Megathrust earthquake occurrence is dependent on the physical properties and stress environments of convergent plate boundaries. Local and regional earthquakes may modulate the stress environment ...fractionally that affects the nucleation of next earthquakes. This study investigates the influence of precedent earthquakes on the induction of forthcoming earthquakes around the circum‐Pacific plate boundaries. We assess the global stress perturbation induced by 1,636 earthquakes combining 1,457 earthquakes with moment magnitudes greater than or equal to MW7.0 and 179 earthquakes with moment magnitudes MW6.4–6.9 in 1900–2020. We stack the induced Coulomb stress changes for optimally oriented reverse faults. The circum‐Pacific region is divided by eight subregions. The cumulative Coulomb stress changes reach up to the order of tens to hundreds of bar at the convergent plate boundaries. Descendant large earthquakes dominantly occurred in the regions with large lateral gradients of cumulative Coulomb stress changes induced by precedent earthquakes. The cumulative Coulomb stress changes for 120 years are comparable among subduction zones. The seismicity in subduction zones may depend on the strength of the stress field as well as the lateral gradient in the stress field. The instability and inhomogeneity in stress fields may play a major role in the nucleation of megathrust earthquakes.
Plain Language Summary
Great earthquakes produce significant damages over large areas. The timely identification of impending earthquakes is crucial for seismic hazard mitigation. We assess the stress induced from neighboring earthquakes for 120 years around the circum‐Pacific plate boundaries. Large earthquakes perturbed the stress fields. We analyze 1,636 earthquakes with moment magnitudes greater than or equal to 6.4 in 1900–2020. The cumulative Coulomb stress changes reach up to the order of tens to hundreds of bar at the plate boundaries. Large earthquakes followed in the regions with large lateral gradients of cumulative Coulomb stress changes induced by precedent earthquakes. Laterally heterogeneous stress fields may play an important role in nucleation of great earthquakes.
Key Points
The stress fields in subduction zones were highly perturbed by precedent large earthquakes
Descendant large earthquakes dominantly occur in the regions of high stress contrasts
The stress perturbation by precedent earthquakes may be a major factor controlling global seismicity
The distance-dependent coseismic and postseismic displacements produced by the 2011 M
9.0 Tohoku-Oki megathrust earthquake caused medium weakening and stress perturbation in the crust around the ...Korean Peninsula, increasing the seismicity with successive M
5-level earthquakes at the outskirts of high seismicity regions. The average M
5-level occurrence rate prior to the megathrust earthquake was 0.15 yr
(0.05-0.35 yr
at a 95% confidence level), and the rate has increased to 0.71 yr
(0.23-1.67 yr
at a 95% confidence level) since the megathrust earthquake. The 2016 M
5-level midcrustal earthquakes additionally changed the stress field in adjacent regions, inducing the 15 November 2017 M
5.4 earthquake. The successive 2016 and 2017 moderate-size earthquakes built complex stress fields in the southeastern Korean Peninsula, increasing the seismic hazard risks in the regions of long-term stress accumulation. The increased seismic risks may continue until the medium properties and stress field are recovered.
We investigated VP/VS changes in the upper crust of the Japanese islands after the 2011 MW9.0 Tohoku‐Oki megathrust earthquake. Abrupt VP/VS changes with azimuth‐dependent variations were observed ...after the megathrust earthquake. The VP/VS changes ranged between −0.0458 (±0.0012) and 0.0422 (±0.0033). Large localized VP/VS changes over regional distances suggested medium‐dependent deformation. Peak VP/VS changes were observed along paths subparallel or subperpendicular to the directions toward the megathrust earthquake. The VP/VS changes displayed characteristic 2θ variations as a function of the azimuth difference, suggesting azimuthal seismic anisotropy. The VP/VS ratio recovered gradually over time. Some regions presented permanent VP/VS changes. The azimuthal seismic anisotropy may have developed from preferential crack orientation as a consequence of the combined effects of lithospheric displacements and a depth‐dependent ambient stress field. It appears that recovery of the medium properties may take decades.
Plain Language Summary
We report a unique observation of azimuthal seismic anisotropy in the upper crust of Japanese islands, which was induced by the 2011 M9.0 Tohoku‐Oki megathrust earthquake. The azimuthal seismic anisotropy was caused by the direction‐dependent discriminative lithospheric displacement during the megathrust earthquake. We analyzed the traveltimes of local seismic waves and estimated the changes in Vp/Vs ratios after the megathrust earthquake. We observed the abrupt Vp/Vs changes after the megathrust earthquake. The Vp/Vs ratios recovered gradually with time for several years. This study may provide important information on the medium property changes after the megathrust earthquake and will be useful for mitigation of potential seismic hazards.
Key Points
Abrupt VP/VS changes occurred in the upper crust of the Japanese islands after the 2011 Tohoku‐Oki megathrust earthquake
Peak VP/VS changes developed along the paths subparallel or subperpendicular to the megathrust
Azimuthal seismic anisotropy developed due to the lithospheric displacements caused by the megathrust earthquake
The seismicity in the Korean Peninsula has increased since the 2011 Mw9.0 Tohoku‐Oki megathrust earthquake. Two strike‐slip earthquakes with magnitudes of ML5.1 and 5.8 occurred in the southeastern ...Korean Peninsula on 12 September 2016. The two events occurred within 48 min. The ML5.8 earthquake was the largest event in the Korean Peninsula since 1978 when national seismic monitoring began. Both events produced strong high‐frequency ground motions. More than 500 aftershocks with local magnitudes greater than or equal to 1.5 followed the events for 2 months. An unreported subsurface strike‐slip fault with a dip of 65° to the east and a strike of N27°E was responsible for the earthquakes. The fault ruptured at depths of 11–16 km, resulting in a rupture plane of ∼26 km2. The aftershock distribution displayed horizontal streaks at a depth of ∼14 km, which was consistent with the focal mechanism solutions from long‐period waveform inversion. The number of aftershocks decreased exponentially with time. The two ML5.1 and 5.8 earthquakes produced regional Coulomb stress changes of −4.9 to 2.5 bar. The spatial distribution of the aftershocks correlated with the Coulomb stress changes. The peak dynamic stress induced by strong ground motions reached 14.2 bar. The groundwater levels changed coseismically in some regions of decreased static stresses. The earthquakes on previously unidentified faults raised attention for the potential seismic hazards by earthquakes with long recurrence intervals.
Key Points
Moderate‐sized earthquakes occurred on a subsurface hidden fault
Midcrustal earthquakes occurred as a consequence of stress perturbation by a megathrust earthquake
The static and dynamic stress changes caused by the main shock induced aftershocks
The earthquakes in the western East Sea (Sea of Japan) mostly occur in the continental margin off the east coast of the Korean Peninsula. The seismic hazard potentials in and around the western Ease ...Sea are studied based on analyses of tectonic structures, seismicity features, earthquake source properties, Coulomb stress changes, and strong ground motions. The earthquake source mechanisms suggest that paleo-rifting structures in the western East Sea were activated by the current stress field. A low stress cumulation rate results in the occurrence of earthquakes with long recurrence intervals. The background seismicity suggests that earthquakes with magnitudes
M
w
5.0, 6.0, and 7.0 may occur within every
∼
44,
∼
336, and
∼
2550 years at 95 % confidence level. The spatial distribution of earthquakes changes with time. Most earthquakes are clustered within
∼
60 km from the coast. The seismicity analysis indicates an apparent increase of moderate-size (
M
w
3–5) earthquakes since the 2011
M
w
9.0 Tohoku-Oki megathrust earthquake. Static stress changes by moderate-size inland earthquakes induce offshore events. The seismicity and Coulomb stress changes suggest high seismic potentials around the western margin of the Ulleung basin. Earthquakes with magnitudes
M
w
6.0–7.0 in the western East Sea may produce peak ground accelerations of 0.2 g within the distance of
∼
40–80 km, which includes the coastal regions.
A deep crustal strike-slip earthquake swarm that occurred in the intracontinental region of the southwestern Korean Peninsula is investigated. The earthquakes are detected using matched filter ...analysis. The hypocenters are then refined, showing more than 500 micro to small strike-slip earthquakes concentrated on a fault plane with dimensions of 500 m by 300 m at a depth of ~21 km. The low temperature in the source depth facilitates the seismicity. The earthquake swarm was transiently active for 15 days from 25 April 2020 to 12 May 2020, presenting a high seismicity rate with a Gutenberg-Richter b value of 2.01 (±0.05). The earthquake swarm began in the central area and expanded outward with time. The spatiotemporal seismicity distribution reflects bilateral rupture. The focal mechanism solutions suggest a left-lateral strike-slip fault with a strike of 103.6∘ ± 3.2∘ and a dip of 73.3∘ ± 11.7∘. The spatial distribution of seismicity generally agrees with the focal mechanism solutions. The orientation of the inverted stress field agrees with the ambient stress field, suggesting that the earthquake swarm occurred as a consequence of ambient lithostatic stress loading. Coulomb stress changes suggest that the stress increased in the directions of WNW-ESE and NNE-SSW from the fault. The transiently active earthquake swarm began to occur after the 2011 MW9.0 Tohoku-Oki megathrust earthquake, suggesting a rapid release of accumulated stress in response to changes in the properties of the medium caused by the megathrust earthquake.
•One-off crustal strike-slip earthquake swarms occur in a stable intracontinental region.•Deep-crustal earthquake swarm expands outward along the fault plane.•The earthquake swarm started to occur in weakened crust after a megathrust earthquake.
Major earthquakes in continental regions may cause significant damage. Preexisting fault system across megacity receives high attention for possible seismic damages. Earthquake occurrence mechanism ...is important to assess the geohazard potentials. Continental‐scale Quaternary fault system is developed across the Seoul metropolitan area where the population is the largest in the Korean Peninsula. Historical seismic‐damage records suggest potential seismic hazards in the Seoul metropolitan area. We investigate the fault motions and spatial distribution of earthquakes in the Seoul metropolitan area using a matched‐filter technique that is based on stacked waveform crosscorrelation functions among densely‐deployed seismic stations. The analysis detects 1103 earthquakes that include 360 events with magnitudes (ML) of −0.6 to 2.0 around the Chugaryeong fault and 34 events with magnitudes of −0.5 to 2.7 around Wangsukcheon, Pocheon, and Yeseonggang faults. The seismicity suggests a set of near‐vertical subparallel (or orthogonal) faults that develop from the major faults. A major fault system behaves as a backbone structure that makes branch faults develop, producing seismicity including major earthquakes. The backbone structure may control the fault development that conforms to the ambient stress field. The backbone faults may play a role to increase geohazard potentials.
Plain Language Summary
The Seoul metropolitan area is the most populated region in the Korean Peninsula. Historical seismic damage records and major Quaternary faults suggest high seismic hazard potentials in the region. The identification of earthquake‐spawning faults is crucial for seismic hazard mitigation. We illuminate the earthquake‐spawning faults from micro to small earthquakes in the Seoul metropolitan. We perform a matched‐filter analysis to find earthquakes in adjacent locations of reported events. Recently‐deployed temporary dense seismic stations allow us to detect microearthquakes in high ambient‐noise environments of the megacity. We detect 1103 earthquakes that include 360 events with magnitudes (ML) of −0.6 to 2.0 around the Chugaryeong fault and 34 events with magnitudes (ML) of −0.5 to 2.7 around Wangsukcheon, Pocheon, and Yeseonggang faults. The earthquake distribution suggests the presence of active near‐vertical subparallel or conjugate faults along the major Quaternary faults that may behave as backbones of earthquake‐spawning faults.
Key Points
Micro to small earthquakes occur around Quaternary faults in the Seoul metropolitan area
Subvertical strike‐slip faults develop along the Quaternary faults, responding to the ambient stress field
Clustered earthquakes occur along conjugate or subparallel faults
The average shear‐wave velocity within the top 30 m from the surface, VS30, represents site characteristics including the soil classification and site amplification that are essential information for ...building codes and seismic design. A novel method to determine a VS30 model based on a composite analysis of borehole standard penetration test numbers (SPT N) and horizontal‐to‐vertical (H/V) spectral ambient noise ratios is introduced. A national VS30 model for South Korea is determined using the method. The shear‐wave velocity structures beneath 20 nationwide broadband seismic stations are determined using the H/V analysis. The SPT N data are collected from 175,619 nationwide densely‐distributed boreholes. The shear‐wave velocity models from SPT N values are calibrated for the local reference velocity models from H/V analysis. A representative relationship between the SPT N values and shear‐wave velocities is introduced. A national VS30 model for South Korea is determined using the calibrated SPT N models at the nationwide boreholes. The VS30 model is verified by comparisons with local field measurements. The proposed model is consistent with the USGS model based on a surface slope analysis. The VS30 structure presents high correlation with geological and topographic features. The VS30 values are low in coastal (low topographic) areas, and high in mountain (high topographic) areas. Apparent linear relationship is observed between VS30 and topography. The western and southeastern coastal regions may be vulnerable to strong seismic shaking.
Plain Language Summary
Seismic ground motions are an important factor to control the seismic damages. The seismic amplification is highly dependent on the shear‐wave velocities at shallow depths ≤30 m, VS30. We introduce a novel method to determine a VS30 model based on the standard penetration test numbers (SPT N) from nationwide 175,619 boreholes and horizontal‐to‐vertical spectral ambient noise ratios (H/V ratios) from 20 broadband seismic stations. We determine a VS30 model for South Korea. The shear‐wave velocity structures beneath broadband seismic stations are used for local reference velocity models. The shear‐wave velocity models from SPT N values are calibrated for the local reference velocity models from the seismic stations. We determine a representative relationship between the SPT N values and shear‐wave velocities. We determine a high‐resolution VS30 model using the calibrated SPT N models at boreholes. The proposed model is verified by comparisons with other results. The VS30 structure presents high correlation with geological and topographic characteristics. The VS30 values are low in coastal areas, and high in mountain areas. The western and southeastern coastal regions may be vulnerable to strong ground motions during earthquakes.
Key Points
A novel method based on SPT N values and H/Vratios is proposed for determination of national VS30 model for South Korea
A high‐resolution national VS30 model can be calculated using the calibrated SPT N data at densely distributed boreholes
The VS30 model presents high correlation with geological and topographic features
Mitochondria are crucial for cellular energy metabolism and are involved in signaling, aging, and cell death. They undergo dynamic changes through fusion and fission to adapt to different cellular ...states. In this study, we investigated the effect of knocking out the dynamin 1-like protein (Dnm1l) gene, a key regulator of mitochondrial fission, in neural stem cells (NSCs) differentiated from Dnm1l knockout embryonic stem cells (Dnm1l−/− ESCs). Dnm1l−/− ESC-derived NSCs (Dnm1l−/− NSCs) exhibited similar morphology and NSC marker expression (Sox2, Nestin, and Pax6) to brain-derived NSCs, but lower Nestin and Pax6 expression than both wild-type ESC-derived NSCs (WT-NSCs) and brain-derived NSCs. In addition, compared with WT-NSCs, Dnm1l−/− NSCs exhibited distinct mitochondrial morphology and function, contained more elongated mitochondria, showed reduced mitochondrial respiratory capacity, and showed a metabolic shift toward glycolysis for ATP production. Notably, Dnm1l−/− NSCs exhibited impaired self-renewal ability and accelerated cellular aging during prolonged culture, resulting in decreased proliferation and cell death. Furthermore, Dnm1l−/− NSCs showed elevated levels of inflammation and cell stress markers, suggesting a connection between Dnm1l deficiency and premature aging in NSCs. Therefore, the compromised self-renewal ability and accelerated cellular aging of Dnm1l−/− NSCs may be attributed to mitochondrial fission defects.
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