term forecast of earthquakes with M≥5.0 as applied to the seismodynamic regime of the Baikal rift zone. The forecast emphasizes the detection of places for 1–11-year earthquake generation cycles.A comprehensive analysis of the collected information made it possible to substantiate the conclusion about an opportunity to prevent earthquake damage by using hydrodynamic damping of seismically hazardous fault segments. In the last section, consideration is being given to one of the most promising methods of such man-made impacts, which uses modern technological advances in drilling deep multil-branch and directionally inclined wells with horizontal deviation. The paper discusses the techniques that make it possible to prevent episodes of unexpected reactivation of fault segments in the form of excitation of earthquakes with M≥6.0. Attention is drawn to conducting tests at selected sites in order to improve the technology as part of the approach to earthquake damping.

Introduction to the special issue on the Hualien earthquake swarms

The Hualien city is located near the pole of rotation induced by the along-strike transition from Ryukyu subduction to the collision between the Luzon arc and the Chinese continental margin in northeastern Taiwan (Rau et al., J Geophy Res 113: B09404, 2008). Such a tectonic regime creates varied seismogenic structures that are responsible for the ruptures of the 1951 M L 7.3 Hualien-Taitung earthquake sequence (Chen et al., J Geophy Res 113: B02304, 2008), the 2018 M w 6.4 Hualien earthquake sequence (e.g., Rau and Tseng, Terr Atmos Ocean Sci 30: 281–283, 2019), the 2021 M L 6.2 Shoufeng earthquake sequence, and numerous and persistent moderate-to-small size earthquake swarms in the Hualien area. This special issue was introduced by the several month-long 2021 episode of moderate-to-small size earthquake swarms occurred in the Hualien area (Figs. 1 and 2, Central Weather Bureau, 2022). The 2021 Hualien earthquake episode includes small-to-moderate-size earthquakes that release minute strain energy on the small asperities and larger events (M > 5.5) that ruptured nearby asperities. Figure 3 shows the spatiotemporal distribution of the Hualien earthquake sequence between April 1 and December 31 of 2021. The 2021 Hualien earthquake sequence provides us a great opportunity to examine the earthquake physics, characteristics, seismogenic processes, and the early warning algorithm of an earthquake swarm, which are the key elements for the understanding of generation of earthquakes. In this special issue, we have collected six papers that detailed various aspects of the 2021 Hualien earthquake sequence, which provide us a better understanding of the nature of the Hualien earthquake swarms.Fig. 1The crustal (≤ 40 km) Hualien earthquake swarm activities observed from April to December of 2021. Star symbols show the epicenters of the largest three earthquakes in this time period. Rainbow color represents the focal depth of each event. The radius of each circle is proportional to the observed MLFig. 2The temporal distribution of seismicity in the upper 40 km beneath this area observed from April to December of 2021. Several swarm activities can be identified, in which intense seismicity happened within a short time interval. The main earthquake swarms occurred from April to July of 2021. Numbers indicate the days after April 1st, 2021Fig. 3The spatiotemporal distribution of the Hualien earthquake swarm activities from April to December of 2021. The symbol color represents the time in days after April 1st, 2021