Wei-Min Wang

Midcrustal low-velocity layer beneath the central Himalaya and southern Tibet revealed by ambient noise array tomography

[1]xa0Ambient noise tomography has been becoming an important tool to image the shallow lithospheric structure of the Earth. Using 2 months of ambient noise data from 20 stations of the Himalayan Nepal Tibet Seismic Experiment, we investigate the upper and middle crustal structure in the central Himalaya and southern Tibet. About 120 interstation Rayleigh wave empirical Greens functions with sufficient signal-to-noise ratio are obtained and used for group velocity dispersion analysis in the period range 6–25 s using frequency-time analysis technique. The obtained dispersion data are then used to construct 2-D group velocity maps. At the short periods from 9 to 15 s, the distribution of Rayleigh wave velocities delineates several distinct low- and high-velocity zones separated mainly by geological boundaries. The high group velocity zone is located mainly around regions with plutonic rocks, and the low group velocity zone is located around regions with sedimentary or metasedimentary rocks. Finally, we invert for the shear velocity structure in the upper and middle crust along a N-S trending cross section at the longitude 86.5°E. We observe a clear low-velocity layer in the middle crust (about 10–25 km depth) distributed on both sides of the Indus Yarlung Suture zone. The existence of this midcrustal low-velocity zone suggests a mechanically weaker middle crust beneath the central Himalaya and southern Tibet, which might decouple the upper crustal deformation from that of the lower crust in the Tibetan-Himalayan orogenic processes.

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.

Rupture history of the 2013 Mw 6.6 Lushan earthquake constrained with local strong motion and teleseismic body and surface waves

[1]xa0The rupture history of the 20 April 2013 Mw 6.6 Lushan (China) earthquake is constrained by inverting waveforms of local strong motion, teleseismic broadband body waves, and long-period surface waves. This earthquake ruptured a blind thrust fault oriented N210°E (along the Longmenshan fault zone) and dipping 40° to the NW. The inverted slip distribution is heterogeneous, dominated by a slip patch with a roughly right triangular shape, which spans a depth range of 5–20 km and accounts for two thirds of the total seismic moment (8.9 × 1018 N m). The rupture initiated roughly at the middle of the triangles hypotenuse and, during the first 4 s, propagated mainly in along-strike and downdip directions, toward a peak slip of 1.2 m. Despite a large number of fatalities and economic loss, the estimated static and apparent stress drops of the Lushan earthquake are 1.5 MPa and 0.35 MPa, considerably low with respect to other similar intraplate earthquakes.

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.

Seismic Lg‐wave Q tomography in and around Northeast China

[1]xa0We investigate regional variations in the Lg-wave quality factor (Q) in Northeast China and its vicinity with a tomographic method. Digital seismic data recorded at 20 broadband stations from 125 regional events are used to extract Lg-wave spectra. Tomographic inversions are independently conducted at 58 discrete frequencies distributed log evenly between 0.05 and 10.0 Hz. We simultaneously invert for the Lg-wave Q distribution and source spectra at individual frequencies without using any a priori assumption about the frequency dependence of the Q model and source function. The best spatial resolution is approximately 1° × 1° in well-covered areas for frequencies between 0.4 and 2.0 Hz. The Lg Q shows significant regional variations and an apparent relationship with regional geology. We use a statistical method to investigate the regional variations of Lg Q and their frequency dependence. The average Q0 (1 Hz Lg Q) in the entire investigated region is 414. Sedimentary basins are usually characterized by lower average Q0 values (from 155 to 391), while volcanic mountain areas have relatively high average Q0 values (from 630 to 675). Lg Q generally increases with increasing frequency. However, the frequency dependence has complex nonlinear features on a double-logarithmic scale, indicating that the commonly adopted power-law relationship may be oversimplified in a broad frequency band. The frequency dependence varies in different geological areas, with larger variations seen at lower frequencies.

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.

Yield Estimation of the 25 May 2009 North Korean Nuclear Explosion

We collect nine vertical component broadband seismograms from the 25 May 2009 North Korean nuclear explosion for a regional seismic network in which eight stations also recorded the 9 October 2006 North Korean nuclear test. Comparing the observed waveforms and spectra from the two events, we estimate that the amplitudes of the records from the second event are approximately five times those from the first one. Additionally, we use 599 vertical broadband seismograms from 82 regional events recorded on the regional network between December 1995 and May 2009 to calibrate the network Lg ‐wave magnitude. The calibrated network is used to calculate the Lg ‐wave body‐wave magnitude m b( Lg )=4.53 for the 25 May 2009 North Korean nuclear explosion. Based upon 15 first arrivals from the two North Korea nuclear explosions, the regional Pn velocity in the northeast China–North Korea region is calculated to be 8.0u2009u2009km/s. This result, along with the regional geological structures, suggests that the North Korean test site (NKTS) is located at a relatively stable continental region. We thus use a modified fully coupled magnitude–yield relation to estimate the explosion yield, and the result shows that the yield of the 25 May 2009 North Korean nuclear test is approximately 2.35xa0kt under the minimum burial depth assumption.nnOnline Material: Epicentral parameters, computed m b( Lg ), and corrected m b( Lg ) for 82 events used in the study.

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.

The 12 February 2013 North Korean Underground Nuclear Test

Online Material: Figures of Pn waveform comparisons and spectral ratios; tables of Pn differential times and parameters for events used in the study.nnOn 12 February 2013, North Korea conducted its third and the largest nuclear test to date in the China–North Korea border area. According to local news, people living in nearby Chinese cities experienced shaking from this explosion. The U.S. Geological Survey (USGS) reported the explosion was located at (41.301°xa0N, 129.066°xa0E) and the magnitude was Mxa05.1. This event trigged abundant regional seismic phases in northeast China, Korea, and Japan. Because of its large magnitude, the seismic records from this event showed better signal‐to‐noise ratios than those from previous two nuclear explosions. Illustrated in Figurexa01 are broadband regional seismograms at station MDJ for three North Korea nuclear tests. These waveforms are highly similar, all are featured with abrupt P ‐wave arrivals, weak Lg phases and well‐developed short‐period Rayleigh waves. We collect the regional waveforms recorded on China National Digital Seismic Network (CNDSN), Global Seismic Network (GSN), and Japan F‐net to investigate the 12 February 2013 North Korean nuclear test.nnnnFigure 1. nSeismograms recorded on MDJ from three North Korean nuclear tests in 2013, 2009, and 2006. Illustrated are normalized vertical displacements. The event date, maximum amplitudes, and epicenter distances are listed on the left. Marks on the waveforms indicate apparent group velocities. Note that the waveforms show similar features and display clear impulsive P ‐wave onset, relatively weak Lg phases, and 3–5xa0s period Rayleigh waves.nnnn### High‐Precision LocationnnBy adopting the relative location method (e.g., Schaff and Richards, 2004; Zhang etxa0al. , 2005; Wen and Long, 2010; Murphy etxa0al. , 2013), and using the first North Korean nuclear test on 9 October 2006 as the master event, we calculate the origin times and locations of the 25 May 2009 and …

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.