黄河北岸兰州段丘陵区土壤生态化学计量与空间变异北大核心CSCD

为探究黄河北岸兰州段丘陵区土壤碳、氮、磷含量及化学计量比的空间变异特征,基于12个样地数据,采集1 m剖面内不同发生层土壤进行土壤含水率(SWC)、pH、有机碳(SOC)、全氮(STN)和全磷(STP)的测定。分析了研究区土壤碳、氮、磷含量及其生态化学计量特征在植被和土层上的差异及空间变异特征。结果表明:(1)研究区SOC、STN、STP均值分别为4.53、0.74、0.13 g·kg^(-1),C∶N、C∶P、N∶P均值分别为7.85、55.17、8.40;(2)研究区SOC、STN、C∶N和N∶P的块基比C_(0)/(C_(0)+C)<0.25,主要受气候、植被、地形等自然因素影响;STP、C∶P则C/(C+C)>0.75,主要受退耕还草等人为因素影响;(3)研究区土壤整体受到氮限制,与此同时有机碳更为匮乏,磷则在短期内不会成为限制因子。在黄河流域兰州段北岸丘陵区北岸的生态治理与恢复中,应重视有机肥和氮肥的配置与施加,同时需减少人为干扰,这对于植被的快速恢复与养分的固定具有重要意义。     

黑龙江省兴凯湖平原土壤pH值时空变化特征分析

土壤酸碱度是影响土壤化学和生物学性质的一个重要因素,会对土地生产力产生重要影响.为掌握兴凯湖平原土壤pH值近年来变化状况,采用空间统计方法,基于全国第二次土壤普查及2019年土壤pH值数据,对工作区近30年土壤pH值时空变化特征进行了分析.结果表明：兴凯湖平原土壤pH值总体呈下降趋势,但整体上土壤pH值变幅不大,主要集中在±0.5～±1区间.从土地利用类型上来看,兴凯湖平原耕地区土壤pH值均呈下降趋势,西部的林地区pH值整体保持不变或略有上升.在兴凯湖平原4种主要土壤类型中,除暗棕壤外,其他3类土壤在近30年间pH平均值均呈下降趋势.因此需要对工作区土壤pH值变化的原因进行多因素、定量分析,以进一步明确具体原因.     

A parametric study of the required seating length for bridge decks during earthquake

During the recent major earthquakes, some bridges suffered severe damage due to the pull-off-and-drop collapse of their decks. This is due to the large differential movements of the adjacent spans of bridges during strong shaking compared to the seating lengths provided. The differential movements are primarily due to the different vibration properties of adjacent spans and non-uniform ground excitations at the bridge supports. This paper analyses the effects of various bridge and ground motion parameters on the required seating lengths for bridge decks to prevent the pull-off-and-drop collapse. The random vibration method is used in the analysis. A two-span bridge model with different span lengths and vibration frequencies and subjected to various spatially varying ground excitations is analysed. Non-uniform spatial ground motions are modelled by the filtered Tajimi–Kanai power spectral density function and an empirical coherency function. Ground motions with different intensities, different cross-correlations and different site conditions are considered in the study. The required seating lengths for bridge decks are calculated. Numerical results are presented and discussed with respect to different bridge vibration and ground motion properties. © 1998 John Wiley & Sons, Ltd.     

变化环境下半干旱草原流域径流变化特征及其影响因子定量分析

地表水文过程在气候波动和人类活动的作用下发生了不可忽视的变化,这种变化对于内蒙古半干旱草原流域来说更为显著。采用改进的M-K趋势检验法、双累积曲线法、累积距平法和小波变换法对典型半干旱草原流域——锡林河流域1963～2015年径流序列的变化特征进行了剖析,并以社会经济指标量化流域内人类活动影响及细化降水特征因素,运用统计检验和主成分分析进行了变化环境下流域径流主要影响因素的定量分析。结果表明,锡林河流域径流量在水文年、季尺度呈现显著(p0.05)减少趋势;流域径流的突变年份为1998年,在其前后影响径流变化的主导因素发生变化,1998年以后径流受到人类活动和气候变化的双重影响;水文年和枯水季径流序列均存在6年、25年左右的周期,而丰水季径流不存在显著周期变化;尽管气候因素中的降水、蒸发、相对湿度及所有人类活动因素与径流显著相关而影响径流,但其中人类活动为径流变化的主导因素。     

基于Seislet-TV双正则化约束的地震随机噪声压制方法

人工地震数据总是受到随机噪声的干扰,地震数据时-空变的特性使得常规去噪方法处理效果并不理想,容易导致有效信号的损失.目前广泛应用的预测滤波类方法存在处理时变数据能力不足的问题.随着压缩感知理论的不断完善,稀疏变换阈值算法能够解决时变地震数据噪声压制问题,但是常规的稀疏变换方法,如傅里叶变换,小波变换等,并不是特殊针对地震数据设计的,很难提供地震数据最佳的压缩特征,同时,常规阈值算法容易导致去噪结果过于平滑.因此开发更加有效的时-空变地震数据信噪分离方法具有重要的工业价值.本文将地震数据信噪分离问题归纳为数学基追踪问题,在压缩感知理论框架下,利用特殊针对地震数据设计的VD-seislet稀疏变换方法,结合全变差(TV)算法,构建seislet-TV双正则化条件,并利用分裂Bregman迭代算法求解约束最优化问题,实现地震数据的有效信噪分离.通过理论模型和实际数据测试本文方法,并且与工业标准FXdecon方法进行比较,结果表明基于seislet-TV双正则化约束条件的迭代方法能够更加有效地保护时-空变地震信号,压制地震数据中的强随机噪声.     

短期重现性地磁日变化感应电流集中分布与地震关系初步研究

通过分析台站之间地磁垂直分量日变化相关性,研究中国大陆南北带地区90°E～110°E,2009～2018年地磁日变化感应电流线状集中分布现象与M_S≥6.0地震关系。统计发现,我国每月平均出现约2次感应电流集中分布现象,且10天左右内有原地重复出现现象——感应电流集中分布重叠异常,原地重现的集中分布感应电流日期间隔一般在10天左右以内,原地重现日期一般不连续,重现次数一般仅有2次。研究发现,我国南北带2009～2018年共出现重叠异常23次,20次重叠异常出现后18个月内重叠段发生M_S≥6.0地震,对应率高达87%,地震基本发生在重叠段端部250km以内,重叠段发生2次M_S≥6.0地震的比例较低,约35%;重叠段2个端部均发生M_S≥6.0地震的比例极小,仅约9%。部分重叠段走向与已经发现的上地幔和地壳高导带埋深走向基本一致,推测重叠异常发生在上地幔和地壳高导带附近,是来自上地幔和地壳内高导带附近的地震异常信息。目前已知,中国大陆强地震主要发生在上地幔高导层的隆起区一带及其壳内高导层发育地区,深部电性测深结果表明,大震易于发生在电导率急剧变化的梯度带,但这些地区何时发生地震还未得知,本文发现的重叠异常解释了这些地区1～2年内是否发生地震这一问题。     

北天山中段时变重力场离散小波多尺度分解

利用北天山地区2016～2019年观测的4期流动重力观测资料,分析研究一年尺度的重力场动态变化特征,并利用小波分析方法,将不同场源深度的重力异常进行分离。通过功率谱分析,获取各阶小波重力细节对应的场源深度。研究结果表明,2017年8月9日精河M_S6.6地震前,震中位于负值集中区,四阶小波重力细节显示震中附近出现明显的四象限分布;2020年1月16日库车M_S5.6地震前,震中位于负值区,小波重力细节整体量值较小;功率谱估算的场源近似深度与2次地震的震源深度相近。     

陕北定边盐湖卤水水位历史演变及其气候指示意义(1265—1949年)

受自然环境和资料限制的原因,中国西北的季风边缘区内的湖泊千-百年时间尺度演变序列重建工作存在不足.历史文献中的湖泊演变信息丰富,有效利用可较好地解决这一问题.本文通过梳理历史文献记录的定边盐湖采盐方式资料,反演历史时期定边盐湖卤水水位,重建公元1265-1949年陕北定边盐湖卤水水位演变序列,该序列较好地指示出公元12...     

VARIATION OF THE TRAVEL TIME DIFFERENCE BETWEEN TELESEISMIC PS CONVERTED WAVE AND PP TRANSMITTED WAVE IN FOCAL REGION BEFORE AND AFTER WENCHUAN MS8.0 EARTHQUAKE

When P waves from distant earthquakes meet a velocity discontinuity in the earth's crust and upper mantle, they give rise to a series of converted PS waves besides PP refracted waves. It is possible to monitor the variation of the physical properties of the medium in the limited formation space above the transition zone of the seismogenic zone by measuring the time difference between the teleseismic PS converted wave and the first arrival P wave, that is, time-variation Δt_PS=t_PS-t_P. The advantage of this method is that the transition point of the teleseismic source with similar source is relatively stable at the transition interface, and the accuracy of the measured relative time is high, and the change of the medium in a small range of the seismogenic zone above the conversion interface can be monitored. This paper studies the variation of the travel time difference Δt_PS in focal region before and after Wenchuan M_S8.0 earthquake. We select 2001 to 2012 as the research period, use teleseismic waveforms which occurred in the southern region of Sumatra and Hindu Kush area recorded by Sichuan station YZP and JJS. These teleseisms satisfy 5.0 ≤ M ≤ 6.5, and their waveform signal-to-noise ratio is high with clear initial P-wave motion. The epicentral distance of teleseisms is less than 3 degrees. Then we obtain the variation of the travel time difference Δt_PS between teleseismic PS converted wave and PP transmitted wave recorded during the study period in the two stations. The results show that there is a slow increasing trend of Δt_PS before 2006, and an obvious low value process of Δt_PS appeared in the period about 2 years before the Wenchuan earthquake. The maximum decline was about 0.2~0.3s, more than 4~5 times the measurement error. The low value has a certain degree of return about 2~3 months before the earthquake. The change of arrival time difference indicates that the medium is in different states in different periods of seismogenic process. The sharp decrease in Δt_PS from 2006 to January 2008 may be due to the strong disturbance caused by the stress accumulation of the medium. At this stage, the velocity of P wave and S wave increases with the increase of stress, and the increase of S wave velocity will result in the decrease of Δt_PS. The change of Δt_PS is greatly affected by S wave velocity, so Δt_PS appears to decrease rapidly. Regarding the low value that has a certain degree of return about 2~3 months before the Wenchuan earthquake, the possible reason is that the release of stress is much higher than the accumulation of stress in meta-instability stage. At this stage, the velocity of S wave decreases and the decrease of S wave causes Δt_PS to increase. Then, the Wenchuan earthquake of magnitude 8.0 occurred. It is shown that the teleseismic converted wave method in this paper can monitor the variation of medium's wave velocity before large earthquakes, and it has a good prospect in seismic monitoring and worth further experimental study.     

2014年康定M_S6.3级地震前后的重力变化特征

利用四川地区2010年8月至2015年3月的流动重力观测数据作出重力场等值线变化图像,研究分析2014年康定MS6.3级地震前后的重力变化特征,结果表明:(1)三岔口地区长期保持区域性重力正值变化异常及重力变化高梯度带;(2)在康定MS6.3级地震前后三岔口地区的区域性重力正值变化异常及重力变化高梯度带变化并不是特别显著;(3)在康定MS6.3级地震前震区重力变化等值线出现四象限区域或者类似于四象限区域的分布特征;(4)康定MS6.3级地震发震位置处于0值线附近且0值线在此处发生明显的转折、畸变;(5)康定MS6.3级地震发生在重力减小后回调增加的过程中,震后重力继续回调增加。