Shear Wave Speed Structure Research Articles

Adjoint tomography of the Italian lithosphere

The evolution and state of geological structure at Earth’s surface is best understood with an accurate characterization of the subsurface. Here we present seismic tomographic images of the Italian lithosphere based on ground motion recordings and characterized by compressional and shear wavespeed structure at remarkable resolution, corresponding to a minimum period of ~10 s. Enhanced accuracy is enabled by state-of-the-art three-dimensional wavefield simulations in combination with an adjoint-state method. We focus on three primary findings of our model Im25. It highlights the distribution of fluids and gas (CO2) within the Italian subsurface and their correlation with seismicity. It illuminates Mt. Etna volcano and supports the hypothesis of a deep reservoir (~30 km) feeding a shallower magma-filled intrusive body. Offshore of the eastern Italian coast, it reveals that the Adriatic plate is made of two distinct microplates, separated by the Gargano deformation zone, indicating a complex lithosphere and tectonic evolution.

Observation and inversion of very-low-frequency seismo-acoustic fields in the South China Sea.

Very-low-frequency (VLF) sound has significant potential for underwater detection and estimation of geoacoustic models of the ocean bottom structure. In marine settings, one type of VLF sound is the interface wave. These waves, trapped near the fluid-solid interface, are called Scholte waves, and this is the subject of this study. A field experiment was carried out in the South China Sea with the objective of exciting Scholte waves and investigating the propagation. The data were acquired by an ocean bottom seismometer, deployed on the seafloor. A large volume airgun array near the sea surface provided the sound source. The fundamental and three higher-order mode Scholte waves were excited. The Scholte waves are investigated by seismograms and a phase velocity inversion. The observed frequencies are in the range of 1.0-2.9 Hz. The energy attenuation is proportional to 1/r at the peak frequency 1.4 Hz. The shear wave speed structure, down to 600 m beneath the seafloor, is revealed from the dispersion curves by a least-squares inversion algorithm. The inversion result shows that the shear wave speed is below 300 m/s in the uppermost layer, which explains well the weak excitation of Scholte waves in this experiment.

The anatomy of uppermost mantle shear-wave speed anomalies in the western U.S. from surface-wave amplification

We build SWUS-amp, a three-dimensional shear-wave speed model of the uppermost mantle of the western U.S. using Rayleigh wave amplification measurements in the period range of 35–125 s from teleseismic earthquakes. This represents the first-ever attempt to invert for velocity structures using Rayleigh wave amplification data alone. We use over 350,000 Rayleigh wave amplitude measurements, which are inverted using a Monte Carlo technique including uncertainty quantification. Being a local seismic observable, Rayleigh wave amplification is little affected by path-averaged effects and in principle has stronger depth resolution than classical seismic observables, such as surface wave dispersion data. SWUS-amp confirms shallow mantle heterogeneities found in previous models. In the top 100 km of the mantle, we observe low-velocity anomalies associated with Yellowstone and the Basin & Range province, as well as a fast-velocity anomaly underneath the Colorado Plateau, where a strong velocity gradient at its edges shows a drastic contrast with its surroundings. SWUS-amp also gives additional insights into the current state of the uppermost mantle in the region. We image a high-velocity anomaly beneath the high-topography Wyoming province with a maximum depth extent of about 150–170 km, which is shallower than in previous tomographic models, and resolves previous inconsistencies with geological information. Beneath the Snake River Plain, a finger-like low-velocity anomaly dips to the west, suggesting lateral flow in the region. Below about 150 km depth, SWUS-amp shows a north-south dichotomy in shear-wave speed structure, with the northern region showing mostly high-velocity anomalies, whereas the southern region shows low-velocity anomalies. This is consistent with the continuous subduction history of the western U.S. and with the recent extension and uplift of the southern region.

3D upper mantle structure beneath Japan and its surrounding region from inter-station dispersion measurements of surface waves

Three-dimensional shear wave speed structure of the upper mantle beneath the Japanese Islands and the surrounding regions is constructed from the measurements of inter-station phase speeds of surface waves. We employ the dense broad-band seismic network (F-net), deployed throughout Japan, as well as permanent stations of global seismic network in East Asia. A temporary broad-band seismic network in Far-East Russia, which has been constructed as a part of the Stagnant Slab Project since 2004, is also used in combination with the permanent stations. Using seismic events from 2005 to 2007 with moment magnitude greater than 6.0 and depth shallower than 100km, we measure the phase speeds of the fundamental-mode Rayleigh and Love waves, working with the conventional two-station technique. Phase speed maps of the Japanese Islands and the Sea of Japan are obtained from the inversions of inter-station phase speeds that are measured for a large number of station-pairs. We have collected over 5300 paths for Rayleigh waves and 3800 paths for Love waves. The phase speed maps of Rayleigh waves, in the period range from 25 to 140s, are inverted to obtain an isotropic 3D shear wave speed model in the depth range from 40 to 200km. The isotropic S wave model represents prominent fast wave speed anomalies in northeastern Japan, associated with the subducting Pacific Plate. Fast anomalies of the lithosphere beneath the Sea of Japan are also imaged, implying that the average thickness of lithosphere beneath the Yamato Rise and Japan Basin is about 60±10km. Immediately below this oceanic lithosphere, slow anomalies are found in the asthenosphere beneath the entire region of the Sea of Japan in the depth down to about 200km. In southwestern Japan, the low-angle subduction of the Philippine Sea Plate is mapped clearly, indicating that the northern end of the plate extends beyond the northern coast of the Chugoku region, reaching beneath the Oki Islands in the Sea of Japan. A strong low velocity anomaly off the west coast of Kyushu is found in the depth down to about 130km, which supports the existence of an upwelling flow beneath this area. We also invert the phase speed maps of Rayleigh and Love waves in the period range from 30 to 80s simultaneously, to construct the radially anisotropic shear wave speed model. A remarkable anisotropy with SH>SV is found in the mantle wedge beneath northeastern Japan, while only weak anisotropy is seen beneath the Sea of Japan. Such anisotropic properties, which cannot readily be obtained from body wave studies only, will be an important key to understanding the dynamic processes in this tectonically actively region.

Seismic structure of the upper mantle beneath the Philippine Sea from seafloor and land observation: Implications for mantle convection and magma genesis in the Izu–Bonin–Mariana subduction zone

We have determined the three-dimensional shear wave speed structure of the upper mantle in and around the Philippine Sea region using seismograms recorded by dense land-based and long-term broadband ocean bottom seismographic stations. We used a surface wave tomography technique in which multimode phase speeds are measured and inverted for a 3-D shear wave speed structure by incorporating the effects of a finite frequency and ray bending. The new ocean bottom data provided us with improved spatial resolution (~ 300 km) in the Philippine Sea region. In the upper 120 km, the shear wave speed structure is well correlated with seafloor age. At depths greater than 160 km, fast anomalies of the subducting Pacific Plate are clearly defined. We also found slow speed anomalies beneath the Kyushu–Palau ridge at depths greater than 120 km. Along the Izu–Bonin(Ogasawara)–Mariana arc, we have detected three separate slow anomalies in the mantle wedge at depths shallower than 100 km beneath the back arc. Each anomaly has a width of ~ 500 km. Moreover, these three anomalies have a close relationship with the three groups of frontal and rear arc volcanoes having distinct Sr, Nd, and Pb isotope ratios. We suggest that each of the anomalies is a site of large-scale flow of upper mantle into the mantle wedge, and that each already contains a component from the adjacent subducting slab.

Estimation of Shear Wave Speed in Ocean-Bottom Sediment Using Electromagnetic Induction Source

An experiment for studying the shear wave speed in ocean-bottom sediment using Scholte pulse waves was conducted on a shelf in the East China Sea. To generate Scholte pulse waves, we developed a new electromagnetic induction source, which was placed at 100-m depth on the seafloor and was remotely controlled on board through a 1000-m-long cable. A Scholte pulse propagating in sandy sediment was then measured using a couple of geophones placed on the seafloor at a distance of 100 m from the source. The dispersion property of the Scholte waves was obtained by applying a multiple filter technique (MFT) to the observed waves. We then obtained the shear wave speed structure, i.e., the depth dependency of the shear wave speed in the sediment, by applying an inversion method based on a genetic algorithm (GA). The estimated speed was about 100 m/s near the bottom and increased with depth. The power was a close match to the estimated depth-speed profile, as reported previously by Hamilton [E. L. Hamilton, Geophysics, vol. 41, pp. 985-996, 1976].

Support of high elevation in the southern Basin and Range based on the composition and architecture of the crust in the Basin and Range and Colorado Plateau

To explore the nature of how the structure and physical properties of the crust vary from extended to relatively unextended domains we present teleseismic receiver functions which measure crustal thickness, shear wavespeed structure and the Vp/Vs ratio at 12 seismic stations in eastern Arizona. The crustal thickness is ∼ 28 km, increases slightly eastward, and remains nearly uniform beneath the varying elevations in the southern Basin and Range. The observed Vp/Vs ratio in the Basin and Range (∼ 1.78) exceeds the global average. The southern Colorado Plateau exhibits thicker crust (∼ 40 km) and a slightly greater observed Vp/Vs (∼ 1.81). A discrete region in the Colorado Plateau generates an unusually high Vp/Vs ratio (1.90) and contains low wavespeed zones which serve as evidence of partial melt related to Quaternary volcanism. The metamorphic core complexes in the southern Basin and Range likewise exhibit anomalously high Vp/Vs values (1.79–1.87) and lack locally compensating crustal roots. Density models show that ∼ 85 kg/m 3 lighter crust or ∼ 35 kg/m 3 lighter mantle than that of the surrounding Basin and Range helps these metamorphic core complexes maintain their high elevation. Compositional modeling of intrusive bodies exposed throughout the Catalina–Rincon metamorphic core complex indicates that the observed high Vp/Vs ratio and modeled low density could result from substantial amounts of a plagioclase-rich, quartz-poor rock. These Vp/Vs data are evidence that significant compositional heterogeneity of the crust can occur over a short distance and provide a clue as to how these areas that underwent significant Tertiary extension may have been preconditioned for orogenic collapse.

Three‐dimensional shear wave structure beneath the Philippine Sea from land and ocean bottom broadband seismograms

We obtained three‐dimensional (3‐D) shear wave speed structure beneath the Philippine Sea and the surrounding region from seismograms recorded by land‐based and long‐term broadband ocean bottom seismographic stations. The ocean bottom data gave us a better station coverage to obtain a higher spatial resolution (about 300–400 km) in the Philippine Sea than in previous studies. We employed a new technique of surface wave tomography, in which multimode phase speeds are measured and inverted for a 3‐D shear wave speed structure by incorporating the effects of finite frequency and ray bending. There is a sharp speed contrast along the Izu‐Bonin‐Mariana trench, across which the Philippine Sea side has a significantly slower upper mantle than the Pacific Ocean side. In the upper 120 km, the shear wave speed structure is well correlated with the age of the provinces. At depths greater than 160 km, the pattern is dominated by fast anomalies of the subducted slabs of the Pacific plate and two slow anomalies to the south of the Daito ridge and in the southernmost part of the Philippine Sea.

Shear wave speed structure beneath the South Pacific superswell using broadband data from ocean floor and islands

We determined three‐dimensional shear wave speed structure beneath the South Pacific superswell down to a depth of 200 km by analyzing Rayleigh wave records from broadband ocean bottom seismograph stations and island stations in the Pacific Ocean. The ocean bottom stations were deployed from 2003 to 2005 on the seafloor in the French Polynesian region, which enabled us to study the upper mantle structure beneath the Superswell with unprecedentedly high resolution. We measured the dispersions of fundamental mode Rayleigh waves at periods between 40 and 140 seconds by the two‐station method. We found pronounced slow anomalies near the hot spots and in the Lau Basin. The slow anomalies beneath the hot spots have deeper‐rooted than those associated with the Lau basin. The slow anomalies near the Society, Macdonald, Marquesas, and Pitcairn hot spots continue down to at least 200 km depth.