We observe a clear seismic arrival at ∼35–45s after the direct P wave in USArray recordings of two deep earthquakes occurring beneath northeast China. Velocity-spectrum and beam-forming analyses ...reveal that this arrival has a lower slowness value than the direct P wave and a back azimuth slightly different from the great-circle direction. The measured slowness and arrival time indicate that it is a transmitted S to P conversion from structures below the sources. We employ the common-conversion-point (CCP) stacking and diffraction migration methods to determine the location and geometric features of the seismic structures. The CCP stacking image indicates that the structure is a localized discontinuity at ∼1000km with a dimension at ∼200km by ∼50km along the E–W and N–S directions, respectively. It is located at ∼150km northeast to the two events. The 2D migrated images, on the other hand, indicate that the sources structure are reflectors dipping northeastwards by ∼17° at a slightly shallower depths. The reflectors have a length scale of ∼100km ant their centers are ∼50km away from the epicenters of the two earthquakes. Forward waveform modeling suggests that the dipping reflectors may be thin layers with a thickness of few kilometers. The layers have a lower shear velocity and a higher density than that of the surrounding mantle, which matches well with those predicted for mid-ocean-ridge basalt (MORB) at mid-mantle depths, according to a recent ab initio study. Combined with the results from previous studies, our observations here suggest that the former oceanic crust may be ubiquitously present in the lower mantle beneath subduction zones.
► USArray recorded anomalous seismic arrivals from two deep quakes in NE China. ► The source structures are seismically distinct thin layers in the lower mantle. ► The thin layers could be subducted oceanic crust. ► Oceanic crust may be ubiquitously present in the lower mantle of subduction zones.
To better understand the subsurface behavior of subducting slabs and their relation to the tectonic evolution of the overriding plate, we conduct a full waveform inversion on a large data set to ...determine a high‐resolution seismic model, FWEA18 (Full Waveform inversion of East Asia in 2018), of the upper mantle beneath eastern Asia. FWEA18 reveals sharper, more intense high‐velocity slabs in the upper mantle under the southern Kuril, Japan, and Ryukyu arcs, than previous studies have found. The subducting Pacific plate is imaged as a roughly 100 km thick high‐velocity slab to near 550 km depth indicating relatively little deformation. Stagnation near 600 km depth is observed over horizontal distances of 600 km or less. The Pacific plate we image accounts for roughly 25 Myr of subduction with older slab likely located in the lower mantle. The Philippine plate, subducting beneath the Ryukyu Islands, has a clear termination at about 450 km depth. This may indicate a tearing event in the past or that less Philippine Sea plate has subducted than previously thought. We found a double‐layer high‐velocity anomaly above and below 660 km under the Yellow Sea and eastern coast of North China. This may correspond to parts of the Philippine Sea plate that detached in the past and Pacific plate that have intersected at depth or a complicated behavior of the Pacific plate at that depth. Slow cylindrical anomalies cross the entire upper mantle are imaged beneath major Holocene volcanoes, which are likely upwellings associated with the edges of deep slabs that are entering the lower mantle.
Key Points
High‐resolution 3‐D full waveform inversion for P‐velocities and S‐velocities reveals sharper and more intense mantle structures beneath East Asia
Subducting Pacific and Philippine Sea plates show little deformation until 550 km depth where the slabs stagnate and thicken
Narrow cylindrical slow anomalies beneath major Holocene volcanoes are rooted adjacent to where deep slabs enter the lower mantle
We analyzed thousands of receiver-function data recorded by 154 national and regional seismic stations to study the crustal structure beneath the Ordos plateau and the NE margin of the Tibetan ...plateau. Moho depth and average crustal Vp/Vs ratio were measured at each station. The Ordos plateau and the Trans-North China Orogen east of the Ordos are underlain by a moderately thick crust of ~42km. The Weihe Graben lying at the southern edge of the Ordos plateau has a thin crust of ~30km, while its southern neighbor, the Qinling orogenic belt shows a thick crust extending to as much as 45km deep. The Moho depth beneath the NE margin of the Tibetan plateau varies from 55 to 65km. We found a remarkable contrast between the Tibetan and Ordos plateaus in the measured Poisson's ratio: the Ordos plateau is featured by a high Poisson's ratio while the Tibetan margin has a very low Poisson's ratio. In general, mafic lower crustal rocks have a higher Poisson's ratio than felsic ones. The measured low Poisson's ratio beneath the NE margin of the Tibetan plateau thus indicates that the crustal column beneath the margin is rather felsic, which seems to be inconsistent with a scenario of an inflated crust due to extrusion of lower crust material from the Tibetan plateau to the margin.
► Crustal thickness and Vp/Vs of the Ordos plateau and NE Tibetan plateau were estimated. ► The Ordos is underlain by a moderately thick crust with an intermediate composition. ► Crust beneath the NE Tibetan plateau varies is thick with a low Vp/Vs ratio. ► The low Vp/Vs ratio is inconsistent with the lower-crust flow model proposed for crustal thickening. ► Seismicity around the Ordos plateau is controlled by crustal and lithosphere structure.
We construct a new 3-D shear wave speed model of the crust and the uppermost mantle beneath Northeast China using the ambient noise adjoint tomography method. Without intermediate steps of measuring ...phase dispersion, the adjoint tomography inverts for shear wave speeds of the crust and uppermost mantle directly from 6–40 s waveforms of Empirical Green's functions (EGFs) of Rayleigh waves, which are derived from interferometry of two years of ambient noise data recorded by the 127 Northeast China Extended Seismic Array stations. With an initial 3-D model derived from traditional asymptotic surface wave tomography method, adjoint tomography refines the 3-D model by iteratively minimizing the frequency-dependent traveltime misfits between EGFs and synthetic Green's functions measured in four period bands: 6–15 s, 10–20 s, 15–30 s, and 20–40 s. Our new model shows shear wave speed anomalies that are spatially correlated with known tectonic units such as the Great Xing'an range and the Changbaishan mountain range. The new model also reveals low wave speed conduits in the mid-lower crust and the uppermost mantle with a wave speed reduction indicative of partial melting beneath the Halaha, Xilinhot–Abaga, and Jingpohu volcanic complexes, suggesting that the Cenozoic volcanism in the area has a deep origin. Overall, the adjoint tomographic images show more vertically continuous velocity anomalies with larger amplitudes due to the consideration of the finite frequency and 3-D effects.
•A new 3-D S model beneath NE China is constructed with adjoint tomography method.•Wavespeed anomalies are spatially correlated with known tectonic units.•Low-V conduits indicative of partial melting exist beneath the Cenozoic volcanoes.
The dehydration of oceanic slabs during subduction is mainly thermally controlled and is often expressed as intermediate‐depth seismicity. In warm subduction zones, shallow dehydration can also lead ...to the buildup of pore‐fluid pressure near the plate interface, resulting in nonvolcanic tremor. Along the Cascadia margin, tremor density and intermediate‐depth seismicity correlate but vary significantly from south to north despite little variation in the thermal structure of the Juan de Fuca Plate. Along the northern and southern Cascadia margin, intermediate‐depth seismicity likely corresponds to increased fluid flux, while increased tremor density may result from fluid infiltration into thick underthrust metasediments characterized by very slow shear wave velocities (<3.2 km/s). In central Cascadia, low intermediate‐depth seismicity and tremor density may indicate a lower fluid flux, and shear wave velocities indicate that the Siletzia terrane extends to the plate interface. These results indicate that the presence of thick underthrust sediments is associated with increased tremor occurrence.
Plain Language Summary
Fluids are released from subducting oceanic lithosphere as temperature and pressure within the Earth increases. The release of these fluids is manifest by seismicity within the subducting lithosphere and nonvolcanic tremor near the subduction interface. Along the northern and southern Cascadia margin, the spatial distribution of seismicity within the slab and nonvolcanic tremor correlates with very slow shear wave velocities in the lower crust of the overriding plate. These low‐velocity zones likely represent underthrust sediments containing slab‐derived fluids. In central Cascadia, however, seismicity and tremor are relatively low, and a low‐velocity zone in the lower crust of the forearc is not observed. Our results suggest that a combination of variations in the distribution of underthrust sediments and fluid flux along the margin may be the ultimate control on the tremor and seismicity distribution along the Cascadia margin.
Key Points
Low‐velocity zones in the Cascadia forearc lower crust correlate with nonvolcanic tremor density and intermediate‐depth seismicity
Low‐velocity zones are thick (>10 km) and indicate the presence of fluid within oceanic crust and underthrust metasediments
Low‐velocity zones, seismicity, and tremor can be linked to fluids sourced from a variably hydrated and/or permeable subducting slab
Summary
In seismic full-waveform inversion (FWI), the choice of misfit function determines what information in data is used and ultimately affects the resolution of the inverted images of the Earth's ...structure. Misfit functions based on traveltime have been successfully applied in global and regional tomographic studies. However, wave propagation through the upper mantle results in multiple phases arriving at a given receiver in a narrow time interval resulting in complicated waveforms that evolve with distance. To extract waveform information as well as traveltime, we use a misfit function based on the normalized correlation coefficient (CC). This misfit function is able to capture the waveform complexities in both phase and relative amplitude within the measurement window. It is also insensitive to absolute amplitude differences between modeled and recorded data, which avoids problems due to uncertainties in source magnitude, radiation pattern, receiver site effects or even miscalibrated instruments. These features make the misfit function based on normalized CC a good candidate to achieve high-resolution images of complex geological structures when interfering phases coexist in the measurement window, such as triplication waveforms. From synthetic tests, we show the advantages of this misfit function over the cross-correlation traveltime misfit function. Preliminary inversion of data from an earthquake in Northeast China images a sharper and stronger amplitude slab stagnant in the middle of the transition zone than FWI of cross-correlation traveltime.
We analyzed 49,592 teleseismic receiver functions (RFs) recorded by 278 CEArray stations to image the mantle transition zone (MTZ) beneath the South China Block to understand the origins of deep ...velocity anomalies and their potential links to subduction and intraplate volcanism. We employed a fast‐marching method and a high‐resolution 3‐D velocity model (FWEA18) derived from full waveform inversion in computing P‐to‐S conversion times to better image the 410‐ and 660‐km discontinuities. Our results indicate that the common‐conversion‐point stacking of RFs using 3‐D conversion times yielded better migration images of the two discontinuities. The images revealed a slightly depressed 410‐km with a few small uplifted patches, and showed that the 660‐km beneath the western Yangtze Craton is depressed by 10–25 km, which is likely caused by the stagnant Paleo‐Pacific slab. The 660‐km beneath the southern Cathaysia Block has a 5–15 km high plateau with a topographic low at its central part. The lateral dimension of the topographic low is ∼150 km and is located beneath the central Pearl River Mount Basin near Hong Kong. We speculate that the topographic low occurs within the Hainan plume with a temperature excess of ∼300–400 K and is caused by the garnet phase transition. The displaced deep plume enters the MTZ and spreads nearly horizontally at the base. The plume evolves into two channels with a minor one toward the northeast and a major one toward the southwest, which keep moving upward to the 410‐km. The southwest channel is likely the source that feeds the Hainan volcanoes.
Plain Language Summary
Using data from seismic stations in the South China Block, we investigated the mantle transition zone (MTZ) to understand the origins of deep velocity anomalies and their associations with subduction and intraplate volcanism. By applying advanced techniques and a ground‐truth reference model, we obtained clearer images of the 410‐ and 660‐km discontinuities. The images showed that the 410‐km discontinuity is slightly depressed with some small uplifted areas. Additionally, the 660‐km discontinuity beneath the western Yangtze Craton is depressed due to the presence of a stagnant slab from the ancient Pacific Ocean. In contrast, beneath the southern Cathaysia Block, the 660‐km discontinuity forms a high plateau with a central low area. This area located near Hong Kong may be related to a plume originating from the lower mantle. The plume, with elevated temperatures, enters the MTZ and spreads horizontally. It then evolves into two channels, with one moving toward the northeast and the other toward the southwest. The southwest channel likely supplies magma to the volcanoes in Hainan. These findings provide insights into the complex processes occurring deep within the Earth's mantle in the South China region.
Key Points
FWEA18 is used in migrating CEArray receiver functions to image the mantle transition zone beneath the South China Block
The 660‐km is depressed ∼10–25 km by stagnant slabs beneath the northwestern part of the block
The Hainan volcanoes are fed by a displaced lower mantle plume beneath the central Pearl River Mouth Basin
Long-standing debates exist over the timing and mechanism of uplift of the Tibetan Plateau and, more specifically, over the connection between lithospheric evolution and surface expressions of ...plateau uplift and volcanism. Here we show a T-shaped high wave speed structure in our new tomographic model beneath South-Central Tibet, interpreted as an upper-mantle remnant from earlier lithospheric foundering. Its spatial correlation with ultrapotassic and adakitic magmatism supports the hypothesis of convective removal of thickened Tibetan lithosphere causing major uplift of Southern Tibet during the Oligocene. Lithospheric foundering induces an asthenospheric drag force, which drives continued underthrusting of the Indian continental lithosphere and shortening and thickening of the Northern Tibetan lithosphere. Surface uplift of Northern Tibet is subject to more recent asthenospheric upwelling and thermal erosion of thickened lithosphere, which is spatially consistent with recent potassic volcanism and an imaged narrow low wave speed zone in the uppermost mantle.
Both seismic and geodetic data suggested that the ∼120‐km long Weifang segment of the Tanlu fault zone, a large‐scale active strike‐slip system at east China, is a seismic gap with no obvious ...along‐strike shear motion at surface. Measuring crustal deformation around the segment is crucial to constrain stress/strain buildup and potential seismic risk at the fault. We measured crustal and upper mantle seismic anisotropy using P‐to‐S converted waves at the Moho (Pms) and core‐mantle boundary (SKS) recorded by broadband arrays across the Weifang fault segment. The measured crustal anisotropy inside the fault zone shows a fast direction of ∼NNE, parallel to the fault orientation. Right east to the fault zone, the fast axis rotates by almost 90° to ESE. The crustal anisotropy within the fault zone could be caused by aligned microcracks and foliated minerals due to long‐lasting shear motion inside the fault zone.
Plain Language Summary
The Tanlu fault zone (TLFZ) is the most prominent fault system in east China, an area with a large population and economy. One of the most devastating earthquakes in recent history of China, the M8+ Tancheng earthquake occurred in the central segment of this fault. The Weifang segment north to the Tancheng earthquake has little seismicity and is considered as a seismic gap region. Surface Global Positioning System (GPS) data, however, suggested that there is very little present‐day crustal deformation across the North China Plain where the Weifang segment is located. This is also supported by shear‐wave splitting measurements of the core phases (SKS). Therefore, it is unclear whether the seismic gap is due to lack of strain/stress buildup, or the GPS/SKS data have no resolution on crustal deformation. In this study, we deployed a dense seismic array across the segment, and chose a seismic wave, a P‐to‐S converted wave at the crust‐mantle boundary (i.e., the Moho), to measure crustal deformation. The measured seismic property indicates the presence of aligned microcracks and foliated minerals within the fault zone, which suggests significant shear deformation is occurring inside the TLFZ.
Key Points
The Weifang segment of the Tanlu fault zone shows a noticeable crustal anisotropy with a fast axis parallel to the NNE oriented fault
East to the fault zone, fast direction rotates rapidly by ∼90° to ESE, parallel to the SKS fast polarization direction
The southeastern part of the study area has a thinned crust with a Vp/Vs ratio measurably lower than a typical cratonic crust
The perfectly matched layer (PML) boundary condition has been proven to be effective for attenuating reflections from model boundaries during wavefield simulation. As such, it has been widely used in ...time-domain finite-difference wavefield simulations. The conventional PML has poor performance for near grazing incident waves and low-frequency reflections. To overcome these limitations, a more complex frequency-shifted stretch (CSF) function is introduced, which is known as the CFS-PML boundary condition and can be implemented in the time domain by a recursive convolution technique (CPML). When implementing the PML technique to second-order wave equations, all the existing methods involve adding auxiliary terms and rewriting the wave equations into new second-order partial differential equations that can be simulated by the finite-difference scheme, which may affect the efficiency of numerical simulation. In this paper, we propose a relatively simple and efficient approach to implement CPML for the second-order equation system, which solves the original wave equations numerically in the stretched coordinate. The spatial derivatives in the stretched coordinate are computed by adding a correction term to the regular derivatives. Once the first-order spatial derivatives are computed, we computed the second-order spatial derivatives in a similar way; therefore, we refer to the method as two-step CPML (TS-CPML). We apply the method to the second-order acoustic wave equation and a coupled second-order pseudo-acoustic TTI wave equation. Our simulations indicate that amplitudes of reflected waves are only about half of those computed with the traditional CPML method, suggesting that the proposed approach has computational advantages and therefore can be widely used for forwarding modeling and seismic imaging.
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