Factors responsible for the high position of the Siberian platform

A new model accounting for the origin of anomalously high elevations of the Siberian platform (SP) topography is presented. It is shown that the traditional interpretation of these topographic anomalies is at variance with the available evidence for the geological history of the SP development. The ideas elaborated in the paper are based on the concept of the formation of a mantle plume that has led to the supply of large volumes of molten material into the upper crust and surface basalt eruptions. A new approach is proposed for the construction of a density model of the Siberian upper mantle. A density model of the crust based on the available seismic and petrological data is constructed at the first stage. The calculated anomalous gravity field produced by this model is then subtracted from the observed field. The resulting residual mantle anomalies are used, together with seismological data, for the construction of an upper mantle density model. The formation of the present SP topography is shown to have been controlled by the thickening of the crust due to underplating caused by the development of a giant mantle plume at 251 Ma.     

Seismicity in the outer rise offshore southern Chile: Indication of fluid effects in crust and mantle

We examine the micro-earthquake seismicity recorded by two temporary arrays of ocean bottom seismometers on the outer rise offshore southern Chile on young oceanic plate of ages 14 Ma and 6 Ma, respectively. The arrays were in operation from December 2004–January 2005 and consisted of 17 instruments and 12 instruments, respectively. Approximately 10 locatable events per day were recorded by each of the arrays. The catalogue, which is complete for magnitudes above 1.2–1.5, is characterized by a high b value, i.e., a high ratio of small to large events, and the data set is remarkable in that a large proportion of the events form clusters whose members show a high degree of waveform similarity. The largest cluster thus identified consisted of 27 similar events (average inter-event correlation coefficient > 0.8 for a 9.5 s window), and waveform similarity persists far into the coda. Inter-event spacing is irregular, but very short waiting times of a few minutes are far more common than expected from a Poisson distribution. Seismicity with these features (high b value, large number of similar events with short waiting times) is typical of swarm activity, which, based on empirical evidence and theoretical considerations, is generally thought to be driven by fluid pressure variations. Because no pronounced outer rise bulge exists on the very young plate in the study region, it is unlikely that melt is accessible from decompression melting or opening of cracks. A fluid source related to processes at the nearby ridge is conceivable for the younger segment but less likely for the older one. We infer that the fluid source could be seawater, which enters through fractures in the crust. Most of the similar-earthquake clusters are within the crust, but some of them locate significantly below the Moho. If our interpretation is correct, this implies that water is present within the mantle. Hydration of the mantle is also indicated by a decrease of P_n velocities below the outer rise seen on a refraction profile through one of the arrays [Contreras-Reyes, E., Grevemeyer, I., Flueh, E.R., Scherwath, M., Heesemann, M., 2007. Alteration of the subducting oceanic lithosphere at the southern central Chile trench-outer rise. Geochem., Geophys. Geosyst. 8, Q07003.]. The deepest events within the array on the 6 Ma old plate occur where the temperature reaches 500–600 °C, consistent with the value observed for large intraplate earthquakes within the mantle (650 °C), suggesting that the maximum temperature at which these fluid-mediated micro-earthquakes can occur is similar or identical to that of large earthquakes.     

Seismicity in the platform regions of Ukraine in the zones of anomalous electrical conductivity

It is established for the first time that there are several regions in Ukraine, in which the earthquakes occurring within platform territory are correlated to the anomalous conductive structures in the Earth’s crust and upper mantle. These regions are identified as (1) Donbass and the eastern part of the Dnieper-Donetsk Depression (DDD); (2) eastern margin of the Ingulets-Krivoi Rog suture zone in the area of the Krivoi Rog-Kremenchug fault zone; (3) the western part of the Cis-Azov megablock; (4) the western boundary of the Ukrainian Shield and its slope; (5) North Dobruja and Pre-Dobrujan Depression. The reconstructed tree-dimensional (3D) geoelectrical models of the Earth’s crust and upper mantle feature anomalously low values of electric resistivity. The earthquake sources in the platform areas of Ukraine are localized above the top and in the upper parts of the crustal anomalies of electrical conductivity.     

川南泸州地区地震重定位与地壳三维速度结构研究

近年来全球范围内页岩气水力压裂开采区地震活动剧增的现象,引起社会公众、政府、产业部门和科学家的广泛关注.利用2010年至2021年期间四川南部区域台网和流动台站记录的近震P波和S波到时数据,采用双差层析成像方法,对泸州及周边地区地震位置和地壳浅部介质结构进行了联合反演.重新定位后,中小地震震源深度主要处于2~10 km范围内.在空间上,地震活动呈现丛集特征,集中在螺观山背斜和古佛山背斜之间的低缓区内,并具有一定的时空迁移特征.背斜以南区域和泸县M6.0地震震中处,可观测到北西走向的地震丛集现象,与区域内已知断层分布并不一致.成像结果显示,在地壳浅部,背斜之间低缓区内的S波(V_S)速度结构存在横向分区特征,中北部地区表现为明显的高速异常,对应的波速比(V_P/V_S) 值较低,且大部分地震分布在这些异常体内.但泸县M6.0地震震源区位于P波和S波的高、低速异常的交界地带.结合其他地质与地球物理研究,推测泸县地震发生在一条北西走向、以逆冲为主高角度的调节断层上.

     

藏北高原地震活动性特征及其大地构造意义

藏北高原自新生代以来不仅发生了强烈的火山作用,而且现今的地震活动性仍然强烈.本文收集了2011年前藏北高原区发生的地震事件(源自NEIC)及相应的震源机制解数据(源自GCMT),结合地质与地球物理等相关资料,初步分析表明藏北高原地壳整体上处于伸展应力状态.然而,因区域构造应力场及构造环境差异,将藏北高原地震活动区分为两个地震活动区,即西昆仑地震活动区和藏北中部火山岩区.西昆仑区的地壳应力状态呈东西向伸展,而岩石圈地幔部分主要以南北汇聚为主,表明西昆仑区域下的地壳与岩石圈地幔应力状态是解耦的,而这种解耦机制需要更进一步的研究.而在藏北中部火山岩区的地壳的主张应力场为NNE-SSW的走滑和正断层性质的伸展,尽管缺乏该区域下的岩石圈结构特征认识,但是依据幔源性质的钾质和超钾质火山岩成因模式,认为其下的岩石圈地幔也处于伸展状态,该区域下的地壳与岩石圈地幔同处于伸展应力环境中,表明藏北火山岩区下的结构特征更加复杂,亟待开展相关探测与研究.     

2003年台湾成功地震前的地震活动特征

通过计算Z值的标准正态偏差及古登堡里克特关系中的b值,研究了2003年台湾成功地震（Mw=6．8）之前台湾地区地震活动性图像的变化。在成功地震震源周围的Z值分布中可以识别出地震活动的Mogi环形变化。在成功地震之前的震源区附近还明显看到b值的下降。相对较低的地震活动速率和b值的下降可能是整个地震活动中与平静相联系并使得成功地震前主震区域中等事件活动加强的前兆现象。     

Seismicity pattern and thermal anomalies in the southern Baltic region before and during the Kaliningrad earthquakes of 2004

The seismic situation in the SE Baltic region that existed during the period 1990–2006, including the unexpectedly strong Kaliningrad earthquakes of 2004, is analyzed. The spatiotemporal variation of seismic events in the region is examined on the basis of a newly compiled catalog of tectonic earthquakes. The analysis revealed outbursts of seismic activity in 1995 and 2004, structurization of the distribution of shocks, and their southward migration. The distribution of hot springs that arose in 2002–2004 is analyzed in relation to seismological data. The seismic process and thermal anomalies are shown to be geodynamically controlled, which provided constraints on the nucleation process and focal mechanism of the Kaliningrad earthquakes. The region, located in the western East European platform, should be regarded as seismically rather active.     

A variety of natural systems of industrial groundwater on Siberian platform

The lithologic-genetic formation conditions of a reservoir of industrial groundwater on Siberian Platform, associated with the existence of a bank reef in the Cambrian. Within this cryohydrogeological structure, the properties and reserves of the hydromineral stock that can be developed parallel to the diamond-mining on Udachnaya pipe without significant environmental damage.     

Magnetic properties of intrusive traps of the Siberian platform: Evidence for a self-reversal of the natural remanent magnetization

A combined magnetomineralogical analysis of a doleritic collection gathered from four natural outcrops in the mouth area of the Stolbovaya River revealed a spinodal pattern of the decomposition of a magnetite-ulvospinel solid solution (the stage of the “modulated structure” formation) in the ferromagnetic fraction of samples. Homogenization of exsolution structures during thermal treatment of the majority of the collected samples is shown to involve the effect of partial self-reversal of thermoremanent magnetization. A distinct dependence of the effect on the level of internal elastic stresses suggests that a reverse component of J n can arise during some phases of the spinodal stage of the solid solution decomposition in natural titanomagnetites.     

Seismicity pattern of the Betic Cordillera(southern Spain) derived from the fractal properties of earthquakes and faults

Several studies on earthquake occurrence and associated faulting have demonstrated that both phenomena have a scale-invariant behavior which can be analyzed by means of a set of non-integer dimensions(Dq) describing their fractal properties and the calculation of multi-fractal spectra.It is the case that the behavior of these spectra is asymptotic at the ends of the variation interval of q,which is a real number that enters into the definition of the partition function of the dataset.The difference between the extreme values,called multi-fractal spectrum slope,is used to investigate the heterogeneity of the spatial distribution of earthquakes and fault systems.In this paper we focus on the Betic Cordillera,southeastern Spain,which is commonly considered the contact between the Eurasian and African plates and has an important seismic activity in the context of the Iberian Peninsula.Some of the most conspicuous Iberian earthquakes,such as the 1829 mb6.3 Torrevieja and the 1884 mb6.1 Alhama de Granada earthquakes occurred in this mountain range and both reached intensity X.The present work implies a new analysis based on the slope of multi-fractal spectra and referred to the historical seismicity of the region,specifically b-value(frequency distribution of earthquakes respect to magnitude),epicentral location,seismic energy and faulting.On this basis we propose a seismotectonic zonation that is contrasted with the stress state and the geodynamical evolution of the Betic Cordillera.     

Seismicity and the subduction process

There is considerable variation between subduction zones in the largest characteristic earthquake within each zone. Assuming that coupling between downgoing and upper plates is directly related to characteristic earthquake size, we have tested for correlations between variation in coupling and other physical features of subduction zones: the lateral extent and penetration depth of Benioff zones, age of subducting lithosphere, convergence rate, and back-arc spreading. Using linear multivariate regression, coupling is correlated with two variables: convergence rate and lithosphere age. Secondary correlations within the data set are penetration depth versus lithosphere age, and lateral extent versus convergence rate. An important additional correlation is that back-arc spreading is found to be associated with subduction zones where coupling is low (those characterised by small earthquakes). Taken together, the observed correlations suggest a simple qualitative model where convergence rate and lithosphere age determine the horizontal and sinking rates, respectively, of slabs: these parameters influence the seismic coupling in the subduction zone. In the limit of a fast sinking rate and slow convergence rate, back-arc spreading occurs and thereby appears to be a passive process.     

阳江Ms 4.9地震活动特征、影响场及应急对策

对2004年9月17日阳江Ms4．9地震进行总结。分析认为,这次地震属1969年阳江Ms6．2主震后的晚期强余震,北东东向的平冈断裂和北西向的洋边海断裂是本次地震的主要发震构造,具有逆冲特征,这次地震的震感范围大,影响波及面广,震中烈度达Ⅵ,造成了一定破坏。震前1个月至临震微震（ML〈1．0）频度密集,部分前兆台站出现中期和短期异常。文中最后概述了地震前后的应急对策和发挥的作用。     

Seismicity of the Sinai subplate region:kinematicimplications

The seismic activity of the Sinai subplate region on the basis of both historical (2200B.C.–1900 A.D.) and recent (1900–1995) earthquake catalogs have been evaluated.Moderateand large earthquakes occurred mainly at the subplate boundaries, Dead Sea Fault (DSF) systemin the east, Cyprean arc in the north, and Suez rift in the southwest.Along the Dead Sea Fault system the activity concentrated at the southern andcentralsegments. The earthquake distribution appears to have a tendency to cluster in time andspace.The swarms (February, 1983; April, 1990; August, 1993 and November, 1995) in the GulfofAqaba indicate that the southern segment of the Dead Sea Fault system is the mostseismogenicthrough the last two decades. North of the Dead Sea depression the seismic activitytends to haveoccurred with NW trend to extend under the Levantine Sea. Although the northernsegment ofthe Dead Sea Fault system is well defined from geological, geophysical and historicalearthquakeactivity recent seismic activity is practically absent especially north of Latitude 34°N.In the eastern Mediterranean the seismicity is much higher in the area of the Hellenicarcthan in the Cyprean arc. Moreover, the activity occurs in a wide belt suggesting that theplateboundary is a deformation zone instead of a single line.The seismic activity in the Gulf of Suez is scattered and does not have any distincttrend.However, three active zones are delineated. At the mouth of the gulf most of activityisconcentrated where the Sinai triple junction (Africa, Arabia, Sinai) is situated. The centralpartand the northern part of the gulf include the adjacent area as far as the river Nile. Actually,theactivity is markedly decreased from south to north.Although there is no seismological evidence that the Suez rift continues into theeasternMediterranean, the activity in the Gulf of Suez region cannot be ignored.The parameters of magnitude-frequency relation (a, b) indicate thatthelevel of earthquake activity in the Sinai subplate region is generally moderate. Moreover,theenergy release curve shows a regular trend and reflects occasional high activity. © 1999ElsevierScience Ltd. All rights reserved.     

北武夷葛山坞萤石矿床地质特征及成因探讨

武夷山成矿带是我国十九个成矿带之一,分布着众多的萤石、叶腊石、石墨等非金属矿床及铜铅锌银等多金属矿床,矿床类型多样。本文在讨论葛山坞萤石矿成矿地质特征的基础上,对该矿床的成因进行了探讨,对北武夷地区下一步寻找萤石矿床具有指导意义。     

闽粤海外历史地震与台湾海峡现今强震活动图像

闽粤两省强烈地震多发生在沿海地区,且福建南日岛至广东南澳一线的泉州－汕头地震带地震活动尤为突出。历史上东南沿海地震带曾发生过４次７级以上大地震,而其中３次都发生在泉－汕段海域,继华南地区本世纪著名的１９１８年广东南澳７．３级地震后,１９９４年９月１６日台湾海峡南部又发生７．３级强震,这在经济发达,人口稠密的闽粤沿海地区引起了极大关注。本文通过历史强震活动资料,分析闽粤沿海与台湾海峡强震在时间进程,     

地震活动涨落、自组织结构和大震临界状态的统计特征

根据非线性动力学理论、运用概率论与数理统计方法,对非平衡地震发生系统的空间分布状态进行了研究。提出用空间变异系数δV判断涨落类型:凝聚集、群集、泊松型和均匀点阵分布。从整体观点综合应用各统计量即协方差μ11、非负量I(x,y)、联合信息熵H(x,y)、离散度σ和δV,对非平衡系统的群体空间自组织结构和临界状态的统计特征做了进一步的探讨。根据首都圈公元前231年～2003年M≥434地震活动及其和华北不同范围ML≥3.0地震活动涨落的统计分析认为,μ11≈0,I(x,y)>0,高H(x,y)值,低σ和δV值是强震和大震发生的自组织空间结构和临界现象的基本特征和必要条件。同时对地震空区、条带、前兆性震群及前震等前兆现象成因提出了新的观点和统计解释,并得出结论:仅当一个远离平衡态的系统(区域)有足够的随机性而又非泊松型涨落时,系统的突变(大震)是不可逆转的。     

A plate tectonic hypothesis for the genesis of porphyry copper deposits of the southern Basin and Range Province

The ages and distribution of porphyry copper deposits of the southern Basin and Range Province suggest an origin influenced by a mantle hot spot as this part of the North American plate moved N35–40°W at 3.57 ± 0.65 cm/yr during the interval of the Laramide Orogeny 72 to 52 my ago. This motion coupled with that of the hot spot interpretation of the Corner Seamounts region of the North Atlantic during the same time interval suggests that the North American plate rotated clockwise at an average rate of 0.35 degrees per million years. The apparent pole of rotation is located near 50°N latitude and 64°W longitude in the region of Anticosti Island in the Gulf of St. Lawrence. The apparent diameter of the hot spot is 450 km. Northwesterly motion and clockwise rotation of the North American plate are compatible with existing concepts of plate tectonics, paleomagnetism, and paleoclimatology. The development of early Tertiary blankets of supergene enrichment and their partial destruction during the late Tertiary can be explained by a change from a warm and wet climate during the early Tertiary to a cooler and drier climate during the late Tertiary as this part of the North American plate moved northwesterly from a more tropical climate into a more temperate and arid zone. Orographic rain shadows produced by middle and late Tertiary mountain building in southern California were undoubtedly influencial in enhancing late Tertiary aridity.     

2008年初江南冻雨过程的湿大气锋生

通过2008年1月10日至2月2日发生在我国江南的冻雨过程和前期降雨过程的对比分析,得到下列结论：这次江南冬季持续的冻雨天气是准静止锋活动的结果,用相当温度梯度表示的湿大气锋生可以有效地描述准静止锋的活动特征及其与降水的关系。湿大气锋对应的降水性质与锋面垂直结构有关,锋区上相当温度的逆温结构是形成冻雨的天气学原因．研究进一步揭示,相当温度梯度可提前5～10d预示降水天气过程的开始,也能提前预示过程的结束,而二者的这种关系反映了大气中能量积累和释放的过程．     

Spatiotemporal characteristics of positive cloud-to-ground lightning discharges and bidirectional leader of the lightning

The negative CG lightning discharges neutralizing negative charges in cloud usually dominate for most of thunderstorms. However, a lot of positive CG light-ning discharges often occur in the disappearing stage of thunderstorms, in the stratiform region of mesoscale convective systems and some supercells producing hail and tornado. Because the positive CG lightning discharges produce larger current of the return stroke and neutralize more charges due to the continuing currents with longer las…     

Thermal evolution of the northeastern Siberian platform in the light of apatite fission-track dating of the deep drill core

Thermal evolution of the continental crust beneath the northeastern Siberian craton was studied based on the interpretation of apatite fission-track ages. The samples selected for AFT dating were collected from depths between 2 and 3 km along a 1000-km-long profile, from the crystalline basement of the Siberian platform. The AFT ages range from 185 to 222 Ma, indicating that in the late Triassic-early Cretaceous, the top of the crystalline basement was cooled below ∼100°C. Once the apatite cooled below this temperature, it began to accumulate and preserve tracks produced by spontaneous fissioning of ²³⁸U, and the number of tracks preserved is effective in determining the ages of events using the apatite fission-track method (AFT). The study showed that the apatite from Archean rocks was largely formed at 1.8–1.9 Ga as a result of a Paleoproterozoic metamorphic overprinting during the terrane collision and the subsequent accretion of the Siberian craton. The last thermal event, the self-heating of the collision prism, was terminated by cooling at ∼1.3 Ga. At that time, the Rb-Sr isotopic system became closed and the upper crust passed the ~300°C isograd. The calculation results showed that on further cooling, the ∼100°C isograd was passed at 1143 Ma. This age estimate could be obtained using AFT dating if the above event had been the last one in the thermal history of the Siberian craton. The obtained track ages indicate the existence of a repeated, significantly younger, heating of the crystalline crust due to some local reason.