Stress drop assessment of the August 8, 2017, Jiuzhaigou earthquake sequence and its tectonic implications

By using a broadband Lg attenuation model developed for the Tibetan Plateau,we isolate source terms by removing attenuation and site effects from the observed Lg-wave displacement spectra of the M 7.0 earthquake that occurred on August 8,2017,in Jiuzhaigou,China,and its aftershock sequence.Thus,the source parameters,including the scalar seismic moment,comer frequency and stress drop,of these events can be further estimated.The estimated stress drops vary from 47.1 kPa to 7149.6 kPa,with a median value of 59.4 kPa and most values falling between 50 kPa and 75 kPa.The estimated stress drops show significant spatial variations.Lower stress drops were mainly found close to the mainshock and on the seismogenic fault plane with large coseismic slip.In contrast,the highest stress drop was 7.1 MPa for the mainshock,and relatively large stress drops were also found for aftershocks away from the major seismogenic fault and at depths deeper than the zone with large coseismic slip.By using a statistical method,we found self-similarity among some of the aftershocks with a nearly constant stress drop.In contrast,the stress drop increased with the seismic moment for other aftershocks.The amount of stress released during earthquakes is a fundamental characteristic of the earthquake rupture process.As such,the stress drop represents a key parameter for improving our understanding of earthquake source physics.

Seismic characteristics of the 15 February 2013 bolide explosion in Chelyabinsk, Russia

The seismological characteristics of the 15 February 2013 Chelyabinsk bolide explosion are investigated based on seismograms recorded at 50 stations with epicentral distances ranging from 229 to 4324 km. By using 8–25 s vertical-component Rayleigh waveforms,we obtain a surface-wave magnitude of 4.17±0.31 for this event. According to the relationship among the Rayleigh-wave magnitude,burst height and explosive yield, the explosion yield is estimated to be 686 kt. Using a single-force source to fit the observed Rayleigh waveforms, we obtain a single force of 1.03×10^(12) N, which is equivalent to the impact from the shock wave generated by the bolide explosion.

Broadband seismic illumination and resolution analyses based on staining algorithm

Seismic migration moves reflections to their true subsurface positions and yields seismic images of subsurface areas. However, due to limited acquisition aperture, complex overburden structure and target dipping angle, the migration often generates a distorted image of the actual subsurface structure. Seismic illumination and resolution analyses provide a quantitative description of how the above-mentioned factors distort the image. The point spread function （PSF） gives the resolution of the depth image and carries full information about the factors affecting the quality of the image. The staining algorithm establishes a correspondence between a certain structure and its relevant wavefield and reflected data. In this paper, we use the staining algorithm to calculate the PSFs, then use these PSFs for extracting the acquisition dip response and correcting the original depth image by deconvolution. We present relevant results of the SEG salt model. The staining algorithm provides an efficient tool for calculating the PSF and for conducting broadband seismic illumination and resolution analyses.

A Wavelet Transform Method to Detect P and S-Phases in Three Component Seismic Data

The discrete time wavelet transform has been used to develop software that detects seismic P and S-phases. The detection algorithm is based on the enhanced amplitude and polarization information provided by the wavelet transform coefficients of the raw seismic data. The algorithm detects phases, determines arrival times and indicates the seismic event direction from three component seismic data that represents the ground displacement in three orthogonal directions. The essential concept is that strong features of the seismic signal are present in the wavelet coefficients across several scales of time and direction. The P-phase is detected by generating a function using polarization information while S-phase is detected by generating a function based on the transverse to radial amplitude ratio. These functions are shown to be very effective metrics in detecting P and S-phases and for determining their arrival times for low signal-to-noise arrivals. Results are compared with arrival times obtained by a human analyst as well as with a standard STA/LTA algorithm from local and regional earthquakes and found to be consistent.

Numerical research on evolvement of submarine sand waves in the Northern South China Sea

Submarine sand waves, vital to seabed stability, are an important consideration for oceanic engineering projects such as oil pipe lines and submarine cables. The properties of surface sediment and the evolvement of submarine sand waves in a specified area in the South China Sea are studied using both a hydrological model and field observational data. The bottom flow field data between 2010 and 2011 in the study area are simulated by the Regional Ocean Model System （ROMS）. The migration of submarine sand waves is calculated using Rubin＇s formula along with typhoon data and bottom flow field data, which allows for the analysis of sand wave response under the influence of typhoons. The migration direction calculated by Rubin＇s formula and bottom flow are very similar to collected data. The migration distance of different positions is between 0.0 m and 21.8 m, which reciprocates cumulatively. This shows that Rubin＇s formula can predict the progress of submarine sand waves with the bottom flow simulated by ROMS. The migration distances of 2 sites in the study area are 2.0 m and 2.9 m during the typhoon ＂Fanapi＂. The proportion of the calculated migration distance by the typhoon is 9.17% and 26.36% of the annual migration distance, respectively, which proves that the typhoon can make a significant impact on submarine sand waves.

Partial anhysteretic remanent magnetization (pARM) of synthetic single-and multidomain magnetites and its paleoenvironmental significance

In order to isolate magnetic signals carried by single-domain (SD) ferrimagnetic (FM) minerals from multi-domain (MD) FM minerals, we developed a few pa- rameters using partial anhysteretic remanent magnetization (pARM). Because MD fraction contains only soft (easy to be demagnetized) coercivity spectrum, pARM(>20 mT) was sensitive in eliminating MD contributions. Ratio of pARM(5 mT, 10 mT)/pARM(0, 5 mT) is useful in quantifying a rela- tive abundance of mass ratio between SD and MD fractions. These new proxies can quickly characterize the details of grain size distribution of magnetic minerals in paleoclimatic and paleomagnetic studies.

青藏高原北缘中生代伸展构造^(40)Ar/^(39)Ar测年和MDD模拟

沿着青藏高原北缘的阿尔金山脉东段 ,发育了长度大于 30 0km、EW走向的拉配泉断裂。中美合作阿尔金课题组的地质填图结果表明 ,该断层实际上是一条南倾的正断层 ,局部倾角可以低至 30°以下。沉积学和热年代学研究控制了拉配泉断裂的活动时代 :早—中侏罗统地层可以解释为断裂上盘的伸展盆地沉积 ;下盘岩石中钾长石40 Ar/ 3 9Ar测年和MDD模拟给出 2个阶段的冷却事件 ,早期事件出现在约 2 2 0～ 187Ma之间 ,晚期事件出现在早白垩世晚期 (约 10 0Ma)。早期事件代表了拉配泉断裂正断作用的主要阶段。

阿尔金山区域热演化历史的初步研究

通过地质年龄数据的综合分析,本文探讨了阿尔金山地区自元古宙以来的区域热演化历史。其中,本区自古太古代以来具有多期岩浆活动,其主要活动时期为早古生代晚期(490～385 Ma),高峰期年龄为442Ma左右。同时,具有新元古代、早古生代、中生代和新生代4次地壳抬升和冷却事件。其中中生代冷却事件启动时间与阿尔金北缘伸展作用早期活动时间相一致;新生代事件出现在7 Ma左右,与青藏高原隆升具有密切关系。

SM ALL项目数字化地磁资料可靠性分析

在对中美合作 SMALL (中国低纬地磁台阵 )项目安装在北京地磁台 (白家疃 )和云南通海地磁台的磁通门磁力仪记录资料与磁变仪模拟记录资料进行了详细的对比分析后 ,得出的结果表明 ,两者的日变形态、日变幅有较好的一致性 ,所选特定事件的变化幅度非常一致 ,说明磁通门磁力仪记录到的变化磁场的资料是真实可靠的。尽管目前磁通门磁力仪的基线值存在温度影响和离散度较大的问题 ,但随温度变化有明显的规律性 ,若能采取改进观测条件和观测方法以及缩短观测时间等措施 ,并对基线值进温度校正 ,磁通门磁力仪完全可以提供长期连续可靠的资料。而其为地震预报 ,空间环境预报 ,为日地物理 ,空间物理和地磁学的研究提供的高精度、高分辨率、连续可靠的数字化变化磁场资料是磁变仪模拟记录无法比拟的。

35个铁陨石化学组成研究及其在分类学中的应用

我们在中国某些铁陨石化学分类的基础土，又对11个南极铁陨石及10个美国和其他国家的铁陨石进行了多元素的分析和研究。除首次对11个南极铁陨石进行化学构造分类外，还将以前划分为ⅢAB的新疆铁陨石重新划分为ⅢE化学群，并增加和更正了中国14个铁陨石的一些数据。故本文一并讨论35个铁陨石化学组成的研究结果。

通海地磁台(L=1.03)Pc3地磁脉动的出现率

利用通海地磁台 ( L=1 .0 3) 1 999年 SMALL磁通门磁力仪记录资料 ,对 Pc3脉动出现率的统计结果表明 ,日出现频次有两个峰值 ,即清晨 0 6:0 0～ 0 7:0 0 LT(地方时 )出现频次最高 ,午后 1 4:0 0～ 1 5 :0 0 LT为次峰值。Pc3脉动月出现频率最高的是 4月和 1 2月 ,其次是 8月和9月 ;最少的是 6月和 7月。按季节统计结果为春秋季出现频次最高 ,冬季次之 。

柴达木盆地新生代演化及其构造重建——基于地震剖面的解释

柴达木盆地是青藏高原内部最大的坳陷。柴达木盆地构造成因的研究，可以揭示青藏高原形成机制和生长历史。本文分析了柴达木盆地区域地震勘探剖面，得到如下认识：柴达木盆地一级构造为新生代宽缓复向斜，其振幅和半波长分别从柴西的〉16km和～170km变化为柴东的〈4km和～50km。褶皱首先在柴西贴近阿尔金断裂附近形成（65～50．5Ma），并向柴东扩展（23．3Ma）。复向斜的形成与较老的柴北缘逆冲断层系（65～50．5Ma）和较年轻的柴南缘逆冲断层系（35．5～23．3Ma）有关。盆地内部新生代上地壳缩短作用，由柴西的〉48％，向柴东减小到〈1％，说明在柴西和柴东之间，存在地壳加厚机制的渐进转换：柴西主要为上地壳缩短，柴东主要为下地壳缩短。

Nd isotopic compositions of the Tethyan Himalayan Sequence in southeastern Tibet

The Himalayan orogen consists of three major lithologic units that are separated by two major north-dipping faults: the Lesser Himalayan Sequence (LHS) below the Main Central Thrust (MCT), the Greater Himalayan Crystalline Complex (GHC) above the MCT, and the Tethyan Himalayan Sequence (THS) juxtaposed by the South Tibet Detachment fault (STD) over the GHC. Due to widespread meta-morphism and intense deformation, differentiating the above three lithologic units is often difficult. This problem has been overcome by the use of Sm-Nd isotopic analysis. The previous studies suggested that the LHS can be clearly distinguished from the GHC and THS by their Nd isotope compositions. However, the lack of detailed and systematic Sm-Nd isotopic studies of the THS across the Himalaya in general has made differentiation of this unit from the nearby GHC impossible, as the two appear to share overlapping Nd compositions and model ages. To address this problem, we systematically sam-pled and analyzed Nd isotopes of the THS in southeastern Tibet directly north of Bhutan. Our study identifies two distinctive fields in a εNd -TDM plot. The first is defined by the εNd(210 Ma) values of -3.45 to -7.34 and TDM values of 1.15 to 1.29 Ga from a Late Triassic turbidite sequence, which are broadly similar to those obtained from the Lhasa block. The second field is derived from the Early Cretaceous meta-sedimentary rocks with εNd(130 Ma) values from -15.24 to -16.61 and TDM values from 1.63 to 2.00 Ga; these values are similar to those obtained from the Greater Himalayan Crystalline Complex in Bhutan directly south of our sampling traverse, which has εNd(130 Ma) values of -10.89 to -16.32 and Nd model ages (TDM) of 1.73 to 2.20 Ga. From the above observations, we suggest that the Late Triassic strata of the southeast Tibetan THS were derived from the Lhasa block in the north, while the Early Cretaceous strata of the THS were derived from a source similar to the High Himalayan Crystalline Complex or Indian craton in the south. Our interpretation is consistent with the existing palaeocurrent data and provenance analysis of the Late Triassic strata in southeastern Tibet, which indicate the sediments derived from a northern source. Thus, we suggest that the Lhasa terrane and the Indian craton were close to one another in the Late Triassic and were separated by a rift valley across which a large submarine fan was transported southward and deposited on the future northern margin of the Indian continent.

白垩纪中期地球磁场与全球地质现象(英文)

白垩纪超静磁带(CNS)是指白垩纪中期约40 Ma内(Aptian Santonian, 121~83 Ma)地球磁场没有出现倒转的异常现象。这一现象引起地球学界的极大关注,原因在于它与白垩纪中期(124~90 Ma)其他一系列事件在时间上非常吻合,包括洋底扩张速率的快速增加和洋底高原、海山链、大型火成岩省等生产速率的快速增加。过去20多年许多研究强调所有这些现象与下地幔对流的联系。近期研究的一个重要的进展在于把地球磁场的变化(反转频率和古强度)与许多重要的地质和地球物理过程结合起来,如周期性地幔对流、地幔柱活动、全球热流、真极移、大型火成岩省的产生和生物群集绝灭。文中回顾了白垩纪地球磁场研究以及与白垩纪中期所发生的全球性的地质事件之间的可能存在的关联,并讨论在这一重要研究领域内未来研究的发展方向。

上游太阳风中高能电子向同步轨道区的传输

通常认为,同步轨道区的电子通量增加是由于磁暴或者上游太阳风高速流的扰动所引起。近来的观测表明,起源于太阳活动的行星际高能电子也是引起同步轨道电子通量增加的重要原因之一。Zhao等在研究2000年7月14日太阳剧烈活动时发现,同步轨道区相对论电子通量巨幅增加时没有观察到上游太阳风高速流的扰动,并且磁暴发生在电子通量事件之后。采用解析磁场模型和实际磁场模型(T96模型)模拟来自太阳的相对论电子在磁尾中的运动特性。计算结果表明,当行星际磁场南向时,进入到磁尾的行星际相对论电子可以从较远的磁尾区域运动到同步轨道区域。这一研究结果从理论上论证了起源于太阳活动的高能电子可以对同步轨道区相对论电子通量的增加产生重要的作用。

Observations of kinetic Alfvén waves by THEMIS near a substorm onset

Low frequency electromagnetic fluctuations in the vicinity of a magnetospheric substorm onset were investigated using simultaneous observations by THEMIS multiple probes in the near-Earth plasma sheet in the magnetotail.The observations indicate that in the vicinity of a substorm onset,kinetic Alfvén waves can be excited in the high-βplasma sheet(β=2μ0nT/B 2 ,the ratio of plasma thermal pressure to magnetic pressure)within the near-Earth magnetotail.The kinetic Alfvén wave has a small spatial scale in the high-βplasma.The parallel electric field accompanying kinetic Alfvén waves accelerates the charged particles along the magnetic field.The kinetic Alfvén waves play an important role in the substorm trigger process,and possibly in the formation of a substorm current wedge.

Pressure gradient evolution in the near-Earth magnetotail at the arrival of BBFs

Using in situ observations from THEMIS A, D and E during the 2008–2011 tail season, we present a statistical study of the evolution of pressure gradients in the near-Earth tail during bursty bulk flow(BBF) convection.We identified 138 substorm BBFs and 2,197 non-substorm BBFs for this study. We found that both the pressure and the BZcomponent of the magnetic field were enhanced at the arrival of BBFs at the spacecraft locations. We suggest that the increase of BZduring non-substorm BBFs is associated with flux pile-up. However, the much stronger enhancement of BZduring substorm BBFs implies the occurrence of magnetic field dipolarization which is caused by both the flux pile-up process and near-Earth current disruption. Furthermore, a bow-wave-like high pressure appears to be formed at the arrival of substorm BBFs,which is responsible for the formation of region-1-sense FACs. The azimuthal pressure gradient associated with the arrival of substorm BBFs lasts for about 5 min. The enhanced pressure gradient associated with the bow waveis caused by the braking and diversion of the Earthward flow in the inner plasma sheet. The results from this statistical study suggest that the braking and azimuthal diversion of BBFs may commonly create azimuthal pressure gradients, which are related to the formation of the FAC of the substorm current wedge.

Small-scale dipolarization fronts in the Earth’s magnetotail

Previous studies suggest that dipolarization fronts (DFs) are 1 to 3RE (RE is the earth radius) wide in the dawn-dusk direction. Recent kinetic simulations have found that DFs may break up into small-scale structures after they are produced by reconnection. Motivated by this simulation, we revisited the scale size of DFs in the dawn-dusk direction by using Cluster observations during the years when the inter-distance among Cluster spacecraft was between 1000 and 2000 km. We selected the DFs that were detected by more than one spacecraft and estimated the radii of these DFs by a simple geometrical analysis, which is based on comparison of DF normals observed by different spacecraft. We found a few DFs that were only a few ion inertial lengths in the dawn-dusk direction. These results point out the importance of multi-scale coupling during the evolution of DFs.

The Effect of Strong Near Surface Scattering on Seismic Imaging: Investigation Based on Resolution Analysis

In land seismic exploration,strong near-surface heterogeneities can cause serious problems in seismic data acquisition and the quality of depth imaging.By introducing random velocity models to simulate velocity fluctuations in the near-surface layer and using the point spread function to characterize image quality,we examine how the scattering generated in near-surface heterogeneities can affect the subsurface image.In addition to the commonly known scattering noises which lower the signal to noise ratio in seismic data,our results also reveal that intermediate scale hetero-geneities generate forward scattering which forms phase or travel time fluctuations.Due to intermediate-scale uncertainty in the shallow part of the migration velocity model,these phase changes are carried to the target by the extrapolated wavefields,breaking the zero phase image condition at the image point.This is a primary reason for deteriorated image quality in regions with strong near-surface scattering.If this intermediate-scale information can be obtained and built into the migration velocity,the subsurface image quality can be largely improved.These results can be the ba-sis for further numerical investigations and field experiments.The proposed analysis method can also be used to evaluate other potential methods for dealing with near-surface scattering.

地震反射剖面揭示阿尔泰山陆内逆冲造山机制

The Altai orogen is a typical intracontinental orogen in Central Asia that experienced far-field deformation associated with Indian-Eurasian plate convergence. This region is characterized by uplift comparable to that of the Tianshan Mountains but has a distinct strain rate. Half of the Indo-Asia strain is accommodated by the Tianshan Mountains, whereas the Altai Mountains accommodates only 10%. To elucidate how the Altai Mountains produced such a large amount of uplift with only one-fifth of the strain rate of the Tianshan Mountains, we constructed a detailed crustal image of the Altai Mountains based on a new 166.8-km deep seismic reflection profile. The prestack migration images reveal an antiform within the Erqis crust, an ~10 km Moho offset between the Altai arc and the East Junggar area, and a major south-dipping(30° dip) thrust in the lower crust beneath the Altai Mountains, which is connected to the Moho offset. The south-dipping thrust not only records the southward subduction of the Ob-Zaisan Ocean in the Paleozoic but also controlled the Altai deformation pattern in the Cenozoic with the Erqis antiform. The Erqis antiform prevented the extension of deformation to the Junggar crust. The southdipping thrust in the lower crust of the Altai area caused extrusion of the lower crust, generating uplift at the surface, thickening of the crust, and steep(~10 km) Moho deepening in the Altai Mountains. This process significantly widened the deformation zone of the Altai Mountains. These findings provide a new geodynamic model for describing how inherited crustal structure controls intraplate deformation without strong horizontal stress.