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Double seismic zones (DSZs), parallel planes of intermediate‐depth earthquakes inside oceanic slabs, have been observed in a number of subduction zones and may be a ubiquitous feature of downgoing oceanic plates. Focal mechanism observations from DSZ earthquakes sample the intraslab stress field at two distinct depth levels within the downgoing lithosphere. A pattern of downdip compressive over downdip extensive events was early on interpreted to indicate an unbending‐dominated intraslab stress field. In the present study, we show that the intraslab stress field in the depth range of DSZs is much more variable than previously thought. Compiling DSZ locations and mechanisms from literature, we observe that the “classical” pattern of compressive over extensive events is only observed at about half of the DSZ locations around the globe. The occurrence of extensional mechanisms across both planes accounts for most other regions. To obtain an independent estimate of the bending state of slabs at intermediate depths, we compute (un)bending estimates from slab geometries taken from the slab2 compilation of slab surface depths. We find no clear global prevalence of slab unbending at intermediate depths, and the occurrence of DSZ seismicity does not appear to be limited to regions of slab (un)bending. Focal mechanism observations are frequently inconsistent with (un)bending estimates from slab geometries, which may imply that bending stresses are not always prevalent, and that other stress types such as in‐plane tension due to slab pull or shallow compression due to friction along the plate interface may also play an important role. Plain Language Summary In subduction zones, a plate of oceanic lithosphere descends into the mantle. This means it gets bent from a horizontal orientation offshore the subduction zone to an inclined orientation. Analogous to the bending of a solid beam, this bending of the oceanic lithosphere creates extension in the upper part and compression in the lower part of the oceanic plate. The orientation of these stresses can be retrieved from earthquake focal mechanisms for events that occur in the outer rise region, that is, offshore the actual subduction zone. At deeper depths, downgoing slabs are thought to straighten, which means they decrease their curvature and “unbend.” This has the opposite signature in earthquake focal mechanisms as the bending. We compiled focal mechanism information from in‐slab earthquakes from global subduction zones, in order to check if such an “unbending” signature is present everywhere at depths of 50–300 km. We find that only about half of the investigated regions show such a signature, while the other ones are extensive everywhere. We then compare these findings with global slab shapes and try to constrain what different processes (e.g., stretching of the entire slab due to its weight, bending forces) influence the stress field inside downgoing plates. Key Points Double seismic zone earthquake mechanisms globally show downdip extensive lower planes, upper planes can be downdip compressive or extensive Slab bending and unbending estimates derived from slab2 grids show no ubiquitous presence of plate unbending at intermediate depths Intraslab stress fields are influenced by in‐plane tension, plate bending and megathrust friction; what dominates where is hard to predict

This study focuses on the empirical assessment of the control period TC for intermediate-depth earthquakes. A ground motion database of about 700 recordings from 25 intermediate-depth seismic events occurring in the Vrancea seismic zone in Romania as well as in other sources in the world is used in the study. A simple empirical model is proposed for the evaluation of the control period TC. The largest part of the variability of the proposed empirical model can be attributed to the intra-event component. The analysis of the residuals has shown no significant bias due to the different tectonic regimes of the earthquakes in the database. A comparison between the seismic hazard results in terms of TC computed using the proposed empirical model and from spectral accelerations according to the procedure from the proposed Eurocode 8 draft is performed for several case study sites in Romania. The probabilistic seismic hazard assessment shows that the control periods TC computed in this study are much larger than the ones obtained using the approach from the proposed Eurocode 8 draft revision and larger, as well, than the TC values proposed in the current version of the Romanian seismic design code P100-1/2013. The study also shows that for Romania, a solution for the seismic hazard zonation is to perform the analysis of Sα according to the provisions from the proposed Eurocode 8 draft and to evaluate the control period TC using an empirical model.

The Vrancea Seismic Zone (VSZ) is an atypical intermediate-depth earthquake nest located in the South-East Carpathians in Romania, often regarded as a relic slab sinking into the mantle. The origin of the slab and the earthquakes it produces are debated because brittle failure is unlikely to occur at mantle depths. Cluster types and statistical properties of seismicity can vary with deformation regime, fluid content and temperature. Here we investigate the spatial–temporal properties of earthquakes recorded in the VSZ from a high-quality local catalogue, identify and classify earthquake clusters, placing constraints on the debated nature of the Vrancea slab, its coupling with the overlying crust, and its potential for triggering crustal earthquakes. We use the nearest-neighbour distance to estimate the correlation between pairs of earthquakes, considering the origin time, magnitude, b -value and fractal dimension of the observed seismicity with M w  ≥ 2.5 from the 1977–2023 period. With this approach, we find a small percentage of clustered seismicity (8%), suggesting that background seismicity dominates. We identified 12 conventional mainshock-dominated sequences and 17 swarms with relatively small (0.92) and large (1.73) b -values, respectively, that may reflect differences in stress or the presence of fluids in the crustal volume. The aftershock decays— p -values of 0.99 for the large-event clusters and 1.42 for the swarms—may reflect typical and, respectively, fast stress relaxations. Swarm clusters are predominantly localised within the crust, whereas conventional mainshock sequences lack foreshocks and prevail in the subcrustal domain, occasionally triggering aftershocks at shallower depths. This partitioning suggests higher heat flow and fluid-modulated seismicity in the crust, as opposed to brittle failure conditions, triggered by slab dynamics in the mantle, including abrupt stress release without preceding signals. The spread of aftershocks from subcrustal large mainshocks to shallower depths indicate a stress transfer process where the descending Vrancea slab influences seismic activity in the overlying crust. Graphical Abstract

The North–South Seismic Zone is a well-known seismotectonic zone in China that frequently experiences major earthquakes. A new probabilistic seismic hazard assessment (PSHA) for this zone is required. In this study, we perform a new PSHA for the North–South Seismic Zone with new models, including a fault source model and a time-dependent seismicity model of major-earthquake seismogenic structures. We demonstrate that the seismic hazard in areas near faults calculated using the fault source model is higher than that calculated using the area model. Faults close to their expected recurrence time for major earthquakes have high seismic hazard. Areas of high seismic hazard in the North–South Seismic Zone between 2016 and 2065 are the Moxi Segment of the Xianshuihe Fault, the Anninghe Fault, and the Daliangshan Fault. High-seismic-hazard areas between 2066 and 2115 are the Luhuo and Zheduotang segments of the Xianshuihe Fault and the Anninghe Fault. The Moxi Segment of the Xianshuihe Fault has the highest seismic hazard over the next 50 years.

Large earthquakes alter the stress in the surrounding crust, leading to triggered earthquakes and aftershocks 1 , 2 , 3 . A number of time-dependent processes, including afterslip, pore-fluid flow and viscous relaxation of the lower crust and upper mantle, further modify the stress and pore pressure near the fault, and hence the tendency for triggered earthquakes 4 , 5 . It has proved difficult, however, to distinguish between these processes on the basis of direct field observations, despite considerable effort 6 . Here we present a unique combination of measurements consisting of satellite radar interferograms 7 and water-level changes in geothermal wells following two magnitude-6.5 earthquakes in the south Iceland seismic zone. The deformation recorded in the interferograms cannot be explained by either afterslip or visco-elastic relaxation, but is consistent with rebound of a porous elastic material in the first 1–2 months following the earthquakes. This interpretation is confirmed by direct measurements which show rapid (1–2-month) recovery of the earthquake-induced water-level changes. In contrast, the duration of the aftershock sequence is projected to be ∼3.5 years, suggesting that pore-fluid flow does not control aftershock duration. But because the surface strains are dominated by pore-pressure changes in the shallow crust, we cannot rule out a longer pore-pressure transient at the depth of the aftershocks. The aftershock duration is consistent with models of seismicity rate variations based on rate- and state-dependent friction laws.

Due to the demand for massive urbanization, more high‐rise buildings need to be constructed in major cities. The framed tube structural system can be an effective framing technique for this case. This system comprises tightly spaced peripheral columns connected by deep spandrel beams. Reinforced concrete structural walls provide substantial lateral strength and stiffness to limit damage when structures are subjected to ground shaking. In this paper, T‐shaped shear walls replace the peripheral columns to obtain better performance. The reduced cost of columns, walls, and beams of square‐shaped framed tube structures with peripheral T‐shaped walls compared to the column is in the range of 8%–18%. The shear lag factor for the ground floor of square‐shaped framed tube structures is also reduced from 4% to 56% for different structural configurations. The concrete quantity of columns, walls, and beams of square‐shaped framed tube structures with peripheral T‐walls is reduced by 6%–19%. The longitudinal reinforcement quantity of columns, walls, and beams is also reduced by 12%–25%. The reaction force for the foundation design of square‐shaped framed tube structures with peripheral T‐shaped walls lies between 2% and 11% less than the square‐shaped framed tube structures with peripheral columns. The seismic performance level of buildings with peripheral T‐walls is immediate occupancy (IO) for the hazard level corresponding to service level earthquake (SLE), design basis earthquake (DBE), and maximum considered earthquake (MCE), while IO to life safety (LS) for the buildings having peripheral columns. The plastic hinge formation of considered buildings with peripheral columns is 1%–5%, 2%–7%, and 7%–12%, SLE, DBE, and MCE, respectively, while the buildings with peripheral T‐walls are insignificant. The drift ratio of buildings with peripheral walls is less than that of those with peripheral columns.

Southern Iceland is one of the main outlets of the Icelandic ice sheet and is subject to seismicity of both tectonic and volcanic origins along the South Iceland Seismic Zone (SISZ). A sedimentary complex spanning Marine Isotopic Stage 6 (MIS 6) to the present includes evidence of both activities. It includes a continuous sedimentary record since the Eemian interglacial period, controlled by a rapid deglaciation, followed by two marine glacioisostasy-forced transgressions, separated by a regression phase connected to an intra-MIS 5e glacial advance. This record has been constrained by tephrostratigraphy and dating. Analysis of this record has provided better insights into the interconnectedness of hydrology and volcanic and tectonic activity during deglaciations and glaciations. Low-intensity earthquakes recurrently affected the water-laid sedimentation during the early stages of unloading, accompanying rifting events, dyke injection, and fault reactivations. During full interglacial periods, earthquakes were significantly less frequent but of higher magnitude along the SISZ, due to stress accumulation, favored by low groundwater levels and more limited magma production. Occurrence of volcanism and seismicity in Iceland is commonly related to rifting events. Subglacial volcanic events seem moreover to have been related to stress unlocking related to limited or full unloading/deglaciation events. Major eruptions were mostly located at the melting margin of the ice sheet.

The Koyna-Warna region in western India is well known around the globe for recurrent reservoir-triggered seismicity soon after the impoundment of the Koyna and Warna reservoirs. The seismicity pattern delineates two distinct seismic zones, Koyna Seismic Zone (KSZ) and Warna Seismic Zone (WSZ). To understand the seismic potential of the region, we estimated the strain budget by analysing the published GPS velocities and earthquake catalogue of the region. Although the KSZ and WSZ are separated by ~25 km only, the rate of strain accumulation in the former (2.55E+16 Nm/year) is estimated to be ~11 times larger when compared to the latter (2.29E+15 Nm/year). However, KSZ releases only ~20% of the accumulated energy per year, whereas, WSZ releases most of the accumulated energy in the form of earthquakes. Best fitting elastic dislocation model for KSZ also shows a left lateral slip of 0.8 mm/year and the fault plane dips at ~77° in NW direction. The distribution of strain accumulation and release rates in the two regions may be attributed to significant spatial variability in the medium properties such as density, elastic constants and fracture density. This proposition is supported by other geophysical studies in the region. A density model constructed from Airborne Gravity Gradiometry (AGG) data also shows relatively higher average density for KSZ compared to the WSZ. The strain budget of the region suggests that the earthquake activity in KSZ may continue for a longer time whereas it may diminish in the WSZ in the near future. Based on the gross strain estimates, the KSZ has accumulated enough strain post the 1967 M6.3 Koyna earthquake to generate an event of Mw5.8, provided the accumulated strain is released in a single event. The seismic hazard scenario in terms of peak ground acceleration (PGA) due to a potential Mw5.8 event is estimated using the stochastic simulation (SS) technique. The simulated PGA at a radial distance of ~40km from the source zone is estimated to range between 0.09 and 0.26g with an expected intensity of V–VII.

The spatial variation of ground motion is determined for high-frequency P- and S-waves in the New Madrid Seismic Zone at a site near Mooring, TN. An L-shaped array consisting of 19 seismometers and having arm lengths of 600 m located on Holocene fluvial sediments of the Mississippi River was used to examine wave coherency appropriate for many sites throughout the Mississippi embayment and other sediment sites associated with large rivers. Data from local and regional earthquakes within a distance range of 300 km show that coherency within the frequency band of 0.5 to 16 Hz degrades with inter-station distance across the array according to an empirical exponential model. Vertical component P-waves are coherent over nearly 2 horizontal wavelengths. However, the coherency for horizontal component S-waves degrades more rapidly than the vertical component P-waves. S-waves become significantly incoherent at distances of only 0.2 horizontal wavelength and become completely incoherent after 0.5 wavelength. The results of this study show clear and quick decay of wave coherence likely to occur in strong ground motions from nearby earthquakes. The observed incoherence can be a significant factor for the response of structures having foundation lengths of even 100 m.

Understanding the seismo–ionospheric coupling mechanism requires a quiet geomagnetic condition, as this represents an ideal situation to detect abnormal variations in the geomagnetic field. In reality, continuous interactions between solar wind and Earth’s magnetosphere create many fluctuations in the geomagnetic field that are more related to sun–magnetosphere interactions than to seismotectonic causes. A triaxial magnetometer was installed at the Muntele Rosu Observatory near the Vrancea seismic zone in 1996 to measure the local magnetic field. Since 2002, the data have become more consistent, allowing for the representation of long time series. Since then, variations have been observed on the eastern component (By) of the magnetic field, which sometimes overlaps with significant earthquakes. Previous studies have shown that high decreases in amplitude recorded on the By component of the magnetic field measured at Muntele Rosu have been accompanied by higher seismicity, while small decreases have been accompanied by lower seismic energy release. This research analyzes the geomagnetic data collected between September 2002 and May 2008 from two geomagnetic observatories, one located in the proximity of the Vrancea seismic zone and another one situated 120 km away. For each geomagnetic anomaly identified, the daily seismic energy released was plotted logarithmically, along with seismicity and Kp indices. Additionally, the daily seismic energy released was also plotted logarithmically for all earthquakes with Mw ≥3. To identify variations in the By component, datasets recorded at Muntele Rosu (MLR) were compared with those recorded at Surlari National Geomagnetic Observatory (SUA), to discriminate between global magnetic variations associated with solar activity and possible seismo–electromagnetic variations. The standard deviation (SDBy) was calculated for each anomaly recorded on the By component of the magnetic field and compared with the cumulative seismic energy release. To determine if this type of variation was present in other components of the magnetic field, the following ratios were calculated for all data recorded at Muntele Rosu: Bz/Bx, Bz/By, and Bz/BH. The size of the anomalies resulting from the standard deviation measured on the By component (SDBy) partially validates the relationship between the size of the anomalies and the seismic energy release during the anomaly. The relationship between the released seismic energy and the anomaly magnitude is vaguely respected, but these variations seem to follow two patterns. One pattern is described by smooth decreases, and the other pattern involves decreases where the By component varies significantly over short periods, generating decreases/increases in steps. It was noticed that seismic activity is greater for the second pattern. Additionally, using standard deviation measured on the magnetic field represents a great tool to discriminate external magnetic field variations from local, possibly seismo–magnetic variations.