Three-dimensional Velocity Model Research Articles

Local earthquake tomography of the Aegean crust: Implications for active deformation, large earthquakes, and arc volcanism

Three-dimensional velocity models can enable accurate earthquake location, improved seismic hazard assessment, and can enhance our understanding of geodynamic processes. This is particularly true for areas such as the Aegean, where the crust is marked by active volcanoes as well as shear and rift zones, all of which make this region highly heterogeneous. This work describes the application of local earthquake tomography in order to elucidate the crustal structure of the Aegean. We utilized a dataset of 2,135,625 P arrivals and 1,095,515 S arrivals from 178,676 events recorded across 324 stations. Results indicate that the boundary separating the slow Vp from the fast anomalies at 20–40 km depth is well correlated with the Aegean Moho obtained from receiver functions. A slow Vp (−5%) zone at depths >30 km along the Hellenic arc is likely associated with basal underplating that forms the base of the crust. On the other hand, fast Vp and moderate to high Vp/Vs (1.77–1.92) in the upper-middle crust may indicate evolving metamorphic core complexes. Steeply dipping clusters of earthquakes could highlight pathways of fluid migration from the base to the middle/upper crust, albeit more detailed seismological studies are needed to confirm this interpretation. The 3D velocity model can be also utilized in order to investigate the amount of melt fraction beneath active volcanoes and the influence of fluids on the rupture zone of large earthquakes. Our results show a melt fraction between 4 and 10% beneath active volcanoes, with the largest volume of melt present beneath Santorini caldera. The ruptures zones of 10 large earthquakes (Mw ≥ 6.0), show slow Vp anomalies and moderate to high Vp/Vs (> 1.77), suggesting that these events were likely triggered by weakening of their source zones by fluid activity.

Seismological evidence for a multifault network at the subduction interface

Subduction zones generate the largest earthquakes on Earth, yet their detailed structure, and its influence on seismic and aseismic slip, remains poorly understood. Geological studies of fossil subduction zones characterize the seismogenic interface as a 100 m–1 km thick zone1–3 in which deformation occurs mostly on metres-thick faults1,3–6. Conversely, seismological studies, with their larger spatial coverage and temporal resolution but lower spatial resolution, often image the seismogenic interface as a kilometres-wide band of seismicity7. Thus, how and when these metre-scale structures are active at the seismic-cycle timescale, and what influence they have on deformation is not known. Here we detect these metres-thick faults with seismicity and show their influence on afterslip propagation. Using a local three-dimensional velocity model and dense observations of more than 1,500 double-difference relocated earthquakes in Ecuador, we obtain an exceptionally detailed image of seismicity, showing that earthquakes occur sometimes on a single plane and sometimes on several metres-thick simultaneously active subparallel planes within the plate interface zone. This geometrical complexity affects afterslip propagation, demonstrating the influence of fault continuity and structure on slip at the seismogenic interface. Our findings can therefore help to create more realistic models of earthquake rupture, aseismic slip and earthquake hazard in subduction zones.

Shallow three-dimensional shear wave velocity model for the Mexico City Basin

In this study, a simplified shallow three-dimensional shear wave velocity (Vs) model is presented for the Mexico City Basin. To this end, previous studies were carefully reviewed to assemble a database of Vs and site period measurements for the region. The new site period measurements obtained at the western edge of the Basin were compared to the existing 2004 Complementary Technical Standards of Mexico site period nominal map, (NTC, 2004) the Lermo et al. (2020) site period map, and Design Seismic Actions System (SASID), 2020 site period predictions. Each site period prediction method was shown to have differences with respect to the new measurements along the western edge of the Basin. However, there was no bias for the prediction methods with the Lermo et al. (2020) and SASID predictions, demonstrating less error between the measured values than the NTC (2004) predictions. To develop the three-dimensional Vs model, the shallow (top 60 m) subsurface was divided into five generalized soil layers. The site period was used as the only input for the three-dimensional Vs model to simplify and maximize the model’s applicability. The performance of the model was assessed by comparing the measured and predicted Vs profiles. Overall, the three-dimensional Vs model developed in this study is a valuable tool that can be used along with geophysical estimates of deeper structure for ground motion modeling and preliminary site response studies along with other benefits to the seismic resiliency of the region.

Multi-Scale Seismic Measurements for Site Characterization and CO2 Monitoring in an Enhanced Oil Recovery/Carbon Capture, Utilization, and Sequestration Project, Farnsworth Field, Texas

To address the challenges of climate change, significantly more geologic carbon sequestration projects are beginning. The characterization of the subsurface and the migration of the plume of supercritical carbon dioxide are two elements of carbon sequestration that can be addressed through the use of the available seismic methods in the oil and gas industry. In an enhanced oil recovery site in Farnsworth, TX, we employed three separate seismic techniques. The three-dimensional (3D) surface seismic survey required significant planning, design, and processing, but produces both a better understanding of the subsurface structure and a three-dimensional velocity model, which is essential for the second technique, a timelapse vertical seismic profile, and the third technique, cross-well seismic tomography. The timelapse 3D Vertical Seismic Profile (3D VSP) revealed both significant changes in the reservoir between the second and third surveys and geo-bodies that may represent the extent of the underground carbon dioxide. The asymmetry of the primary geo-body may indicate the preferential migration of the carbon dioxide. The third technique, cross-well seismic tomography, suggested a strong correlation between the well logs and the tomographic velocities, but did not observe changes in the injection interval.

Solidified magma reservoir derived from active source seismic experiments in the Aira caldera, southern Kyushu, Japan

The Aira caldera, located in southern Kyushu, Japan, originally formed 100 ka, and its current shape reflects the more recent 30 ka caldera-forming eruptions (hereafter, called the AT eruptions). This study aimed to delineate the detailed two-dimensional (2D) seismic velocity structure of the Aira caldera down to approximately 15 km, by means of the travel-time tomography analysis of the seismic profile across the caldera acquired in 2017 and 2018. A substantial structural difference in thickness in the subsurface low-velocity areas in the Aira caldera between the eastern and western sides, suggest that the Aira caldera comprises at least two calderas, identified as the AT and Wakamiko calderas. The most interesting feature of the caldera structure is the existence of a substantial high-velocity zone (HVZ) with a velocity of more than 6.8 km/s at depths of about 6–11 km beneath the central area of the AT caldera. Because no high ratio of P- to S-wave velocity zones in the depth range were detected from the previous three-dimensional velocity model beneath the AT caldera region, we infer that the HVZ is not an active magma reservoir but comprises a solidified and cool remnant. In addition, a poorly resolved low-velocity zone around 15 km in depth suggests the existence of a deep active magma reservoir. By superimposing the distribution of the known pressure sources derived from the observed ground inflation and the volcanic earthquake distribution onto the 2D velocity model, the magma transportation path in the crust was imaged. This image suggested that the HVZ plays an important role in magma transportation in the upper crust. Moreover, we estimated that the AT magma reservoir in the 30 ka Aira caldera-forming eruptions has the total volume of 490 km3 DRE and is distributed in a depth range of 4–11 km.Graphical

Magma and hydrothermal sources below the northern part of Paramushir Island (Kuril Arc) inferred from ambient noise tomography

Paramushir is the northernmost island of the Kuril Arc comprising several Holocene volcanoes, of which two are presently active: Ebeko and Chikurachki. We present the first crustal-scale three-dimensional seismic shear-wave velocity model of the northern part of Paramushir derived with the use of data of 20 temporary stations operated in 2021–2022 and one permanent station. The continuous seismic data were used to obtain the Rayleigh wave dispersion curves for periods ranging from 0.5 s to 12 s through computing ambient seismic noise cross-correlation functions. Then the 3D shear wave velocity distribution was constructed by seismic tomography. The synthetic tests demonstrate that the model has sufficient resolution down to 7–10 km depth. We observe a series of low-velocity anomalies at the depths of 4–6 km below the volcano-origin Vernadsky Ridge. They may likely represent magma reservoirs that are responsible for Holocene eruptions along the ridge. These anomalies are overlain by the high-velocity anomalies associated with the rigid cover consisting of an upper-crustal granite-granodiorite body and consolidated magma intrusions. Along the eastern flank of Ebeko we reveal a thin low-velocity anomaly at a depth of ∼1 km that can be interpreted as a water-saturated layer. The contact of this layer with hot magma intrusions leads to the formation of a large amount of steam that is ejected during the frequent phreatic eruptions of Ebeko and from numerous fumaroles in the summit area. On the other hand, this layer, which is following down to the Pacific Coast, might be promising for the geothermal energy exploitation.

Joint inversion of Rayleigh group and phase velocities for S-wave velocity structure of the 2021 MS6.0 Luxian earthquake source area, China

On September 16, 2021, a MS6.0 earthquake struck Luxian County, one of the shale gas blocks in the Southeastern Sichuan Basin, China. To understand the seismogenic environment and its mechanism, we inverted a fine three-dimensional S-wave velocity model from ambient noise tomography using data from a newly deployed dense seismic array around the epicenter, by extracting and jointly inverting the Rayleigh phase and group velocities in the period of 1.6–7.2 s. The results showed that the velocity model varied significantly beneath different geological units. The Yujiasi syncline is characterized by low velocity at depths of ∼ 3.0–4.0 km, corresponding to the stable sedimentary layer in the Sichuan Basin. The eastern and western branches of the Huayingshan fault belt generally exhibit high velocities in the NE-SW direction, with a few local low-velocity zones. The Luxian MS6.0 earthquake epicenter is located at the boundary between the high- and low-velocity zones, and the earthquake sequences expand eastward from the epicenter at depths of 3.0–5.0 km. Integrated with the velocity variations around the epicenter, distribution of aftershock sequences, and focal mechanism solution, it is speculated that the seismogenic mechanism of the main shock might be interpreted as the reactivation of pre-existing faults by hydraulic fracturing.

Role of subduction dynamics on the unevenly distributed volcanism at the Middle American subduction system

A typical subduction of an oceanic plate beneath a continent is expected to be accompanied by arc volcanoes along the convergent margin. However, subduction of the Cocos plate at the Middle American subduction system has resulted in an uneven distribution of magmatism/volcanism along strike. Here we construct a new three-dimensional shear-wave velocity model of the entire Middle American subduction system, using full-wave ambient noise tomography. Our model reveals significant variations of the oceanic plates along strike and down dip, in correspondence with either weakened or broken slabs after subduction. The northern and southern segments of the Cocos plate, including the Mexican flat slab subduction, are well imaged as high-velocity features, where a low-velocity mantle wedge exists and demonstrate a strong correlation with the arc volcanoes. Subduction of the central Cocos plate encounters a thick high-velocity feature beneath North America, which hinders the formation of a typical low-velocity mantle wedge and arc volcanoes. We suggest that the presence of slab tearing at both edges of the Mexican flat slab has been modifying the mantle flows, resulting in the unusual arc volcanism.

Study on the reduction of Fe2O3 pellets by CO-CO2 and H2-H2O: Reaction kinetics and pore network model

The steel industry contributes to creating an environmentally friendly society by providing high-quality steel and working to develop low-carbon processes. Using hydrogen produced from renewable energy to replace part of coke in blast furnaces is one of the measures to reduce fossil carbon dioxide emissions in the process of steelmaking. In this paper, a series of reduction experiments of Fe2O3 pellets at different temperatures were carried out under three conditions of hydrogen-rich in blast furnace. The variation law between reduction degree and apparent activation energy was calculated. The result shows that when the reduction degree reaches 11% and 70%, the apparent activation energy reaches the peak, which are 38.9 kJ/mol and 19.5 kJ/mol, respectively. This suggests that the reduction process is affected by two or more mechanisms. Then, on the basis of the typical single interface reaction kinetics model, the multistep reduction kinetics model of hydrogen-rich reduction Fe2O3 pellets was established. In addition, based on the results of industrial computerized tomography (CT) analysis, the three-dimensional pore network model and velocity model of Fe2O3 pellets after reduction were established. The coupling relationship between the fractal dimension of pellet pores and reduction atmosphere was studied by introducing fractal theory. Under hydrogen-rich reduction condition, the volume fractal dimension of pore structure of Fe2O3 pellets increases from 2.41 to 2.58. Therefore, the pore structure formed by hydrogen-rich reduction is conducive to the diffusion of reducing gas in the pellet, so as to improve the reduction degree of pellet.

3D Near-surface Velocity Model Building by Integrating Surface, Borehole Seismic, and Well-logging Data in a Small-scale Testbed

This study aims to build a 3D velocity model for investigating and evaluating underground contaminants flowing through groundwater pathways. To accomplish this goal, we performed seismic exploration at a test site with fractured rock layers within a depth of 100 m. The acquired seismic data was then used to construct a three-dimensional (3D) P-wave velocity model of the test site. Although comprehensive geophysical exploration may be useful for investigating the near-surface structure, this paper focuses only on constructing a P-wave velocity model using the seismic method. The primary information of the model consists of extracted velocities from a two-dimensional surface, borehole first-arrival traveltime tomography results, and full waveform sonic log data. Since the test site has the spatial restriction of the survey line, we intended to improve the geological structure analysis results using various quantitative and qualitative analysis methods. First, to increase the reliability of velocity information, we performed a traveltime analysis on the zero vertical interval (ZVI) gather from the borehole seismic data. Then, we identified the qualitative information of the fracture zone's location by analyzing the amplitude variation of the ZVI gather. Moreover, we extracted structural information using the common reflection point gather from the borehole seismic data to supplement the obtained lithological information. However, the information for constructing a 3D P-wave velocity model was still insufficient due to the spatial constraints of the survey line and the limited depth of the borehole seismic survey. We filled this gap using the radial basis function interpolation method. We could verify the completed 3D P-wave velocity model by comparing it with core log data. Overall, the integrated interpretation of the final velocity model and the analysis results could provide a probable pathway that indicates hydraulic connectivity.

Tackling the challenges of tectonic tremor localization using differential traveltimes and Bayesian inversion

SUMMARY Episodic tremor and slip events in Northern Cascadia produce low-frequency, emergent seismic signals, often referred to as tectonic tremor. Methods designed to locate tremor face two challenges that increase the likelihood of producing low quality catalogues: first, signal arrival and duration are often poorly defined; second, high rates of tremor activity during episodes can cause multiple signals to be indiscernible from each other. In this paper, we present a new method of locating tectonic tremor using differential traveltimes from a waveform-envelope cross-correlation in a grid-based Bayesian inversion. To address the aforementioned challenges, we use a recently developed three-dimensional shear wave velocity model to compute traveltimes, and include processes to remove data outliers, estimate data error statistics, and quantify uncertainties within the Bayesian framework. Although this method is designed for tremor, to test the approach we consider a set of 58 local earthquakes between magnitudes −0.07 and 2.6 in the Southern Vancouver Island region and obtain well-constrained relocations. Residuals between official catalogue values and our relocations are quantized with respect to the 1 km grid resolution of the inversion, and average 2.7 km in epicentre and 5.2 km in depth. Analysis shows that depths of relocations are sensitive to horizontal variations and simplifications in velocity models. We then present our catalogue of tremor events during the 2004 episodic tremor and slip event beneath Southern Vancouver Island, Canada. Median uncertainties of tremor events quantified by 95 per cent credibility interval widths in a 1 km grid are 5 km and 9.km in horizontal and depth directions, respectively (1.2 km and 2.3 km using traditional standard deviation-based uncertainties). Comparison of our catalogue with previously published work demonstrates that our new method yields a good detection rate, a greater degree of epicentral clustering, and better depth resolution of tremor events. Catalogues produced using this new method may help to provide insight into the spatial extent of tremor, especially in depth, by yielding enhanced constraints on source locations on a regional scale.

3D velocity and anisotropy of the southeastern Tibetan plateau extracted by joint inversion of wave gradiometry, ambient noise, and receiver function

With the deformation pattern of the southeastern Tibetan Plateau remaining controversial, its anisotropic structure and the three-dimensional (3D) velocity model are powerful tools for understanding this issue. The surface wave dispersion curve (DC) and receiver function (RF) are sensitive to absolute velocity and elasticity contrasts, respectively. In this work, we propose a new method for the joint inversion of 3D shear velocity (Vs) and anisotropy using the datasets of RFs and DCs, which are obtained by combining the wave gradiometry method and ambient noise tomography. Because DCs and RFs are divided into back-azimuth bins, and both datasets in each bin are jointly inverted to obtain velocity in the corresponding direction, the method is called the azimuth-dependent joint inversion method. By applying this new method to the Temporary West Sichuan Array, we obtained a high-resolution 3D Vs model and anisotropic structure of the southeastern Tibetan Plateau. In our model, the vertically consistent fast propagation direction in the Songpan-Ganzi block (SGB) and the north Chuandian block (NCDB) suggest that the deformation in this region is largely coupled vertically. Our model provides anisotropic evidence of the Yangtze crystalline basement extending to the SGB. The low-velocity zone (LVZ) was much thicker and more prominent in the NCDB than in the SGD. Anisotropy in the middle lower crust of the NCDB does not show any sign of an LVZ flowing from the SGB or the Qiangtang block to the NCDB. These observations are contrary to those of the crust flow model, suggesting that the LVZ was generated locally. The mushroom-shaped high-velocity structure beneath the southern Chuandian block and boundary fault zone may be a relic of the Emeishan paleo-mantle plume.

Multi-Mode Surface Wave Tomography of a Water-Rich Layer of the Jizhong Depression Using Beamforming at a Dense Array

Urban structure imaging using noise-based techniques has rapidly developed in recent years. Given the complexity of the cross-correlation function in high-frequency signals, here, the beamforming (BF) method was used to analyze one data set taken from a dense array in the Jizhong Depression and obtain multi-mode dispersion curves. Multi-mode surface waves improved inversion stability, reduced non-uniqueness, and yielded a one-dimensional shear wave (S-wave) velocity model. Interpolation yielded a high-resolution three-dimensional (3D) S-wave velocity model for the study area. The model shows that velocity gradually changed in the horizontal direction and greatly increased in the vertical direction, which is largely consistent with changes in the sedimentary environment related to the continuous subsidence of the Jizhong Depression since the Quaternary. A low-velocity anomaly at a depth of ~300–400 m was revealed and determined to be caused by either a deep-buried ancient river course or low-lying area. This study demonstrates the potential of the BF method for processing dense array data sets of urban exploration. The high-resolution 3D S-wave velocity model provides a new reference for studying the Quaternary structure of the Jizhong Depression, as well as groundwater resources, urban infrastructure, and underground spaces.

Probabilistic approach for seismic microzonation integrating 3D geological and geotechnical uncertainty

A novel probabilistic methodology for regional seismic site characterization is proposed and applied to a region with highly heterogeneous surficial geology and varying soil sediment thickness and stiffness. The method combines various sources of geological and geotechnical uncertainties to develop a three-dimensional (3D) shear-wave velocity ( Vs) model and evaluate the associated uncertainties. A 3D geological model of the unconsolidated deposits was developed using geostatistical interpolation and simulation methods. Sequential indicator simulations produced a quantitative geologic model that explicitly quantified geological uncertainties based on the likelihood of specific soil types occurring. In situ measurements and multivariate statistical analysis allowed the development of empirical correlations between Vs, geotechnical parameters, depth, and soil types. The resulting 3D Vs values were estimated on the basis of Vs-depth correlations and the probability of occurrence of each soil type. In this approach, the propagated uncertainty was also quantified by considering the combined variance. Seismic microzonation mapping was then conducted by transforming the 3D Vs model into two-dimensional (2D) maps that represent the spatial distributions of the time-averaged shear-wave velocity of the top 30 m ( Vs,30) and the fundamental site period ( T0), along with their respective uncertainties using Monte Carlo simulations. The results indicate that microzonation maps and their uncertainties are influenced by the thickness, occurrence probability, and geotechnical properties of soils. The proposed method can be used to assess the probabilistic seismic risk at local and regional scales in areas with geologically and geotechnically complex soil properties.

CMT inversion for small-to-moderate earthquakes applying to dense short-period OBS array at off Ibaraki region

We conducted centroid moment tensor (CMT) inversion for small-to-moderate aftershocks in the off Ibaraki region of the 2011 off the Pacific coast of Tohoku earthquake. In this study, we used high-frequency (0.4–1.0 Hz) seismograms from a dense array of short-period ocean bottom seismometers (OBSs) and a reliable three-dimensional (3-D) seismic velocity model. Higher frequency analysis and dense OBS arrays offer CMT solutions with high spatial resolutions. Since our OBS array observed aftershocks occurring immediately following the 2011 off the Pacific coast of Tohoku earthquake, we determined 536 CMT solutions for small-to-moderate aftershocks in the off Ibaraki region (JMA-scale magnitudes of 2.5–4.0). According to our CMT solutions, characteristics of the aftershock activities in the off Ibaraki region are classified into 5 groups: (1) thrust earthquakes, which are considered as interplate earthquakes, separated by the large slip area of the 2011 Ibaraki-oki earthquake (the largest {mathrm{M}}_{mathrm{w}} 7.6 aftershock of the 2011 off the Pacific coast of Tohoku earthquake) and the tectonic tremors; (2) intraslab strike-slip earthquakes located at the north of the fault area of the 2011 Ibaraki-oki earthquake; (3) intraslab normal-fault earthquakes, which suggest a tensional stress field within the subducting Pacific Plate due to the plate bending by the overriding Philippine Sea Plate; (4) various earthquake focal mechanisms above a subducting seamount, which suggest the 3-D complex fractures; and (5) normal-fault earthquakes shallower than the interplate earthquakes, which were possibly caused by the heterogeneity of the subducting seamount.Graphical

Three-dimensional Crustal Radial Anisotropy Velocity Model in Northeastern Tibet

Three-dimensional Crustal Radial Anisotropy Velocity Model in Northeastern Tibet

Developing a Consistent Travel-Time Framework for Comparing Three-Dimensional Velocity Models for Seismic Location Accuracy

Location algorithms have historically relied on simple, one-dimensional (1D) velocity models for fast seismic event locations. 1D models are generally used as travel-time lookup tables, one for each seismic phase, with travel-times pre-calculated for event distance and depth. These travel-time lookup tables are extremely fast to use and this fast computational speed makes them the preferred type of velocity model for operational needs. Higher-dimensional (i.e., three-dimensional—3D) seismic velocity models are becoming readily available and provide more accurate event locations over 1D models. The computational requirements of these 3D models tend to make their operational use prohibitive. Additionally, comparing location accuracy for 3D seismic velocity models tends to be problematic, as each model is determined using different ray-tracing algorithms. Attempting to use a different algorithm than the one used to develop a model usually results in poor travel-time prediction. We demonstrate and test a framework to create first-P and first-S 3D travel-time correction surfaces using an open-source framework (PCalc + GeoTess, https://www.sandia.gov/salsa3d/software/geotess) that easily stores 3D travel-time and uncertainty data. This framework produces fast travel-time and uncertainty predictions and overcomes the ray-tracing algorithm hurdle because the lookup tables can be generated using the exact ray-tracing algorithm that is preferred for a model.

Optimization of local scale seismic networks applied to geothermal fields. The case of the Acoculco caldera, Mexico

The exploitation of a geothermal field can be accompanied by both natural and induced seismicity. Hence the installation of a seismic network suitable for also locating low-magnitude earthquakes, is of great interest for geothermal development, especially for monitoring the activity related to the injection or production.Most of the aspects that affect the location of earthquakes are data quality and phase picking, the inverse method chosen to solve the problem, the velocity model, and the seismic network configuration. We focus on the optimization of network configuration and the reduction of the location error is one of the alternatives to finding optimal station positions. Here we propose an improvement of the D-OPTIMAL algorithm (Tramelli et al., 2013) that tries and find optimal station positions minimizing the amount of the error ellipsoid of the event location using the D-criterion. This algorithm is computationally efficient and easy to be implemented. In this version, we also introduced the possibility to account for several prior information that is generally available when implementing a monitoring site permanently or temporarily. These a priori parameters introduced are: i) three-dimensional seismic velocity models, ii) seismic noise levels, iii) topographic gradient, and iv) H/V ratio values. The last three parameters are introduced in the station position selection using a weighted system scheme.We applied this methodology to the Acoculco geothermal field (Mexico) where an injection test was planned and executed in 2021. The comparison between the network defined using the standard approach, which considers a 1D velocity model and the installation costs, and this improved version shows the importance of introducing a priori information during the selection of the network. Installation sites showed better distribution in the region, resulting in an overall increase in the sensitivity and a decreasing of the error location estimation in the target region.The methodology presented here is easy to apply to other study cases such as active volcanoes, anthropogenic activities, or other kinds of location studies at local scale.

Seismic Tomography and Earthquake Relocation in North China Craton From 2008 to 2017

We present a newly developed three-dimensional seismic velocity model and high-precision earthquake relocations in North China using seismic data recorded by the National Earthquake Data Center of China from 2008 to 2017 through the double-difference tomography (tomoDD) method. The Vp model in the upper crust (below 15 km) generally agrees with those in previous studies, and it fits well with the geological structure. The result shows that the vast majority of earthquakes in North China occurred at the junction of the low-velocity zone (depression zone) and the high-velocity zone (uplift zone). This situation is difficult to change in the short term, so it is highly unlikely that another earthquake of the magnitude of 7.2 in Xingtai in 1966 and 6.2 in Zhangjiakou North in 1998 will occur in these areas in the near future. But in the Tangshan earthquake area, the situation is completely different. Our joint inversion results of 3D P-wave velocity structure and earthquake relocation show that there is a narrow low-velocity anomaly perpendicular to the surface at 20–25 km in the Tangshan area and there are no earthquakes in this anomaly area. The formation of this low-velocity zone is most likely due to the remnants of the delayed subduction of the Pacific plate to the Eurasian plate, and in the context of the Kobe earthquake in Japan, we believe that this anomaly will continue to erode the Tangshan subsurface structure and may cause strong earthquakes in the future. Our study provides the groundwork for future earthquake prevention and mitigation in North China.

A Three-Dimensional Slip Velocity Model for Water-Lubricated Hydrodynamic Journal Bearings

Hydrodynamic journal bearings, coated with polytetrafluoroethylene (PTFE) and lubricated by water, have been widely used in ships and large-scale pumps, and the function is to maintain the stability of rotor system. However, slip velocity exists on the PTFE-coated surface, whose effect is still an open question. This study aims to investigate the static characteristics of water-lubricated hydrodynamic journal bearings under three-dimensional slip velocity boundary conditions. Firstly, under the non-slip boundary condition, the CFD (computational fluid dynamics) method with ANSYS Fluent is verified based on the Reynolds lubrication equation and the open literature. Then, a three-dimensional slip velocity equation that is based on the Navier slip velocity boundary condition is proposed and embedded into Fluent. Finally, the effects of slip length on the static characteristics are analyzed. Under the same eccentricity ratio, with the increase in slip length, the load capacity decreases due to the decrease of the pressure circumferential gradient, and the friction power decreases. Under the same eccentricity ratio and the same slip length, with the increase in the attitude angle, the load capacity and friction power increase. However, under the non-slip boundary condition, the effects of attitude angle on the load capacity and friction power are insignificant. This paper could provide a reference for studying slip velocity in the hydrodynamic journal bearing.