Low-velocity Layer Research Articles

Crustal structure beneath the east side of Pearl River Estuary from onshore-offshore seismic experiment

ABSTRACT The land-sea transition zone in the northern South China Sea (SCS) records important information from the continental rifting to the seafloor spreading. The crustal structure is the key to explore the deep tectonic environment and the evolution of the SCS. In 2015, the onshore-offshore 3D deep seismic experiment was carried out on the Pearl River Estuary (PRE). Explosions and air guns were used as sources on land and at sea respectively in this experiment.Onshore seismic stations and Ocean Bottom Seismographs (OBSs) synchronously recorded the seismic signals. We focus on an onshore-offshore seismic profile (L2, SE-trending) along the eastern side of the PRE. By modelling the seismic travel times, we constructed a P-wave velocity model along the profile. The model shows that the sediment on land is thin and has seismic velocities of 4.5–5.5 km/s. In contrast, thickness of the offshore sediment gradually increases to more than 4.0 km, and the velocities vary between 2.0 km/s and 4.5 km/s. The onshore and offshore crustal velocities are 5.8–6.8 km/s and 5.5–6.8 km/s, respectively. At depth between 15 km and 20 km, a low-velocity layer (LVL; only 5.9 km/s) is detected, pinching out under the Littoral Fault Zone (LFZ). The LVL has probably accommodated the crustal extension beneath the land area, resulting in low extent of the crustal thinning. A slightly uplifted Moho exists beneath the Dongguan fault depression zone, representing a place where hot mantle materials ascend. Localized thickening of the sediments and rapid thinning of the crust characterize the LFZ, and it can be regarded as a tectonic boundary between the South China (SC) with normal continental crust and the northern SCS margin with extended continental crust.

Verification of the seismic P-wave velocities under Moho boundary: Central Poland case study, LUMP profile

The tectonic settings investigated by several seismic projects in previous research targeting the structure in Central Poland mainly focused on the Earth’s crust. In this paper, we present P-wave velocity verification in the uppermost mantle beneath LUMP profile towards SSE-NNW. Using recordings of 36 DATA-CUBE recorders from ca. 300–490 km far earthquake in coal mine “Janina” in southern Poland, we calculated travel times to verify P-wave velocity below the Moho boundary from previous studies. It shows that a significantly lower mean velocity value should be used for the upper mantle while counting these offsets of travel times in the SSE-NNW direction than that used on previous profiles. We present two possible models: first, the most simple one that fits the observed first arrivals, and the second with a low-velocity layer beneath the Moho boundary. In both cases, we used a priori crustal model focusing only on P-wave velocity in the uppermost mantle. Both of them significantly improved adjustment of travel times to the observed data. To evaluate the tendency of adopting too high velocities beneath the Moho, we used also 11 broadband stations, Reftek 151-121 “Observer”, from “13 BB Star” passive experiment and 6 STS-2 seismometers from permanent stations of the Polish Seismological Network (PLSN).

Lithospheric structure based on integrated analysis of geological-geophysical data along the DOBREfraction’99/DOBRE-2 profile (the East European Platform —the East Black Sea Basin)

The continuous DOBREfraction’99/DOBRE-2 WARR and CDP profile of 775 km length with about 100 km overlap was acquired by an international team from Ukraine, Poland, Denmark, USA, Netherlands, Germany, Great Britain and Norway in 1996 and 2007 respectively. It crosses southeastern East European Craton (the southern slope of the Voronezh Massif, Donbas, the Priazov Megablock), the North Azov Trough, the Middle Azov High, the Indolo-Kuban Depression, the Crimea-Caucasus inversion zone, the Sorokin Trough, the Shatsky and Andrusov Ridges in the Eastern Black Sea Basin. In terms of the number of tectonic structures of different ages and origin, the profile has no analogues in world practice. Along the profile there has first been carried out an interdisciplinary geological and geophysical study of the entire lithospheric cross-section with the use of seismic data, magnetic, gravity and thermal fields, information on seismic tomography and spontaneous electric emission of the Earth. Fundamentally new information has been obtained for the structure of the lithosphere and a number of controversial problems have been convincingly solved. Tectonically, the lithosphere is a complex collage of structures arised in different geodynamic conditions from the Archean to the Neogene as a result of successive stages of its formation. The regional regularities in the structure of the lithosphere are the decrease in the thickness of the crust from north to south, from ancient structures to young ones with simultaneous elevation of the top of asthenosphere from 210 km under the Voronezh Massif to 90 km under the East Black Sea Basin. The standard continental crystalline crust has been mapped on the southern slope of the Voronezh Massif and in the Priazov Megablock. It has been reworked by the Paleozoic rifting in Donbas. The analysis of the heterogeneity of the crystalline crust and mantle has first been used to assess the position of the actual boundary of the East European Platform and the transition zone to the Scythian Plate, which corresponds to the deepest position of the asthenosphere top and the division between domains of different seismic velocities in the lithospheric mantle. The changes in the crystalline crust type under the Main Azov Thrust fixes the buried boundary of the East European Platform. The relationship has been established between large sedimentary structures and the areas of change in the composition of the crystalline crust along suture zones. The main faults of the sedimentary cover as the sutures of different ages have been traced in the cross-section of the crystalline crust and in the upper mantle. The subcrustal decomposition of the mantle and the crust—mantle mixture at the base of the crust of the maximum thickness between the South Crimean and Mesozoic sutures and in Donbas have resulted from the lithosphere reworking during its development. Different types of crystalline crust and gentle inclined intracrustal disturbances sometimes causing doubling different layers of the crystalline crust provide evidence of repeated subduction of the oceanic crust of the Paleo-, Mezo- and Neotethys. The low-velocity mantle layer between the South Crimean and Mesozoic sutures indicates a present-day manifestation of the total effect of post-Paleozoic subductions. A set of possible rocks has been determined for the crystalline crust within the southern slope of the Voronezh Massif, Donbas and the Azov Mega block. In the Crimea-Caucasus inversion zone at a depth of 30 km a complex of rocks has been documented to be inherent in the standard continental crust which is an alternative to the speculation about the local elevation of the upper mantle due to the serpentinization of mafic rocks.

Formula omitted] model for Nepal Himalaya

We investigate the spatial variations of Pg attenuation structure in Nepal Himalaya. We implement the “Two-Station method” on 2325 waveforms obtained from 435 events recorded at 151 stations deployed across Nepal Himalaya to measure the Pg-wave quality factor (Q). These station-station measurements are inverted further for investigating the spatial variations of Pg attenuation using a back projection algorithm. The model contains variations across the region, with much of the region being dominated by high attenuation. It is observed that the areas around the existing faults and lineaments exhibit very low Q values. A few exceptional high Q pockets are observed to be limited predominantly in the north western side of our study area. We hypothesize that intrinsic attenuation plays a major role in causing the high apparent Pg attenuation in the crust of Nepal Himalaya. This may be mainly caused by highly pressurized fluids trapped within a thin low velocity layer (LVL) at shallow depths. Moreover it is interesting to observe that all the past earthquakes of Mw>7 since 1934, fall into the regions of low Q values but not the lowest. It may be anticipated that the very low Q areas are too weak to sustain the accumulation of high energy release during a large magnitude earthquake. Therefore the observed low Q structures across the study area may imply a weak crust and subsequently a favorable zone for future rupture.

Genetic causes of oil-rich and oil-poor reservoirs: Implications from two Cenozoic basins in the eastern North China Craton

The Bohai Bay and Hehuai (southern North China) rift basins in the eastern part of the North China Craton are south-north-adjacent. They have shown synchronous evolutionary processes, and possess generally identical superficial and shallow structural characteristics as well as similar basin areas. However, there is a large difference in the richness of oil resources between the two basins. The Bohai Bay Basin has extremely abundant oil reserves, while commercial oil reserves have not been found in the Hehuai Basin. The deep tectonic structures, magmatic activities, and modern and paleo-geothermal fields of the two basins are significantly different. Compared with the Hehuai Basin, the Bohai Bay Basin has a thinner crust and more complex structure with multiple low-velocity layers. It is also characterized by intense magmatic activity, high modern and paleo-geothermal fields, frequent seismic activity, and active deep interactions, small effective elastic thickness of the isotropic lithosphere, and shorter balanced transformation wavelength of the lithosphere with a high likelihood of local compensation. The Hehuai Basin has a simple deep structure and homogeneous crustal composition, with a high likelihood of regional compensation. The characteristics of the deep structures mentioned above are generally similar to those of the southern part of the stable Ordos Basin, except for the smaller crust thickness. This indicates the presence of differences in Mesozoic destruction between the southern and northern zones in the eastern part of the North China Craton. The northern zone was subjected to significant destruction, while the southern zone was subjected to modifications, primarily in the form of local changes in the structures and/or properties of the crust or lithospheric mantle, with the overall structure and stability of the craton kept intact. The formation of high-quality source rock is primarily influenced by the abnormal flourishment of organisms in water bodies during the syndepositional period, and is also strongly associated with the high geothermal setting of basins and nutrients from hydrothermal solutions and volcanoes. In other words, it is mainly controlled by deep processes and deep-major fault activity. The differences in the deep structures and modern and paleo geothermal fields of the two basins correspond to the difference in richness of oil resources, suggesting that there is an important internal or causal relationship between the two aspects. This viewpoint coincides with the conditions and environments required for the development of high-quality source rock in hydrocarbon-rich basins (sags) in China and other countries, and is evidenced by the modern lake basin of the East African Rift. A new hydrocarbon generation model is proposed in this work: petroleum is a comprehensive product of the integration of bioenergy, thermal energy, and other related energies (such as chemical and kinetic energy) and their interactions; the degree of richness of petroleum is generally controlled by the regional tectonic structure, thermal environment, and deep processes; nonmarine basins or depressions with abundant resources are closely related to active deep processes, intense exchange of material between the deep and shallow layers, participation of external hydrocarbons, and energy integration and conversion.

SH-Mode Seismic-Reflection Imaging of Earthfill Dams

Abstract Assessing subsurface characteristics and imaging geologic features (e.g., faults, cavities, low-velocity layers, etc.) are typical problems in near-surface geophysics. These questions often have adverse geotechnical engineering implications, and can be especially acute when associated with high-hazard structures such as large earthen flood-control dams. Dam-related issues are becoming more frequent in the United States, because a large part of this major infrastructure was designed and constructed in the early- to mid-twentieth century; these dams are thus passing into the latter stages of their design life, where minute flaws that were overlooked or thought to be insignificant in design/construction are now proving problematic. The high-hydraulic heads associated with these structures can quicken degradation of weak areas and compromise long-term integrity. Addressing dam-related problems solely with traditional invasive drilling techniques is often inadequate (i.e., lack of lateral resolution) and/or economically exorbitant at this scale. However, strategic geotechnical drilling integrated with the broad utility of near-surface geophysics, particularly the horizontally polarized shear-wave (SH-mode) seismic-reflection technique for imaging the internal structural detail and geological foundation conditions of earthfill embankment dams can cost-effectively improve the overall subsurface definition needed for remedial engineering. Demonstrative evidence for this supposition is provided in the form of SH-wave seismic-reflection imaging of in situ and engineered as-built components of flood-control embankment dams at two example sites in the central United States.

Parametric models for 3D topographic amplification of ground motions considering subsurface soils

Amplification of seismic waves due to surface topography and subsurface soils has often been observed to cause intensive damage in past earthquakes. Due to its complexity, topographic amplification has not yet been considered in most seismic design codes. In this study, we simulate ground-motion amplification based on 3D Spectral Element Method, using Hong Kong Island as a local testbed site. The analyses revealed that topographic amplification of ground motions is frequency-dependent. If the site is made of homogenous rock, the amplification factor is best correlated with the curvature smoothed over a characteristic length equal to half of the wavelength in rock. Amplification of high frequency wave is correlated with small-scale features, and amplification of long-period waves is correlated with large-scale features in horizontal dimension. The maximum topography amplification generally ranges from 1.6 to 2.0 on the protruded areas. When a low-velocity subsurface soil layer is considered, the topographic amplification pattern is significantly influenced by the thickness of the soil layer, as wavelength in soil is relatively short. The characteristic length reduces as soil thickness increases, and the amplification pattern becomes closely correlated to smaller-scale topographic features as well as slope angles. Results also show that the effect of material damping can be decoupled from the topographic effects and modeled using a theoretical attenuation factor. The study proposed parametric models to predict 3D topographic amplification using simple proxies considering subsurface soils, material damping and input wave frequencies, which gives accurate results with a standard deviation of residuals within 0.1–0.15.

Seismic wave propagation in floating ice sheets – a comparison of numerical approaches and forward modelling

Seismic data processing collected from arctic sea ice is often challenging. Large amplitudes of coherent surface waves interfere with the reflected seismic waves and can make the reflection data virtually unusable. In order to understand this noise, forward modelling of the full wavefield is a useful approach. I present and test a forward modelling workflow by comparing the modelling results with common numerical approaches. The main differences between numerical curves and modelling results depend on simplifications in the assumptions of the numerical approaches. Forward modelling is less dependent on rough assumptions and can give a more accurate understanding of the surface wave propagation and its interaction with reflected waves. Furthermore, a thin elastic layer with high velocity located between two low velocity layers, like sea ice, generates guided waves. The numerical approaches focus only on the flexural waves and ignore other wave types. The presented results demonstrate that forward modelling can provide improved detail in the generated wavefield. In addition, the forward modelling results in this work show that leaky Rayleigh and Scholte waves can exist simultaneously.

Offshore Fault Geometrics in the Pearl River Estuary, Southeastern China: Evidence from Seismic Reflection Data

The Pearl River Estuary (PRE) is located at the onshore-offshore transition zone between South China and South China Sea Basin, and it is of great significant value in discussing tectonic relationships between South China block and South China Sea block and seismic activities along the offshore active faults in PRE. However, the researches on geometric characteristics of offshore faults in this area are extremely lacking. To investigate the offshore fault distribution and their geometric features in the PRE in greater detail, we acquired thirteen seismic reflection profiles in 2015. Combining the analysis of the seismic reflection and free-air gravity anomaly data, this paper revealed the location, continuity, and geometry of the littoral fault zone and other offshore faults in PRE. The littoral fault zone is composed of the major Dangan Islands fault and several parallel, high-angle, normal faults, which mainly trend northeast to northeast-to-east and dip to the southeast with large displacements. The fault zone is divided into three different segments by the northwest-trending faults. Moreover, the basement depth around Dangan Islands is very shallow, while it suddenly increases along the islands westward and southward. These has resulted in the islands and neighboring areas becoming the places where the stress accumulates easily. The seismogenic pattern of this area is closely related to the comprehensive effect of intersecting faults together with the low velocity layer.

Numerical investigation on heat transfer and flow characteristics in helically coiled mini-tubes equipped with dimples

Heat transfer and flow in dimpled helically coiled mini-tubes are numerically examined. The effects of Reynolds number Re, ratio of tube diameter to coil diameter t, dimple density d and dimple size s are examined. It is shown the dimple case provides a higher Nusselt number Nu upon the smooth case. The highest and lowest dimensionless heat transfer coefficients Nu/Nu0 are 2.281 and 1.788, respectively. The Nu/Nu0 firstly rises and then declines against the increase in Re, producing a peak value. Greater Nu/Nu0 is achieved when a lower t is used. A decrease in d or s results in a higher Nu/Nu0 with Re lower than particular-Re, while an opposite trend occurs with Re higher than particular-Re. The highest and lowest evaluation parameter (Nu/Nu0)/(f/f0)1/3 are 1.830 and 1.437, respectively. A better performance from energy saving viewpoint is achieved with a lower t. Besides, great reduction in wall temperature level is achieved due to dimples. With the increase in Re, the flow pattern develops accompanied with more obvious separation and reattachment behavior. Utilization of dimples produces significant promotion in velocity level close to the boundary layer, which is principally responsible for heat transfer enhancement. The boundary velocity coefficient shows great similarity to Nu/Nu0. Tangential velocity is greatly promoted when dimples are fixed, which is believed to generate intensified secondary flow. The thickness of the low velocity layer and the high temperature region in the fluid domain are greatly reduced due to dimples. Furthermore, a lower temperature gradient level and a more uniform fluid temperature field are achieved owing to dimples. The dimple case brings about great reduction on average heat transfer entropy generation level, with enhanced effect at a lower t. An evident decrease in maximum wall temperature is identified when dimples are used. Correlations for predicting Nusselt number and friction factor are obtained.

Numerical simulation of the impact of atmospheric turbulence on a wind turbine in complex terrain

In this study, which was part of the project AssiSt, numerical analyses of wind turbine power, loads and wake development in the extreme wind conditions found on a complex terrain site in northern Spain have been performed. The turbine is located near the edge of a characteristic cliff. Realistic atmospheric inflow conditions are prescribed by propagation of highly resolved meteorologic data of the LES code PALM in the aerodynamic solver FLOWer. The global features of the flow field in the complex terrain site were investigated showing significant inclination angles, high turbulence intensities and a characteristic mixing mechanism of high and low momentum velocity layers. Further, the wake development and its deflection and deformation was analyzed. Finally, the turbine’s load response was investigated, showing severe fatigue loading as consequence of the highly unsteady inflow conditions.

Exploration of S-wave velocity profiles at strong motion stations in Eskisehir, Turkey, using microtremor phase velocity and S-wave amplification

We have explored 1D S-wave velocity profiles of shallow and deep soil layers over a basement at strong motion stations in Eskisehir Province, Turkey. Microtremor array explorations were conducted at eight strong motion stations in the area to know shallow 1D S-wave velocity models. Rayleigh wave phase velocity at a frequency range from 3 to 30 Hz was estimated with the spatial autocorrelation analysis of array records of vertical microtremors at each station. Individual phase velocity was inverted to a shallow S-wave velocity profile. Low-velocity layers were identified at the stations in the basin. Site amplification factors from S-wave parts of earthquake records that had been estimated at the strong motion stations by Yamanaka et al. (2017) were inverted to the S-wave velocities and Q-values of the sedimentary layers. The depths to the basement with an S-wave velocity of 2.2 km/s are about 1 km in the central part of the basin, while the basement becomes shallow as 0.3 km in the marginal part of the basin. We finally discussed the effects of the shallow and deep sedimentary layers on the 1D S-wave amplification characteristics using the revealed profiles. It is found that the shallow soil layers have no significant effects in the amplification at a frequency range lower than 3 Hz in the area.

Seismic imaging of mantle wedge corner flow and arc magmatism.

I reviewed studies on the inhomogeneous seismic structure of the mantle wedge in subduction zones, in relation to corner flow and its implications for arc magmatism. Seismic studies in Tohoku clearly imaged the descending flow portion of the corner flow as a thin seismic low-velocity layer right above the slab. Slab-derived H2O is fixed to the layer as hydrous minerals, which are brought down by the slab and eventually decompose. The released H2O rises and encounters the ascending flow, formed to fill the gap caused by the descending flow. The combination of H2O addition and adiabatic decompression causes partial melting within the ascending flow. For many subduction zones, seismic tomography has distinctly imaged the ascending flow of the corner flow as a seismic low-velocity and/or high-attenuation layer in the mantle wedge inclined nearly parallel to the slab. These observations indicate that the volcanic front in subduction zones is formed both by the ascending flow and the addition of slab-derived H2O.

Joint Inversion of 1-D Magnetotelluric and Surface-Wave Dispersion Data with an Improved Multi-Objective Genetic Algorithm and Application to the Data of the Longmenshan Fault Zone

Magnetotellurics and seismic surface waves are two prominent geophysical methods for deep underground exploration. Joint inversion of these two datasets can help enhance the accuracy of inversion. In this paper, we describe a method for developing an improved multi-objective genetic algorithm (NSGA–SBX) and applying it to two numerical tests to verify the advantages of the algorithm. Our findings show that joint inversion with the NSGA–SBX method can improve the inversion results by strengthening structural coupling when the discontinuities of the electrical and velocity models are consistent, and in case of inconsistent discontinuities between these models, joint inversion can retain the advantages of individual inversions. By applying the algorithm to four detection points along the Longmenshan fault zone, we observe several features. The Sichuan Basin demonstrates low S-wave velocity and high conductivity in the shallow crust probably due to thick sedimentary layers. The eastern margin of the Tibetan Plateau shows high velocity and high resistivity in the shallow crust, while two low velocity layers and a high conductivity layer are observed in the middle lower crust, probably indicating the mid-crustal channel flow. Along the Longmenshan fault zone, a high conductivity layer from ~ 8 to ~ 20 km is observed beneath the northern segment and decreases with depth beneath the middle segment, which might be caused by the elevated fluid content of the fault zone.

Crystal structure and elasticity of Al-bearing phase H under high pressure

Al has significant effect on properties of minerals. We reported crystal structure and elasticity of phase H, an important potential water reservoir in the mantle, which contains different Al using first principles simulations for understanding the effect of Al on the phase H. The crystal and elastic properties of Al end-member phase H (Al2O4H2) are very different from Mg end-member (MgSiO4H2) phase H and two aluminous phase H (Mg0.875Si0.875Al0.25O4H2 (12.5at%Al) and Mg0.75Si0.75Al0.5O4H2 (25at% Al)). However differences between Mg end-member phase H and aluminous phase H are slight except for the O-H bond length and octahedron volume. Al located at different crystal positions (original Mg or Si position) of aluminous phase H has different AlO6 octahedral volumes. For three Al-bearing phase H, bulk modulus (K), shear modulus (G), compressional wave velocity (Vp) and shear wave velocity (Vs) increase with increasing Al content. Under high pressure, density of phase H increases with increasing Al content. The Al content affects the symmetry of the phase H and then affects the density and elastic constants of phase H. The total ground energy of phase H also increases with increasing Al content. So an energy barrier for the formation of solid solution of phase H with δ-phase AlOOH is expected. However, if the phase H with δ-phase AlOOH solid solution does exit in the mantle, it may become an important component of the mantle or leads to a low velocity layer at the mantle.

Ps mantle transition zone imaging beneath the Colorado Rocky Mountains: Evidence for an upwelling hydrous mantle

We analyze teleseismic P-to-S conversions for high-resolution imaging of the mantle transition zone beneath the Colorado Rocky Mountains using data from a dense PASSCAL seismic broadband deployment. A total of 6,021 P-to-S converted receiver functions are constructed using a multi-channel minimum-phase deconvolution method and migrated using the common converted point technique with the 3-D teleseismic P- and S-wave tomography models of Schmandt and Humphreys (2010). The image finds that the average depths of the 410-km discontinuity (the 410) and 660-km discontinuity (the 660) at 408±1.9 km and 649±1.6 km respectively. The peak-to-peak topography of both discontinuities is 33 km and 27 km respectively. Additionally, prominent negative polarity phases are imaged both above and below the 410. To quantify the mean properties of the low-velocity layers about 410 km, we utilize double gradient layer models parameterization to fit the mean receiver function waveform. This waveform fitting is accomplished as a grid-search using anelastic synthetic seismograms. The best-fitting model reveals that the olivine–wadsleyite phase transformation width is 21 km, which is significantly larger than anhydrous mineral physics prediction (4–10 km) (Smyth and Frost, 2002). The findings of a wide olivine–wadsleyite phase transformation and the negative polarity phases above and below the 410, suggest that the mantle, at least in the 350–450 km depth range, is significantly hydrated. Furthermore, a conspicuous negative polarity phase below the 660 is imaged in high velocity region, we speculate the low velocity layer is due to dehydration flux melting in an area of convective downwelling. Our interpretation of these results, in tandem with the tomographic image of a Farallon slab segment at 800 km beneath the region (Schmandt and Humphreys, 2010), is that hydrous and upwelling mantle contributes to the high-standing Colorado Rocky Mountains.

Investigation of sediment thickness effect to the receiver function through forward modelling

In this study, we investigated sediment thickness effect to the receiver function waveform. We conducted forward modelling of receiver function using different thickness of low velocity sediment layers. We varied low velocity layer from 0 - 4 km thickness. We applied Gaussian width parameter of 1.5 and 2.5, which corresponds to low pass filter with a corner frequency of 0.75 and 1.25 Hz, respectively. In this numerical analysis, we applied halfspace model with low velocity layer near the surface representing sedimentary layer. We generated synthetic receiver function using Thomson-Haskell matrix method. From our modelling, changes occur in the first few seconds of calculated synthetic receiver functions. Such changes occur due to reverberation in the sedimentary layer near the surface. Such changes differ with different sedimentary thickness used in the model. We applied H-k stacking method to the calculated synthetic receiver functions to see the effect of sedimentary layers to the results of H-k stacking.

S-velocity structure in Cimandiri fault zone derived from neighbourhood inversion of teleseismic receiver functions

The availability information about realistic velocity earth model in the fault zone is crucial in order to quantify seismic hazard analysis, such as ground motion modelling, determination of earthquake locations and focal mechanism. In this report, we use teleseismic receiver function to invert the S-velocity model beneath a seismic station located in the Cimandiri fault zone using neighbourhood algorithm inversion method. The result suggests the crustal thickness beneath the station is about 32-38 km. Furthermore, low velocity layers with high Vp/Vs exists in the lower crust, which may indicate the presence of hot material ascending from the subducted slab.

S-to-P Conversions from Mid-mantle Slow Scatterers in Slab Regions: Observations of Deep/Stagnated Oceanic Crust?

The fate of a subducted slab is a key ingredient in the context of plate tectonics, yet it remains enigmatic especially in terms of its crustal component. In this study, our efforts are devoted to resolve slab-related structures in the mid-mantle below eastern Indonesia, the Izu–Bonin region, and the Peru area by employing seismic array analysing techniques on high-frequency waveform data from F-net in Japan and the Alaska regional network and the USArray in North America. A pronounced arrival after the direct P wave is observed in the recordings of four deep earthquakes (depths greater than 400 km) from three subduction systems including the Philippines, the Izu–Bonin, and the Peru. This later arrival displays a slightly lower slowness compared to the direct P wave and its back-azimuth deviates somewhat from the great-circle direction. We explain it as an S-to-P conversion at a deep scatterer below the sources in the source region. In total, five scatterers are seen at depths ranging from ~930 to 1500 km. Those scatterers appear to be characterised by an ~7 km-thick low-velocity layer compared to the ambient mantle. Combined evidence from published mineral physical analysis suggests that past subducted oceanic crust, possibly fragmented, is most likely responsible for these thin-layer compositional heterogeneities trapped in the mid-mantle beneath the study regions. Our observations give a clue to the potential fate of subducted oceanic crust.

Crustal structure revealed by a deep seismic sounding profile of Baijing-Gaoming-Jinwan in the Pearl River Delta

The Pearl River Estuary area, located in the middle part of the southern China coastal seismic belt, has long been considered a potential source of strong earthquakes above magnitude 7.0. To scientifically assess the potential strong earthquake risk in this area, a three-dimensional artificial seismic sounding experiment, consisting of a receiving array and seabed seismograph, was performed to reveal the deep crustal structure in this region. We used artificial ship-borne air-gun excitation shots as sources, and fixed and mobile stations as receivers to record seismic data from May to August 2015. This paper presents results along a line from the western side of the Pearl River Estuary to the western side of the Baijing-Gaoming-Jinwan profile. A two-dimensional velocity structure was constructed using seismic travel-time tomography. The inversion results show that the Moho depth is 27 km in the coastal area and 30 km in the northwest of the Pearl River Estuary area, indicating that the crust thins from land to sea. Two structural discontinuities and multiple low-velocity anomalies appear in the crustal section. Inside both discontinuity zones, a low-velocity layer, with a minimum velocity of 6.05 km s−1, exists at a depth of about 15 km, and another, with a minimum velocity of 6.37 km s−1, exists at a depth of about 21.5 km between the middle and lower crust. These low velocities suggest that the discontinuities may consist of partly molten material. Earthquakes with magnitudes higher than 5.0 occurred in the low-velocity layer along the profile. The deep Kaiping-Enping fault, rooted in the crust, may be one of the most important channels for deep material upwelling and is related to tectonic movement since the Cretaceous in the Pearl River Delta tectonic rift basin.