ALBERT

Description: 〈span〉〈div〉Summary〈/div〉Dispersion curve inversion is one of the core components of Rayleigh wave surveys, which mainly include linear and nonlinear inversion theoretical systems. Damped least squares (DLS) is the most mature and commonly used method of linear optimization, but it relies heavily on more accurate initial models, otherwise it can easily fall into a local minimum or can even result in an incorrect inversion. As a representative method of nonlinear optimization, genetic algorithm (GA) may be more feasible to obtain a global optimal solution for the geophysical inversion in theory. However, the GA algorithm is less stable, as well as less efficient in the later period of the inversion. In the past, the above two systems have been used independently to perform inversion processing. Faced with complex seismic geological conditions, they often display poor adaptability and lack balance between speed and accuracy. For this reason, we made a reasonable and effective improvement to the generation of the initial population and the coding of the classic GA to overcome the time-consuming and memory-intensive computational issues. We redefined the selection and crossover function to prevent the ‘premature convergence’ phenomenon in genetic iterations. Simultaneously, the DLS and the steepest descent method (SD) are embedded in the GA inversion process to linearly optimize the dominant individuals in the population (i.e. local extrema) in the local space to guide the population to move quickly and stably advance towards the global optimal direction. Next, a robust DLS inversion is used to obtain the final S-wave velocity model using the adaptive GA inversion results as the input velocity model. The model and actual dataset processing results show that our proposed nested joint inversion can combine the advantages of linear and nonlinear inversion that can effectively suppress the multi-solution problem of the dispersion curve inversion and significantly improve the inversion efficiency and accuracy.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉Dispersion curve inversion is one of the core components of Rayleigh wave surveys, which mainly include linear and non-linear inversion theoretical systems. Damped least squares (DLS) is the most mature and commonly used method of linear optimization, but it relies heavily on more accurate initial models, otherwise it can easily fall into a local minimum or can even result in an incorrect inversion. As a representative method of non-linear optimization, genetic algorithm (GA) may be more feasible to obtain a global optimal solution for the geophysical inversion in theory. However, the GA algorithm is less stable, as well as less efficient in the later period of the inversion. In the past, the above two systems have been used independently to perform inversion processing. Faced with complex seismic geological conditions, they often display poor adaptability and lack balance between speed and accuracy. For this reason, we made a reasonable and effective improvement to the generation of the initial population and the coding of the classic GA to overcome the time-consuming and memory-intensive computational issues. We redefined the selection and crossover function to prevent the ‘premature convergence’ phenomenon in genetic iterations. Simultaneously, the DLS and the steepest descent method are embedded in the GA inversion process to linearly optimize the dominant individuals in the population (i.e. local extrema) in the local space to guide the population to move quickly and stably advance towards the global optimal direction. Next, a robust DLS inversion is used to obtain the final 〈span〉S〈/span〉-wave velocity model using the adaptive GA inversion results as the input velocity model. The model and actual data set processing results show that our proposed nested joint inversion can combine the advantages of linear and nonlinear inversion that can effectively suppress the multisolution problem of the dispersion curve inversion and significantly improve the inversion efficiency and accuracy.〈/span〉

Description: We revisit the problem of coseismic rupture of the 2008 M w 7.9 Wenchuan earthquake. Precise determination of the fault structure and slip distribution provides critical information about the mechanical behaviour of the fault system and earthquake rupture. We use all the geodetic data available, craft a more realistic Earth structure and fault model compared to previous studies, and employ a nonlinear inversion scheme to optimally solve for the fault geometry and slip distribution. Compared to a homogeneous elastic half-space model and laterally uniform layered models, adopting separate layered elastic structure models on both sides of the Beichuan fault significantly improved data fitting. Our results reveal that: (1) The Beichuan fault is listric in shape, with near surface fault dip angles increasing from ~36° at the southwest end to ~83° at the northeast end of the rupture. (2) The fault rupture style changes from predominantly thrust at the southwest end to dextral at the northeast end of the fault rupture. (3) Fault slip peaks near the surface for most parts of the fault, with ~8.4 m thrust and ~5 m dextral slip near Hongkou and ~6 m thrust and ~8.4 m dextral slip near Beichuan, respectively. (4) The peak slips are located around fault geometric complexities, suggesting that earthquake style and rupture propagation were determined by fault zone geometric barriers. Such barriers exist primarily along restraining left stepping discontinuities of the dextral-compressional fault system. (5) The seismic moment released on the fault above 20 km depth is 8.2 x 10 21  N m, corresponding to an M w 7.9 event. The seismic moments released on the local slip concentrations are equivalent to events of M w 7.5 at Yingxiu-Hongkou, M w 7.3 at Beichuan-Pingtong, M w 7.2 near Qingping, M w 7.1 near Qingchuan, and M w 6.7 near Nanba, respectively. (6) The fault geometry and kinematics are consistent with a model in which crustal deformation at the eastern margin of the Tibetan plateau is decoupled by differential motion across a decollement in the mid crust, above which deformation is dominated by brittle reverse faulting and below which deformation occurs by viscous horizontal shortening and vertical thickening.

Description: Recently constructed models of crustal structure across Tibet based on surface wave data display a prominent mid-crustal low velocity zone (LVZ) but are vertically smooth in the crust. Using six months of broad-band seismic data recorded at 22 stations arrayed approximately linearly over a 440 km observation profile across northeastern Tibet (from the Songpan–Ganzi block, through the Qaidam block, into the Qilian block), we perform a Bayesian Monte Carlo joint inversion of receiver function data with surface wave dispersion to address whether crustal layering is needed to fit both data sets simultaneously. On some intervals a vertically smooth crust is consistent with both data sets, but across most of the observation profile two types of layering are required: a discrete LVZ or high velocity zone (HVZ) formed by two discontinuities in the middle crust and a doublet Moho formed by two discontinuities from 45–50 km to 60–65 km depth connected by a linear velocity gradient in the lowermost crust. The final model possesses (1) a mid-crustal LVZ that extends from the Songpan–Ganzi block through the Kunlun suture into the Qaidam block consistent with partial melt and ductile flow and (2) a mid-crustal HVZ bracketing the south Qilian suture coincident with ultrahigh pressure metamorphic rocks at the surface. (3) Additionally, the model possesses a doublet Moho extending from the Qaidam to the Qilian blocks which probably reflects increased mafic content with depth in the lowermost crust perhaps caused by a vertical gradient of ecologitization. (4) Crustal thickness is consistent with a step-Moho that jumps discontinuously by 6 km from 63.8 km (±1.8 km) south of 35° to 57.8 km (±1.4 km) north of this point coincident with the northern terminus of the mid-crustal LVZ. These results are presented as a guide to future joint inversions across a much larger region of Tibet.

Description: This paper describes an alternative acceleration approach for determining GRACE monthly gravity field models. The main differences compared to the traditional acceleration approach can be summarized as: (1) The position errors of GRACE orbits in the functional model are taken into account; (2) The range ambiguity is eliminated via the difference of the range measurements and (3) The mean acceleration equation is formed based on Cowell integration. Using this developed approach, a new time-series of GRACE monthly solution spanning the period January 2003 to December 2010, called Tongji_Acc RL01, has been derived. The annual signals from the Tongji_Acc RL01 time-series agree well with those from the GLDAS model. The performance of Tongji_Acc RL01 shows that this new model is comparable with the RL05 models released by CSR and JPL as well as with the RL05a model released by GFZ.

Description: The crustal and upper mantle velocity structure in the northeastern Tibetan Plateau is obtained from joint analysis of receiver functions and Rayleigh wave dispersions. The resulting velocity model reveals a close correlation between the thick (〉60 km) crust and the presence of an intracrustal low-velocity zone beneath the Qiangtang and Songpan-Ganzi terranes as well as the northwestern Qilian orogen. However, the high V p / V s ratio of the crust is found only beneath the Qiangtang and Songpan-Ganzi terranes. The crustal low velocity zone does not appear in the west Qinling and southeastern Qilian orogens, which have a relatively thin (~50 km) crust, indicating that crustal channel flow is not the primary mechanism by which the northeastern Tibetan Plateau grows. A continuous low velocity zone from the mid-to-lower crust down to 160 km beneath the eastern Kunlun fault suggests an induced local mantle upwelling after partial detachment of the lithosphere.

Description: The EarthScope USArray provides an opportunity to obtain detailed images of the continental upper mantle at an unprecedented scale. The majority of mantle models derived from USArray data to date contain spatial variations in seismic-wave speed; however, in many cases these data sets do not by themselves allow a non-unique interpretation. Joint interpretation of seismic attenuation and velocity models can improve upon the interpretations based only on velocity and provide important constraints on the temperature, composition, melt content, and volatile content of the mantle. The surface wave amplitudes that constrain upper-mantle attenuation are sensitive to factors in addition to attenuation, including the earthquake source excitation, focusing and defocusing by elastic structure, and local site amplification. Because of the difficulty of isolating attenuation from these other factors, little is known about the attenuation structure of the North American upper mantle. In this study, Rayleigh wave traveltime and amplitude in the period range 25–100 s are measured using an interstation cross-correlation technique, which takes advantage of waveform similarity at nearby stations. Several estimates of Rayleigh wave attenuation and site amplification are generated at each period, using different approaches to separate the effects of attenuation and local site amplification on amplitude. It is assumed that focusing and defocusing effects can be described by the Laplacian of the traveltime field. All approaches identify the same large-scale patterns in attenuation, including areas where the attenuation values are likely contaminated by unmodelled focusing and defocusing effects. Regionally averaged attenuation maps are constructed after removal of the contaminated attenuation values, and the variations in intrinsic shear attenuation that are suggested by these Rayleigh wave attenuation maps are explored.