Explore the vast range of titles available.

Compared to continental strike‐slip faults, oceanic transform faults (OTFs) are thought to mainly slip aseismically and host significantly more foreshocks triggered by precursory aseismic slip which enhance the mainshocks' short‐term predictability. However, long‐term high‐resolution observational constraints remain limited. In December 2024, one of the largest ever OTF earthquakes occurred offshore California on the Mendocino OTF. Here we show that foreshock activity is very limited within the rupture zone of this moment magnitude (Mw) 7.0 earthquake and does not indicate accelerating aseismic slip in the preceding month. The 2016 Mw 6.6 and 1994 Mw 7.0 Mendocino OTF earthquakes share similar characteristics. The 15 historical Mw ≥ 5.5 mainshocks also have significantly fewer foreshocks on average compared to continental strike‐slip earthquakes. Therefore, there is no clear evidence of aseismic‐nucleation‐related seismicity preceding large Mendocino OTF earthquakes. Our results further demonstrate that enhanced foreshock activity is not a general characteristic of OTFs despite prevalent aseismic slip.

The article presents a new type of morphostructural objects defined by the author: intratransform lenticular extensions confined to active troughs of transform faults. A morphostructural analysis is carried out using geophysical data. It has been revealed that such structures are formed in active zones of transform faults with maximum offset of segments of the mid-ocean ridge. Intratransform lenticular extension zones have been classified, according to which types I‒III of morphostructures were identified, which illustrate not only various evolutionary stages, but also various kinematic scenarios. Type I is a lenticular zone. This type includes small, young (<10 Ma) lenticular extension zones of the local level, occurring mainly in the transform fault of the Southern Hemisphere. Type II is a lenticular transformed basin. It is a lenticular formation at the regional level with a large (~100 to ~300 km) offset formed on a large transform fault with a block of highly deformed lithosphere in the axial part of the lens and shear movements along one of their fringing arc troughs, but without internal spreading segments. Type III is a multitransform lenticular system. This is a global level lenticular formation, located on a large transform boundary with an extreme offset, limited by passive arcuate scarps and enclosed between them by several closely spaced transform fault trough valleys and internal spreading zones. A kinematic model is constructed for identified morphostructure types I‒III in accordance with the classification of intratransform lenticular extension zones.

The mantle plumes modify geophysical and geochemical features along and across mid‐ocean ridges. Despite abundant studies of plume‐ridge interaction, few geodynamic studies focus on the Arctic Ocean. The Jan Mayen Hotspot is located at the southern end of the Mohns Ridge and offset by the Jan Mayen Transform Fault, which creates an ideal area to study plume‐ridge‐transform fault interaction at the ultra‐slow spreading ridge. Through analyzing geophysical observations, we revealed that the M factor and crustal thickness decrease and the axial relief increases northeastward along the Mohns Ridge within a distance of ∼370 km to the Jan Mayen Hotspot. Combined with modeling results, the properties of the Jan Mayen plume were estimated, which has a diameter of 75 km, a temperature anomaly of 100°C, and a buoyancy flux of 0.22 Mg/s. Additionally, our model results indicate that the along‐ridge dispersion of plume is slightly enhanced by the transform fault. Plain Language Summary Mid‐ocean ridges are places in the ocean where the Earth's plates separate and new crust is formed. Hotspots and oceanic transform faults modify the seafloor topography and chemical composition of the igneous crust. In the Arctic Ocean, the Mohns Ridge is located between 70°N and 74°N, with the nearby Jan Mayen Hotspot and Jan Mayen Transform Fault to the south, making it a natural laboratory for the investigation of plume‐ridge‐transform interaction. This study combines different types of geophysical data and numerical models to quantify how plumes and transform faults influence the seafloor topography and crustal structure at the ultra‐slow spreading Mohns Ridge. The findings reveal that the Jan Mayen mantle plume affects the seafloor togography and crustal thickness over several hundreds of kilometers along the Mohns Ridge. The presence of an oceanic transform fault above the mantle plume reduces the thermal anomaly for adjacent ridge segments but enhances the along‐ridge plume dispersion. By comparing the observed data with model results, the properties (i.e., diameter, temperature amomaly, and buoyancy flux) of the Jan Mayen mantle plume were estimated. This study provides valuable insights into how plumes and transform faults modify ridge spreading processes. Key Points The crustal thickness, axial relief, and the fraction of magma emplacement to plate separation (M) along the Mohns Ridge are calculated Numerical models estimate a diameter of 75 km, a thermal anomaly of 100°C, and a buoyancy flux of 0.22 Mg/s for the Jan Mayen mantle plume Plume‐ridge interactions affect along‐ridge topography and crustal thickness, and transform faults could enhance the interaction distance

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

The depths of most moderate‐sized oceanic earthquakes are poorly constrained because of the lack of local recording stations and noisy teleseismic recordings. This hampers our understanding of slip behaviors along oceanic faults and the mechanical properties of the oceanic lithosphere. In this study, we develop a new method to enhance the weak body‐wave signals, particularly the depth phases, associated with earthquakes on oceanic transform faults using large‐aperture arrays in the teleseismic range. We simulate the enhanced teleseismic signals to refine the centroid depths of moderate‐sized earthquakes. We validate the new approach using synthetic waveforms and show it outperforms conventional methods when dealing with noisy signals. We obtain the depth estimates for three moderate‐sized earthquakes on the Chain transform fault in the equatorial Atlantic Ocean and find two of them are consistent with a local catalog derived using oceanic bottom seismometers. Application of the new method to the past decades of teleseismic recordings of moderate‐sized earthquakes on the large and slow‐slipping transform faults will provide significantly improved constraints on the width of the seismogenic zone, thus advancing our understanding of the rheology of oceanic lithosphere and earthquake processes in oceanic settings and, by comparison, their more dangerous continental counterparts. The new method is not limited to oceanic transform earthquakes, but can be easily adapted to other seismological studies in which noisy but coherent signals could be revisited for better usage. Plain Language Summary Depth, along with epicenters and other parameters, is one of the most fundamental parameters that characterize earthquakes. We often determine earthquake depths using nearby seismic stations. For large earthquakes, distant recordings can also be used to obtain accurate depths. However, for moderate‐sized earthquakes, especially those on oceanic faults, the depths are very difficult to constrain due to the lack of local stations and noisy distant recordings. In this study, we develop a method that can use the abundant global sensors to increase the signal‐to‐noise ratio of recordings associated with earthquakes on oceanic transform faults (OTF). We model the enhanced signals to refine the depths of several moderate‐sized earthquakes and validate them using results derived from local ocean bottom recordings. We find the depths for two out of all three moderate‐sized earthquakes are consistent with depths derived using local observations. Our results highlight the possibility of using decades of global recordings to obtain a much better image of the seismogenic zone of the large OTF. Key Points We develop a new large‐array‐based approach for stacking and enhancing weak teleseismic body wave phases We validate and apply the method to the long‐term problem of resolving the seismogenic width of oceanic transform faults This technique has the potential to enhance weak signals needed for many other seismological studies

Oceanic transform faults (TFs) offset mid‐ocean ridges, juxtaposing different‐aged crust and causing fracture zones (FZ) crossing ocean basins. Mounting evidence challenges the consensus that TFs are conservative plate boundaries, instead supporting the existence of extensional tectonics and two phases of magmatism. These processes should affect the magnetic signature of these systems, yet it is still a matter of debate. Here, we use magnetic data over the Hayes and Oceanographer TFs and six archive data sets to explore the magnetization of TFs on a global scale. Magnetization at ridge‐transform intersections and FZs is stronger than within transform valleys. Weak magnetization in valleys supports tectonic deformation. Strong magnetization, either normal or reversed, traced from the outside corner into FZs, sometimes even overprinting the adjacent older seafloor, documents a second magmatic phase. Both features show a spreading‐rate dependence. Magnetic data comply with other data in supporting that TFs are more dynamic than previously considered.

Partial melting beneath a mid‐ocean ridge creates a chemically depleted layer in the uppermost mantle. This chemical depletion lowers the density of the lithosphere compared with the unmelted mantle. Furthermore, dehydration associated with depletion leads to an increase in mantle viscosity that may affect the structure and dynamics of the oceanic plate. Previous studies have mainly considered the formation of this depleted upper‐mantle layer in a two‐dimensional mid‐ocean ridge setting, leaving the dynamics of mid‐ocean ridge–transform fault systems largely unexplored. In this study, spatial variations in the degree of depletion in the uppermost mantle are predicted for a mid‐ocean ridge–transform fault system using a three‐dimensional thermomechanical model. The degree of depletion generally increases with increasing half‐spreading rate. Less depletion is predicted beneath the transform fault and fracture zone compared with the surrounding mantle. Lateral differences in the degree of depletion in the ridge‐parallel direction are reduced when plastic yielding is considered. The degree of variation in the predicted depletion is related to the transform fault length especially at a low spreading rate, thereby suggesting that the large scatter in observed abyssal peridotite compositions with slow spreading rates could be partly attributed to the length of the fault. Plain Language Summary Oceanic plates are a key component in the theory of plate tectonics. Partial melting beneath a mid‐ocean ridge, where oceanic plates are formed, results in residual melt‐depleted mantle that is less dense and more viscous than the unmelted mantle. It is therefore important to understand the degree of depletion in this tectonic setting to better constrain the structure and dynamics of oceanic plates. Previous studies have mainly considered the formation of depleted mantle in a two‐dimensional mid‐ocean ridge setting. However, this layer of depleted mantle likely undergoes a more complex formation process if transform faults, which connect neighboring mid‐ocean ridges, are located nearby. In this study, spatial variations in the degree of depletion in the uppermost oceanic mantle are predicted for a mid‐ocean ridge–transform fault system using three‐dimensional computational simulations. The degree of depletion generally increases with the rate of plate motion. The degree of depletion is lower beneath transform faults and their extension compared with the surrounding mantle. Key Points Thermomechanical modeling is used to predict the degree of mantle depletion in a mid‐ocean ridge–transform fault system The degree of depletion can be significantly lower beneath the transform fault and fracture zone compared with the surrounding mantle Modeling results indicate that chemical heterogeneity due to depletion changes the structure and dynamics of the oceanic plate

Oceanic transform faults connect spreading centers and are imprinted with previous tectonic events. However, their tectonic interactions are not well understood due to limited observations. The Discovery transform fault system at 4°S, East Pacific Rise (EPR), represents a young transform system, offering a unique opportunity to study the interplay between faulting and other tectonic events at an early phases of an oceanic transform system. Discovery regularly hosts M 5–6 characteristic earthquakes, and the seafloor north of Discovery includes a 35 km‐long rift zone that records a complex history of rifting, faulting and volcanism, suggesting that the transform faults likely interact with regional tectonic activity. We apply a machine‐learning enabled workflow to locate 21,391 earthquakes recorded during a 1‐year ocean bottom seismometer experiment in 2008. Our results indicate that seismicity on the western Discovery fault is separated into seven patches with distinct aseismic and seismic slip modes. Additionally, we observe a patch of off‐fault seismicity near where seafloor abyssal hills intersect the rift zone. This seismicity may have been caused by varying opening rates as spreading rate decreases from north to south in the rift zone. Our findings suggest that the Discovery system is still evolving, and that system equilibrium has not been reached between rifting and faulting. These results reflect the complex yet rarely observed interactions between fault slip, plate rotation, and rifting which are likely ubiquitous at oceanic transform systems. Oceanic transform faults are major plate boundaries connecting mid‐ocean ridges. Despite their important role in plate tectonics, their interactions with adjacent mid‐ocean ridges and surrounding oceanic plates are not well understood. The Discovery transform fault system at 4°S, East Pacific Rise, is a young oceanic transform system formed approximately 1 My ago, offering a unique opportunity to study the interplay between faulting and other tectonic events at an early phase of an OTF. Discovery faults have quasi‐periodical magnitude ( M ) 5–6 earthquakes. Using ocean bottom seismometer data recorded over 1 year, we find that seismicity of the western Discovery fault can be grouped into seven patches, indicating division of alternating slip modes that either releases tectonic strain by M  > 5 earthquakes or creep steadily. North of the western Discovery fault, a ∼10 km wide rift zone, abundant seamounts, and abyssal hills form an interactive tectonic complex. We observe a patch of off‐fault seismicity coinciding with seafloor abyssal hills near their intersection with the rift zone. This off‐fault seismicity indicates ongoing deformation within the oceanic plate and possible spatial variations in rifting rates. Our results suggest that the Discovery system is still evolving with rifting and faulting accommodating plate spreading simultaneously. The western Discovery transform fault has seven patches that are likely dominated by alternating seismic and aseismic slip modes Machine‐learning method helps to identify off‐fault seismicity along abyssal hills, indicating ongoing deformation within the oceanic plate The Discovery transform system is young and still evolving, forming an interactive system with faulting, rifting, and plate rotation

Origin of the continental-scale Tan-Lu fault zone (TLFZ), East China, remains controversial. About 550 km sinistral offset of the Dabie orogenic belt (DOB) and Sulu orogenic belt (SOB) is shown along the NE-NNE-striking TLFZ. Syn-collisional, sinistral ductile shear belts in the TLFZ have been identified. Thirteen phengite bulk separates from the mylonites were dated by the 40 Ar/ 39 Ar method. They gave cooling ages of the 198–181 Ma for the shear belts along the eastern margin of the DOB and 221–210 Ma from the western margin of the SOB. Distribution of the foreland basin deposits suggests that sinistral offset of the DOB and SOB by the TLFZ took place prior to deposition of the Upper Triassic strata. The marginal structures around the DOB and SOB support syn-collisional faulting, and indicate anticlockwise rotation of the DOB during the displacement. The folding and thrust faulting related to crustal subduction, coeval with the Tan-Lu faulting, is older than the foreland basin deposition related to the orogenic exhumation. Several lines of evidence demonstrate that the TLFZ was developed as a syn-collisional transform fault during latest Middle to earliest Late Triassic time when the DOB and SOB experienced crustal subduction of the South China Block (SCB). Eastward increase of the crustal subduction rates is believed to be responsible for the sinistral transform faulting.

Earthquakes that rupture several faults occur frequently within the shallow lithosphere but are rarely observed for intermediate‐depth events (70–300 km). On 29 November 2007, the Mw7.4 Martinique earthquake struck the Lesser Antilles Island Arc near the deep end of the Wadati‐Benioff‐Zone. The sparse regional seismic network of 2007 previously hampered a detailed examination of this unusually complex event. Here, we combine seismic data from different studies with regional moment tensor inversion results and 3D full‐waveform modeling. We show that the earthquake is a doublet consisting of dip‐slip and strike‐slip motion along two oblique structures, both activated under extensional stress along the strike of the slab. Comparison with tectonic reconstructions suggests that the earthquake ruptured along a re‐activated ridge‐transform segment of the subducted Proto‐Caribbean spreading ridge. The unprecedented resolution of the source process highlights the influence of pre‐existing structures on localizing slab deformation also at intermediate‐depth. Plain Language Summary Some earthquakes in continents and near the ocean floor are known to break multiple, differently oriented, faults. Such compound earthquakes are rarely observed in subducted plates in the intermediate‐depth region between 70 and 300 km. Intermediate‐depth earthquake mechanics and stress state are possibly different from shallower earthquakes, and maybe they hinder complex events. On 29 November 2007, the Mw7.4 Martinique earthquake occurred at a depth of ∼150 km, near the deep end of the regional seismic zone below the Lesser Antilles Arc. The regional seismic network in 2007 was relatively sparse; it revealed the earthquake had a complex mechanism but did not previously allow for an in‐depth study. In this study, we combine different types of data and methods, including full waveform information, based on a recently derived 3D regional velocity model. Our analyses shows that the Martinique earthquake consisted of at least two distinct sub‐events on perpendicular faults in the subducted plate—a source doublet. By comparing the orientations of the doublet faults with plate tectonic reconstructions, we infer that this intermediate‐depth earthquake broke along a fossil plate‐boundary. This indicates that such structures remain structural weaknesses even after subduction. Key Points Moment tensor solutions, aftershock activity, back‐projection, and source‐time function suggest a complex rupture of the 29 November 2007, Mw7.4 Martinique earthquake Regional Moment Tensor modeling and aftershock cross‐correlation identified the event as a source doublet Earthquake likely re‐activated a fossil ridge‐transform structure associated with the subducted Proto‐Caribbean spreading ridge