Temporal changes of the inner core over several years have been well observed by different studies, especially those using high-quality repeating earthquakes (i.e., doublets). The phenomenon has commonly been interpreted as the differential rotation of the inner core shifting its interior heterogeneities. However, estimates of the inner-core rotation rate vary by an order of magnitude and an alternative interpretation, the rapid growing or shrinking at the inner core boundary (ICB), is favored by some studies. In this study, we used high-quality doublets from our previous systematic global search and analyzed the temporal changes (in terms of arrival times and waveforms) of inner core waves (both the refractive PKIKP and the reflective PKiKP). Using SKP (or PP) phase as a reference to eliminate possible clock errors, we found that the temporal changes are mostly from the PKIKP arrivals and always start before the onset of the late-arriving PKiKP. The observation is consistent with the proposal of differential rotation and rules out the ICB as the sole source of the temporal changes. On the other hand, we discovered compelling evidence of the differential rotation. Stations AAK and KZA in Kyrgyzstan are virtually the same distance to the doublets along the South Sandwich Islands (SSI) and hence are referred to as twin stations by us. The fortuitous geometry captures the underlying local structures, which have complex lateral velocity gradients. The yearly temporal change from different doublets also varies a lot, but surprisingly, it strongly correlates with the underlying velocity gradient, providing unequivocal evidence for the rotation of the inner core. The rotation rate could be accurately determined as 0.127° ± 0.006° per year at 95% confidence level in 1991-2010.

The Earth’s inner core is primarily composed of solid iron exposed to pressures of ~330-360 GPa and to temperatures corresponding to that of the surface of the sun. Its rheological properties determine deformation and the rotational dynamics of the inner core. The rheology of the inner core however, remains poorly understood. In a recent published paper (¹Ritterbex & Tsuchiya 2020), we propose a theoretical mineral physics approach based on the density functional theory to constrain the viscosity of hexagonal close packed (hpc) iron, the most likely phase of iron stable in the inner core. Since plastic deformation is rate-limited by atomic diffusion at the extreme conditions of Earth’s center, we quantify self-diffusion in hcp iron non-empirically. Results are used to model the rate-limiting creep behavior of hcp iron, suggesting dislocation creep to be a potential mechanism driving inner core deformation which might contribute to the observed seismic anisotropy of the inner core. The associated viscosity agrees well with geodetic estimates supporting that the inner core is significantly less viscous than Earth’s mantle. We demonstrate that the predicted low viscosity of hcp iron is consistent with a strong gravitational coupling between the inner core and mantle compatible with seismic observations of small fluctuations in the inner core rotation rate. We will discuss why the inner core is too weak to undergo translational motion, one of the hypotheses to explain the hemispherical patterns of seismic anisotropy in the inner core. Instead, our results provide evidence that mechanical stresses of tens of Pa are sufficient to deform hcp iron by dislocation creep at extremely low geological strain rates, comparable to the candidate forces driving inner core convection. ¹S. Ritterbex and T. Tsuchiya (2020). Viscosity of hcp iron at Earth's inner core conditions from density functional theory. Scientific Reports 10, 6311. [doi:10.1038/s41598-020-63166-6]

Although several experiments have been conducted to measure high-pressure metal/silicate partitioning of sulfur, results scatter considerably at high pressure and it is still largely unclear. While earlier experiments reported a highly siderophile character of sulfur at high-P,T, more recent experiments showed a less siderophile one, suggesting sulfur could be excluded from the major light element candidates of the outer core. Since this property is a key to understanding the effects of the core-mantle interaction on the distribution of sulfur in the Earth's deep interior, we perform ab initio determination of partitioning behavior of sulfur between liquid metal and silicate melt in the deep mantle pressure condition.

Determination of lattice thermal conductivities (κ_lat) of lower mantle (LM) minerals is a key to understanding the dynamics and evolution of the Earth’s deep interior. Recent experimental studies have shown that κ_lat of MgO, MgSiO₃ bridgmanite (Brg) and MgSiO₃ post-perovskite (PPv) are reduced by Fe incorporation (e.g., Manthilake et al., 2012; Goncharov et al., 2015; Ohta et al., 2017; Hsieh et al., 2017, Okuda et al., 2017; Okuda et al., 2020). However, there are no experiments measured directly under the deep LM conditions, which makes the thermal transport property of the lowermost mantle unclear. Meanwhile, we established an ab initio technique to compute κ_lat of based on the density-functional theory (DFT) combined with fully solving the phonon Boltzmann transport equation, which was successfully applied to the Fe-free systems, MgO (Dekura and Tsuchiya, 2017), Brg and PPv (Dekura and Tsuchiya, 2019). Recently, the technique has been extended further to the Fe-bearing LM minerals, (Mg,Fe)O ferropericlase (FP), (Mg,Fe)SiO₃ Brg and PPv, combined with the internally consistent DFT+U technique for more precisely describing the Fe-O bond (e.g., Wang et al., 2015). Our calculations demonstrate negative solid solution effects of the low-spin Fe on κ_lat of FP and high-spin Fe on κ_lat of Brg and PPv. Our detailed analyses indicate that such strong effects occur primarily due to the substantial changes in harmonic properties and are found to be Brg > PPv >FP. The present results improve the conventional estimation of the effective LM conductivity (e.g., Stacey 1992). It is estimated to be ~2-3 Wm^-1K^-1 for the pyrolytic aggregate (FP + Brg) and ~4-5 Wm^-1K^-1 (FP+PPv) at 136 GPa and 4000 K, which are ~60-80% smaller than the conventional value of ~10 Wm^-1K^-1.

Heat flux pattern at the Earth's core mantle boundary (CMB) imposes a heterogeneous boundary condition on core dynamics that likely affects the geodynamo. Because of the temperature dependence of seismic velocities, this pattern is usually assumed to be proportional to the lowermost layer of global models of mantle seismic tomography. Two biases, however, undermine such a simple linear relationship. First, other contributions than thermal (mainly, compositional and phase changes) may influence seismic velocities. Second, the radial average inherent to tomographic models might distort the local thermal state at the CMB. Here, we analyze mantle simulations of thermochemical convection where, due to their spatial characteristics, specific mantle components are easily identified. These include hot thermochemical piles of chemically differentiated material, the “normal” mantle and, in some cases, lenses of post-peroskite (pPv) associated with coldest regions. We first calculate synthetic seismic velocities based on the thermal, compositional, and phase fields predicted by simulations and on seismic sensitivities built from mineral physics data. We then derive a formalism to infer CMB heat flux from seismic shear velocity anomalies. Finally, using of a specific tomographic model, we illustrate how our formalism applies to actual seismic shear velocity anomalies. Maps of CMB heat flux derived with this approach vary, depending on whether pPv is included or not in the interpretation. A feature present in all models, however, is an attenuation of low heat flux anomalies beneath large low shear velocity provinces (LLSVPs), while large heat flux patches present elsewhere are enhanced, both with respect to the classical linear interpretation.

Teleseismic PP waves and their precursors have been routinely used to study the upper mantle discontinuities as well as lower mantle scatterers. In most studies, the slowness of precursor is close to that of PP. Here, we present observations of a new PP precursor with the epicenter distance larger than 100º, called PdpP phase (“d” indicate the depth of heterogeneity), which represents the P wave “top” reflected by the heterogeneity and a following second reflection under surface. The PdpP arrives 20-50s before PP but with a slowness between PP and Pdiff, which is not present for reference Earth models. We collect the vertical-component records of China National Seismic Network of large earthquakes (Mw>5.8), that occurred between 2010 and 2020 within the epicentral distance range from 80° to 130°, and identify PdpP  from earthquakes that occurred in the region of Central America, Tonga-Fiji, and Africa. We also find that the PdpP phases are often observed as a wave train with gradually increasing slowness, which may reflect a series of discontinuities (410 km and 660 km) or scatterers from the mantle transition zone to mid-lower mantle. We locate the scatterers by calculating the semblance coefficient. To further determine physical properties of the heterogeneities generating the PdpP, we perform 1D and 2D waveform simulations. Our results indicate that the PdpP phases may originate from small-scale low-velocity structures at mid-mantle depth, which may represent a chemically distinct region related to the accumulation of subducted lithosphere.

Deuterium is the heavy stable isotope of hydrogen. The D/H ratio shows large variation in various astronomical bodies such as protosolar nebula (2x10^-5), Earth (SMOW= 1.5 x 10^-4, average seawater), Venus (1.6x10^-2) and carbonaceous chondrites (~2x10^-4). Many studies are conducted to determine the D/H ratio in various rocks with different origins of the Earth, since this may be the key to understand the evolutional history and the origin of water of the Earth. Many studies suggest that hydrous minerals in subducting cold slabs can transport water into deep Earth’s interiors. There is a possibility that several times of sea water exist in the transition zone at depth between 410km and 660 km if the constituent minerals are largely hydrated (e.g. Smyth 1994).  Therefore, the D/H ratio may be changed by the partitioning behaviors of D and H among these mantle minerals by the circulation of water in deep interiors. In this study, we determined the free energy of D and H bearing forsterite, wadsleyite and ringwoodite by ab initio calculation in order to determine the equilibrium constants of D and H isotopic exchange reactions between them. First, we determined the stable structures of hydrous forsterite, wadsleyite and ringwoodite with Mg vacancy with two hydrogen atoms or Si vacancy with four hydrogen atoms by first principles calculation based on density functional theory. Then, the phonon frequencies are calculated based on density functional perturbation theory (Baroni et al. 2001) and also by the finite displacement method (Parlinski et al. 1997, Togo and Tanaka 2015). Then, we used quasi-harmonic approximation to calculate the Gibbs free energy of H and D baring phases. In this presentation, we report the Gibbs free energy of isotopic exchange reaction between forsterite and wadsleyite, and also between wadsleyite and ringwoodite.

The mantle transition zone (MTZ) is potentially a geochemical water reservoir because of the high H2O solubility in its dominant minerals, wadsleyite and ringwoodite. Whether the MTZ is wet or dry fundamentally impacts our understanding of the deep-water distribution, geochemical recycling, and the pattern of mantle convection. The high-quality elastic data of at pressure and temperature (PT) conditions of MTZ, which are crucial, were calculated using the method of Wu and Wentzcovitch (2011), which reduces the computational workload to tenth of the conventional method. These calculated elastic data agree well with the available experimental data. The iron and water effect on the elasticity are also well described.  With these high-quality elastic data  we analyze the water content at the top and the bottom of MTZ. We found that the water content of wadsleyite at the top of MTZ can be well constrained when density and velocities jumps are considered together. For a pyrolitic mantle composition with ~60% olivine, our best fit is ~ 0.5 wt% water at the top of MTZ (Wang et al., 2019). The seismic tomography and the topography of the 660-km discontinuity of the bottom of MTZ can be explained by varying water content and temperature, suggesting an average water concentration of ~0.2 wt% in the bottom of MTZ (Wang et al., 2021). Thus, we conclude that the water concentration in the MTZ likely decreases with depth globally and the whole MTZ contains the equivalent of about one ocean mass of water.  Wang, W-Z., Walter, M.J., Peng, Y., Redfern, S., Wu, Z-Q., 2019. Earth Planet. Sci. Lett. 519, 1–11. Wang, W-Z., Zhang, H., Brodholt, J., Wu, Z-Q., 2021. Earth Planet. Sci. Lett. 554,116626.Wu, Z-Q., Wentzcovitch, R.M., 2011. Phys. Rev. B 83, 1–8.

Correcting the effects of the sphericity of the Earth in the results of the interpretation of gravimetric data is a topical issue in modern gravimetry. We consider regional density models of the Earth’s crust and upper mantle. A method is proposed for transforming the plane density models into spherical ones and vice versa. Algorithms for calculating the vertical component of gravity field for both model types are presented. We suggest calculating the gravity integral by approximating discretization elements by polyhedrons. Using this idea we developed an efficient algorithm for calculating the gravity field of an arbitrary shape object (e.g. a tessiroid).Comparison of the speed and errors of computing the field of the regional density model with the method of approximating polyhedrons and with the five-pointGauss–Legendre (GL) method shows that the presented method is far more computationally faster (up to a 100 times). Based on this result we created a fast algorithm for gravity data inversion. The described methods are realized as software that implements transformation of the plane density models into the spherical ones and computes the vertical component of the gravity field. The software is based on the NVidia CUDA library which allows for effective program parallelization. The computation can be conducted on several GPUs simultaneously.For the two regional plane models of the Earth’s crust, their transformation into spherical models is carried out and the resulting gravity fields are compared. The RMS of the residual between the fields is at most 5%. Thus, firstly it is reasonable to obtain a plane regional model of the Earth crust. Then, we can convert this model into the spherical model and continue fitting the field by the full-scale spherical methods.

In this study, we have inverted dense focal mechanism solutions and high-resolution stress field in the Longmenshan fault. The dense focal areas are divided into 9 segments and named S1~S9 from south to north by applying the “sliding window” scanning method. Our results show: (1) Proportions of thrust, strike-slip and normal faulting are the highest in S1, S8 and S9, respectively. While the proportion of strike-slip faulting in the late Wenchuan aftershocks has generally increased, which is closely related to the stress supplements and adjustments along the strike of Longmenshan fault. In addition, post-seismic adjustment processes near the Wenchuan mainshock and in the far end of aftershock zone are longer than that in the middle section of aftershock zone, and the far end of aftershock zone has the most complicated and intensive post-seismic adjustment processes. (2) The fault plane structures reveal that the buried faults near the Wenchuan mainshock and in the far end of aftershock zone as well as the south end of Huya fault participated in the post-seismic activities. The dip angles show good relationships with strike-slip components, which the dip angles range of 50°~70° in the areas with obvious thrust components and are generally larger than 60° in the areas with obvious strike-slip components. In addition, the increase of dip angles is coincided with narrowing of the width of Wenchuan aftershock zone. (3) The segmentation differences of stress directions lead to the crustal tearing and mantle material upwelling in the seismic gap between Wenchuan and Lushan earthquakes, the seismicities in the buried faults near the Wenchuan mainshock and in the far end of aftershock zone. Combined stress filed and previous geodetic surveys, we deduce the uplift of Longmenshan may be mainly caused by shortening and thickening of the upper crust.

Both conventional and increasingly popular machine leaning approaches have made big progresses in first arrival picking of earthquakes. Among them, deep learning technologies show more robustness and capacity of discriminating earthquake signals from noises. But all these phase picking methods work on single station, and inevitably produce redundant phase arrivals when increasing the sensitivity of phase pickers. Multi-station or network based earthquake detection approaches is the cornerstone of eliminating phase arrival redundancy, which consists of false picks and mixing phase types. It is also the key to locate seismic sources. Phase association bridges the single station phase picking and multi station earthquake detection. We propose a new phase linking and source locating method: Phase association and source searching (PASS) method, which combines the concepts of both the source scanning algorithm and the back projection method. PASS applies both norm grid search and the beam-search strategy to find the global optimal solution of event locations and origin times. It associates phases of a common event and locates the source precisely, simultaneously and automatically. The source searching process is centered at the anchor station with the first P arrival in a phase arrival group nor the whole study region. For each potential location, a confidence score of the matched phase number as the first weighting factor and the time residual as the second one is used to evaluate the detection. Synthetic tests show that PASS can locate earthquakes with common precision compared to that by linear method. Then the method is applied to one month data in the southern Longmenshan fault belt. All manually picked and located events are detected.

Seismic-wave gradiometry is a brand-new array data processing technology introduced by Charles A. Langston (2007). This method using subarray including a main station or reference station, and auxiliary stations within a certain range. Based on the waveform difference between the main station and the auxiliary station, the spatial gradient of the waveform can be calculated, and then the phase velocity, azimuth angle of the ray, geometric spreading and radiation pattern of the main station can be calculated. The surface wave data recorded by the seismic arrays located around the Anninghe-Zemuhe-Xiaojiang fault zone are used to study the structure and geometry of the fault zone. The Anninghe-Zemuhe-Xiaojiang fault zone is located in the southwestern part of Sichuan. The northern section is connected with the Xianshuihe fault, and the overall strikes is nearly north-south. The fault zone is a major active fault on the eastern boundary of the Sichuan-Yunnan active block. In the past ten years, 12 strong earthquakes of magnitude 5 or above have occurred on this fault zone. The fault zone has strong seismicity and its future seismicity is of big concern. This study uses the Seismic-wave gradiometry to extract phase velocity, wave directionality, geometrical spreading, and radiation pattern by vertical component surface-wave data. The seismic network deployed along the Anninghe–Zemuhe-Xiaojiang fault zone in Yunnan and Sichuan with an average spacing of 5km is used to extract spatial gradients. The Wave gradiometry parameters are used together with velocity change, GPS measurement to analyze the properties of fault zone and to shade light on seismicity in the future. 

Longquan fault, a > 200 km long NNE-SSW striking anticline in the center of the Sichuan basin, is currently active and poses a serious threat to adjacent highly populated Chengdu metropolis. It has six earthquakes instrumentally recorded in the past century. The two largest are 1967 Ms5.5 Renshou earthquake and 2020 Ms 5.1 Qingbaijiang earthquake. The former event with hypocenter of 4 km depth with lost lives and houses damage. The hypocenter depth of the recent 2020 event is actually controversial according to different studies. The elastic depth of the Longquan fault is itself controversial as well. One view supports that Longquan fault merges into a shallow decollement and the decollement extends all the way from Longmenshan to the west and outcrops at the Longquan Shan. Oppositely, the other view demonstrates that Longquan fault is a deep-rooted fault zone that reaches the Moho, with two branches at the top. Conceivably, whether the Longquan fault extends only to a shallow depth or cut the whole crust is an important question, relating to evaluate the potential damage level and building seismic design. In order to closely probe into the deep structure of the Longquan fault, we conduct a dense seismic array to cover this fault along its strike from south to north. High qualified receiver functions and surface wave dispersion curves extracted from the ts-PWS stacked empirical Green’s function, are jointly inverted. Based on the detailed crustal structure image, we can hopefully identify the Longquan fault deep extent, and further estimate the seismic potential associated with this fault.

The Tibetan Plateau uplifted rapidly with crust shortening and eastward extrusion since the Cenozoic. Blocked by the stable Yangtze block, the crust has been strongly deformed in the southeastern margin of the Tibetan Plateau. Seismic anisotropy is helpful to understand the fine crust structure and the inner dynamic process. We collected seismic data from high-density seismic array, extracted the Rayleigh wave dispersion based on the ambient noise, and adopted an azimuth-dependent dispersion curve inversion method to obtain high-resolution S-wave velocity and anisotropy image in the crust and uppermost mantle in this region. The seismic fast wave directions in the upper crust are basically consistent with the strikes of the adjacent faults and the GPS horizontal velocities, rotating clockwise wrap around the Eastern Himalayan Syntaxis (EHS). However, the anisotropy of the mid-lower crust is obviously different from the upper crust. For example, in both the Muli-Yanyuan basin and Central Yunnan block, the seismic fast wave directions change from NE direction in the upper crust to the NW direction in the mid-lower crust. The anisotropy directions in the middle and lower crust are consistent with the distribution trends of the low velocity zone. In the range of the bottom of lower crust and the top of the upper mantle, the direction of the seismic fast wave change again, which is consistent with the anisotropic distribution of the upper crust. This may indicate that the upper crust and lower crust were coupled in the early historical period. The low velocity viscous fluid squeezed into the mid-lower crust in the Miocene, leading to decoupling between the original upper and lower crust in the southeastern margin of the Tibetan Plateau. Material extrusion may lead to the crustal deformation in the southeastern margin of the Tibetan Plateau since the Cenozoic.

The uplifting mechanism of eastern Tibetan plateau is still a hot and highly controversial topic，a better understanding of lithosphere structure may help us to better understand its uplifting mechanism. Ambient noise tomography is often useful in estimating dispersion curves with periods less than 40s, therefore it is a powerful tool to estimate the crustal structure. On the other hand, the wave gradiometry analysis can extract dispersion curves with periods up to the maximum capacity of the instruments, and therefore are useful to estimate the structure down to the mantle. In this study, the Rayleigh wave phase velocity are calculated using ambient noise at short periods and wave gradiometry at long periods. In the eastern margin of Tibetan plateau, the Rayleigh wave phase velocity extracted by wave gradiometry are generally increased from west to east and are largely separated by the block boundaries, the distributions of anisotropy are seemly divided by block boundaries or relative to the distribution of fault zones or the orientation of orogenic belts. Liang et al. (2020) shows the Rayleigh wave phase velocity and anisotropy extracted by ambient noise cross correlation in short periods. Those two groups of dispersions can be combined to invert shear velocity and anisotropy of lithosphere using the ADDCI (Azimuth-Dependent Dispersion Curve Inversion) method. The 3D shear wave velocity and anisotropies of the lithosphere within eastern margin Tibetan plateau will be obtained using the combined wide-band dispersion curves.

Structure of transition zone seismic discontinuities at the depth of 410- and 660-km provides important constrains on mantle dynamics and helps address fundamental questions as the origin of mantle plumes and fate of subducted slabs. Teleseismic P-to-S receiver function technique, which has been widely used to map the depths of the two discontinuities, suffers from trade-offs between the depth of the interfaces and overlying velocity anomalies. In this study, we propose and test a procedure that utilizes the multiply reflected waves PpPdp, PpPds, and SsSds, to simultaneously determine the depths of transition zone discontinuities, the velocity anomalies above the discontinuities, and both P- and S-wave velocity jumps across the discontinuities. The procedure includes deconvolving the direct P waveform and SH waveform from the P-SV components and transverse components, respectively, to produce the vertical, radial and transverse receiver functions. The discontinuity depths and velocity anomalies in the overlying layer are solved simultaneously through the differential travel times of multiply reflected P- and S-waves relative to direct P- or S-wave. The jump in P- and S-velocity across the discontinuities are inverted from the amplitude of the reflected waves. We collect seismic records for M≥6.0 earthquakes in the distance range of 30-95 degrees from permanent and temporary arrays deployed near the Tibetan Plateau and calculate radial receiver functions. Stacking of receiver functions shows robust detection of the multiply reflected P- and S-waves. After showing the validity of the method, we present the preliminary results for the transition zone discontinuities beneath the Tibetan Plateau. 

The seismic discontinuities at the depths of 410- and 660-km caused by mineral phase transitions have many attributes that provide important constraints on the mantle composition and heat flow state. Studying the thickness and jump in velocity and density near the 660-km discontinuity can help us understand the tectonic activities and convection processes under the Tibetan Plateau. The undulations of the topography reveal the thermal state, and the velocity and density changes reflect the water content and material composition near the base of transition zone. These are of great help to the study of slab subduction and the formation and evolution of the Qinghai-Tibet Plateau. While earlier studies mainly focused on changes of discontinuity depth, we investigate the velocity and density jump across the 660-km discontinuity beneath the Tibetan Plateau using the amplitude of P660s converted. We collect seismic waveform data for M≥6.0 earthquakes in the distance range of 30-95 degrees from permanent and temporary arrays deployed near the Tibetan Plateau, and calculate radial receiver functions by deconvolving the radial waveform from the vertical P-component. Stacking of receiver functions shows robust detection of the P660s converted wave. The multi-frequency amplitude of P660s is inverted for the thickness of and the shear velocity jump across the 660-km discontinuity through a Bayesian approach. Preliminary shear velocity structure near the 660-km discontinuity indicate that ringwoodite to perovskite+ferropericlase is the dominant cause of the 660 beneath Tibetan Plateau.

Using ambient noise cross-correlation to detect the velocity change of subsurface structure is a useful method to detect physical changes in materials associated with seismogenic process. This study focuses on the velocity change of the seismic gap between the 2008 Wenchuan and 2013 Lushan earthquakes. Chen et al. (2013) divided the unruptured area into the seismic gap I in the north between Wenchuan and Lushan earthquakes and the seismic gap II in the south from Tianquan-Rongjing-Luding-Kangding. This study focus on the seismic gap I. Based on the background noise data of 40 stations near the seismic gap from December 2017 to June 2018, the code wave data are obtained by cross-correlating ambient noise, and the velocity change in this region are obtained by calculating the time delay by stretching method. Combining the velocity change with the stress and strain state around it, the velocity change in this area are then analyzed together with factors such as precipitation, temperature, microseismicity and other possible factors.

The Dabie-Sulu orogenic belt, a collision zone between the South China and North China plates is the largest ultrahigh pressure metamorphic belt in the world. Since Cenozoic, the tectonic activity has been increased and the basins has developed (Li et al., 2017; Zheng, 2008). Seismic Lg waves propagate in the continental crust and their amplitude attenuation is closely related to temperature, partial melting, basin sediments, tectonic activities and crustal thickness changes (Zhao et al., 2010, 2013). To investigate the tectonic evolution such as that Dabie-Sulu belt was staggered by the Tan-Lu fault for hundreds kilometers, we conducted the Lg wave Q tomography to explore the lateral variation of the crustal attenuation. Our dataset is compose of 7,202 vertical-component broadband digital seismograms recorded at 458 stations from 19 regional seismic events. A broadband model of Lg wave Q model was obtained between 0.05 and 10.0 Hz for the Dabie-Sulu orogenic belt. The model resolution reaches 1º×1º for most of the areas. Our results show that the Bohai Basin, the South Yellow Sea Basin and the Ordos Basin are characterized by strong attenuation, whereas the Dabie-Sulu orogenic belt exhibits relatively weak attenuation. This research was supported by the National Key Research and Development Program of China (2017YFC0601206) and the National Natural Science Foundation of China (grants 41674060, 41630210, 41974054, and 41974061).

The deep structure of active reverse faults is generally difficult to constrain from surface observations and may conceal shortening within the hinterland.  As one of the most widely used geomorphological markers, fluvial terraces are usually limited to interpret the foreland basin structural settings and cannot be easily extended into orogenic highlands, due to poor terrace preservation. This makes it difficult to identify the ongoing crustal scale deformation, and limits resolution of deeper fault geometry and kinematics within the hinterland of compressional orogens. Here we investigate the geometry of active, crustal-scale reverse faulting and how it varies along strike, from deformations of unusually extensive river terraces of the Beida, Baiyang, and Shiyou River through the northwestern Qilian Shan, China. These terraces are well preserved from the foreland basin to tens of kilometers into the hinterland, recording both faulting and folding of the North Qilian Fault. Kinematic and elastic modeling of these deformed terraces suggest that the main body of the western North Qilian Fault is composited of a steep reverse fault (~50^o) that soles into a ~10^o décollement at ~15 km depth, while the near-surface fault geometry varies greatly along strike. The terrace profiles of the Baiyang and Shiyou River reveal a young, deep-seated folding that has expanded across both the foreland and the hinterland, suggesting the emergence of a new North Qilian mountain front. Combining geochronology and terrace deformation of the Beida River, we find the shortening rate of the North Qilian Shan is 1.4±0.4 mm/yr, 23±10% of the geodetic shortening rate across Qilian Shan orogen. The structures revealed by Beida, Baiyang, and Shiyou River represent the maturity and infant stage of the North Qilian Shan, respectively.

The crustal attenuation structure can significantly reveal the rheology and thermal property of the geological blocks, and provide seismological constraints on tectonic evolution. Seismic Lg waves usually propagates within continental crust (e.g., Zhang and Lay, 1995). However, some large earthquakes generate Lg phases which can be observed across the oceanic crust (Kennett,1986). Therefore, Lg data recorded at onshore station can be used to estimate the attenuation structure for oceanic crust. A total of 16,678 vertical-component broadband digital seismograms was collected to extract Lg spectra from 444 earthquakes with magnitudes larger than 5.2 and recorded by 523 stations between 1999 and 2020. We conducted the Lg wave Q tomography in the Gulf of Mexico-Caribbean area where is a mixture of continents, islands and marginal seas. A broadband Lg-wave Q model was constructed between 0.05 and 10.0 Hz and with a resolution of approximately 1.0°×1.0° for this region. The resultant Q maps are well consisting of geological settings and previous studies (Laske and Masters, 1997; Blackwell et al., 2011; Meghan and Thorsten, 2012; Zhao and Mousavi, 2018). Prominent low attenuation anomalies are located in the Southern North America, especially in the Yucatan and Bahamas-Florida minor plates. Strong attenuation regions are associated with complex tectonic conditions, such as Cocos Ridge, Antilles subduction zone and the Americas, which are linked by a transform plate boundary between continental South America. Strong attenuation can be observed in the northern Caribbean Plate, whereas the southern part is characterized by relatively weak attenuation. This research was supported by the National Key Research and Development Program of China (2017YFC0601206, 2018YFC1503402) and the National Natural Science Foundation of China (41974061, 41974054, 41630210, and 41674060).

Based on a dense seismic array deployed to monitor the seismic gap between the Wenchuan and Lushan earthquakes, a joint inversion of dispersion curves and receiver functions yielded a fine velocity structure in this region with the highest resolution of approximately 20 km. The results show that the upper crust of the Songpan-Ganzi terrane overthrusted onto the basement of the Sichuan Basin, indicating that the crustal shortening model may be the major mechanism that accounts for the growth of the Longmenshan. The results also show a low-velocity zone (Vs < 3.5 km/s) and thickened crust (>67 km) beneath the seismic gap, which could be associated with partial melting. The heat transferred from the partial melting at the base of the seismic gap may increase the temperature of the faults to as high as 300–400 °C and cause the segment of the Longmenshan Fault in the seismic gap to become “aseismic.” In contrast, the Shuangshi-Dachuan Fault, Dayi Fault, and other blind faults to the east-southeast of the seismic gap are located above a high-velocity body. The seismogenic environment of these faults is similar to the segments ruptured during the Wenchuan and Lushan earthquakes, both of which occurred above the westward extension of the strong Sichuan Basin. Higher seismic activity was also observed along these faults than that above the low-velocity zone. Therefore, we deduce that a major earthquake on these faults is possible and that the magnitude could be larger than that of the Dayi earthquake (M 6.2)