Core mantle boundary topography from short period PcP, PKP, and PKKP data

We use a total of 839,369 PcP, PKPab, PKPbc, PKPdf, PKKPab, and PKKPbc residual travel times from [Bull. Seism. Soc. Am. 88 (1998) 722] grouped in 29,837 summary rays to constrain lateral variation in the depth to the core-mantle boundary (CMB). We assumed a homogeneous outer core, and the data were corrected for mantle structure and inner-core anisotropy. Inversions of separate data sets yield amplitude variations of up to 5 km for PcP, PKPab, PKPbc, and PKKP and 13 km for PKPdf. This is larger than the CMB undulations inferred in geodetic studies and, moreover, the PcP results are not readily consistent with the inferences from PKP and PKKP. Although the source-receiver ambiguity for the core-refracted phases can explain some of it, this discrepancy suggest that the travel-time residuals cannot be explained by topography alone. The wavespeed perturbations in the tomographic model used for the mantle corrections might be too small to fully account for the trade off between volumetric heterogeneity and CMB topography. In a second experiment we therefore re-applied corrections for mantle structure outside a basal 290 km-thick layer and inverted all data jointly for both CMB topography and volumetric heterogeneity within this layer. The resultant CMB model can explain PcP, PKP, and PKKP residuals and has approximately 0.2 km excess core ellipticity, which is in good agreement with inferences from free core nutation observations. Joint inversion yields a peak-to-peak amplitude of CMB topography of about 3 km, and the inversion yields velocity variations of ±5% in the basal layer. The latter suggests a strong trade-off between topography and volumetric heterogeneity, but uncertainty analyses suggest that the variation in core radius can be resolved. The spherical averages of all inverted topographic models suggest that the data are best fit if the actual CMB radius is 1.5 km less than in the Earth reference model used (i.e. the average outer core radius would be 3478 km).     

地球内核及其边界的结构特征和动力学过程

现代地震学展现出了一个复杂的地球内核内部和表面结构.地球内核内部结构的主要特征表现为其地震波速度和衰减呈现各向异性,且各种结构（速度、衰减和各向异性）均呈现东西半球差异,而内核表面的新发现则包括其局部区域存在起伏的地形和固液并存的糊状层.地球内核压缩波速度和衰减均呈现以地球旋转轴为轴的柱对称各向异性,沿地球旋转轴方向传播的压缩波比沿赤道方向传播的压缩波传播更快且衰减更强烈.同时,内核各向异性结构随深度而变化：内核顶部约100~400 km接近各向同性,而在内核最深处300~600 km内则可能存在一个具有不同各向异性特征的内内核.地球内核的东西半球差异表现在多方面：在内核顶部~100 km厚度内,东半球的各向同性速度比西半球快约0.8%,东半球具有较强的衰减（Q=250）,而西半球则具有较弱的衰减（Q=600）;西半球的顶部各向同性层厚度约为100 km,而东半球顶部各向同性层厚度则约为400 km;在各向同性层底下,西半球具有较强的各向异性（~4%）,而东半球则具有较弱的各向异性（~0.7%）.地球内核边界在菲律宾海、黄海、西太平洋以及中美洲下方存在1~14 km高的地形起伏,在鄂霍次克海西南部下方存在4~8 km厚的糊状层.地球内核的这些新发现引发了对许多可能的新物理机制的探讨,也促使我们重新评估我们对外核成分、外核热化学对流、内核凝固过程和地球磁场驱动力的认识.这些结果表明内核凝固过程和地球磁场的热和化学驱动力远比传统观念认为的横向均匀分布复杂得多.内核西半球可能不断凝固并释放潜热和轻元素,而东半球则可能不断熔化并吸收潜热和轻元素,外核对流的驱动力在东西半球可能截然不同,甚至呈现相反方向.这些凝固与熔化交替过程也发生在局部地形起伏区域.在糊状层区域,地球内核凝固释放潜热和化学能,而在大部分无糊状地区,内核凝固只释放潜热.

     

利用远震地震波形畸变现象探测地幔低速体

地幔低速异常体的精确探测对于研究热点/地幔柱的成因至关重要.但是,由于观测手段以及波前愈合现象的影响,远震走时层析成像在探测地球深部的低速异常体时具有极大的限制.因此,越来越多的学者利用地震波穿过低速体后产生的绕射波和直达波干涉造成的波形畸变信息来研究地幔低速体.本文通过三维地震数值模拟来拟合长白山、黄石公园地区宽频地震记录,分析绕射波和直达波相互干扰造成的波形畸变现象.模拟结果显示,绕射波/波形畸变对于探测地幔深处的低速体具有较高分辨率;目前常用的基于互相关测量的有限频层析成像方法对于深部低速体速度异常低估现象明显.如果将有限频层析成像结果在深部的速度异常值放大到原来的2倍,数值模拟可以很好的拟合观测到的地震波形畸变现象.通过对长白山地区、黄石公园地区记录的地震波形畸变现象的拟合,我们进一步确认了长白山、黄石地区下方分别在400~600 km、700~1000 km处存在地幔低速体,并对低速体速度异常值进行了矫正.

     

Isomer specific determination of hexabromocyclododecanes (HBCDs) in small cetaceans from the South China Sea--Levels and temporal variation

A diastereoisomer (alpha, beta, and gamma) specific analytical method for measuring hexabromocyclododecanes (HBCDs) was developed using liquid chromatography-tandem mass spectrometry (LC-MS-MS). The method developed was applied to analyze blubber of small cetaceans to divulge the distribution and temporal variation of HBCDs in the Asian marine environment. HBCDs were detected in all the blubber samples of finless porpoises (Neophocaena phocaenoides) and Indo-Pacific humpback dolphins (Sousa chinensis) collected from the South China Sea during 1990-2001. Concentrations of HBCDs were higher in humpback dolphins (31-380 ng/g lipid) than in finless porpoises (4.7-55 ng/g lipid), which can be attributed to habitat differences. Average concentrations of alpha-HBCD in finless porpoises increased from 9.5 ng/g lipid in 1990 to 35 ng/g lipid in 2000/01. alpha-HBCD was predominant among the three isomers with some exceptions in finless porpoises collected in 1990. This is the first report on HBCDs contamination in marine mammals from Southeast Asia.     

Assessing the temporal variance of evapotranspiration considering climate and catchment storage factors

Understanding the temporal variance of evapotranspiration (ET) at the catchment scale remains a challenging task, because ET variance results from the complex interactions among climate, soil, vegetation, groundwater and human activities. This study extends the framework for ET variance analysis of Koster and Suarez (1999) by incorporating the water balance and the Budyko hypothesis. ET variance is decomposed into the variance/covariance of precipitation, potential ET, and catchment storage change. The contributions to ET variance from those components are quantified by long-term climate conditions (i.e., precipitation and potential ET) and catchment properties through the Budyko equation. It is found that climate determines ET variance under cool-wet, hot-dry and hot-wet conditions; while both catchment storage change and climate together control ET variance under cool-dry conditions. Thus the major factors of ET variance can be categorized based on the conditions of climate and catchment storage change. To demonstrate the analysis, both the inter- and intra-annul ET variances are assessed in the Murray-Darling Basin, and it is found that the framework corrects the over-estimation of ET variance in the arid basin. This study provides an extended theoretical framework to assess ET temporal variance under the impacts from both climate and storage change at the catchment scale.     

利用GRACE时变重力场探测2010年中国西南干旱陆地水储量变化

GRACE(Gravity Recovery And Climate Experiment)卫星计划为监测陆地水储量变化提供了有效技术手段.本文采用2003至2010年共计8年的GRACE月重力场模型反演中国西南区域陆地水储量变化,与GLDAS(Global Land Data Assimilation System)全球水文模型进行对比分析,其结果在时空分布上均符合较好,同时在2009年秋至2010年春该区域陆地水储量均呈现明显减少,与该时段云贵川三省的干旱事件相一致;比较分析了2009年秋至2010年春GRACE反演陆地水储量变化与TRMM(Tropical Rainfall Measuring Mission)合成数据计算的月降雨量的时空分布,两组结果均与西南干旱事件对应时段与区域十分吻合;对近8年的陆地水储量变化与月降雨量数据进行相关性分析,其结果表明陆地水储量变化与降雨量强相关,即降雨量是导致陆地水储量变化的主要因素;分析该区域地表温度变化,结果显示2009年9月至2010年3月地表温度均比历史同期高,地表温度的升高加剧了陆地水储量的减少.     

Sediment dynamics and temporal variation of runoff in the Yom River,Thailand

The Yom River is one of the four major sediment sources to the Chao Phraya River in Thailand. Human activities and changes in climate over the past six decades may have affected the discharge and sediment load to some extent. In the current study, the river discharge and sediment characteristics in the mainstream of the Yom River were investigated using the field observation data from 2011 to 2013 and the historical river flow and sediment data from 1954 to 2014 at six hydrological stations operated by the Royal Irrigation Department of Thailand (RID). The non-parametric Mann-Kendall test and double mass curve were used to analyze the sediment dynamics and temporal changes in the discharge of the Yom River. The results revealed that the sediment was mainly transported in suspension, and the bed-to-suspended sediment loads ratio varied between 0 and 0.05. The daily suspended sediment load (SSL) in the upper and middle basins had a strong correlation with the daily discharge and could be represented by power equations with coefficients of determination higher than 0.8. The daily suspended sediment load in the lower basin did not directly depend on the corresponding discharge because of the reduction in river slope and water diversion by irrigation projects. It also appeared that the river discharges and sediment loads were mainly influenced by climate variation (floods and droughts). Moreover, the average sediment transport of the upper, middle, and lower reaches were 0.57, 0.71, and 0.35 million t/y, respectively. The sediment load in the lower basin decreased more than 50% as a result of changes in the river gradient (from mountainous to floodplain areas). The results from sediment analysis also indicated that the construction of the Mae Yom Barrage, the longest diversion dam in Thailand, and land-use changes did not significantly affect the sediment load along the Yom River.     

华北地震时空多重分形的时间演化特征研究

应用分形理论,采用扩充数盒子方法EBA对华北地区1960~ 1982年发生的地震时间分布进行了多重分形谱计算。结果表明,在这期间该地区的几次较强地震在分形谱上都有明显的反映,强震年份的分形谱发生较大幅度变化,正q区D_q 值下降,负q区D_q 值上升,具有一定的预报意义。文中进而计算了几次较强地震的地震序列(1966年邢台、1969年渤海、1975年海城、1976年唐山)的空间多重分形谱,研究其随时间的演化现象,表明强震前后分形谱变动幅度较大,震群型地震序列和主震型地震序列的分形谱呈现出不同的变化趋势。     

Monitoring of organic contaminants in sediments from the Korean coast: Spatial distribution and temporal trends (2001-2007)

Spatial and temporal trends of organic contaminants in sediments along the Korean coast were estimated through a nationwide environmental monitoring program from 2001 to 2007. The concentrations of organic contaminants in sediments along the Korean coast were relatively low or moderate compared with foreign studies. The mean detection frequencies of organic contaminants during the seven years were highest for PAHs and PCDD/Fs, followed by PCBs, DDTs, TBT and HCHs in decreasing order. Based on published sediment quality guidelines, the ecological risks of persistent organic pollutants in sediments along the Korean coast were low, despite exceedances of the ERL at 2-6 sites for DDTs, and the TEL at 9-18 sites for PCDD/Fs. Nonparametric tests to assess temporal trends revealed significant decreasing trends for PCBs and PCDD/Fs at four and three sites, respectively (p < 0.05). These results reflect the effects of regulations on the use of those contaminants.     

民航气象观测的中国中东部大气边界层廓线数据的评估与应用

本文从民航气象报告(AMDAR)数据库中提取了2008—2019年期间我国中东部18个机场飞机起飞和降落期间的大气边界层(ABL)廓线数据,并利用常规探空数据评估了不同季节和高度范围的AMDAR数据准确性及其随探空站与机场距离和飞行状态的变化.与常规探空数据相比,低层(P>850 hPa)的AMDAR温度和风分量的均方根误差(RMSE)均比较高层(850~500 hPa)小,且各高度层上的温度和风分量的RMSE都随间隔距离的增加而增大.相对而言,飞行状态对AMDAR数据准确性的影响很小.在间隔距离小于50 km且P>850 hPa高度层范围内,温度、纬向风和经向风的平均RMSE分别为0.68~0.83 K、1.48~1.62 m·s^-1和1.73~1.81 m·s^-1.相比于RMSE,对应的平均偏差都显著更小,且随间隔距离无明显变化.数据应用示例表明,AMDAR廓线数据由于其较高的时间分辨率,不仅可以反映ABL结构的日变化特征,还可用于研究ABL局地环流特征.基于18个机场AMDAR廓线数据分析发现,我国远内陆、干燥地区大气边界层高度(ABLH)具有较强的日变化,尤其是在春夏两季,而沿海和近内陆地区ABLH的日变化强度和季节差异都较小.

     

Spatial distribution and temporal variation of reference evapotranspiration in arid and semi‐arid regions of Iran

Analysis of spatial and temporal variations of reference evapotranspiration (ET_o) is important in arid and semi‐arid regions where water resources are limited. The main aim of this study was to analyse the spatial distribution and the annual, seasonal and monthly trends of the Penman–Monteith ET_o for 21 stations in the arid and semi‐arid regions of Iran. Three statistical tests the Mann‐Kendall, Sen's slope estimator and linear regression were used for the analysis. The analysis revealed that ET_o increased from January to July and deceased from July to December at almost all stations. Additionally, higher annual ET_o values were found in the southeast of the study region and lower values in the northwest of the region. Although the results showed both positive and negative trends in annual ET_o series, ET_o generally increased, significantly so in six (～30%) of the stations. Analysis of the impacts of meteorological variables on the temporal trends of ET_o indicated that the increasing trend of ET_o was most likely due to a significant increase in minimum air temperature, while decreasing trend of ET_o was mainly caused by a significant decrease in wind speed. At the sites where increasing ET_o trends were statistically significant, the rate of increase varied from (+)8·36 mm/year at Mashhad station to (+)31·68 mm/year at Iranshahr station. On average, an increasing trend of (+)4·42 mm/year was obtained for the whole study area during the last four decades. Seasonal and monthly ET_o have also tended to increase at the majority of the stations. The greatest numbers of significant trends were observed in winter on the seasonal time‐scale and in September on the monthly time‐scale. Copyright © 2011 John Wiley & Sons, Ltd.     

Levels and temporal trends of PCDD/PCDFs and non-ortho PCBs in ringed seals from East Greenland

The levels and temporal trend of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) and non-ortho substituted PCBs (c-PCBs, i.e. CB77, CB126 and CB169) were determined in ringed seal blubber from central East Greenland collected in 1986, 1994, 1999 and 2003, respectively. Since 1986 the concentrations of PCDDs, PCDFs and c-PCBs all show a decreasing trend. The annual decreases were estimated to 5.2% and 5.3% for pg/g WHO-TEQ ww of PCDD/Fs and c-PCB, respectively. The annual median concentrations of PCDDs ranged from 5.4 to 24.4 pg/g WHO-TEQ ww and those of PCDFs from 2.5 to 5.1 pg/g WHO-TEQ ww. Compared to PCDD/Fs concentrations in ringed seals from other Arctic areas the levels of PCDD/Fs found in 1986 were the highest recorded. The annual median concentrations of c-PCBs decreased 24.2 to 9.1 pg/g WHO-TEQ ww. The levels of c-PCBs observed in 1986 are similar to levels found in ringed seals from Svalbard in 1990 and from eastern Hudson Bay in 1989–1992. The dominant and most TEQ-contributing PCDD congener was 1,2,3,7,8-PeCDD. CB126 was the dominating and most TEQ-contributing c-PCB congener. The concentrations of PCDDs, PCDFs and c-PCBs were highly significantly inter-correlated. Principal component analysis of the PCDD/PCDF congeners and c-PCBs was performed to analyse the pattern of compounds during time.     

A note for the mechanism of high-frequency oscillation instability resulted from absorbing boundary conditions

In this paper the explanation of the mechanism of high-frequency oscillation instability resulted from absorbing boundary conditions is further improved. And we analytically prove the proposition that for one dimensional discrete model of elastic wave motion, the module of reflection factor will be greater than 1 in high frequency band when artificial wave velocity is greater than 1.5 times the ratio of discrete space step to discrete time step. Based on the proof, the frequency band in which instability occurs is discussed in detail, showing such high-frequency waves are meaningless for the numerical simulation of wave motion.     

Timing of the terrestrial Permian-Triassic boundary biotic crisis: Implications from U-Pb dating of authigenic zircons

The Late Permian to Early Triassic transition represents one of the most important Phanerozoic mass extinction episodes. The cause of this event is still in debate between catastrophic and gradual mechanisms. This study uses the U-Pb method on zircons from the uppermost Permian/lowermost Triassic clay deposits at Chahe (Guizhou Province, SW China) to examine time constraints for this event. The results of both this and previous studies show that the ages of Bed 68a and 68c (the upper clay bed of the terrestrial Permian-Triassic boundary (PTB)) respectively are 252.6±2.8 and 247.5±2.8 Ma. This age (within the margin of error) almost accords with the upper clay bed (Bed 28) age of Meishan and the eruption age of Tunguss Basalt, and is so far the most accurate age obtained from terrestrial PTB. The claystone of Bed 68 was formed in the earliest Triassic. The biotic crisis occurred at nearly the same time in terrestrial and marine environments during Permian-Triassic interval; however the extinction patterns and processes are different. The extinction pattern of the terrestrial plants shows a major decline at the PTB after long-term evolution, followed by a retarded extinction of the relicts in the earliest Triassic.     

Accuracy of deflections of vertical determined from discrete solution of the geodetic boundary value problem

Summary The general problem of determining the figure of the earth leads to the solution of the geodetic boundary value problem. By its discrete approximation we obtain the discrete disturbing potential that maintains all properties of the original problem. Thus, the discrete approximation of the disturbing potential can be used in studying the behaviour of the earth's gravity field outside the disturbing masses. The deflections of the vertical are one of the quantities describing the behaviour of the earth's gravity field. A method for their computation from the discrete solution of the geodetic boundary value problem is put forth and estimates for its accuracy are given.     

Regional stresses along the Eurasia-Africa plate boundary derived from focal mechanisms of large earthquakes

The focal mechanism solutions of 83 European earthquakes withM>6, selected from a total of 140, have been used to derive the directions of the principal axes of stress along the plate boundary between Eurasia and Africa from the Azores islands to the Caucasus mountains. Along most of the region, the horizontalP-axes are at an angle of 45° to 90° with the trend of the plate boundary. HorizontalT-axes are concentrated in central Italy and northern Greece in association with normal faulting. Large strike-slip motion of right-lateral character takes place at the center of the Azores-Gibraltar fault and the North Anatolian fault. From Gibraltar to the Caucasus the boundary is complicated by the presence of secondary blocks and zones of extended deformations with earthquakes spread over wide areas. Intermediate and deep earthquakes are present at four areas with arc-like structure, namely, Gibraltar, Sicily-Calabria, Hellenic arc and Carpathians.     

High-resolution cyclostratigraphy of geochemical records from Permo-Triassic boundary section of Dongpan, southwestern Guangxi, South China

Spectral analysis is applied to geochemical data from a deep-water Permian-Triassic Boundary (PTB) section across PTB in Dongpan, Guangxi for high-resolution cyclostratigraphic research. The characteristics of the Milankovitch Cycles have been well recorded in the strata of the Dongpan section. Spectrum on Ce/La records shows especially that the ratios among its three preponderant cycles in the section are 5:2:1, which well displays the phenomenon of the sedimentary cycles driven by orbital force. The Milankovitch theory is further confirmed by the Paleozoic records.     

Topography on the D′′ region from analysis of a thin dense layer beneath a convecting cell

In this study, we examine the development of topography on a thin dense layer at the base of the lower mantle. The effect of the convecting mantle above is represented as a traction acting on the upper surface of the layer. Topography on the layer boundaries is predicted by a balance of dynamic flow stress and external traction. The nature of boundary topography depends on the magnitude of the driving tractions and the density variation within the layer. If we assume that the layer density is greatest beneath areas of mantle downwelling and decreases to a minimum beneath areas of mantle upwelling (the layer is thermally coupled to the convection in the overlying mantle) then its upper boundary develops a cusp-like peak beneath the upwelling mantle. The height of this peak is potentially much greater than the layer thickness. If, however, the layers are effectively coupled by viscous shear then internal density gradients of the opposite sign may be established. In this case, we observe solutions where the layer is completely swept away beneath areas of mantle downwelling leaving steep-sided ‘islands’ of dense material. This mechanism therefore provides a possible explanation for steep-sided anomalously slow regions at the base of the mantle observed by seismic methods (e.g. beneath south Africa) or for discrete ultralow velocity zones detected at the core-mantle boundary beneath locations of surface hotspots. The magnitude of the upper boundary driving tractions compared to the density gradient within the layer is the key parameter that determines the nature of flow in, and consequently boundary topography of, the layer. The deflection of the core-mantle boundary is small compared with that of the top of the dense layer, but a change in sign of the ratio of these deflections is observed as the magnitude of the driving tractions changes relative to the magnitude of the internal density gradient. We compare seismic measurements of core-mantle boundary topography and D′′ topography with the predictions of this model in an attempt to constrain model parameters, but no clear correlation seems to exist between D′′ thickness and CMB topography.     

基于sP前驱震相叠加研究南美中部地区岩石圈-软流圈边界形态

岩石圈-软流圈边界（lithosphere-asthenosphere boundary）是上地幔内具有负速度梯度的地震波速度界面.对俯冲带区域LAB开展地震学探测有助于进一步认识岩石圈和软流圈的相互作用以及与板块俯冲有关的地球动力学过程.本文收集了2006-2012年发生于南美中部地区4个深源地震的垂向宽频带波形资料,利用线性倾斜叠加处理得到了相对走时-慢度域的灰度图,并成功提取了sP在LAB底反射的前驱震相s_LABP.基于改进的一维速度模型IASP91-SA计算得出了6个s_LABP震相反射点的水平分布,并划分为西部（Ⅰ）和东部（Ⅱ）分区：分区Ⅰ内LAB深度位于60~63 km,平均深度为61 km,起伏为3 km;分区Ⅱ内LAB深度位于78~82 km,平均深度为80 km,起伏为4 km.研究结果显示南美中部地区LAB深度自西向东呈变大的趋势,这可能反映了大陆岩石圈受改造程度的差异.我们推测在靠近海沟的地区,软流圈内部分熔融程度较高且熔体较为富集,对大陆岩石圈的侵蚀作用较强;在远离海沟的地区,软流圈内部分熔融程度降低且熔体分布减少,对大陆岩石圈的侵蚀作用减弱.

     

基于MODIS数据的2000-2010年乌梁素海"黄苔"时空变化

利用2000-2010年每年5-9月MODIS数据根据比值算法提取乌梁素海湖区黄苔的面积和空间分布信息并进行统计分析,探求乌梁素海黄苔产生的时空分布规律及特征,从而为黄苔的预防和治理提供支持.结果表明:(1)黄苔面积变化的年际和月际特征方面,2000、2001、2005、2006、2008、2010年黄苔面积超过了多年平均值(24 km2).5—7月份黄苔面积较小,保持在20 km2左右;8月黄苔面积迅速增长(约28 km2),9月黄苔面积最大,达到40 km2左右.(2)黄苔发生频率方面,2001年黄苔的规模和频率最高,发生频率达到0.58;2005、2006、2010年次之,发生频率在0.25附近波动(多年年均黄苔暴发频率为0.19);其他年份黄苔的发生频率处于低于0.10的水平.黄苔发生规模较大、次数较多的月份集中在8、9月,发生频率分别达到0.27、0.52,超过多年月均黄苔暴发频率0.19;其他月份黄苔的发生频率处于低于0.10的水平.(3)黄苔出现的空间分布方面,西大滩为东大滩的北部至中部,以及乌梁素海南部明水区排干口附近的西部沿岸是黄苔出现频率较高的区域.(4)2个月前的日均温度、降雨和营养盐浓度及当月风速与黄苔的产生具有极显著相关性;营养盐含量(TN、TP)的空间分布与黄苔的空间分布表现出较好的相关性.乌梁素海黄苔面积的年际变化受人类活动特别是生态补水的影响明显.