Zhenxing Yao

Optimized explicit finite-difference schemes for spatial derivatives using maximum norm

We propose an optimized scheme using the maximum norm and the simulated annealing.The maximum norm offers the largest set of possible solutions for solvers to search.The explicit finite-difference operator is greatly improved by our optimized scheme.We use a tight error limitation to make accuracy improvement to be high and solid.Our optimized scheme allows greater saving of computational cost and memory demand. Conventional explicit finite-difference methods have difficulties in handling high-frequency components due to strong numerical dispersions. One can reduce the numerical dispersions by optimizing the constant coefficients of the finite-difference operator. Different from traditional optimized schemes that use the 2-norm and the least squares, we propose to construct the objective functions using the maximum norm and solve the objective functions using the simulated annealing algorithm. Both theoretical analyses and numerical experiments show that our optimized scheme is superior to traditional optimized schemes with regard to the following three aspects. First, it provides us with much more flexibility when designing the objective functions; thus we can use various possible forms and contents to make the objective functions more reasonable. Second, it allows for tighter error limitation, which is shown to be necessary to avoid rapid error accumulations for simulations on large-scale models with long travel times. Finally, it is powerful to obtain the optimized coefficients that are much closer to the theoretical limits, which means greater savings in computational efforts and memory demand.

Regional Seismic Characteristics of the 9 October 2006 North Korean Nuclear Test

We investigate the regional seismic signature of the 9 October 2006 North Korean nuclear test. Broadband regional data for the nuclear test and a group of earthquakes close to the test site were obtained between December 2000 and No- vember 2006. Epicentral distances from the stations to the test site are between 371 and 1153 km. We first use these regional events to calibrate the Lg-wave magnitude in the network. Then the network is used to calculate mbLg �� 3:93 for the North Ko- rean nuclear explosion. Using a modified fully coupled magnitude-yield relation, the yield of the North Korean nuclear test is estimated to be 0.48 kt. Because of large uncertainties in the source depth, the estimate is preliminary. The P=S-type spectral ratios Pg=Lg, Pn=Lg, and Pn=Sn are calculated for the nuclear explosion and a group of earthquakes close to the test site. At frequencies above 2 Hz, the network-averaged P=S spectral ratios clearly separate the 9 October 2006 explosion from the regional earthquakes. Our result indicates that a single-blast explosion in the North Korea re- gion shows different seismic characteristics from an earthquake. Any well-coupled single-blast explosion detonated in this region with yield similar to that for the North Korean nuclear test has a large probability of being identified by a regional seismic network such as the one adopted in this study.

Accelerating 3D Fourier migration with graphics processing units

Computational cost is a major factor that inhibits the practicalapplicationof3Ddepthmigration.Wehavedevelopeda fastparallelschemetospeedup3Dwave-equationdepthmigration on a parallel computing device, i.e., on graphics processing units GPUs. The third-order optimized generalized-screen propagator is used to take advantage of the builtinsoftwareimplementationofthefastFouriertransform.The propagator is coded as a sequence of kernels that can be called from the computer host for each frequency component. Moving the wavefield extrapolation for each depth leveltotheGPUsallowshandlingalarge3Dvelocitymodel,but this scheme can be speeded up to a limited degree over the CPU implementation because of the low-bandwidth data transfer between host and device. We have created further speedupinthisextrapolationschemebyminimizingthelowbandwidth data transfer, which is done by storing the 3D velocity model and imaged data in the device memory, and reducing half the memory demand by compressing the 3D velocity model and imaged data using integer arrays instead of float arrays. By incorporating a 2D tapered function, timeshiftpropagator,andscalingoftheinverseFouriertransform intoacompactkernel,thecomputationtimeisreducedgreatly. Three-dimensional impulse responses and synthetic data exampleshavedemonstratedthattheGPU-basedFouriermigration typically is 25 to 40 times faster than the CPU-based implementation. It enables us to image complex media using 3D depth migration with little concern for computational cost.Themacrovelocitymodelcanbebuiltinamuchshorter turnaroundtime.

Rupture process of the M w 7.9 Nepal earthquake April 25, 2015

On April 25, 2015, a magnitude Mw7.9 earthquake occurred in the southern Himalaya, Nepal, at 14:11 local time (UTC 2015-04-25 06:11). Its epicenter was at 28.147°N, 84.708°E with a source depth of 15 km, as determined by the United States Geological Survey (USGS). The earthquake hazard and secondary disasters, including landslides and avalanches, resulted in serious damage to Nepal and surroundings (including Kathmandu and the northern Himalaya of China) and caused huge loss of life and considerable destruction of property. The April 25 earthquake occurred on the active tectonic arc of the Himalaya, which defines the subduction thrust interface between the Indian and the Eurasian plates. Across the Nepalese Himalaya, the northward motion of the Indian Plate relative to the Eurasian Plate is estimated to be about 40 mm/yr, which results from the northward under thrusting of the India Plate beneath the Eurasian Plate. The convergence between India and the Himalaya proceeds at a rate of about 18 mm/yr (Bilham et al., 1997; Bettinelli et al., 2006; Ader et al., 2012). The continental collision of the Indian and the Eurasian plates generates numerous and frequent earthquakes, and makes this area one of the most seismically hazardous regions in the world (Figure 1). The ongoing collision between the Indian and the Eurasian plates has built the Tibetan Plateau (the highest plateau region on Earth) and has induced the imbricate thrust belt as the plate boundary. From south to north, the major faults comprise the Main Frontal Thrust fault (MFT), the Main Boundary Thrust fault (MBT), and the Main Central Thrust fault (MCT). In deeper depth, the fold-thrust belt is connected with the Main Himalaya Thrust fault (MHT) with a low dip angle (Cattin and Avouac, 2000; Lavé and Avouac, 2000; Bettinelli et al., 2006). Research of historical earthquakes and GPS measurements has revealed the high-risk potential of generating great earthquakes in the Nepalese Himalaya (Bilham et al., 1998; Ambraseys and Douglas, 2004; Ader et al., 2012; Sapkota et al., 2013). Based on the convergence rate between the Indian and Eurasian Plates, and on historical seismicity, Ader et al. (2012) warned of the high possibility of the occurrence of a large earthquake in the central Nepal seismic gap; the occurrence of the April 25 earthquake confirmed this concern. Following the earthquake, preliminary results of the source mechanism and rupture process for this earthquake were prepared using the fast source inverse approach with real-time far-field seismograms (http://www.itpcas.ac.cn/ xwzx/zhxw/201504/t20150426_4344080.html). Here, a listric finite fault model is constructed to simulate the earthquake fault according to the tectonic setting. A new source process model is estimated by joint inversion of far-field seismograms and GPS coseismic displacements. The inverted results might help both in disaster mitigation and in research into seismotectonic and dynamic simulations of this region.

Volcanic history of the Imbrium basin: A close-up view from the lunar rover Yutu.

Significance After the Apollo and Luna missions, which were flown about 40 years ago, the Moon was explored only from orbit. In addition, no samples were returned from the young and high-FeO and TiO2 mare basalt in the northern Imbrium basin. Such samples are important to understand the formation and evolution of the Procellarum KREEP [potassium (K), rare earth elements (REE), and phosphorus (P)] terrain, a key terrain highly enriched in radioactive nuclides. The Chang’e-3 mission carried out the first in situ analyses of chemical and mineral compositions of the lunar soil and ground-based measurements of the lunar regolith and the underlying basalt units at this specific site. The lunar regolith layer recorded the surface processes of the Moon, whereas the basalt units recorded the volcanic eruption history. We report the surface exploration by the lunar rover Yutu that landed on the young lava flow in the northeastern part of the Mare Imbrium, which is the largest basin on the nearside of the Moon and is filled with several basalt units estimated to date from 3.5 to 2.0 Ga. The onboard lunar penetrating radar conducted a 114-m-long profile, which measured a thickness of ∼5 m of the lunar regolith layer and detected three underlying basalt units at depths of 195, 215, and 345 m. The radar measurements suggest underestimation of the global lunar regolith thickness by other methods and reveal a vast volume of the last volcano eruption. The in situ spectral reflectance and elemental analysis of the lunar soil at the landing site suggest that the young basalt could be derived from an ilmenite-rich mantle reservoir and then assimilated by 10–20% of the last residual melt of the lunar magma ocean.

Static Deformation Due to Shear and Tensile Faults in a Layered Half-Space

Based on the generalized reflection and transmission coefficient matrix method, formulations for surface static displacements in a layered half-space are extended to include tensile and inflation point sources from a point pure shear dis- location source. Equations for calculating internal displacement fields from these sources are also derived. The validity of the formula and precision of the new method are illustrated by comparing the consistency of our results and the analytical solutions given by Okadas (1985, 1992) code in a homogenous half-space and Wang et al.s (2003) numerical solutions in a multilayered half-space. We also study the effect of a layered half-space on the surface displacement created by various finite faults. Several typical velocity structures in reality are selected. For strike-slip, reverse dip- slip, and tensile finite-fault models, the focal depth is very sensitive to the presence of the layered model. The slip displacement is more sensitive to the layered model in the case of the normal dip-slip sources. More numerical tests show that the sen- sitive slip is mainly due to the ultralow-velocity topsoil. For inflations, the source depth and volume change also altered due to the layered model.

The faulting process of Tangshan earthquake inverted simultaneously from the teleseismic waveforms and geodesic deformation data

Abstract It is well known that for natural earthquakes the faulting processes are highly complicated. To study such processes, all of the different types of data that can be acquired should be incorporated into the investigations. In accordance with this consideration, the faulting process of the Tangshan earthquake ( M S = 7.8) on 27 July 1976 is investigated. First, the geometrical attitude of the fault is determined by analysing the geological structure, intensity distribution, locations of the aftershocks and geodesic data collectively. Then the faulting process is simultaneously inverted from 14 WWSSN long-period teleseismic waveforms and 70 horizontal geodesic static deformation data. The results show that the Tangshan earthquake is one of the strongest intraplate earthquakes with a complex faulting process. The dimension of the main fracture is approximately 100 km by 15 km with a change in direction at the central part of the fault; the southern segment strikes at about NE30°, becoming 50–60° for the northern segment. The total seismic moment inverted is 2.0 × 10 20 Nm and the dislocation was relatively concentrated near the central part of the fault, where the slip elapsed for a longer time. These results suggest that near the city of Tangshan there might be a strong unbroken block and the fault changes its trend there. Hence the fault system was locked before the earthquake. When the regional stress increased and exceeded the strength limit, the fracture initiated from this location and extended bilaterally.

Optimized Chebyshev Fourier migration: A wide-angle dual-domain method for media with strong velocity contrasts

A wide-angle propagator is essential when imaging complex media with strong lateral velocity contrasts in one-way waveequation migration. We have developed a dual-domain one-way propagator using truncated Chebyshev polynomials and a globally optimized scheme. Our method increases the accuracy of the expanded square-root operator by adding two high-order terms to the traditional split-step Fourier propagator. First, we approximate the square-root operator using Taylor expansion around the reference background velocity. Then, we apply the first-kindChebyshevpolynomialstoeconomizetheresultsofthe Taylor expansion. Finally, we optimize the constant coefficients using the globally optimized scheme, which are fixed and feasible for arbitrary velocity models. Theoretical analysis and numericalexperimentshavedemonstratedthatthemethodhasvery highaccuracyandexceedstheunoptimizedFourierfinite-difference propagator for the entire range of practical velocity contrasts. The accurate propagation angle of the method is always about 60° under the relative error of 1% for complex media with weak, moderate, and even strong lateral velocity contrasts. The method allows us to handle wide-angle propagations and strong lateral velocity contrast simultaneously by using Fourier transform alone. Only four 2D Fourier transforms are required for each step of 3D wavefield extrapolation, and the computing cost is similar to that of the Fourier finite-difference method. Compared with the finite-difference method, our method has no twowaysplittingerrori.e.,azimuthal-anisotropyerrorfor3Dcases and almost no numerical dispersion for coarse grids. In addition, it has strong potential to be accelerated when an enhanced fast Fouriertransformalgorithmemerges.

Lateral Variation in the Sedimentary Structure of West Bohai Bay Basin Inferred from P-Multiple Receiver Functions

The widely distributed Cenozoic sediments in the Bohai Bay Basin give rise to noticeable modification of broadband teleseismic P waveforms. At one station in the west Bohai Bay Basin, the observed amplitude of tangential P-receiver func- tions is significantly above the noise level, and sedimentary reverberations (e.g., the P-type wave PpPp) remain one of the most prominent features in both the radial and tangential components. To investigate the lateral heterogeneity structure under this site, a 3D raytracing technique is used to compute the teleseismic P-wave response, and a fast simulated annealing algorithm is applied to the simultaneous inversion of radial receiver functions for different backazimuths. An upper crustal structure con- sisting of shallow dipping sedimentary layers with low seismic velocities and large Poissons ratios is proposed to interpret the observed seismic data. The west-dipping interfaces we obtained are consistent with the north-northeast-south-southwest sur- face geology in North China, and Tertiary extensional fault structures may be re- sponsible for the formation of dipping sedimentary layers.

The 14 November 1986 Taiwan earthquake - an event with isotropic component

Abstract Investigating the possibility that some earthquakes may significantly differ from the double-couple mechanism is always a subject of concern to seismologists. In this paper we give a detailed study of the 14 November 1986 Taiwan earthquake (Ms = 7.8) using the long-period body-wave waveform inversion. A hybrid global inversion method was used to derive the best-fitting solution and estimate the extreme models of non-double-couple components. The regional P-waveforms provide high resolution to the isotropic component. The inversion result shows that the Taiwan earthquake is an event composed of the reverse dip-slip dislocation source and an isotropic component. A notable source parameter in the result is the relative intensity of the isotropic component, which is defined as S P = (σ 1 + σ 2 + σ 3 ) 3M 0 , where σ1, σ2 and σ3 are the eigenvalues of the seismic moment tensor with σ1 > σ2 > σ3 and M 0 = (σ 1 − σ 3 ) 2 . The best-fitting solution of SP is −0.33, and the extreme values of SP are −0.48 and −0.13, respectively. This result suggests the existence of an obvious compressional isotropic component at the source process of this event.