Earthquake Source Region Research Articles

Postseismic processes in the region of the july 29, 2021 Chigni earthquake, Alaska. Part I: modeling results

We analyze the postseismic processes in the region of the Mw 8.2 Chignik earthquake, which occurred on July 29, 2021. Using the seismic rupture surface model constructed in our previous paper (Konvisar et al., 2023), we have simulated the viscoelastic relaxation process. The results of the simulation have shown that reducing the viscosity of the asthenosphere to 1018 Pa ‧ s in the calculations gives displacement velocities close to those recorded at the GPS coastal points. However, the displacements on islands close to the earthquake source region differ significantly not only in magnitude but also in direction. At the same time, the constructed model of the postseismic creep is closely consistent with the GPS displacement data and with the LOS (line-of-sight) displacement map derived from radar images acquired from the descending orbit of Sentinel 1A satellite. We also analyze temporal variations of the gravity field in the earthquake region. The obtained coseismic anomaly agrees with the anomaly calculated from the rupture surface model. Due to the insufficiently long series of gravity models after the earthquake, it is not yet possible to isolate the postseismic anomaly. The analysis of the postseismic processes is continued in the second part of this work (Smirnov et al., 2024), where we present the compare the time evolution of the postseismic displacements of various GPS sites with the aftershock activity, which allows us to draw conclusions about the creep nature of the postseismic processes in the source region of the Chignik earthquake.

High‐Frequency Ground Motions of Earthquakes Correlate With Fault Network Complexity

AbstractUnderstanding the generation of damaging, high‐frequency ground motions during earthquakes is essential both for fundamental science and for effective hazard preparation. Various theories exist regarding the origin of high‐frequency ground motions, including the standard paradigm linked to slip heterogeneity on the rupture plane, and alternative perspectives associated with fault complexity. To assess these competing hypotheses, we measure ground motion amplitudes in different frequency bands for 3 ≤ M ≤ 5.8 earthquakes in Southern California and compare them to empirical ground motion models. We utilize a Bayesian inference technique called the Integrated Nested Laplace Approximation (INLA) to identify earthquake source regions that produce higher or lower ground motions than expected. Our analysis reveals a strong correlation between fault complexity measurements and the high‐frequency ground motion event terms identified by INLA. These findings suggest that earthquakes on complex faults (or fault networks) lead to stronger‐than‐expected ground motions at high frequencies.

Analysis of the Stress Field Around Concealed Active Fault From Minor Faults‐Slip Data Collected by Geological Survey: An Example in the 1984 Western Nagano Earthquake Region

AbstractEarthquakes with magnitudes of 6–7 have been reported even in various active tectonic settings where fault deformation topography have not been detected. Therefore, delineating concealed active faults generating such earthquakes is necessary to reduce earthquake damage; however, few studies exist to provide its clues regarding such faults. The 1984 Western Nagano Earthquake in Japan was a main shock with a magnitude of Mj 6.8 and a depth of 2 km at the source. Solid bedrocks are well‐exposed in the earthquake source region; however, no surface rupture have been identified, and the active fault is known to be concealed. In this study, we collected data on striations observed in fractures by geological survey around the source area of the 1984 Western Nagano Earthquake. Using the collected data, the multiple inverse method was used to estimate the stresses that affected the striation formation. Consequently, stresses similar to acting faults in this area were detected in minor faults around the known concealed active fault. This suggests that the minor faults might be part of the damage zone that has been developed around the concealed active fault. Some minor faults were recognized in Quaternary volcanic rocks, confirming that they experienced displacements recently. This study indicates the possibility of detecting concealed active faults in the bedrock by geological survey.

Seismic Velocity Response to the Ocean Waves Based on Borehole Array

The ambient noise correlation in recent years has been widely used to study the seismic velocity temporal variation related to volcanoes, geothermal fields, earthquake source regions. However, in some regions and times, seasonal seismic velocity changes which could be attributed to environmental factors, including temperature, atmospheric pressure and groundwater, can also be observed. Eliminating the influence of these environmental factors in the temporal variation monitoring results of subsurface media will be helpful to obtain the clear changes of shallow media due to tectonic activities. We used the ambient noise correlation technique of multi-component and multi-frequent-band to obtain the seismic velocity variation of crustal media in Hailing Island, Guangdong Province, China. The results show that the seismic velocity variation of crustal medium in the study area has a strong correlation with the ocean wave height

Probabilistic Tsunami Hazard Assessment Considering the Sequence of the First and Second Earthquakes Along the Nankai Trough

The Earthquake Research Committee (ERC) of the Headquarters for Earthquake Research Promotion conducted a probabilistic tsunami hazard assessment due to large earthquakes along the Nankai Trough for the next 30 years. Utilizing the basic data of earthquake source models and calculated maximum tsunami heights, the authors propose a method to evaluate the hazard curves of the first and second earthquakes separately, considering the sequence of earthquake occurrence in one cycle of large earthquake activities along the Nankai Trough. First, based on the relative weights allocated to the 176 occurrence patterns of 79 earthquake source regions, the weights for the 2,720 characterized earthquake fault models (CEFMs) are calculated. The hazard curve of the first earthquake is evaluated using the 2,720 sets of maximum tsunami heights and weights under the condition that one of the CEFMs causes an earthquake. Next, the conditional hazard curves for the possible second earthquakes conditional on each individual first earthquake are calculated. Finally, the hazard curve for the second earthquake is evaluated as a weighted average of the conditional hazard curves. Numerical examples are shown for 15 sites. The first earthquake accounts for about 60% or more of the total hazard evaluated by ERC, and its contribution increases with increasing maximum tsunami height. The first and second earthquakes account for 80%–90% of the total hazard.

Apparent stress as an indicator of stress meta-instability: The 2021 MS6.4 Yangbi earthquake in Yunnan, China

Investigating spatiotemporal changes in crustal stress associated with major earthquakes has implications for understanding seismogenic processes. However, in individual earthquake cases, the characteristics of the stress after it reaches its maximum value are rarely discussed. In this study, we use the 2021 MS6.4 Yangbi earthquake in Yunnan, China and events of magnitudes ML ≥ 3.0 occurred in the surrounding area in the previous 11 years to investigate the spatiotemporal evolution of apparent stress. The results indicate that apparent stress began to increase in January 2015 and reached a maximum in January 2020. Apparent stress then remained at a high level until October 2020, after which it declined considerable. We suggest that the stress was in the accumulation stage from January 2015 to January 2020, and entered the meta-instability stage after October 2020. During the meta-instability stage, the zone of decreasing stress expanded continuously and the apparent stress increased around the Yangbi earthquake source region. These features are generally consistent with the results of laboratory rock stress experiments. We propose that apparent stress can be a good indicator for determining whether the stress at a specific location has entered the meta-instability stage and may become the epicenter of an impending strong earthquake.

Coseismic Deformation of the 2023 Türkiye Earthquake Doublet from Sentinel-1 InSAR and Implications for Earthquake Hazard

Abstract The 6 February 2023 Türkiye earthquake doublet occurred on the east Anatolian fault system, which marks the tectonic boundary between the Arabia plate and the Anatolian microplate. This earthquake doublet consists of the Mw 7.8 Pazarcik earthquake along the east Anatolian fault and the Mw 7.6 Çardak earthquake along the Savrun–Çardak fault. Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR) satellite successfully imaged the surface deformation caused by this earthquake doublet. The pixel offset from cross correlation of two Synthetic Aperture Radar images complements the interferograms in mapping the surface ruptures and the near-field deformation. We inverted for a coseismic slip model in elastic half-space using the InSAR phase and the range offset data. The variance reduction of the inversion reaches ∼90%. The coseismic slip model shows that the 2023 Türkiye earthquake doublet are left-lateral strike-slip events. The peak slip is located near Nurhak in southern Türkiye along the Savrun–Çardak fault. From measuring discontinuities in the pixel offset images we found that the surface rupture length of the Pazarcik earthquake is ∼300 km and the surface rupture length of the Çardak earthquake is ∼100 km. To first order, the faults are dipping vertically. “Slip gaps” are identified by our modeling, and they might be the source regions of future large earthquakes.

Subdaily Slow Fault Slip Dynamics Captured by Low‐Frequency Earthquakes

AbstractGeodetic positioning is the geophysical record of reference for slow slip events, but typical daily solutions limit studies of the evolution of slow slip to its long‐term dynamics. Accompanying seismic low‐frequency earthquakes located precisely in time and space provide an opportunity to image slow slip dynamics at subdaily time scales. Here we show that a high‐resolution time history of low‐frequency earthquake fault slip alone can reproduce the geodetic record of slow slip that we observe to be dominated by subdaily fault slip dynamics. However, a simple linear model cannot accommodate the complex dynamics present throughout the slow slip cycle, and an analysis of different phases of the slow slip cycle shows that the ratio of geodetic to seismic fault slip varies as a function of time. This suggests that the low‐frequency earthquake source region saturates as slow slip grows in moment and area. We propose that rheological heterogeneities at the plate boundary associated with low‐frequency earthquakes do not play a significant role in the slow slip rupture process, thus implying that their activity is incidental to the driving aseismic slip.

Crustal structure and seismicity characteristics based on dense array monitoring in northwestern Yunnan, China

Owing to the complex geological environment and frequent seismic activity in northwestern Yunnan, China, the northwest Yunnan dense array was installed. The dense array monitored the Mw6.1 Yangbi earthquake that occurred on May 21, 2021, on the western boundary of the Sichuan-Yunnan block. The earthquake exhibited a typical foreshock-mainshock-aftershock sequence and presented a clear process of earthquake migration. From April 1, 2018 to December 31, 2021, 11,623 earthquakes were recorded by 72 stations in the dense array. Based on their arrival time data, the Vp/Vs model consistency-constrained double-difference seismic tomography method was employed. We obtained high-precision crustal 3-D Vp, Vs, Vp/Vs models, and relocation results for northwestern Yunnan. A wide distribution of low-velocity anomalies was observed in the middle and upper crust of northwestern Yunnan. Based on GPS measurements, the contour of the low-Vp zone is aligned with the clockwise movement of crustal material caused by the compression of the Tibetan Plateau. A seismic cluster with a banded distribution pattern was identified in an intersection area of multiple faults. And its formation might be related to the infiltration of geothermal fluid in a fluid channel composed of unknown fractures. The geothermal fluid originates from the intersection of the fault-enriched hot springs. The flow and seepage from the fluid channels spread to the Yangbi earthquake source region, reactivating the previously existing unmapped faults, ultimately leading to the occurrence of Yangbi foreshocks. Because of the stress transmission, the Coulomb stress of the foreshock triggered the high-strength part of the main fault, which triggered the occurrence of the main earthquake.

A method for revealing seismic event precursores in geomonitoring data

The paper presents an overview of the methodological and algorithmic results obtained in the framework of long-term polygon studies of source regions of strong earthquakes. Methods for identifying precursors of seismic events in geomonitoring data are considered. Algorithmic schemes are presented for identifying precursors of strong earthquakes by analyzing the time series of previous seismic activity. To represent the relationship of various geodynamic factors that are associated with the studied geophysical fields resonances of gravitating factors are considered which are understood as the intersection of normalized graphs of individual components of gravitational forces. The proposed algorithms and methodology for constructing the “energy predictive wedge” model are based on this principle.

Gravity yearly transient change around the epicenter of 2022 MS 6.9 Menyuan earthquake, China and seismotectonic implications

The MS 6.9 Menyuan earthquake occurred in the northeastern Tibetan Plateau on 8 January 2022. High-precision absolute gravity observations provide a unique perspective on seismotectonic feature of strong continental earthquakes. An obvious and coincident gravity increase about 5 years before the Menyuan earthquake with the rate up to 3.94 μGal/yr is observed at four absolute gravity stations within 350 km away from the epicenter. These four stations have been carried out repeat gravity observations by the Crustal Movement Observation Network of China (CMONOC) for more than a decade. The observed gravity changes could not be reasonably explained by the land water storage change nor crustal vertical deformation and thus might be related to the processes in the crust. A disc-shaped equivalent source region with a radius of 175 km is then modeled, based on the residual gravity rates. We find that the seismicity increases gradually in the modeled source region from about 2017 to 2021, similar to the pattern of absolute gravity change. Furthermore, the spatiotemporal distribution of epicenters is consistent with the fluids diffusion model, indicating that this could be related to the redistribution of deep fluid mass. With consideration of the near-surface processes, regional crustal structure, and seismicity, multiple lines of evidence consistently refer to the presence of deep fluids migration before the Menyuan earthquake. Our results suggest that high-precision absolute gravimetry can be used to monitor crustal fluid environment and determine mass changes in the source regions of potential large earthquakes.

Complex fault geometry of the 1976 Ms 7.8 Tangshan earthquake source region in North China

Investigating the fine structure of fault zones is essential for understanding the mechanism of large earthquake nucleation. In this paper we study the complex fault geometry of the Tangshan fault zone in North China using a new seismic data set from 2010 to 2020 that spans ∼10 years. Based on the well-located hypocenters and focal mechanism solutions, we obtained more detailed information on the fault zone structural features: (1) the fault zone is divided into five segments according to different dip angles of the fault plane; (2) the spatial distribution of seismicity reveals conjugate fault branches and a flower-shaped structure at the northern part of the fault zone; and (3) the fault zone is dominated by a strike-slip type, but their intersecting segments are characterized by a normal-fault type. We further obtained high-resolution seismic images of Vp, Vs and Vp/Vs around the Tangshan source region. Combining the fault geometry with the low-velocity anomalies below 10 km depth, we infer that the ascending of hot mantle material into the lower crust may have weakened the Tangshan fault zone and generate the 1976 Ms. 7.8 Tangshan earthquake.

THE MARCH 25, 2020 M<sub>W</sub>=7.5 PARAMUSHIR EARTHQUAKE AND ITS IMPACT ON RECENT GEODYNAMICS OF THE ADJACENT SECTION OF THE KURIL-KAMCHATKA SUBDUCTION ZONE

An intraplate tsunamigenic earthquake with МW=7.5 occurred on March 25, 2020 southeast of the Paramushir Island (Kuril Islands) beneath the outer slope of the Kuril-Kamchatka Trench. Since 1900, this earthquake has been the largest event for an 800-km long oceanic slope and a 300-km long segment of the Kuril seismofocal zone located near the epicenter. Sub-horizontal compression stresses generated in the earthquake source region were oriented across the seismofocal zone. A type of motion is represented by reverse faulting along the both nodal planes.The compressive stress state in which there occurred the Paramushir earthquake reflects the present-day geodynamics in the subduction zone near the hypocenter. The paper shows that the earthquake occurrence is due to a strong mechanical contact surface between the Pacific and North American lithospheric plates in the subduction zone. The analysis of coseismic displacement of the nearest Global Navigation Satellite System (GNSS) station served as confirmation of the determination of fault plane solution of the earthquake. A seismogenerating motion occurred along the plane oriented to the southwest and dipping towards the trench. For Finite fault source models, there were calculated the increments of the Coulomb stress in the subduction zone. For the main fault plane, the increment of the Coulomb stress in the interpolate contact area propagates to a depth of ~30 km and reaches 1 bar.Coseismic stress increment in the subduction zone at the northern flank of the Kuril island arc, which has a high seismic potential at the present stage of the tectonic cycle, increases the likelihood of the largest interplate earthquake occurrence therein.

A Preliminary Result: Characteristic and Mechanism of Deep earthquake in Java subduction zone

The occurrence of deep-focus earthquakes is unusual and has become one of mysterious seismic phenomena. The behavior of deep earthquake occurrence is still poorly understood. The geodynamical process of a deep earthquake must have a fundamental difference of physical processes from a shallow or intermediate earthquake. There are two main processes of deep earthquake occurrence, water infiltration into the subduction zone (dehydration embrittlement) and transformational faulting of metastable olivine. In Indonesia, the Java subduction zone is known as the deep earthquake source region. We will present preliminary results from the study of characteristic and geodynamical processes of deep earthquake events. We used catalog data from ISC-EHB and mechanism data from global CMT catalog within study area 3° – 13°S and 104,5° – 115°E. The activity of deep earthquakes in Java subduction showed 3 main characteristics i.e the seismicity decreases exponentially in 60 – 300 km depth, lack of earthquake activity due to hole structure in 330 – 500 km and increasing earthquake activity in 500-600 km depth due to transformational faulting of metastable olivine. Earthquakes’ focal mechanism most varies at shallow depth, subsequently, focal mechanism transition occurred from reverse at 60-300 km to normal at >300 km depth.

Seismicity modulation in a 3-D rate-and-state interacting fault population model

SUMMARY Seismicity rate is sometimes observed to correlate with periodic stress oscillations, such as seasonal changes in hydrological loads, or solid-earth and oceanic tidal stresses. However, seismicity modulation is far from being systematic, often weak and the mechanical control is not yet fully understood. Here, a 3-D interacting fault population loaded by a periodic stress has been studied. For that a new earthquake simulator coupling rate-and-state friction and 3-D quasi-dynamic elasticity has been developed, and used to explore the mechanical conditions leading to seismicity modulation. It is first shown that seismicity modulation is maximized when the amplitude of periodic loading stress approaches aσ, a being the viscous parameter of the rate-and-state friction and σ the normal stress in the earthquake source region. The mode of modulation is frequency dependent: short loading periods lead to a stress control (seismicity rate follows the Coulomb stress) while longer periods lead to stressing rate control (seismicity rate tracks Coulomb stressing rate variations). An analytical expression for the critical period Tc separating the two regimes is derived from Dieterich’s theory, showing a dependence of the transition on Coulomb stress amplitude. Finally, the average seismicity rate either scales exponentially with Coulomb stress (stress control), or linearly with Coulomb stressing rate (stressing rate control), as suggested by Dieterich’s theory. These features indicate that for the range of parameters explored, stress redistribution within the fault population is a second order effect in the seismicity modulation, which is primarily controlled by the properties of remote loading. This paper therefore confirms with numerical simulations the Coulomb stress and stressing rate control of seismicity rate, and the limited importance of stress redistribution in controlling earthquake production anticipated by previous theoretical studies.

Simulation of the grace space system observation data of the focal areas of the strong simushir and okhotsk sea earthquakes

Over the past decades studies of the gravitational field and its anomalies using space technologies have significantly intensified. Satellite data allow at the global and local level with sufficient accuracy to study the geodynamics of source zones of strong earthquakes. In this work for the analysis of seismically active regions of the Kuril-Kamchatka arc and the geodynamic processes occurring here measurements made by the GRACE are used. Additionally, to analyze seismic activity in the study areas we used geophysical information contained in the USGS catalog for the period 2006–2013. Digital maps of the distribution of the EWH parameter relative to the source regions of strong earthquakes with magnitude Mw>8 for background and disturbed conditions have been constructed. The modified anomaly index was calculated and on its basis digital maps of observed gravitational anomalies were built.

Complex Fault Geometry of the 1976 Ms 7.8 Tangshan Earthquake Source Region in North China

Complex Fault Geometry of the 1976 Ms 7.8 Tangshan Earthquake Source Region in North China

Complex Fault Geometry of the 1976 Ms 7.8 Tangshan Earthquake Source Region in North China

Complex Fault Geometry of the 1976 Ms 7.8 Tangshan Earthquake Source Region in North China

Changing impacts of Alaska-Aleutian subduction zone tsunamis in California under future sea-level rise

The amplification of coastal hazards such as distant-source tsunamis under future relative sea-level rise (RSLR) is poorly constrained. In southern California, the Alaska-Aleutian subduction zone has been identified as an earthquake source region of particular concern for a worst-case scenario distant-source tsunami. Here, we explore how RSLR over the next century will influence future maximum nearshore tsunami heights (MNTH) at the Ports of Los Angeles and Long Beach. Earthquake and tsunami modeling combined with local probabilistic RSLR projections show the increased potential for more frequent, relatively low magnitude earthquakes to produce distant-source tsunamis that exceed historically observed MNTH. By 2100, under RSLR projections for a high-emissions representative concentration pathway (RCP8.5), the earthquake magnitude required to produce >1 m MNTH falls from ~Mw9.1 (required today) to Mw8.0, a magnitude that is ~6.7 times more frequent along the Alaska-Aleutian subduction zone.

Mica kink-band geometry as an indicator of coseismic dynamic loading

Recognizing that exhumed shear zones had a seismogenic history is an important prerequisite for microstructural studies of energy partitioning in the earthquake source region. Using optical measurements of kinked micas along the Sandhill Corner shear zone, a seismogenic fault/shear zone exhumed from the base of the frictional-to-viscous transition, we demonstrate that their geometric properties are statistically different from kinks in micas that underwent regional deformation and metamorphism. Kink-band asymmetries and the angles of external rotation are larger in rocks from the Sandhill Corner shear zone, whereas kink-band widths are smaller. Our data show remarkable similarities to kink bands from dynamic deformation experiments, meteorite impacts, and nuclear explosion sites, suggesting a dynamic origin for their formation. Our results suggest that mica kink bands may be a useful indicator of dynamic loading and paleoseismicity in the rock record.