1.08 - Crust and Lithospheric Structure – Active Source Studies of Crust and Lithospheric Structure

Crust and Lithospheric Structure - Active Source Studies of Crust and Lithospheric Structure

We combine observations of group and phase velocity dispersion of Rayleigh waves, of the waveform of a long‐period PL phase, of Pn and Sn velocities from unreversed refraction profiles using earthquakes, and of teleseismic S‐P travel time residuals to place bounds on the seismic wave velocity structure of the crust and upper mantle under Tibet. From surface wave measurements alone, the Tibetan crustal thickness can be from 55 to 85 km, with corresponding uppermost mantle shear wave velocities of about 4.4 to 4.9 km/s, respectively. The Pn and Sn velocities were determined to be 8.12±0.06 and 4.8±0.1 km/s, respectively, using travel time data at Lhasa from earthquakes in and on the margins of Tibet. Combining these results, the crustal thickness is most likely to be between 65–80 km with an average shear wave velocity in the upper crust less than 3.5 km/s. A synthesis of one PL waveform does not provide an additional constraint on the velocity structure but is compatible with the range of models given above. In contrast to observations obtained for eight earthquakes in the Himalaya, measurements of both teleseismic S and P wave arrival times for nine earthquakes within Tibet show unusually large intervals between P and S compared with the Jeffreys‐Bullen Tables. Thus the Pn and Sn velocities apparently do not reflect high velocities in the mantle to a great depth beneath Tibet. From the dependence of the seismic velocities of olivine on pressure and temperature and from the similarity of the measured Pn and Sn velocities beneath Tibet and beneath shields and platforms, the velocities at the Moho beneath Tibet are compatible with the temperature being 250°–300° higher than beneath shields and platforms, i.e., 750°C if the temperature beneath the platforms is close to 500°C. Such a temperature could reach or exceed the solidus of the lower crust. Simple one‐dimensional heat conduction calculations suggest that the volcanic activity could be explained by the recovery of the geotherm maintained by a mantle heat flux of about 0.9 HFU at the base of the crust. If the distribution of radioactive heat production elements were not concentrated at the top of the crust, radioactive heating could also contribute significantly to the recovery of the geotherm and thus lower the required mantle heat flow. Thus the idea of a thickened crust in response to horizontal shortening is compatible both with these data and with these calculations.Appendices are available with entire article on microfiche. Order from the American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J81‐004; $1.00. Payment must accompany order.

Research Article| October 01, 2013 Three‐Dimensional Seismic Velocity Structure and Precise Earthquake Relocations in the Salton Trough, Southern California Guoqing Lin Guoqing Lin Division of Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149 Search for other works by this author on: GSW Google Scholar Author and Article Information Guoqing Lin Division of Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149 Publisher: Seismological Society of America First Online: 14 Jul 2017 Online ISSN: 1943-3573 Print ISSN: 0037-1106 Bulletin of the Seismological Society of America (2013) 103 (5): 2694–2708. https://doi.org/10.1785/0120120286 Article history First Online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Guoqing Lin; Three‐Dimensional Seismic Velocity Structure and Precise Earthquake Relocations in the Salton Trough, Southern California. Bulletin of the Seismological Society of America 2013;; 103 (5): 2694–2708. doi: https://doi.org/10.1785/0120120286 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search Abstract I present a new 3D seismic velocity model and high‐precision earthquake relocations between 1981 and 2010 near the Salton Trough and the San Jacinto fault zone. The simul2000 tomography algorithm is applied to derive the 3D VP and VP/VS models using first arrivals of a composite event data set. The final VP model correlates with geological features at shallow depths and is more consistent with the refraction studies than the starting 3D regional‐scale model. The VP/VS model shows strong variations in the upper crust and is dominated by relatively low values (below 1.7) in the middle crust except for the Salton Trough and the Imperial Valley. The newly resolved model is used to relocate all the seismicity in the study area. I then apply the similar‐event cluster analysis and differential‐time relocation approach based on waveform cross‐correlation data to the 3D relocated events. A dramatic sharpening of seismicity patterns is obtained after using these methods. The new locations are more tightly clustered than the previous catalogs mainly due to the different absolute locations. The depth distribution along a basal surface may suggest the existence of a detachment fault from the north end of the Salton Trough to United States–Mexico border. The fine‐scale velocity structure and precise earthquake relocations will be helpful for investigating the fault geometry, seismicity, stress interaction, and other seismological studies for southern California.Online Material: Figures of model resolution tests, cross‐sectional comparisons of different P‐velocity models, and earthquake depth distributions. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Seismic velocity structure across the Fennoscandia–Sarmatia suture of the East European Craton beneath the EUROBRIDGE profile through Lithuania and Belarus

The Central and East Java region, which is part of the Sunda Arc, has an important role in producing destructive earthquakes and volcanic complexes in Indonesia as a result of the convergence between the Indo-Australian plate that subducts under the Eurasian plate. In this study, the 3-D seismic velocity structure (Vp, Vs, and Vp/Vs) of the crust and upper mantle was determined to reveal the presence of the subducted slab, volcanic sources, and seismogenic features in the Central and East Java region. We have manually re-picked P- and S-arrivals of 1,488 events from January 2009 to September 2017 recorded at 27 stations of the BMKG network. An iterative damped least-squares inversion method was applied to simultaneously calculate both hypocenter relocations and velocity structure beneath this high-risk region to a depth of 200 km. We then compare the tomographic results and seismicity to interpret structural features in the seismic zones. The subducted slab is dipping toward the north, imaged by the high-velocity regions with low Vp/Vs at depths of about 50–100 km. Low-velocity anomalies with high Vp/Vs above the slab at a depth of ∼100 km, imply the possible location of partial melting from slab dehydration. Fluids and melts are ascending to feed the volcanoes i. e., Merapi-Merbabu, Wilis, Pandan, Semeru, Bromo, and Ijen that also have similar low-velocity anomalies at 10–30 km depths, suggesting the presence of the sedimentary basin or magma reservoir. We also have redetermined the hypocenter location of the 2021 (Mw 6.1) Malang earthquake at 8.94oS, 112.45oE, with a depth of 59.7 km. The location error in the x, y, and z directions are 3.08, 6.39, and 11.91 km, respectively. This intraslab event with a thrusting mechanism is located in the high-velocity region and close to the intermediate-depth seismic clusters, which indicates the geometry of the oceanic slab. In the region of 1994 (Mw 7.8) Banyuwangi earthquake, we found a low-velocity anomaly at ∼ 50 km depth that might be associated with the presence of subducting seamount that is more hydrated than the surrounding slab. The slip over this subducting seamount caused the tsunamigenic earthquake.

From tectonic reconstruction to upper mantle model: An application to the Alpine-Mediterranean region

Understanding the thermal and compositional structure of the Antarctic lithosphere is fundamental for assessing its tectonic stability, geodynamic evolution, and mantle processes beneath East and West Antarctica. However, interpretations based on single geophysical observables remain highly non-unique due to the coupled effects of temperature and composition on seismic velocity and density. Here we present a multi-physics framework that integrates gravity, seismic velocity, heat flow, and thermodynamic modeling to derive high-resolution density, temperature, and compositional models of the Antarctic lithosphere and lithospheric mantle. We first perform a three-dimensional parallel gravity inversion constrained by seismic shear-wave velocity structure, using a structurally coupled objective function that combines data misfit, model regularization, and Gramian-based structural consistency. Structural similarity between density and velocity is enforced in the mantle, where seismic constraints are strongest, while thermally corrected density relationships are incorporated within the crust. The inversion is accelerated through a matrix-free implementation with CUDA-enabled forward and adjoint modeling and MPI–GPU parallelization, enabling continental-scale imaging at a resolution of 5 km × 5 km. The resulting absolute density model reproduces observed Bouguer gravity anomalies with low residuals and reveals pronounced lateral heterogeneity across Antarctica. Building on these results, we further decouple temperature (T) and composition (Mg#) in the upper mantle through joint simulation of seismic velocity and density. Forward models are constructed using Gibbs free energy minimization with Perple_X, incorporating phase equilibria, anelastic attenuation, and rheological effects. A probabilistic grid-search approach with Monte Carlo uncertainty analysis enables robust estimation of T and Mg# and identification of regions where standard solid-state physics fails to explain observations. Our results indicate a cold, thick, and chemically depleted lithospheric root beneath East Antarctica, consistent with a stable cratonic mantle, while West Antarctica is characterized by elevated temperatures, fertile compositions, and widespread regions exceeding solid-state limits, suggesting active asthenospheric upwelling and possible decompression melting beneath the West Antarctic Rift System. This study demonstrates the power of integrated geophysical–thermodynamic approaches for resolving the thermo-compositional state of continental lithosphere.

Regional variations of the intrinsic shear wave quality factor Qµ in both the upper crust and upper mantle of continents are large, with values in old, stable cratons exceeding those in tectonically active regions in both depth ranges by as much as an order of magnitude or more. Qµ depends upon frequency, at least near 1 Hz, and that frequency dependence also varies regionally in the upper crust. It is typically low in tectonically active regions and higher in stable regions. Because of the large variations in Qµ from region to region, it is easy to map regional variations of both upper crustal Qµ and Q estimated from the coda of Lg waves (QLgc), even though both measurements may be marked by large uncertainties. Although coda Q of direct body waves may be strongly affected by scattering, QLgc appears to be primarily governed by intrinsic Qµ in the upper crust. Both upper crustal Qµ and QLgc values correlate with the time that has elapsed since the most recent tectonic activity in continental regions. A tomographic image of the variation of QLgc values across Africa shows reduced Q values which correspond to recent tectonic activity in the East African rift system and other regions of Mesozoic or younger age. Reductions of QLgc that correlate with tectonic activity that occurred in the early Paleozoic during the coalescence of the cratons which formed that continent can also be detected. Qµ increases rapidly at midcrustal depths, in a range which appears to coincide with the transition to the plastic lower crust. In the lower crust and upper mantle, Qµ decreases with increasing depth, possibly by progressive unpinning of dislocations with increasing temperature. Observed regional variations in upper mantle Qµ at depths of about 150 km can be explained by differences in temperature alone, but those at crustal depths cannot. Regional variations of Qµ in the upper crust are most easily explained by differences in the density of fluid‐filled fractures in which fluids can move during the propagation of seismic waves. Studies of the regional variation of Qµ and QLgc indicate that crack density is greatest during and immediately following tectonic activity in a region and that it decreases with time. Permeability determinations in deep wells show that fluid movements in those cracks may be largely restricted to zones of crustal fracturing. That situation will produce widely differing values of Q in local studies, depending on the location of the study relative to the fractures. The fluid volume in cracks appears to decrease with time by loss to the surface or by retrograde metamorphism, causing a reduction in the number of open cracks and a concomitant increase in Qµ.

Data from a 260‐km‐long seismic refraction profile provide a detailed look at the crustal structure beneath the central Columbia Plateau (CCP). The CCP profile, centered on the Pasco Basin in eastern Washington, trends approximately N50° E between the towns of Wasco, Oregon, and Warden, Washington. The upper crust above the crystalline basement consists of between 5 and 12 km of the Columbia River Basalt Group (CRBG) and underlying sediments. The CRBG is laterally variable in thickness, ranging from 3 to 6 km over the length of the CCP seismic profile, but is thickest near the center of the Columbia Plateau. A thick, but laterally varying, accumulation of sediments underlies the CRBG with the thickest accumulation of sediments (5.0 km/s) occurring beneath the Pasco Basin in what we interpret to be a graben. Sediments there extend from the base of the CRBG to basement (6.3 km/s) at depths between 10 and 12 km. We recognize two layers between the basement and the upper mantle with seismic velocities of 6.8 and 7.5 km/s, located at 18‐ and 25‐km depths, respectively. The 6.8‐km/s layer thins beneath the graben, where it has apparently been replaced by the 7.5‐km/s layer, creating the characteristic geometry and velocity of the “rift pillow” layer previously observed in some other recognized continental rifts. The Mono (8.4 km/s upper mantle) is located at about 40‐km depth; an apparent localized, low‐velocity layer is observed within the upper mantle at a depth of about 50 km. Thus the deep crustal and upper mantle structure beneath the central Columbia Plateau is complex and is atypical of normal deep continental crust. The presence of an upper crustal graben and lower crustal rift pillow, as inferred from the seismic refraction data, combined with gravity, earthquake, electrical, and geologic data, suggest that Eocene (and possibly later) continental rifting occurred prior to deposition of the CRBG.

DOI = 10.3126/hjs.v5i7.1269 Himalayan Journal of Sciences Vol.5(7) (Special Issue) 2008 p.69

Crustal shear-wave velocity structure beneath northeast India from teleseismic receiver function analysis

Subsurface seismic velocity structure is essential for earthquake source studies, including hypocenter determination. Conventional hypocenter determination methods ignore the inherent uncertainty in seismic velocity structure models, and the impact of this oversight has not been thoroughly investigated. Here, we address this issue by employing a physics-informed deep learning (PIDL) approach that quantifies uncertainty in two-dimensional seismic velocity structure modeling and its propagation to hypocenter determination by introducing neural network ensembles trained on active seismic survey data, earthquake observation data, and the physical equation of wavefront movement. An analysis of an earthquake in southwest Japan using our method revealed that accounting for such uncertainty propagation significantly reduced the bias and uncertainty underestimation in the hypocenter determination, enabling quantitative evaluation of the focal depth relative to the plate boundary. Our results highlight the potential of PIDL for various geophysical inverse problems, such as investigating earthquake source parameters, which inherently suffer from uncertainty propagation.

The structure and evolution of the lithosphere and underlying mantle control tectonics, magmatism, and natural resource development and distribution. Seismic tomography offers essential information on mantle temperature, the thickness and strength of the lithosphere, and the convection below it. Here, we report on the completion of a series of continent- and plate-scale waveform tomography studies using the automated multimode inversion of surface and S-wave waveforms. Together, the models cover all of the Earth’s continents and their surroundings and reveal a fascinating diversity of structures, while also indicating common mechanisms of lithospheric dynamics and evolution. Each model used global data coverage that was maximised witin the hemisphere centred at the continent. Structural information was extracted from Rayleigh waves and S and multiple S waves on over a million vertical-component-seismogram waveform fits. The effects of errors were minimised by statistical and targeted outlier analyses and the removal of the least mutually consistent data. The models advance the resolution of the imaging compared to the state of the art at the scale of the continents.Cratonic lithosphere on all continents shows complexities and fragmentation. The lithosphere beneath most diamondiferous kimberlites—originally emplaced on thick cratonic lithosphere—is still thick at present. Relatively low seismic velocities at kimberlite locations are indicative of craton-lithosphere thinning and are detected at multiple locations in all continents with known kimberlites. Cenozoic basalts are found exclusively where the lithosphere is observed to be thin. Beneath some of the volcanic regions, low-velocity anomalies extend deep into the mantle, consistent with deep-mantle upwellings feeding the magmatism. Sediment-hosted metal deposits tend to be located near contrasts in the thickness of the lithosphere, including both craton boundaries and other substantial heterogeneities. Intraplate seismicity is controlled by plate-boundary stresses and by the lateral variations of the lithospheric thickness and strength. Areas with relatively thin lithosphere localise deformation and seismicity, provided that sufficient tectonic stress is transmitted into the plate interior.ReferencesChagas de Melo, B, Lebedev, S, Celli, NL, et al., 2025. The lithosphere of South America from seismic tomography: Structure, evolution, and control on tectonics and magmatism, Gondwana Research, GR Focus - Invited Review, 138, 139–167. doi:10.1016/j.gr.2024.10.012.Chua, EL, Lebedev, S. Waveform tomography of the Antarctic Plate, Geophysical Journal International, submitted.Celli, NL, Lebedev, S, Schaeffer, AJ, Gaina, C, 2021. The tilted Iceland Plume and its effect on the North Atlantic evolution and magmatism, Earth and Planetary Science Letters 569, doi:10.1016/j.epsl.2021.117048.Celli, NL, Lebedev, S, Schaeffer, AJ, et al., 2020. The upper mantle beneath the South Atlantic Ocean, South America and Africa from waveform tomography with massive data sets, Geophysical Journal International 221, 178–204. doi:10.1093/gji/ggz574.de Laat, JI, Lebedev, S, Celli, NL, et al., 2023. Structure and evolution of the Australian plate and underlying upper mantle from waveform tomography with massive data sets, Geophysical Journal International 234, 153–189. doi:10.1093/gji/ggad062Dou, H, Xu, Y, Lebedev, S, et al., 2024. The upper mantle beneath Asia from seismic tomography, with inferences for the mechanisms of tectonics, seismicity, and magmatism, Earth-Science Reviews, 255, doi:10.1016/j.earscirev.2024.10484

SUMMARY The lithospheric structure in Oman has been determined by analyzing teleseismic P-receiver functions recorded at broad-band and short-period seismic stations of the Oman Seismological Network. Lithospheric structure is obtained by jointly inverting receiver functions and Rayleigh wave group velocities derived from continental-scale surface wave tomography of Pasyanos. We observe relatively thick crust (40–48 km) within the ophiolite formed mountains in northern Oman. The crustal thickness is about 35 km within the passive continental margin of the southern Oman region. Uppermost (<5 km) crustal shear wave velocities are faster in the northern ophiolite regions compared to the southern Oman region, while shear velocities in the middle crust are faster in the Southern Oman region compared to the ophiolite region. This observation coincides well with cretaceous to Eocene marine platform sequences overlying Precambrian to Cambrian basement of the southern part. Joint inversion analysis shows that the Moho depth of Oman varies from 34 km in the southern region to 48 km in the northern part.

Deep drill holes bored into in situ oceanic crust were reviewed. Since the beginning of the Deep Sea Drilling Project, 45 drill holes have penetrated over ca. 50 m into normal oceanic crust, however, most are concentrated in the north Atlantic and east Pacific Oceans. Basement ages of most holes are younger than 20 Ma, and are rather biased toward younger crust compared to the average age of the oceanic crust (61 Ma). Only five deep drill holes have penetrated over 500 m into the basement, three of which drilled into slow spread crust formed at 8 cm/yr and one in intermediately spread crust.Three deep holes (332B, 395A, 418A) drilled into slow spread crust formed at Mid-Atlantic Ridge gave the first evidence of magnetic reversals through the vertical oceanic crust, and showed that the slow spread upper oceanic crust away from hot spots is dominantly composed of pillow lavas with a normal MORB-like affinity. Younger 332B and 395A holes (3.5 and 7.3 Ma) gave poor core recovery (18-21%), while the oldest Hole 418A (110 Ma) yielded a fairly high recovery of 72%.Hole 504B is the only hole to penetrate the extrusive rocks and most of the way through the sheeted dike complex (1836.5 m sub-basement). Average core recovery dropped from 29.8-25.3% in the lava and transition zone down to 14.3% in the sheeted dike complex. Unfortunately, the 504B lava is the depleted extremity of MORBs from intermediate-fast spread ridges. One of the most important findings of Hole 504B is a discrepancy between the seismic velocity structure and the downhole lithology in that the Layer 2/3 boundary resides in the middle of the sheeted dikes, as interpreted by the difference in porosity and bulk density.Hole 1256D is dedicated to coring typical oceanic crust and ultimately penetrates the entire crust into the upper mantle. The site is located on the 15-Ma Cocos plate generated at a superfast rate (22 cm/yr). 502-m-long cores of basement (48% recovery) are lavas showing moderately evolved MORB-like compositions similar to those from the present fast spread ridges. The hole has been cleaned and left ready for future drilling, possibly into Layer 3.The above examples of deep drill holes show that the major obstacles to ultradeep drilling are hole collapse and poor core recovery. Riser drilling is expected to overcome these obstacles for “the 21-century Mohole”.