Source properties of the 1997–98 Central Italy earthquake sequence from inversion of long-period and broad-band seismograms

We study source properties of the main earthquakes of the 1997–98 Umbria-Marche (central Italy) sequence by analysis of regional-distanceand teleseismic long period and broadband seismograms recorded by MedNet and IRIS/GSN stations. We use a modified Harvardcentroid-moment tensor (CMT) algorithm to allow inversion of long period waveforms, primarily Rayleigh and Love waves, for small earthquakes (4.2 M_W 5.5) at local to regional distances (<15^°). For the seven largest earthquakes (M_W>5.2) moment tensors derived from local and regional data agree well with those determined using teleseismic waveforms and standard methods of analysis. We also determine moment tensors for a foreshock and 12 other aftershocks, that were too small for global analysis. Focal depth and rupture propagation are analyzed for three largest shocks by inversion of teleseismic broadband body waves. The earthquakes are generally located at shallow depth (5 km or shallower) and are characterized by normal faulting mechanisms, with a NE-SW tension axis. The presumed principal fault plane dips at a shallow angle towards the SW. Only one of the events analyzed has an entirely different faulting geometry, indicating instead right-lateral strike-slip motion on a plane approximately E-W, or left-lateral faulting on a N-S plane. The other significant exception to the regular pattern of mechanisms is represented by the March 26, 1998, event, located at 51 km depth. Its connection with the shallow earthquake sequence is unclear and intriguing. The time evolution of the seismic sequence is unusual,with the mainshock accounting for only approximately 50% of the total moment release. The broadband teleseismic waveforms of the main, September 26, 09:40, earthquake are very complicated for the size of the event and suggest a complex rupture. In our favored source model, rupture initiated at 5 km depth, propagated updip and was followed, 3 seconds later, by a shallower subevent with a slightly rotated mechanism.     

湿润亚热带典型白云岩流域的岩溶无机碳汇强度

岩溶作用积极参与全球碳循环过程,进一步弄清岩溶碳汇机制对于精确评估碳循环过程意义重大。中国碳酸盐岩分布面积广大,尤以西南地区最为广泛与集中,虽然本区已开展了较多以石灰岩为主流域的岩溶碳汇研究,但是与白云岩流域相关的研究较为薄弱,这不利于精确评价中国岩溶碳汇潜力和服务于国家碳中和战略。为此,本文以中国西南岩溶区内具代表性的贵州施秉黄洲河白云岩流域作为研究区,对湿润亚热带典型白云岩小流域的水-岩-气相互作用及其无机岩溶碳汇进行了为期1年(2018年5月至2019年5月)的水文水化学自动监测研究。获得了黄洲河白云岩流域的降雨量、流量、水温、电导率、pH值等高时间分辨率的连续监测数据,并采用化学热力学模型计算了方解石与白云石饱和指数及河水中CO₂分压等水化学指标。结果显示:(1)该白云岩流域属典型的山区雨源型河流,洪水流量过程虽然呈现显著的暴涨暴落特征,但是流域出口河水的化学动态仍受水化学稳态行为的控制;(2)流域上游高原面上浅切割地表河两侧发育溶隙型含水介质,中下游排水系统为地表河,由于地表河水中CO₂的释放,使该流域出口河水的方解石饱和指数较高,并伴有河床钙华沉积;(3)采用白云石开放系统化学热力学模型,结合不同土地利用类型的土壤CO₂数据,计算出泄流于地表河的岩溶地下水 {\mathrm{HCO}}_3^{-}浓度为5.1 mmol/L,该值要高于黄洲河出口河水 {\mathrm{HCO}}_3^{-}平均浓度16%,表明该岩溶流域内的地表河因受紊流脱气和水生生物光合作用的影响,水中的溶解无机碳有所降低;(4)由水化学-径流法准确计算了该白云岩流域的岩溶碳汇通量(以CO₂计),为36.43 t/(km²·a),该值并未显著高于中国南方其他碳酸盐岩流域的岩溶碳汇通量,表明白云岩流域覆被中CO₂浓度仍与流域径流量呈反比关系,这两个主控因子的共同作用,使湿润亚热带岩溶流域的岩溶碳汇通量不至于发生较大变化。     

Study of seasonal snow cover influencing the ground thermal regime on western flank of Da Xing'anling Mountains, northeastern China

Although many studies relevant to snow cover and permafrost have focused on alpine, arctic, and subarctic areas, there is still a lack of understanding of the influences of seasonal snow cover on the thermal regime of the soils in permafrost regions in the mid-latitudes and boreal regions, such as that on the western flank of the Da Xing'anling(Hinggan) Mountains, northeastern China. This paper gives a detailed analysis on meteorological data series from 2001 to 2010 provided by the Gen'he Weather Station, which is located in a talik of discontinuous permafrost zone and with sparse meadow on the observation field. It is inferred that snow cover is important for the ground thermal regime in the middle Da Xing'anling Mountains. Snow cover of 10-cm in thickness and five to six months in duration(generally November to next March) can reduce the heat loss from the ground to the atmosphere by 28%, and by 71% if the snow depth increases to 36 cm. Moreover, the occurrence of snow cover resulted in mean annual ground surface temperatures 4.7–8.2 °C higher than the mean annual air temperatures recorded at the Gen'he Weather Station. The beginning date for stable snow cover establishment(SE date) and the initial snow depth(SDi) also had a great influences on the ground freezing process. Heavy snowfall before ground surface freeze-up could postpone and retard the freezing process in Gen'he. As a result, the duration of ground freezing was shortened by at least 20 days and the maximum depth of frost penetration was as much as 90 cm shallower.     

菲律宾海及其邻近海区的水文特征

根据国内外的有关资料,对非律宾海及其邻近的水文特征进行了分析与研究,得出了菲律宾海及其邻近海区的潮汐、海流、海浪、盐度、水温、透明度及水色等水文要素的特点和特征,进一步闸明菲律宾及其邻近海区水文要素的分布特征及季节变化规律,为船舶在海上航行和海洋学研究,提供可靠的参考数据。     

太湖水情特征

根据太湖湖区及环湖河道主要测站的历年水文气象资料,分析探讨了太湖的径流、水位、增减水现象和河湖流向等水情特征,为合理利用和保护湖泊水资源提供依据。     

西洞庭湖沉水植物分布格局对环境因子及水文情势差异的响应

沉水植物是湖泊生态系统的重要组成部分,其生产力和分布格局受环境因子特别是水文情势决定.洞庭湖是长江流域重要的大型通江湖泊,近年来受人为干扰和气候变化影响,水文节律与水质等环境因子发生改变,导致沉水植物出现衰退现象,急需开展科学恢复,因此有必要对洞庭湖沉水植物深入研究.本研究选取西洞庭湖为研究区域,于2018年和2019年夏季调查了12处典型生境、98个样点的沉水植物与水深、透明度等11个环境因子,采用独立样本T检验和冗余分析方法对沉水植物与环境因子的关系进行分析,对比年际水文情势变化的影响.结果表明：1）西洞庭湖沉水植物主要在水深较浅、水质更优、水体更为稳定的半阻隔子湖和自由连通的湖湾区分布,在河道及水位波动较大的区域分布较少,有、无沉水植物分布样点间存在显著差异的环境因子为水深、透明度、底泥总磷和pH;2）在有沉水植物分布的样点,沉水植物生物量与pH、水深和水体总磷呈显著相关关系;3）自然连通的季节性淹没湖泊沉水植物生物量在2018年高于2019年,可能与2019年5—8月沉水植物关键生长期出现的涨水过程有关,持续的高水位对沉水植物的生长产生了不利影响.维持自然水文节律、湖泊生境异质性与自由连通性、健康的水质等是恢复西洞庭沉水植物的关键,建议在水深低于3 m、营养盐浓度适中、流速及风浪较小的湖湾区或半阻隔湖泊开展沉水植物恢复.     

近30年洞庭湖季节性水情变化及其对江湖水量交换变化的响应

江湖水量交换的变化影响着通江湖泊洞庭湖的水情,进而影响湖区社会经济及生态的可持续发展.以洞庭湖城陵矶站、南咀站以及长江干流宜昌站、螺山站1981-2012年逐日水位、流量观测数据为基础,采用单位根检验、方差分析和水位-流量绳套曲线等方法对洞庭湖季节性水情变化特征进行提取,并探究江湖水量交换变化对其产生的影响.研究表明:近30年来洞庭湖水情呈阶段性特征,与相对稳定的1981-2002年相比,2003-2012年湖泊水位总体呈下降趋势,年均水位下降0.43 m;枯、涨、丰、退水期各季水情变化特征为:2003年以后洞庭湖丰水期水位平均下降0.60 m,呈现出"高水不高"现象;退水期水位平均下降1.49 m,退水加快;枯水期水位略有上升,平均上升0.18 m;涨水期水位变化不明显.湖泊退水期水位降幅最为明显,尤其是10月大幅下降,平均下降2.03 m,有提前进入枯水期的趋势.水情变化与江湖水量交换变化密切相关:丰水期,三口(松滋、太平和藕池)分流量减小在一定程度上降低湖泊水位;退水期,三口分流量减小叠加城陵矶出口长江水位下降对洞庭湖产生拉空作用,湖泊出流加快水位被拉低;枯水期,主要是1 3月,城陵矶出口长江水位上升对湖泊顶托作用增强,湖泊出流减缓水位略有抬升.     

Stormy geomorphology: an introduction to the Special Issue

The degree to which the climate change signal can be seen in the increasing frequency and/or magnitude of extreme events forms a key part of the global environmental change agenda. Geomorphology engages with this debate through extending the instrumental record with palaeogeomorphological research; studying resilience and recovery of geomorphic systems under extreme disturbance; documenting the mediation by catchment organisation of transport processes during extreme events; applying new monitoring methods to better understand process‐response systems; and illustrating how process, experimental and modelling insights can be used to define the buffering of geomorphic systems and human assets from the effects of extremes, providing practical outcomes for practitioners. Copyright © 2016 John Wiley & Sons, Ltd.     

Tipping points in Texas rivers

Anticipating geomorphic tipping points requires that we learn from the past. Major geomorphic changes in coastal plain rivers of Texas resulting in river metamorphosis or regime shifts were identified and the major driving factors determined. Eleven such transformations – possible tipping points – were identified from contemporary observations, historical records, and Quaternary reconstructions. Two of the tipping points (between general aggrading and degrading valley states) are associated with reversals in a fundamental system control (sea‐level). One (stable or aggrading versus degrading channels) is associated with an abrupt change in sediment supply due to dam construction, and two others (changes from meandering to anastomosing channel patterns, and different anastomosis styles) are similarly related to changes in sediment supply and/or transport capacity, but with additional elements of historical contingency. Three tipping points are related to avulsions. One, from a regime dominated by re‐occupation of former channels to one dominated by progradation into flood basins, is driven by progressive long‐term filling of incised valleys. Another, nodal avulsions, is triggered by disturbances associated with tectonic uplift or listric faults. The third, avulsions and related valley metamorphosis in unfilled incised valleys, is due to fundamental dynamical instabilities within the fluvial system. This synthesis and analysis suggests that geomorphic tipping points are sometimes associated with general extrinsic or intrinsic (to the fluvial system) environmental change, independent of any disturbances or instabilities. Others are associated with natural (e.g. tectonic) or human (dams) disturbances, and still others with intrinsic geomorphic instabilities. This suggests future tipping points will be equally diverse with respect to their drivers and dynamics. Copyright © 2018 John Wiley & Sons, Ltd.