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Seismic surveys along subduction zones have identified anomalously high ratio of P‐ to S‐wave velocity (VP/VS) in the subducting oceanic crust that are possibly due to the presence of pore water. Such interpretations postulate that the pore structure is homogeneous at the scale of the seismic wavelength. Here we present the first statistical evidence of a heterogeneous pore structure in oceanic crust at scales larger than laboratory samples. The spatial correlation of measured bulk density profiles of the crustal section of the Samail ophiolite suggests that the pore structure is heterogeneous at scales smaller than ∼1 m. Wave‐induced fluid flow cannot follow the loading during the seismic wave propagation at this estimated heterogeneity, which implies that fluid‐filled microscopic pores and cracks have a limited impact on the observed high VP/VS anomalies in the subducting oceanic crust. Large‐scale cracks may therefore play an important role in shaping these anomalies. Plain Language Summary Seismic studies along subduction zones have identified unusually high ratios of P‐ to S‐wave velocity (VP/VS) in the subducting oceanic crust, which indicates the presence of water‐filled cracks and pores. The close link between pore water and local seismic activity highlights the importance of quantitatively interpreting these seismic anomalies in terms of pore characteristics. Previous interpretations have assumed that the microscopic pore structure is quite homogeneous, even at macroscopic scales as large as the seismic wavelength. However, our analysis of a bulk density profile of the crustal section of the Samail ophiolite, Oman, which is a fossilized oceanic plate preserved on land, indicates that the pore structure is more heterogeneous than previously assumed. This means that the fluid flow within the unit volume that represents the macroscopic physical properties of the rock cannot follow the wave‐induced loading during seismic wave propagations. This results in a relatively small impact of water on the seismic velocity, as inferred from theoretical models that predict the effective elastic properties of rock containing fluid‐filled cracks. Therefore, microscopic cracks may not have a large impact on the high VP/VS values of subducting oceanic crust, whereas large‐scale cracks may play a more significant role. Key Points The bulk density of the crustal section of the Samail ophiolite is more spatially heterogeneous than previously assumed The effect of fluid‐saturated microcracks on low‐frequency seismic velocities is modeled as an unrelaxed condition for this heterogeneity The high VP/VS anomaly in the subducting oceanic crust can be explained by both microcracks and large‐scale cracks

The water cycle at subduction zones remains poorly understood, although subduction is the only mechanism for water transport deep into Earth. Previous estimates of water flux 1 – 3 exhibit large variations in the amount of water that is subducted deeper than 100 kilometres. The main source of uncertainty in these calculations is the initial water content of the subducting uppermost mantle. Previous active-source seismic studies suggest that the subducting slab may be pervasively hydrated in the plate-bending region near the oceanic trench 4 – 7 . However, these studies do not constrain the depth extent of hydration and most investigate young incoming plates, leaving subduction-zone water budgets for old subducting plates uncertain. Here we present seismic images of the crust and uppermost mantle around the central Mariana trench derived from Rayleigh-wave analysis of broadband ocean-bottom seismic data. These images show that the low mantle velocities that result from mantle hydration extend roughly 24 kilometres beneath the Moho discontinuity. Combined with estimates of subducting crustal water, these results indicate that at least 4.3 times more water subducts than previously calculated for this region 3 . If other old, cold subducting slabs contain correspondingly thick layers of hydrous mantle, as suggested by the similarity of incoming plate faulting across old, cold subducting slabs, then estimates of the global water flux into the mantle at depths greater than 100 kilometres must be increased by a factor of about three compared to previous estimates 3 . Because a long-term net influx of water to the deep interior of Earth is inconsistent with the geological record 8 , estimates of water expelled at volcanic arcs and backarc basins probably also need to be revised upwards 9 . Seismic images of Earth’s crust and uppermost mantle around the Mariana trench show widespread serpentinization, suggesting that much more water is subducted than previously thought.

The Eastern Himalayan Syntaxis (EHS) serves as a natural laboratory for the study of intense continental collision and lateral extrusion tectonics. By aiming at the intricate tectonic dynamics south and southeast of the EHS, we integrate seismic data from new broadband stations in central Myanmar with permanent stations in southeastern Tibet to establish a high‐resolution 3‐D shear wave velocity model through ambient noise surface wave tomography. Our imaging results reveal distinct differences in crustal seismic velocity structures between the West Burma Block, Chuan‐Dian Block, and the Shan Plateau, highlighting the extent of oblique subduction and restricted crustal extrusion. Notably, two north‐south oriented low‐velocity zones in the mid‐to‐lower crust of southeastern Tibet are mainly confined within the Chuan‐Dian Block and terminate near the Red River Fault, with limited extension into the Shan Plateau. Plain Language Summary The Eastern Himalayan Syntaxis (EHS) marks the point where the Indian and Asian plates collide most intensely in the eastern Himalayas. Geoscientists have observed significant mountain‐building activities around the EHS extending into southeastern Tibet (SE Tibet), driven by the intense collision of these continental plates. In SE Tibet, materials are squeezed sideways, a process known as lateral extrusion due to the collision of these plates. This is evident from seismic studies showing slower wave speeds in two distinct regions of the middle to lower crust. We analyzed seismic data from new stations in central Myanmar and permanent stations in SE Tibet to better understand the zone transitioning from intracontinental collision to lateral extrusion. Using a technique called ambient noise tomography, we developed a high‐resolution 3‐D model of the crust and uppermost mantle. Our findings reveal significant differences in the crust's structure between areas of subduction, extrusion, and transition, enhancing our understanding of the geodynamic processes beneath the EHS and its surrounding areas. Particularly, two slow‐speed belts related to the lateral extrusion are confined within SE Tibet, supporting a model of limited crustal extrusion. Key Points We obtain an integrated 3‐D Versus model by ambient noise tomography based on the data from both central Myanmar and southeastern Tibet Distinct crustal structures in the West Burma Block, southeastern Tibet, and the Shan Plateau coincide with distinct tectonic responses Two north‐south low‐velocity zones in mid‐to‐lower crust end near Red River Fault, indicating restricted crustal extrusion

Geodynamic modeling and seismic studies have highlighted the possibility that a thin layer of low seismic velocities, potentially molten, may sit atop the core‐mantle boundary but has thus far eluded detection. In this study we employ normal modes, an independent data type to body waves, to assess the visibility of a seismically slow layer atop the core‐mantle boundary to normal mode center frequencies. Using forward modeling and a data set of 353 normal mode observations we find that some center frequencies are sensitive to one‐dimensional kilometer‐scale structure at the core‐mantle boundary. Furthermore, a global slow and dense layer 1–3 km thick is better‐fitting than no layer. The well‐fitting parameter space is broad with a wide range of possible seismic parameters, which precludes inferring a possible composition or phase. Our methodology cannot uniquely detect a layer in the Earth but one should be considered possible and accounted for in future studies. Plain Language Summary Normal modes are long‐period oscillations of the whole Earth as it vibrates after large earthquakes. The frequency that a mode oscillates at depends on the interior structure of the Earth. Research suggests a global and thin layer of anomalous composition and low seismic wave speeds may have formed at the base of Earth's mantle, but would be difficult to observe seismically. We test and quantify the effect of this layer on the frequencies at which normal modes vibrate. We then compare these predictions to a large data set of normal mode frequency measurements to examine whether such a layer is consistent with observed data. We find that not only is a layer of 1–3 km thickness permitted by the modes, but that a layer being present improves the fit to the data. There are a wide range of parameters that adequately fit the data set so we cannot be specific about its properties. Furthermore a layer is likely not a unique way to improve the model. A seismically slow layer at the core‐mantle boundary has implications for processes in the mantle and outer core and the interaction between them. Key Points Normal mode center frequencies are a sensitive data type to detect seismically anomalous thin layers at the core‐mantle boundary A slow and dense layer on the order of 1 km thick atop the core‐mantle boundary can improve the fit to normal mode data The inclusion of a layer is likely not a unique way to improve the 1D model and layer properties remain uncertain

While 3D seismic reflection is well established in hydrocarbon exploration at the kilometer scale in relatively simple offshore settings, its application to shallow faulting in continental basins is rare, owing to difficulties in adapting acquisition and processing to rugged terrains and complex near-surface conditions. We present the first high-resolution 3D seismic study of a seismogenic fault in a structurally complex intramontane basin at depths < 200 m. The survey focuses on the Pantano–Ripa Rossa Fault, ruptured during the 1980 Mw 6.9 Irpinia earthquake, the largest Italian event of the past century. This fault cuts across the Pantano di San Gregorio Magno, a small basin filled with Quaternary sediments and showing modest cumulative displacement. Our results demonstrate that in such environments, where morphotectonic analysis and 2D geophysics provide limited constraints, high-resolution 3D seismic imaging is crucial to resolve fault geometry and to assess surface-faulting hazard. The 3D volume reveals a ~35–40 m wide intra-basin deformation zone beneath the 1980 rupture, composed of synthetic and antithetic splays, and highlights lateral variations in fault geometry and stratigraphy. Deformation is distributed and complex, with fault-controlled depocenters, variable sedimentary architectures, and rapid basement-depth changes—features unresolved by 2D data. We infer that the Pantano–Ripa Rossa Fault is relatively young, active since the late Middle Pleistocene, and developed in the hanging wall of the NE-dipping southern basin-bounding fault, challenging previous models that located the master fault along the northern basin margin.

Using recently collected high‐resolution seismic data along a dense linear transect across Ohio, West Virginia, and Virginia (called Mid‐Atlantic Geophysical Integrative Collaboration (MAGIC) profile), we analyze P‐to‐S receiver functions to investigate the undulations of the mantle transition zone (MTZ) discontinuities (410‐ and 660‐km) beneath the central Appalachian region. Our results incorporating the effects of local crustal and mantle structures suggest shallowing of both the 410‐ and the 660‐km discontinuities from the northwest (inland) to the southeast (coast) along MAGIC profile. Hydro‐thermal upwelling beneath the eastern U.S. coastal plain due to a hydrated MTZ and hot upwelling return flow associated with the descending lower mantle Farallon slab is consistent with our observations of MTZ structure considering 3D velocity heterogeneity. The inferred hydrous hot upwelling rising into the upper mantle may trigger dehydration melting atop the 410‐km discontinuity, which may help to explain the presence of a low velocity upper mantle anomaly beneath the region today. Plain Language Summary The dynamic and tectonic processes of passive continental margins that do not lie on plate boundaries are poorly understood compared with active continental margins. The mantle flow patterns beneath the eastern North America, a long‐lived passive continental margin, are debated, due to limited local high‐resolution seismic studies. Using a dense seismic array through Ohio, West Virginia, and Virginia (called MAGIC profile) with unprecedented resolution, we investigate the undulations of 410‐ and 660‐km discontinuities (boundaries of the mantle transition zone) beneath the eastern U.S. by analyzing seismic waves that have been converted at these discontinuities. After correcting for local crustal and mantle structures, we identify positively correlated shallowing of the 410‐ and 660‐km discontinuities from the inland to the coast, which are hard to explain by thermal heterogeneity alone. Our findings suggest the existence of hot upwelling return flow associated with the neighboring descending Farallon slab in the lower mantle beneath the eastern U.S. When this passive hot return flow rises through the hydrated transition zone into the upper mantle, dehydration melting atop 410‐km discontinuity may be triggered. The buoyant melt may further rise through the upper mantle, providing a potential explanation for upper mantle velocity anomalies beneath the region. Key Points Seismic receiver functions image mantle transition zone discontinuities from a regional high‐resolution transect across the eastern U.S The results are difficult to reconcile with either thermal or hydration variations alone in the transition zone Local hydro‐thermal upwelling due to hydrated transition zone and hot return flow associated with the descending Farallon slab may exist

The Valles Caldera (VC), one of the largest Quaternary silicic calderas in North America, formed by explosive rhyolitic eruptions. Seismic studies suggest a crustal magmatic reservoir beneath the caldera with low‐velocity anomalies, but resolving the detailed geometry of localized melt requires constraints from seismic anisotropy. To image P‐wave velocity and radial anisotropy using dense nodal array data, we develop a teleseismic tomography method that integrates an eikonal solver with the adjoint‐state approach. Our results reveal a pronounced low‐velocity anomaly (>20% reduction) extending laterally across the resurgent dome and down to 15–20 km depth, consistent with a crustal magma chamber. We also identify a colocated zone showing a previously unrecognized pattern of strong positive P‐wave radial anisotropy (up to 8%) where horizontally polarized P‐waves travel faster than vertically polarized ones. This anisotropy indicates a laterally extensive magmatic sill complex and provides new constraints on magma distribution and reservoir architecture beneath the VC.

Seismic studies of glaciers yield insights into spatio-temporal processes within and beneath glaciers on scales relevant to flow and deformation of the ice. These methods enable direct monitoring of the bed in ways that complement other geophysical techniques, such as geodetic or ground penetrating radar observations. In this work, we report on the analysis of passive seismic data collected from the interior of the North East Greenland Ice Stream, the Greenland ice sheet's largest outlet glacier. We record thousands of basal earthquakes, many of which repeat with nearly identical waveforms. We also record many long-duration glacial tremor episodes that migrate across the seismic network with slow velocities (e.g. ~4–12 m s −1 ). Analysis of the basal earthquakes indicates a transition between times of individual event activity and times of tremor activity. We suggest that both processes are produced by shear slip at localized asperities along the bed. The transition between discrete and quasi-continuous slipping modes may be driven by pore-water pressure transients or heterogeneous strain accumulation in the ice due to strength contrasts of the underlying till.

Seismic microzonation is performed to assess the seismic risk in an area. In this paper, seismic microzonation for Lahore, Pakistan has been carried out. Firstly, the Geotechnical and geological properties of soils in the region were classified based on 119 boreholes. Two downhole tests were performed to measure the dynamic in-situ properties of soil. The design spectra for Lahore city from BCP 2007 and 2021 were used as target spectra to develop two synthetic acceleration time histories respectively. Afterward, one-dimensional non-linear site response analysis was performed at 33 sites having depth of 30 m for evaluation of parameters such as peak ground acceleration and spectral acceleration at the ground surface. Major seismic hazards considered for the seismic risk assessment are (1) peak ground acceleration at the ground surface, (2) surface spectral acceleration and (3) spectral amplification in the top 30 m of soil. All the major hazards estimated above were also used to prepare a seismic risk map of Lahore. Additionally, two site-specific design spectra were proposed in accordance with the soil classes D and E. The results of this study demonstrate the importance of micro-scale seismic studies to quantify the seismic risks associated with earthquakes.

A crisp step function is not an adequate threshold for studies of Markovian occurrence of large earthquakes, because it can lead to missing or pseudo links in an observed sequence that should be a Markov chain. A more realistic threshold is a fuzzy one where there is a transition magnitude band, located between those magnitudes that are too small for the earthquakes to be part of a Markovian process and those who are certainly large enough for the earthquakes to be part of it, where earthquakes may or may not be part of the process. This fuzzy threshold is described by a membership function that gives the probability of an earthquake with a given magnitude belonging to the process. We propose a membership function with probabilities in the transition band proportional to the seismic moment. To estimate empirical transition probabilities when considering a fuzzy magnitude threshold, we propose a counting strategy for the observed transitions and justify it through Monte Carlo simulations. The counting strategy is illustrated by application to the model from a previous seismic study of the Japan area by testing, through Monte Carlo simulations, how well the counting strategy results resemble optimum estimations of the transition probabilities. The simulations are also used to study the behavior of three Markovianity measures, and it is found that the peak values of these measures are not useful in identifying the true transition band, but that this band may be better identified by using the whole set of values taken by each measure for different transition band models. As an illustration, the measures were applied to real data from the previous study, a short set corresponding to a single realization, and found that the behavior of the measures does not agree with those expected from a crisp threshold, but agree, within the limitations of the data, with either a fuzzy threshold going from zero probability for magnitudes M≤6.9 to probability one for M≥7.2 or from zero probability for magnitudes M≤7.0 to probability one for M≥7.2.