Stepwise joint inversion of surface wave dispersion,Rayleigh wave ZH ratio,and receiver function data for 1D crustal shear wave velocity structure

Accurate determination of seismic velocity of the crust is important for understanding regional tectonics and crustal evolution of the Earth. We propose a stepwise joint linearized inversion method using surface wave dispersion, Rayleigh wave ZH ratio （i.e., ellipticity）, and receiver function data to better resolve 1D crustal shear wave velocity （Vs） structure. Surface wave dispersion and Rayleigh wave ZH ratio data are more sensitive to absolute variations of shear wave speed at depths, but their sensi- tivity kernels to shear wave speeds are different and complimentary. However, receiver function data are more sensitive to sharp velocity contrast （e.g., due to the existence of crustal interfaces） and Vp/Vs ratios. The stepwise inversion method takes advantages of the complementary sensitivities of each dataset to better constrain the Vs model in the crust. We firstly invert surface wave dispersion and ZH ratio data to obtain a 1D smooth absolute vs model and then incorporate receiver function data in the joint inver- sion to obtain a finer Vs model with better constraints on interface structures. Through synthetic tests, Monte Carlo error analyses, and application to real data, we demonstrate that the proposed joint inversion method can resolve robust crustal Vs structures and with little initial model dependency.

Determination of the local magnitudes of small earthquakes using a dense seismic array in the Changning-Zhaotong Shale Gas Field,Southern Sichuan Basin

With the development of unconventional shale gas in the southern Sichuan Basin,seismicity in the region has increased significantly in recent years.Though the existing sparse regional seismic stations can capture most earthquakes with ML≥2.5,a great number of smaller earthquakes are often omitted due to limited detection capacity.With the advent of portable seismic nodes,many dense arrays for monitoring seismicity in the unconventional oil and gas fields have been deployed,and the magnitudes of those earthquakes are key to understand the local fault reactivation and seismic potentials.However,the current national standard for determining the local magnitudes was not specifically designed for monitoring stations in close proximity,utilizing a calibration function with a minimal resolution of 5 km in the epicentral distance.That is,the current national standard tends to overestimate the local magnitudes for stations within short epicentral distances,and can result in discrepancies for dense arrays.In this study,we propose a new local magnitude formula which corrects the overestimated magnitudes for shorter distances,yielding accurate event magnitudes for small earthquakes in the Changning-Zhaotong shale gas field in the southern Sichuan Basin,monitored by dense seismic arrays in close proximity.The formula is used to determine the local magnitudes of 7,500 events monitored by a two-phased dense array with several hundred 5 Hz 3 C nodes deployed from the end of February 2019 to early May 2019 in the Changning-Zhaotong shale gas field.The magnitude of completeness(MC)using the dense array is-0.1,compared to MC 1.1 by the sparser Chinese Seismic Network(CSN).In addition,using a machine learning detection and picking procedure,we successfully identify and process some 14,000 earthquakes from the continuous waveforms,a ten-fold increase over the catalog recorded by CSN for the same period,and the MC is further reduced to-0.3 from-0.1 compared to the catalog obtained via manual processing using the same dense array.The proposed local magnitude formula can be adopted for calculating accurate local magnitudes of future earthquakes using dense arrays in the shale gas fields of the Sichuan Basin.This will help to better characterize the local seismic risks and potentials.

High resolution tomography of the Tanlu fault zone near Hefei with passive seismic and magnetotelluric linear array data

As the largest fault trending NNE-SSW to NE-SW in the eastern Eurasia Continent,the Tanlu fault zone(TLFZ)extends over 2,400 km within China,roughly from Wuxue,Hubei Province,to Russia.Since the Quaternary period,the TLFZ has been an earthquake-prone area in eastern China where several major earthquakes resulted by tectonic compression occurred,causing tremendous casualties and significant economic losses.Many studies on different segments of the TLFZ have been carried out in the past few decades.However,numerous key questions regarding the fault zone remain unanswered due to a lack of clear subsurface characterization and fault delineation.In this study,we present high-resolution tomographic results across the TLFZ to the east of Hefei,where one 22-km-long passive seismic array with densely spaced short-period nodes,and a 24-km-long magnetotelluric array were deployed adjacent to each other.We find the velocity and resistivity variations are highly consistent with known surface geology.Sharp property contrasts in both the seismic shear wave velocity and electrical resistivity profiles clearly delineate the Tanlu F1 fault(TLF-1)near Hefei.More interestingly,an upwelling with distinct high velocity is imaged within the Hefei Basin to the west of the TLF-1,whereas a slanted block with lowvelocity and low-resistivity seems to cut into or thrust upon the high-grade to low-grade middle-pressure rocks in the Zhangbaling uplift right below the Tanlu F2 ductile shear fault(TLF-2).The presented results show a new approach to characterize deep subsurface structure of the TLFZ beyond 2-km depths using passive data,which it is often difficult for active seismic surveys with refracted and reflected waves to image.

3D v_P and v_S models of southeastern margin of the Tibetan plateau from joint inversion of body-wave arrival times and surface-wave dispersion data

A new 3D velocity model of the crust and upper mantle in the southeastern （SE） margin of the Tibetan plateau was obtained by joint inversion of body- and sur- face-wave data. For the body-wave data, we used 7190 events recorded by 102 stations in the SE margin of the Tibetan plateau. The surface-wave data consist of Rayleigh wave phase velocity dispersion curves obtained from ambient noise cross-correlation analysis recorded by a dense array in the SE margin of the Tibetan plateau. The joint inversion clearly improves the Vs model because it is constrained by both data types. The results show that at around 10 km depth there are two low-velocity anomalies embedded within three high-velocity bodies along the Longmenshan fault system. These high-velocity bodies correspond well with the Precambrian massifs, and the two located to the northeast of 2013 Ms 7.0 Lushan earthquake are associated with high fault slip areas during the 2008 Wenchuan earthquake. The aftershock gap between 2013 Lushan earthquake and 2008 Wenchuan earthquake is associated with low-velocity anomalies, which also acts as a barrier zone for ruptures of two earthquakes. Generally large earthquakes （M 〉 5） in the region occurring from 2008 to 2015 are located around the high-velocity zones, indicating that they may act as asperities for these large earthquakes. Joint inversion results also clearly show that there exist low-velocity or weak zones in the mid-lower crust, which are not evenly distributed beneath the SE margin of Tibetan plateau.

Seismic anisotropy beneath the Chinese Mainland: Constraints from shear wave splitting analyses

We use earthquakes recorded by the China National Seismic Network from 2015 to 2019 and measure shear wave splitting parameters of SKS to study the anisotropic characteristics beneath the mainland of China.In general,the fast directions change from nearly E-W in western China(northwest China and Qinghai-Tibetan Plateau)to nearly N-S in central China(Ordos and Sichuan-Yunnan),and then turn to approximately E-W in eastern China(North and South China).The delay times of slow wave in eastern China are about 1.0-1.7 s,larger than those in central and western China(about 0.6-1.0 s).In addition,the fast directions in eastern China are highly consistent with the plate motion direction and horizontal GPS velocities with respect to Eurasia,indicating that the observed anisotropy is mainly from the asthenosphere which is strongly coupled to the overlying lithosphere.However,the fast directions in western China are mostly in accord with the strike of the surface structures(such as faults),possibly due to the directional arrangement of crystal lattices caused by shear deformation under tectonic activities.

Pervasion degree of convective mixing and investigations of mantle mixing

Geodynamic studies have shown that mantle convection is like a giant blender to make the original heterogeneous mantle mixing and homogenizing. However, some models, especially from geochemical data show that the modem mantle may still contain a number of reservoir bodies with different chemical composition. Then, the modern mantle is homogeneous？ Authors have defined a box replacement degree of convective mantle mixing and pervasion degree of convective mantle mixing （that equals to initial density of tracing elements divided by final density of tracing elements） to investigate the mantle mixing. The previous results have shown that after four billion years the mantle is basic uniform and the box replacement of the convective mantle is more than 80% in steady-state convection models. This paper calculates and discusses the pervasion degree of convective mantle mixing in detail. For the initial state we will set some 10°× 10°（spacing 0.25°） tracing boxes, which includes 1 681 tracer, at the top and bottom of the mantle, and then track the motions of these tracers, and investigate the convective mixed pervasions. The results show that at the initial stage though the convective mixing pervasions are very different from mode to mode, after running for some time, the convective mixing pervasions for most modes are going to be a constant and the tracers are more evenly distributed in the whole mantle.

Ambient Noise Tomography of Jinan:The Migration of Groundwater and the Formation of Geothermal Water

Jinan is an important city in eastern China,with rich groundwater in the region.There are four famous springs in the urban area and an abundance of geothermal water in the northern part,which makes the migration of groundwater in this area a very important issue.To study the shallow shear wave velocity structure and groundwater migration in Jinan,we utilized almost a month of continuous waveform data from 175 short period seismometers deployed by the Chinese Academy of Geological Sciences,in order to calculate the cross-correlation function.We picked 7749 group dispersion curves and 6117 phase dispersion curves with a period range of 0.2–2 s.Through inversion,we obtained the fine threedimensional shear wave velocity and azimuthal anisotropy structure(0–2.4 km).Combining the results with local geological and hydrological data,the following conclusions were reached.(1)There are widespread high velocity anomalies in the region between the Qianfoshan and Wenhuaqiao faults,as well as to the east of the Wenhuaqiao Fault,which may be related to the intrusive gabbro known as the Jinan Intrusive Rock.(2)The two distinct high velocity anomalies in our model(referred to as west and east Jinan Intrusive Rock in this paper)may indicate that the Jinan Intrusive Rock was broken through crustal movement.(3)There is an obvious low velocity layer under the intrusive rock,which could be the channel of groundwater migration.The precipitation in the southern mountain region seeps down into the ground,then is blocked by the Jinan Intrusive Rock and can only progress downwards to a deeper part,where the groundwater is heated by the geothermal gradient.The heated water finally arrives at the northern part and forms geothermal water.(4)The depth of the low velocity layer beneath the Jinan Intrusive Rock varies laterally,which may indicate that the depth of the groundwater migration is different beneath the west and east Jinan Intrusive Rock.(5)There is strong azimuthal anisotropy in southern Jinan,with nearly E-W fast orientation,which may be related to the tilt limestone layering structure.

Spatial and Temporal Variations of Daily Ranges of the Geomagnetic Vertical Component in the Chinese Mainland

In this paper, we use the daily ranges of the vertical magnetic intensity of approximately 76 geomagnetic stations from January 1, 2008 to December 31, 2010 to analyze the spatial and temporal characteristics of Z ranges. The results are summarized as follows: (1) Temporally, we use regressive analysis and FFT analysis to analyze the data. The results show that the Z component daily ranges of all stations have an obvious cyclical variation, the computed Fourier spectra of all data sets have clearly resolved the required periodicities in the data, in the form of distinct peaks at days 365, 183, 22, and 73, and the power spectra of day 365 is the highest in all periods. (2) In terms of spatial variation, the daily ranges show nonlinear variation with latitude in China. The results show the existence of a point of inflexion (maximal value point) nearby at about 25°N, the daily ranges of Z rise from 15°~25°N and have a good linear decrease variation along with 25°~50°N. (3) Compared with the spatial and temporal variations of Z daily ranges with the Sq current inversion, we found that the spatial and temporal characteristics of Z ranges are decided mainly by the spatio-temporal evolution of the Sq current system. (4) If the latitudes of the maximum amplitudes of variation of the vertical component in the geomagnetic quiet days are roughly taken as the latitudes corresponding to the foci of Sq overhead current system, we can see that these latitudes of foci become higher in summer, are lowest in winter and highest during Equinoxes, displaying conspicuous monthly and daily variations. For two successive geomagnetic quiet days, the latitudes of foci may vary ten degrees.

Effect of hydration on the elasticity of mantle minerals and its geophysical implications

Recent studies have shown that major nominally anhydrous minerals in the Earth's mantle, such as olivine, pyroxene and garnet, can incorporate considerable amounts of water as structurally bound hydroxyl. Even a small amount of water is present in mantle minerals, it can strongly affect a number of physical properties, including density, sound velocity, melting temperature, and electrical conductivities. The presence of water can also influence the dynamic behavior, lead to lateral velocity heterogeneities, and affect the material circulation of the Earth's deep interior. In particular, seismic studies have reported the existence of low-velocity zones in various locations of the Earth's upper mantle and transition zone, which has been expected to be associated with the presence of water in the region. In the past two decades, the effect of water on the elasticity and sound velocities of minerals at relevant pressure-temperature(P-T) conditions of the Earth's mantle attracted extensive interests. Combining the high P-T experimental and theoretical mineralogical results with seismic observations provides crucial constraints on the distribution of water in the Earth's mantle. In this study, we summarize recent experimental and theoretical mineral physics results on how water affects the elasticity and sound velocity of nominally anhydrous minerals in the Earth's mantle, which aims to provide new insights into the effect of hydration on the density and velocity profile of the Earth's mantle, which are of particular importance in understanding of water distribution in the region.

A method for inversion of layered shear wavespeed azimuthal anisotropy from Rayleigh wave dispersion using the Neighborhood Algorithm

Seismic anisotropy provides important constraints on deformation patterns of Earth＇s material. Rayleigh wave dispersion data with azimuthal anisotropy can be used to invert for depth-dependent shear wavespeed azimuthal anisotropy, therefore reflecting depth-varying deformation patterns in the crust and upper mantle. In this study, we propose a two-step method that uses the Neighborhood Algorithm （NA） for the point-wise inversion of depth-dependent shear wavespeeds and azimuthal anisotropy from Rayleigh wave azimuthally anisotropic dispersion data. The first step employs the NA to estimate depth- dependent Vsv （or the elastic parameter L） as well as their uncertainties from the isotropic part Rayleigh wave dispersion data. In the second step, we first adopt a difference scheme to compute approximate Rayleigh-wave phase velocity sensitivity kernels to azimuthally anisotropic parameters with respect to the velocity model obtained in the first step. Then we perform the NA to estimate the azimuthally anisotropic parameters Gc/L and Gs/L at depths separately from the corresponding cosine and sine terms of the azimuthally anisotropic dispersion data. Finally, we compute the depth-dependent magnitude and fast polariza- tion azimuth of shear wavespeed azimuthal anisotropy. The use of the global search NA and Bayesian analysis allows for more reliable estimates of depth-dependent shear wavespeeds and azimuthal anisotropy as well as their uncertainties.We illustrate the inversion method using the azimuthally anisotropic dispersion data in SE Tibet, where we find apparent changes of fast axes of shear wavespeed azimuthal anisotropy between the crust and uppermost mantle.

Elasticity of high-pressure clinoenstatite under mantle conditions:Implications for the origin of the X-discontinuity

The X-discontinuity is characterized by 2–8% wave impedance contrasts and indistinguishable seismological Clapeyron slopes. Its origin is not yet entirely understood and attributed to a variety of plausible causes, among which the coesite-stishovite transition is a popular mechanism due to its large impedance contrasts. However, the sole coesite-stishovite transition is insufficient to explain indistinguishable seismological Clapeyron slopes of the X-discontinuity. The orthopyroxene(OPX) to high-pressure clinopyroxene(HPCPX) transition has been excluded as a candidate mechanism in recent seismic studies because it can only cause small impedance contrasts based on the first-order estimate from the Birch’s law without direct sound velocity measurements. In this study, we performed first-principles calculations to obtain the elasticity of high-pressure clinoenstatite at high pressure and temperature. Our results show that the impedance contrast caused by the OPX-HPCPX transition is ~5.7% for P wave and ~6.9% for S wave, which are much larger than the previous empirical estimation and hence cannot be ignored. Given that eclogite is subject to partial melting in hot or wet regions, which will promote the enrichment of orthopyroxene by consuming silica, we suggested that both the coesite-stishovite transition and the OPX-HPCPX transition may be dominant mechanisms for the X-discontinuity, with the former dominating where eclogite is hard to melt and the latter dominating where partial melting of eclogite occurs. The model is consistent with seismological observations, indicating the important role of the OPX-HPCPX transition in the X-discontinuity and extensive occurrence of partial melting of eclogite. The proposed origin of the X-discontinuity provides a plausible way to illuminate the melting situation of eclogite in the deep earth.

High-resolution 3D crustal S-wave velocity structure of the MiddleLower Yangtze River Metallogenic Belt and implications for its deep geodynamic setting

The Middle-Lower Yangtze River Metallogenic Belt(MLYMB) is an important mineral resource region in China.High-resolution crustal models can provide crucial constraints to understand the ore-forming processes and geodynamic setting in this region. Using ambient seismic noise from 107 permanent and 82 portable stations in the MLYMB and the adjacent area,we present a new high-resolution 3D S-wave velocity model of this region. We first extract 5–50 s Rayleigh wave phase velocity dispersion data by calculating ambient noise cross-correlation functions(CFs) and then use the surface wave direct inversion method to invert the mixed path travel times for the 3D S-wave velocity structure. Checkerboard tests show that the horizontal resolution of the 3D S-wave velocity model is approximately 0.5°–1.0° and that the vertical resolution decreases with increasing noise and depth. Our high-resolution 3D S-wave velocity model reveals:(1) AV-shaped high-velocity zone(HVZ) is located in the lower crust and the uppermost mantle in the study region. The western branch of the HVZ passes through the Jianghan Basin,the Qinling-Dabie orogenic belt and the Nanxiang Basin. The eastern branch, which almost completely covers the main body of the MLYMB, is located near the Tanlu Fault. The low-velocity anomalies between the western and eastern branches are located in the area of the Qinling-Dabie orogenic belt.(2) High-velocity uplifts(HVUs) are common in the crust of the MLYMB,especially in the areas near the Tanlu Fault, the Changjiang Fault and the Yangxin-Changzhou Fault. The intensities of the HVUs gradually weaken from west to east. The V-shaped HVZ in the lower crust and uppermost mantle and the HVUs in the middle and lower crust likely represent cooled mantle intrusive rocks. During the Yanshanian period, fault systems formed in the MLYMB due to the convergence between the South China Plate and the North China Plate, the multiple-direction drifting of the PaleoPacific Plate and its subduction beneath the Eurasian Plate. The dehydration of the cold oceanic crust led to partial melting in the upper mantle. Temperature differences caused strong convection of the upper mantle material that underplated the lower crust and rose to near the surface along the deep fault systems. After mixing with the crustal materials, mineralization processes, such as assimilation and fractional crystallization, occurred in the MLYMB.

Progress in numerical modeling of subducting plate dynamics

The core concerns of plate tectonics theory are the dynamics of subducting plates, which can be studied by integrating multidisciplinary fields such as seismology, mineral physics, rock geochemistry, geological formation studies, sedimentology,and numerical simulations. By establishing a theoretical model and solving it with numerical methods, one can replicate the dynamic effects of a subducting plate, quantifying its evolution and the surface response. Simulations can also explain the observations and experimental results of other disciplines. Therefore, numerical models are among the most important tools for studying the dynamics of subducting plates. This paper provides a review on recent advances in the numerical modeling of subducting plate dynamics. It covers various aspects, namely, the origin of plate tectonics, the initiation process and thermal structure of subducting slab, and the main subduction slab dynamics in the upper mantle, mantle transition zone, and lower mantle. The results of numerical models are based on the theoretical equations of mass, momentum, and energy conservation. To better understand the dynamic progress of subducting plates, the simulation results must be verified in comparisons with the results from natural observations by geology, geophysics and geochemistry. With the substantial increase in computing power and continuous improvement of simulation methods, numerical models will become a more accurate and efficient means of studying the frontier issues of Earth sciences, including subducting plate dynamics.

Effects of chemical,mineralogical,and temperature variations on the density and bulk sound velocity of the bottom lower mantle

In this study, we have modeled the density(ρ) and bulk sound velocity(VΦ) profiles of the bottom lower mantle using the experimental thermal equation of state(EoS) parameters of lower-mantle minerals, including bridgmanite, ferropericlase,CaSiO3-perovskite, and post-perovskite. We re-evaluated the literature pressure-volume-temperature relationships of these minerals using a self-consistent pressure scale in order to avoid the long-standing pressure scale problem and to provide more reliable constraints on the thermal EoS parameters. With the obtained thermal EoS parameters, we have constructed the ρ and VΦ profiles of the bottom lower mantle in different composition, mineralogy, and temperature models. Our modelling results show that the variations of chemistry, mineralogy, and temperature have different seismic signatures from each other. The Fe and Al enrichment at the bottom lower mantle can cause an increase in ρ but greatly lower VΦ. A change in mineralogy needs to be considered with the lateral variation in temperature. The cold slabs will be shown as denser regions compared to the normal mantle because of the combined effect of a lower temperature and the presence of a denser post-perovskite at a shallower depth,whereas the hot regions will have a 1–2% lower ρ than the normal mantle. VΦ of both cold slabs and hot regions will be lower than the normal mantle when bridgmanite is the dominant phase in the normal mantle, yet they will be greater once bridgmanite transforms into post-perovskite in the normal mantle. Our modeling also shows that the presence of a(Fe, Al)-enriched bridgmanite thermal pile above the core-mantle boundary will exhibit a seismic signature of enhanced ρ and VΦ, but a reduced VS,which is consistent with the observed seismic anomalies in the large-low-shear-velocity-provinces(LLSVPs). The existence of such a(Fe, Al)-enriched bridgmanite thermal pile thus can help to understand the origin of the LLSVPs. These results provide new insights for the chemical and structure of the deepest lower mantle.

High-SNR Staining Algorithm for One-Way Wave Equation-Based Modelling and Imaging

Staining algorithms based on two-way wave equation migration methods have been applied to improve the signal-to-noise ratio(SNR)of poorly illuminated structures such as those in subsalt zones.In regular staining algorithms,when a source wavefield reaches the stained area that is associated with the target structures,a new wavefield called stained wavefield is excited,and this stained wavefield forward extrapolates synchronously with the real source wavefield.The forward-extrapolated stained and real source wavefields are cross-correlated with the backward-extrapolated receiver wavefield,and we obtain the stained and the real reverse time migration(RTM)images.The staining algorithms for RTM can suppress the noise of non-target regions and obtain high SNR images of the target structures.Whereas RTM methods are limited by the low computational efficiency and SNR,by contrast,one-way wave equation migration(OWEM)methods have the advantages of high efficiency and no interference from multiples.Thus,we developed a new staining method based on the generalised screen propagator(GSP)as a case of OWEM methods for subsalt imaging.Furthermore,a new stained wavefield called stained receiver wavefield is proposed here,forming two new staining strategies for seismic imaging,in which forward-propagated source and backward-propagated receiver wavefields can be conveniently selected to be stained at the stained area.Numerical experiments demonstrated that this staining GSP method is more effective in improving the SNR of subsalt structures compared to conventional GSP migration and RTM methods;moreover,these new staining strategies as applied to the OWEM methods can greatly improve the SNR of weakly illuminated structures in subsalt zones,in comparison with regular staining algorithms for one-way methods.

Elastic properties of Fe-bearing Akimotoite at mantle conditions: Implications for composition and temperature in lower mantle transition zone

The pyrolite model,which can reproduce the upper-mantle seismic velocity and density profiles,was suggested to have significantly lower velocities and density than seismic models in the lower mantle transition zone(MTZ).This argument has been taken as mineral-physics evidence for a compositionally distinct lower MTz.However,previous studies only estimated the pyrolite velocities and density along a one-dimension(1D)geotherm and never considered the effect of lateral temperature heterogeneity.Because the majorite-perovskite-akimotoite triple point is close to the normal mantle geotherm in the lower MTz,the lateral low-temperature anomaly can result in the presence of a significant fraction of akimotoite in pyrolitic lower MTZ.In this study,we reported the elastic properties of Fe-bearing akimotoite based on first-principles calculations.Combining with literature data,we found that the seismic velocities and density of the pyrolite model can match well those in the lower MTZ when the lateral temperature heterogeneity is modeled by a Gaussian distribution with a standard deviation of~10o K and an average temperature of dozens of K higher than the triple point of MgsiOg.We suggest that a harzburgite-rich lower MTZ is not required and the whole mantle convection is expected to be more favorable globally.

Geodynamic modeling of thermal structure of subduction zones

During subduction processes, slabs continuously have heat exchange with the ambient mantle, including both conduction and advection effects. The evolution of slab thermal structure is one of the dominant factors in controlling physical and chemical property changes in subduction zones. It also affects our understanding of many key geological processes, such as mineral dehydration, rock partial melting, arc volcanism, and seismic activities in subduction zones. There are mainly two ways for studying thermal structure of subduction zones with geodynamic models: analytical model and numerical model. Analytical model provides insights into the most dominant controlling physical parameters on the thermal structure, such as slab age, velocity and dip angle, shear stress and thermal conductivity, etc. Numerical model can further deal with more complicated environments, such as viscosity change in the mantle wedge, coupling process between slabs and the ambient mantle, and incorporation of petrology and mineralogy. When applying geodynamic modeling results to specific subduction zones on the Earth, there are many factors which may complicate the process, therefore it is difficult to precisely constrain the thermal structure of subduction zones. With the development of new quantitative methods in geophysics and geochemistry, we may obtain more observational constraints for thermal structure of subduction zones, thus providing more reasonable explanations for geological processes related to subduction zones.

Frequency-dependent rupture process,stress change,and seismogenic mechanism of the 25 April 2015 Nepal Gorkha M_w 7.8 earthquake

On 25 April 2015,an M_w 7.8 earthquake occurred on the Main Himalaya Thrust fault with a dip angle of^7° about77 km northwest of Kathmandu,Nepal.This Nepal Gorkha event is the largest one on the Himalayan thrust belt since 1950.Here we use the compressive sensing method in the frequency domain to track the seismic radiation and rupture process of this event using teleseismic P waves recorded by array stations in North America.We also compute the distribution of static shear stress changes on the fault plane from a coseismic slip model.Our results indicate a dominant east-southeastward unilateral rupture process from the epicenter with an average rupture speed of ~3 km s^(-1).Coseismic radiation of this earthquake shows clear frequency-dependent features.The lower frequency(0.05-0.3 Hz) radiation mainly originates from large coseismic slip regions with negative coseismic shear stress changes.In comparison,higher frequency(0.3-0.6 Hz) radiation appears to be from the down-dip part around the margin of large slip areas,which has been loaded and presents positive coseismic shear stress changes.We propose an asperity model to interpret this Nepal earthquake sequence and compare the frequency-dependent coseismic radiation with that in subduction zones.Such frequency-dependent radiation indicates the depth-varying frictional properties on the plate interface of the Nepal section in the main Himalaya thrust system,similar to previous findings in oceanic subduction zones.Our findings provide further evidence of the spatial correlation between changes of static stress status on the fault plane and the observed frequency-dependent coseismic radiation during large earthquakes.Our results show that the frequency-dependent coseismic radiation is not only found for megathrust earthquakes in the oceanic subduction environment,but also holds true for thrust events in the continental collision zone.

含水地幔柱与太古宙大陆起源

The origin of continents,which is fundamentally important in understanding the evolution of Earth and its habitability,remains a highly debatable issue.The dominant rock types of Archean cratons are tonalite-trondhjemite-granodiorite(TTG)plutons.Extensive evidence suggests that Archean TTG rocks were formed from a hydrous mafic source[1-3].Subduction has been considered to be the only effective means of transporting a great amount of water to the mantle,leading to partial melting to generate a hydrous mafic crust,which can be regarded as the most possible source of the Archean TTG rocks.Hence,Arndt[2]argued that subduction zones are probably the sole setting in Earth to provide the three main ingredients of the Archean TTG。