Chinese Journal of Geophysics最新文献

It is very common to use the self-potential methods in environmental and engineering applications, especially in some monitoring services. However, the monitored data of each time step are always inverted and interpreted independently. That means the valuable correlation information of time-lapse data is totally ignored. In order to take full advantage of the correlation information, a time-lapse inversion was proposed to promote the reliability of data interpretation. Based on the Darcy's law and Archie's formulas, a dynamic geoelectric model was built to simulate the transportation of contaminant plume in underground porous medium. Then this dynamic model can be used as a state model for the Kalman filtering. And the corresponding observation model can be obtained from conventional self-potential forward calculation. Thus, a Kalman filter recursion can be constructed by using the state model and observation model. During the recursion, the information of geoelectric model evolution and observed self-potential data are fused to achieve a time-lapse inversion of self-potential data. The time-lapse inversion algorithm was tested by both noise added synthetic self-potential data and laboratory observation data from self-potential monitoring over a sandbox. The numerical test shows the validity, robustness, and tolerance to noise of the time-lapse inversion. And the results of physical data test also demonstrate that the time-lapse inversion can invert real time-lapse self-potential data successfully and retrieve the dynamic geoelectric model exactly.

The Tangshan 1976 M7.8 earthquake was followed by two large aftershocks (the Luanxian M7.1 earthquake and the Ninghe M6.9 earthquake). The earthquake sequence occurred in an area with good geodetic observation, and the horizontal and vertical co-seismic displacements were obtained by triangulation net and leveling net. In this study, by using raw leveling data (not the subsidence image processed from raw leveling data) and triangulation observation data, we obtain the rupture distribution of strike slip and dip slip component on seismogenic faults. The geometry and size of the seismic faults of the Luanxian and Ninghe earthquake are considered in our model construction. The result shows that the seismic fault of the Tangshan mainshock manifested right-lateral strike slip with a maximum value greater than 6 m in the southern segment. The strike slip component on the northern segment is much less than that of the southern segment. The total seismic moment of the Tangshan mainshock is 2.58 × 10²⁰ N·m, which is consistent with that determined by seismic waves. The seismic fault of the Luanxian earthquake shows as a left-lateral normal fault with total moment of 4.95 × 10¹⁹ N·m. The seismic fault of the Ninghe earthquake shows as a left-lateral normal fault with total moment of 3.94 × 10¹⁹ N·m, which is an order of magnitude greater than the moment determined by seismic waveform. It is so inferred that the aseismic slip of the Tangshan earthquake occurred on the western part of the Ninghe earthquake fault, demonstrated nearly normal fault property, which is of great significances for geodynamic process and mechanism of aftershock occurrence after the Tangshan earthquake.

On 13th November 2016, the Kaikoura region of New Zealand was struck by a major M_w7.8 earthquake. In this study, coseismic deformation field is derived from 1 Hz high-rate GPS observations based on the GAMIT track solution module, combining with PCA spatial filtering method firstly. Then the 5 s peak displacement of P-wave (P_d) and the peak ground displacement (PGD) are extracted from the real-time simulated kinematic displacements of the trackRTr module. Finally, the warning magnitude is calculated from the statistical regression model. Our results indicate that the duration time of the kinematic deformation is up to 2 min. The KAIK and HANM stations, which are closest to the epicenter, have secondary severe deformation. And the deformation amplitude of the north stations of the epicenter is larger than that of the south. The static coseismic deformation field from high-rate GPS observations shows the characteristics of the focal mechanism of strike-slip after thrust. In addition, the P_d warning magnitudes from different stations have a significant difference, with the maximum magnitude difference of M_w2.5. Considering the timeliness and reliability of the warning magnitude jointly, the warning magnitude from the four-station PGD joint warning method of reasonable spatial distribution can reach its initial stability (M_w7.56) at 23 s after the event, while final stability (M_w7.78) at 110 s, which is consistent with the USGS moment tensor magnitude (M_w7.8).

Based on the three modes of lunar spacecraft tracking techniques: Earth-based tracking mode, high-low satellite-to-satellite tracking mode and low-low satellite-to-satellite tracking mode, the development of lunar gravity models could be divided into 4 stages. First, we introduce the principle, technical characteristics of the different tracking modes, and the representative gravity models, and then make comments on these models’ precision. Further, through the comparison of gravity anomaly precision, characteristics and orbit determination precision from different stages’ lunar gravity field models, we conclude that: the advancement of space tracking techniques has significantly improved the precision of lunar gravity model and effectively promotes the understanding of lunar interior structure and the reliability of lunar satellite orbit determination. Finally, we analyze the deficiency of current lunar gravity models and give a perspective of future space tracking techniques.

Compared with other imaging algorithms (e.g., ray-based, one-way wave equation), reverse time migration (RTM) based on the two-way wave equation exhibits greater superiority, especially in handling steeply dipping structures. However, imaging with conventional single-component seismic data is unsuited for some complicated structures (e.g., gas clouds). Elastic RTM, which is based on the elastodynamic equation and uses multi-component seismic data to extract PP and PS reflectivity and subsurface information, can more consistently reproduce the characteristics of elastic wave propagation in real Earth media, resulting in seismic images that more accurately characterize the subsurface. To begin with, we exploit the first order stress-velocity equations to extrapolate the elastic vector wavefield, then the P- and S-wavefields are separated by computing the divergence and curl operator of the extrapolated particle-velocity wavefield. Then, imaging profiles with pure wave modes are computed by applying the source normalized cross-correlation imaging condition, thus avoiding crosstalk between unseparated wave modes. To address the polarity reversal problem of the converted image, we propose an alternative method in the common-shot domain. We also develop an efficient method that reconstructs the source wavefield in the reverse time direction to save storage in the GPU and to avoid large input/output in the elastic reverse time migration. During the forward modeling, the method only saves the particle-velocity wavefield of all time intervals within an efficient absorbing boundary and the total wavefields in the final time interval. When we extrapolate the receiver wavefield in the reverse time direction, we simultaneously reconstruct the total source wavefields via the saved wavefields. Numerical examples performed with the graben and Marmousi2 models have shown that the polarity reversal correction method works, and elastic reverse time migration can accurately characterize complicated structures.

On 13 November, 2016, an M_w7.8 earthquake occurred in Kaikoura, northern South Island, New Zealand. The M_w 7.8 earthquake caused strong surface deformation, massive landslides and tsunami. Based on the fault slip model released by United States Geological Survey, the co-seismic deformation and stress changes by Kaikoura M_w7.8 earthquake were computed with the global heterogeneous ellipsoid Earth model and high precision topography. The preliminary results show that the hanging wall of the Mw7.8 earthquake uplifted to the northeast, while the footwall subducting southwest. The co-seismic deformation caused by this earthquake is up to several centimeters from Kaikoura to Campbell and the capital city of Wellington. The maximum co-seismic horizontal displacement is about 1.2 m whereas the vertical is about 1.1 m. Although the accumulated strain along seismogenic faults had been released by this earthquake, the compressional force at the both ends of the seismogenic faults was increased due to co-seismic stress changes and the maximum value of the co-seismic stress changes reaches the order of MPa. The Coulomb failure stress changes are also up to MPa that are concentrated in the vicinity of epicenter. At the same time, the risk for NE-SW dextral slip faults increases due to the co-seismic shear stress. We also calculated the Coulomb Failure Stress changes in the North Island and South Island with local fault systems respectively. Our results show that Coulomb failure stress changes are positive in both regions, which means subsequent earthquakes are possible.

Modern geophysical techniques provide great amount data of geophysical field that contain rich information about crustal structures, which can be extracted for geo-mapping based on theory of mathematical physics. This paper shows that the high-order spectral moments of regional gravity data include information of geometrical contour surface, which can be used for selection, positioning and classification of some types of geological structures. By using second-order spectral moments of Bouguer gravity data, one can calculate their statistical unvaried quantities and the ridge-like parameter, which characterizes the crustal deformation belts and can be used for mapping regional distribution of the deformation belts. The boundaries between areas of strong or weak ridge-like parameters represent boundaries of different structural units. We can further compute the second-order spectral moments of the statistical unvaried quantities to calculate the boundary parameter, which may be used for tracing and positioning the structural boundaries after some enhancement are added. This paper gives all the formula for computing the ridge and the boundary parameters, and testing results on crustal structural analysis in Tarim basin, Northwest China. As the results, deep High- and Low-density belts within the Tarim basin are located precisely, providing important basic images for oil/gas exploration in the basin.

The Qinling-Dabie orogenic belt is located between the northeastern margin of the Tibetan Plateau and the Tan-Lu fault; it was formed by the collision of Yangtze and North China blocks. We obtain the phase velocity of Rayleigh wave at the periods of 8∼35 s in the Qinling-Dabie orogenic belt using ambient seismic noise recorded at 160 broad-band stations from China digital seismic network. 24 month data have been cross-correlated to yield the empirical Rayleigh wave Green's functions. Phase velocity dispersion curves are measured for each interstation path by frequency-time analysis. The Rayleigh wave phase speed maps agree well with each other and show clear correlations with major tectonic structures. The Dabie is characterized by high velocity anomaly at 8 s but slow velocity at 14 s, indicating the influence of high pressure (HP)/ultrahigh pressure (UHP) metamorphic regions in the upper crust. At 25 s, the velocity varies from slow in the west to high in the east across the gravity gradient zone in the Taihang-Wuling Mountains. This pattern mainly reflects the effect of crust thickness which is thicker in the west and thin in the east. The southern segment of Tan-Lu fault shows different features across it at 14∼35 s, suggesting that the fault zone may extend down to the crust-mantle boundary. The slow velocity close to the fault is probably caused by the hot material upwelling. More constraints are needed by further study. Obvious slow velocities at periods of 14∼25 s in South Qinglin and northeast Sichuan basin are observed. We could not determine whether this low velocity zone is due to the Tibetan lower crustal flow and/or the delamination of the South Qinglin now. Study of the 3-D shear wave velocity structure in the crust and upper mantle is necessary to constrain the geodynamics of this region in the future.

Based on the new generation model of relative plate motions, the MOVEL model, an absolute plate motion model is built by least squares inversion of the observed hotspot trend data. A systematic test for the consistency between the MOVEL model and all the available hotspot data adopted by previous studies of absolute plate motion models shows there are outliers in the data set. Accordingly, two new methods to reject outliers, such as, step-wise rejection and global search rejection, are proposed. Combined with the chi-square test for the total residual and the normal test for the residual frequency distribution, the best model is selected step by step. A T87 model is eventually obtained. This model provides reasonable fitting to 87 globally distributed hotspot trends, but the predicted plate velocities are systematically lower than the observed rates of volcanic migration along hotspot tracks with the deviation up to 4 cm·a⁻¹. Such a deviation may be explained as a consequence of systematic errors possibly associated with the observed hotspot rate data, or due to relative motions between hotspots caused by mantle return flow. For either case, however, this study's results demonstrate that the hotspot trend data are capable of defining a global hotspot reference frame independently and effectively, which could be conveniently applied to studies on absolute plate motion, mantle convection and true polar wander.

Based on conventional matching pursuit (MP) algorithm, an improved approach of MP is presented in this paper. By introducing a new strategy of building complete library, the Ricker wavelet was taken as atoms and the maximum correlation of original seismic data was used to estimate the location and atomic energy of complete library. The decomposition precision was increased further through the method so that the adaptability of the algorithm was greatly improved. To avoid the atomic spacing being too small, the smallest atomic spacing was introduced, so the decomposition efficiency and quality were further improved. Model trial and application of practical data showed that not only the quality of the signal sparse representation through the algorithm was improved and the convergence speed was accelerated, but also the adaptability and decomposition precision of the algorithm were enhanced. Effective seismic information can be obtained better and the precision of seismic interpretation can be improved by the method.