Chemistry and metal contents of magmatic gases: the new Tolbachik volcanoes case (Kamchatka)

The gaseous products of new Tolbachik volcanoes were studied during 1975 to 1977 throughout all eruptive stages and during the post eruptive activity. In investigations the northern break-out gases emitted during the eruption from the moving and consolidated lava flows there have been detected H₂O (the main component), H₂, HF, HCl, SO₂ and H₂S, CO₂, CO, NH₃, CH₄ and other hydrocarbons, NH₄Cl predominated in compositions of condensates and subtimates on lava flows and the most characteristic microcomponents were Zn, Cu, Pb, Sn, Ag and others. Sampling of gases and condensates at the southern break-out was conducted immediately from the flowing melt. In gases there have been detected H₂O (98 mol. %). HCl and H₂ (0.9 mol. % each) as well as HF, SO₂, H₂S, CO₂ and in small quantities O₂ and N₂, Gases reached the equilibrium state atT andP sampling and were characteristic of gas composition of the southern break-out magma. HCl, HF and H₂SO₄ were predominant during condensate and sublimate mineralization. The major raicrocomponents were represented by Pt, Sb, As, Zn, Cu, Pb, Ni, Co and others. Comparison of compositions of gases and of products of their reactions at the northern and at the southern break-outs allows us to assume the presence of the deeper magma source at the northern break-out and of shallow magma source at the southern break-out.     

Paleomagnetism, geochemistry and Ar/Ar dating of the North-eastern Paraná Magmatic Province: tectonic implications

Seventy sites of sills, flows and dikes from Northeastern Paraná Magmatic Province (PMP), were submitted to paleomagnetic, chemical and radiometric analyses. The rocks are high in TiO₂ content, and similar in composition to the rocks from the northern region of PMP. The sills intrude mainly Paleozoic sediments, and can be subdivided into two domains; the northern being characterized by sills showing reversed polarities, and the southern essentially by sills of normal polarities. ⁴⁰Ar/³⁹Ar dating of three distinct sills gave plateau ages (129.9 ± 0.1, 130.3 ± 0.1 and 131.9 ± 0.4 Ma) that are similar to surface-outcropping flows of the Northern Paraná Basin, and the Ponta Grossa dikes. The new paleomagnetic data combined with existing data from the northern PMP allowed the calculation of a paleomagnetic pole at 71.4° E and 83.0° S (N = 92; α₉₅=2.4°; k = 39). This pole is in good agreement with poles for central and southern PMP, which are slightly older than the northern PMP, as well as for the contemporaneous Central Alkaline Province (Paraguay) on the western side of PMP. In contrast, the coeval pole for the Ponta Grossa dikes (eastern border of PMP), however, is slightly displaced from that group of poles, suggesting that dikes in that area may have undergone some tectonic tilting.     

由构造应力场研究汶川地震断层的分段性

采用构造应力场均匀性对断层或板块边界进行分段的方法, 利用ldquo;裁剪-粘贴rdquo;法给出的余震震源机制解资料, 进一步从应力场角度确定汶川地震发震断层南、 北段分界点位置及南、北段震后应力场. 研究结果表明, 发震断层南、北段分界点位于北川附近, 与先前的研究结果较为一致. 南、 北段震后应力场反演结果显示, 南、北段的最大主应力轴方位均呈北东东向, 且近水平. 南、北段应力场反演的平均拟合残差和置信区间均较大, 主要原因可能是因为南、北段内应力场不均匀性造成的. 因为本文仅由大余震给出了震源断层的一级分段, 南、北段内应做进一步的细分.      

Mantle convection pattern and subcrustal stress field under Asia

Gravitational field models derived from satellite tracking and surface gravity data have been used to derive the forces in the earth's mantle under Asia. Based on studies of tectonic forces from these models, a subcrustal stress field under China has been obtained. The stresses are due to mantle convection. According to the stress patterns, the east and west China blocks and five seismic zones are identified. The tensional stresses exerted by the upwelling mantle convection flows under the crust of Tibet seem to be related to the Tibetan uplift. The compressional orogenic region from the southern tip of Lake Baikal, through Tien Shan, Hindu Kush and the Himalayas to northern Burma appears to be connected with the downwelling mantle convection flows. It is found that the directions of the subcrustal stresses under China are disposed perpendicularly to the major fault systems and seismic belts. The results of stress calculations show that the crust of north China should be in compression and that stresses within it should be sufficient to form the Shansi Graben and Linfen Basin Systems and fracture the lithosphere. This gives a possible explanation of why strong earthquakes occurred in north China which is an isolated narrow region of highest seismicity far from plate boundaries. The tensional stress fields, caused by the upwelling mantle convection flows, are found to be regions of structural kinship characterized by major concentrations of mineral and metal deposits in China.     

Using large dynamite shots to image the structure of the Moho from deep seismic reflection experiment between the Sichuan basin and Qinling orogen

The Qinling orogen was formed as a result of the collision between the North and South China blocks. The Qinling orogen represents the location at which the southern and northern parts of the Chinese mainland collided, and it's also the intersection of the Central China orogen and the north-south tectonic belt. There is evidence of strong deformation in this orogen, and it has had a long and complex geological history. We investigated the structure of the Moho in the southern Qinling orogen using large dynamite shot imaging techniques. By integrating the analysis of the single-shot and the move-out corrections profile, we determined the structure of the Moho beneath the northern Dabashan thrust belt and the southern Qinling orogen, including the mantle suture beneath Fenghuang mountain. The Moho is divided into two parts by the mantle suture zone beneath Fenghuang mountain:(1) from Ziyang to Hanyin, the north-dipping Moho is at about45–55 km depth and the depth increases rapidly; and(2)from Hanyin to Ningshan, the south-dipping Moho is at about 40–45 km depth and shallows slowly. The mantle suture is located beneath Fenghuang mountain, and the Moho overlaps at this location: the shallower Moho is connected to the northern part of China, and the deeper Moho is connected to the southern part. This may indicate that the lithosphere in the Sichuan basin subducts to the Qinling block and that the subduction frontier reaches at least as far as Fenghuang mountain.     

汶川MS8.0地震余震震源机制时空分布特征

本文利用CAP波形反演方法,获取了汶川M_S8.0地震序列中312个具有较高信噪比波形资料的4级以上余震的震源机制解和震源深度. 基于震源深度空间分布与震源机制时空分布,分析了主震后余震区断层行为特征与应力场时空变化,并对龙门山断裂带中北段的发震断层面几何形态进行了初步探讨. 获得的主要认识如下:(1)余震震源深度分布存在显著的空间分段差异. 绵竹以西的余震区南段与平武以东的北段余震深度范围大于中段(绵竹-平武段),但深度小于5 km的5级以上超浅源地震主要分布在明显偏离龙门山断裂带走向的理县NW向分支与余震区北端NNE向分支,而中段余震主要分布在7~19 km深度. (2)余震机制类型存在明显的时空差异. 余震区中段逆冲型地震占绝对优势,理县NW向分支余震则以走滑型为主,机制类型随时间变化不显著. 沿龙门山断裂带走向的余震区南段,早期(2008年8月底前)逆冲型地震比例高于走滑型、晚期走滑型地震比例显著升高并超过逆冲型;而余震区北段早期走滑型地震占绝对优势、晚期逆冲型地震比例大幅上升且超过走滑型. 南、北两段余震机制类型比例的显著变化,可能是余震区两端断层调整性运动的表现. (3)节面走向及P轴方位优势方向均存在显著的空间差异. 南段NWW向P轴方位与区域应力场一致,中段及理县NW向分支P轴优势方向NEE,而北段具NWW和NEE两个优势方向,这种差异反映了余震活动除了受区域应力场控制外,还受到主震引发的局部应力场的控制. 节面走向的多方位分布则反映不同走向的构造参与了主震后的余震活动. (4)沿龙门山断裂带走向,余震区南段具深部缓倾角、浅部高倾角的铲形断面特征;中段深部倾角均值较稳定、浅部倾角均值随深度减小而增大;北段倾角均值相对稳定,显示其断面几何形态相对简单. 上述不同区段倾角均值随深度的变化揭示龙门山断裂带中北段断层面几何形态复杂.     

Movement of theSq foci in 1958

Summary Latitudinal locations of the foci of theSq ionospheric current systems are estimated for each month using geomagnetic data for five international quiet days of each month of the year 1958. It is found that: 1) The pattern of the monthly movement is different in the three sectors chosen for this investigation (North-South American Sector, European-African Sector and Asian-Australian Sector). 2) Regardless of the sector the foci generally seem to move together in the same direction like a rigid system; during the northern summer they shift towards a more northerly latitude than during the northern winter. 3) The positions of the foci differ considerably in adjacent months. 4) The largest movements of the foci are noted in the American sector where the magnetic dip equator is farthest south.Contribution from the Earth Physics Branch No. 453.     

雁荡山湖泊沉积物记录的中国东部季风区小冰期以来气候干湿变化

中国东部季风区过去千年降水变化特征已有大量研究并取得重要进展,但多数研究集中在中国北方和中部地区,南方地区重建记录相对较少.本文对浙江雁荡山雁湖沉积物的总有机碳、色度和粒度等多指标进行分析,重点探讨小冰期以来研究区域的降水变化,结果显示:1)1400-1600 AD,湖区集水发育,研究区气候偏湿润;2)1600-1650 AD,降水量减少,气候偏干旱;3)1650-1750 AD,气候偏湿润;4)1750-2000 AD,研究区气候处于干旱状态,其中1500-1650 AD期间的干湿变化与历史文献记录的雁湖水位有良好的对应关系.在年代误差范围内,雁湖沉积物与浙江天目山泥炭、福建仙山泥炭以及湖光岩玛珥湖沉积物等记录的干湿变化同步,指示了小冰期期间中国东南地区气候波动的一致性.另外,中国东部南、北地区古气候记录反映的降水变化趋势相反,表明在百年时间尺度上,中国东部南北地区降水存在着"南涝北旱"和"北涝南旱"的空间特征.通过与太平洋年代际振荡(PDO)的对比,显示PDO对中国东部地区小冰期期间出现的南-北"偶极型"空间降水特征可能具有重要影响.     

Tsunami Generated by the Late Bronze Age Eruption of Thera (Santorini), Greece

—Tsunami were generated during the Late Bronze Age (LBA) eruption of the island of Thera, in the southern Aegean Sea, by both caldera collapse, and by the entry of pyroclastic surges/flows and lahars/debris flows into the sea. Tsunami generated by caldera collapse propagated to the west producing deep-sea sedimentary deposits in the eastern Mediterranean Sea known as homogenites; open-ocean wave heights of about 1.9–17 m are estimated. Tsunami generated by the entry of pyroclastic flows/surges and lahars/debris flows into the sea propagated in all directions around the island; wave heights along coastal areas were about 7–12 m as estimated from newly identified tsunami deposits on eastern Thera as well as from pumice deposits found at archaeological sites on northern and eastern Crete.     

Contrasting phytoplankton community structure and associated light absorption characteristics of the western Bay of Bengal

Absorption spectra, particulate pigments, and hydrochemical constituents were measured in the western Bay of Bengal (BoB) during July-August 2010 when influence of river discharge is at peak. Chromophoric dissolved organic matter (CDOM) absorption coefficient (a_CDOM(440)) displayed a significant inverse linear relationship with salinity in the surface waters implying conservative mixing of marine and terrestrial end members. The northern part of the study area is influenced by discharge from the river Ganga and a dominant terrestrial CDOM signal is seen. The southern part receives discharge from peninsular rivers with corresponding signals of higher CDOM than the linear model would indicate and higher UV-specific absorption coefficient (SUVA) indicating more aged and humified DOM. Lower contribution of CDOM to total non-water absorption and higher phytoplankton biomass (chlorophyll a absorption coefficient, a_ph(440)) but lower chlorophyll a specific phytoplankton absorption coefficient (a _ph ^* (440)) characterize the northern part, compared to the southern part. Chlorophyll b had a distinct linear relationship with chlorophyll a in the latter. The size index (SI) indicated dominance of microphytoplankton in the northern and nano and picophytoplankton in the southern parts. Chlorophyll a is significantly related to a _ph ^* (440) by an inverse power model in the northern part but by an inverse linear model in the southern part. Our study suggests that knowledge of the phytoplankton community structure is essential to improve chlorophyll a algorithm in the coastal Bay of Bengal.     

Coupling patterns between para‐glacial and permafrost degradation responses in Antarctica

The recently deglaciated environments in maritime permafrost regions are usually affected by very active paraglacial processes. Elephant Point is an ice‐free area of 1.16 km2 located in the SW of Livingston island (South Shetland Islands, Antarctica). Between 1956–2010 the retreat of the ice cap covering most part of this island has exposed 17.3% of the land surface in this peninsula. Two geomorphological units were identified in this new ice‐free area: a moraine extending from the western to the eastern coastlines and a relatively flat proglacial surface. The glacier in 1956 sat in contact with the northern slope of the moraine, but its accelerated retreat ‐ in parallel to the warming trend recorded in the Antarctic Peninsula ‐ left these areas free of glacier ice. Subsequently, the postglacial evolution was controlled by the relaxation phase typical of paraglacial systems. The typology and intensity of geomorphological processes show a significantly different dynamics between the southern and northern slopes of the moraine. This pattern is related to the different stage of paraglacial adjustment in both slopes. In the southern side, on coarser sediments, pronival ramparts, debris flows and alluvial fans are distributed, with a low to moderate activity of slope processes. In the northern side, mass wasting processes are extremely active on fine‐grained unconsolidated sediments. Ice‐rich permafrost is being degraded by thermokarst processes. Landslides and mudflows transfer large amounts of sediments down‐slope. The surface affected by retrogressive‐thaw slumps in the moraine has been quantified in 24,172 m2, which accounts for 9.6% of its surface. The abundance of kettle‐lakes is also indicative of the degradation of the ground ice. Paraglacial processes are expected to continue in the moraine and proglacial area in the near future, although their intensity and duration will depend on the magnitude and rate of future climate trends in the northern Antarctic Peninsula. Copyright © 2015 John Wiley & Sons, Ltd.     

高纬向阳侧磁层顶通量传输事件的特性研究--通量管轴线方位及运动分析

本文分析了2001年2月和3月期间Cluster Ⅱ穿越磁层顶前后的观测资料,检测到13个通量传输事件（FTEs）.用多颗卫星磁场测量资料的最小方差分析（MVAB）方法确定FTE的管轴方向（其中6个方向较可靠）.FTE管轴方向的分布和低纬处不同,在磁顶法线坐标系LMN中对M轴有较大偏离,比较靠近L轴.deHoffmann Teller(HT)分析指出,13个FTEs都存在一个很好的HT参考系,表明它们以一个准稳的MHD结构运动.对垂直于管轴方向的运动分析表明FTEs并不一定和背景等离子体一起对流,它们可快于或慢于背景流,但FTEs的运动和背景流基本沿相同方向,其间可有一不大的夹角.在HT坐标系中,10个FTEs的等离子体速度接近零, 其他3个FTEs的等离子体速度约为局地Alfven波速的14%,都不符合Walen关系.其中北半球事件的Walen曲线为正斜率,南半球事件为负斜率,这说明等离子体沿磁力线（北半球顺着磁场,南半球逆着磁场）流向磁层.     

HYDROLOGICAL CYCLES ON THE NORTH AND SOUTH PERIPHERIES OF MOUNTAIN–GLACIAL BASINS OF CENTRAL ASIA

Four high mountain glacial basins of the northern and southern periphery of central Asia were studied to determine their interaction with the external hydrological cycle over the Eurasian continent. Two of them located in the northern periphery are closed drainage basins with continental climate and the other two are open basins located in the southern periphery. Calculations of mass energy exchange, glacial runoff and components of the hydrological cycles were conducted. For glaciers with a continental climate, the calculations of snow–ice melt and runoff were based on solar parameters. For glaciers with a marine climate regime, glacier melt and runoff were based on air temperature. The relative errors of simulated annual flows were, on average, 8–14%. The components of the regional hydrological cycles (precipitation, condensation, runoff and evaporation) were quantified for each glacial system and their share in total atmospheric moisture was determined. The closed basins of the northern periphery in central Asia stored annually about 0·1–2·4% of the total external atmospheric moisture in the Aralo-Caspian and Tarim hydrographic systems. About 0·22–0·24% of the external water cycle is transferred annually in open glacial basins of the southern periphery. The glaciers of these regions return 0·25–0·30% of the external water cycle per year to the Pacific and Indian oceans, 0·03% and 0·06% of this external moisture is taken from the glacial resources of the Gongga and Xixibangma glaciers. © 1997 by John Wiley & Sons, Ltd.     

A transition from strombolian to phreatomagmatic activity induced by a lava flow damming water in a valley

The Golan Heights is a Plio-Pleistocene volcanic plateau. Cinder cones of Late Pleistocene age are very common in the eastern and northern Golan, while phreatomagmatic deposits are relatively rare and occur just in two structures — the maar of Birket Ram and the tuff ring of Mt. Avital. The complex of Mt. Avital includes two large cinder cones, a tuff ring with an elongated central depression and several basaltic flows, some of them breach the cinder cones. The (exposed) eruptive history of the complex includes (1) an early stage of basaltic lava flows, (2) strombolian activity and the buildup of the southern cinder cone, (3) a second stage of basaltic flows and the buildup of the northern cinder cone, and then a transition to (4) phreatomagmatic explosions. The phreatomagmatic deposits include surges, lapilli fallout deposits and coarse-grained lithic tuff breccias, which were found up to 200 m above the central depression. Basaltic and scoriaceous clasts are the main component of all deposits, while juvenile material is usually a minor component, almost absent in the lapilli deposits.It is suggested that the phreatomagmatic events in Mt. Avital were induced by the infiltration of water from a lake that existed in a nearby topographic low (Quneitra Valley). The lake was formed or significantly expanded at about 300 ka due to a lava flow that blocked the drainage of the valley to the west. The interlayering of tuff and scoria at the top of the northern cinder cone and the good preservation of a lava flow top breccia under the surges imply that the phreatomagmatic activity immediately followed and even coincided with the last stages of strombolian activity. It is suggested that the dry–wet transition was triggered by the effusion of the second stage lavas and the buildup of the northern cinder cone, which probably caused a reduction of pressure in the magmatic system and allowed the lake water an access to the magmatic system. The minimum age of the phreatomagmatic events is determined by a 54 ka Musterian site which lies directly on top of the tuff in the Quneitra Valley.     

Erosional unloading,hillslope geometry,and the height of the Cascade Range,Washington State,USA

Peaks in the Cascade Range in northern Washington State are on average ～800 m higher than in southern Washington. The influences of differential valley excavation and variations in hillslope length and average slope on these altitudinal trends were tested using a 3‐dimensional model for isostatic rock uplift and calculations of hillslope length and slope respectively. The magnitude of isostatic peak uplift calculated by the model is highly dependent on the flexural rigidity (D) and the related effective elastic thickness (T_e) of the crust of this region. Crustal rigidity was constrained using published estimates and by estimating the depth of the seismogenic zone in the area (D > 1 × 10²³ Nm and T_e > 24 km). With these constraints, isostatic compensation due to differential erosion added < 700 m and 300 m, or < 25% overall, of height to peaks in the northern and southern Cascades, respectively. Deeper valley incision in the northern Cascades accounts for < 300 m of the 800 m difference in peak altitudes between north and south. Similarly, variation in valley spacing and slope account for < 350 m of the difference in mean altitude between northern and southern regions. Hence, at least several hundred m difference in altitude between the northern and southern regions of the Cascades in Washington must be due to tectonic, geologic, or geophysical factors rather than surficial and geomorphic effects like isostatic response to valley incision and hillslope geometry. Copyright © 2009 John Wiley & Sons, Ltd.     

Present-day seismicity,stress field and crustal deformation of Egypt

In this study we investigate present-day seismicity and crustal deformation of Egypt based on a comprehensive earthquake catalog from 1900 to 2004 by focal mechanism stress inversion and by recent GPS observations. Spatial distribution of earthquake epicenters indicates that Egypt has been suffered from both interplate and intraplate earthquakes. Most earthquake activity (more than 70%) has been concentrated in northern Egypt along the geologically documented borders of Sinai subplate (northern Red Sea and its two branches Suez rift and Aqaba–Dead Sea transform). The majority of inland earthquake focal mechanisms in Egypt are normal with strike-slip component or strike-slip faulting events. Only a small minority, namely four events, exhibits reverse faulting. The inversion method of Gephart and Forsyth (1984) was applied to calculate the orientation of the principle stress axes and the shape of the stress tensor. The best fitting tensor in Egypt shows homogeneity stress field. The tension stress regime is dominant in northern Egypt. The stress directions are well resolved by the 95% confidence limits, the relative stress magnitude has a value of about 0.3. However, along southern Egypt the strike-slip regime is dominant. The shape factor (R-value) is 0.5, which means that the deviatoric components of σ₁ and σ₃ are of the same magnitude, but of opposite signs. The average horizontal velocity of GPS stations in Egypt is 5.15± 1.1 mm/year in mostly NNW direction. The results of deformation analysis indicate that the northern Egypt is deformed more than the southern part. Only the Egyptian-Mediterranean coastal–Nile Delta zone dominates as a compression deformation area. However, an extensional deformation has been observed throughout the rest of country. This means that the relative motion of African plate with respect to both Eurasian and Arabian has highly controlled the deformation processes in Egypt.     

Two examples of subaqueously welded ash-flow tuffs: the Visean of southern Vosges (France) and the Upper Cretaceous of northern Anatolia (Turkey)

Pyroclastic deposits interpreted as subaqueous ash-flow tuff have been recognized within Archean to Recent marine and lacustrine sequences. Several authors proposed a high-temperature emplacement for some of these tuffs. However, the subaqueous welding of pyroclastic deposits remains controversial.The Visean marine volcaniclastic formations of southern Vosges (France) contain several layers of rhyolitic and rhyodacitic ash-flow tuff. These deposits include, from proximal to distal settings, breccia, lapilli and fine-ash tuff. The breccia and lapilli tuff are partly welded, as indicated by the presence of fiamme, fluidal and axiolitic structures. The lapilli tuff form idealized sections with a lower, coarse and welded unit and an upper, bedded and unwelded fine-ash tuff. Sedimentary structures suggest that the fine-ash tuff units were deposited by turbidity currents. Welded breccias, interbedded in a thick submarine volcanic complex, indicate the close proximity of the volcanic source. The lapilli and fine-ash tuff are interbedded in a thick marine sequence composed of alternating sandstones and shales. Presence of a marine stenohaline fauna and sedimentary structures attest to a marine depositional environment below storm-wave base.In northern Anatolia, thick massive sequences of rhyodacitic crystal tuff are interbedded with the Upper Cretaceous marine turbidites of the Mudurnu basin. Some of these tuffs are welded. As in southern Vosges, partial welding is attested by the presence of fiamme and fluidal structures. The latter are frequent in the fresh vitric matrix. These tuff units contain a high proportion of vitroclasis, and were emplaced by ash flows. Welded tuff units are associated with non-welded crystal tuff, and contain abundant bioclasts which indicate mixing with water during flowage. At the base, basaltic breccia beds are associated with micritic beds containing a marine fauna. The welded and non-welded tuff sequences are interbedded in an alternation of limestones and marls. These limestones are rich in pelagic microfossils.The evidence above strongly suggest that in both examples, tuff beds are partly welded and were emplaced at high temperature by subaqueous ash flows in a permanent marine environment. The sources of the pyroclastic material are unknown in both cases. We propose that the ash flows were produced during submarine fissure eruptions. Such eruptions could produce non-turbulent flows which were insulated by a steam carapace before deposition and welding. The welded ash-flow tuff deposits of southern Vosges and northern Anatolia give strong evidence for existence of subaqueous welding.     

Subauroral red arcs as a conjugate phenomenon: comparison of OV1-10 satellite data with numerical calculations

This study compares the OV1-10 satellite measurements of the integral airglow intensities at 630 nm in the SAR arc regions observed in the northern and southern hemisphere as a conjugate phenomenon, with the model results obtained using the time-dependent one-dimensional mathematical model of the Earth ionosphere and plasmasphere (the IZMIRAN model) during the geomagnetic storm of the period 15–17 February 1967. The major enhancements to the IZMIRAN model developed in this study are the inclusion of He+ ions (three major ions: O⁺ H⁺ and He⁺ and three ion temperatures), the updated photochemistry and energy balance equations for ions and electrons, the diffusion of NO⁺ and O⁺₂ ions and O(¹D) and the revised electron cooling rates arising from their collisions with unexcited N₂, O₂ molecules and N₂ molecules at the first vibrational level. The updated model includes the option to use the models of the Boltzmann or non-Boltzmann distributions of vibrationally excited molecular nitrogen. Deviations from the Boltzmann distribution for the first five vibrational levels of N₂ were calculated. The calculated distribution is highly non-Boltzmann at vibrational levels v > 2 and leads to a decrease in the calculated electron density and integral intensity at 630 nm in the northern and southern hemispheres in comparison with the electron density and integral intensity calculated using the Boltzmann vibrational distribution of N₂. It is found that the intensity at 630 nm is very sensitive to the oxygen number densities. Good agreement between the modeled and measured intensities is obtained provided that at all altitudes of the southern hemisphere a reduction of about factor 1.35 in MSIS-86 atomic oxygen densities is included in the IZMIRAN model with the non-Boltzm-ann vibrational distribution of N². The effect of using of the O(¹D) diffusion results in the decrease of 4–6% in the calculated integral intensity of the northern hemisphere and 7–13% in the calculated integral intensity of the southern hemisphere. It is found that the modeled intensities of the southern hemisphere are more sensitive to the assumed values of the rate coefficients of O⁺(⁴S) ions with vibrationally excited nitrogen molecules and quenching of O⁺(²D) by atomic oxygen than the modeled intensities of the northern hemisphere.     

Fault mechanisms and tectonic implication of the 1985–1987 earthquake sequence in south-western Ethiopia

Integrated inversions of short-period P, broadband P, and long-period P &s waves are done for fault mechanisms, focal depths, seismic moments, and source-time functions from the largest four earthquakes of the 1985 and 1987 earthquake sequence in south-western Ethiopia. These earthquakes had similar normal-faulting mechanisms. The general trends of the fault planes follow the Main Ethiopian Rift which is in agreement with foreshock-aftershock distribution, surface breaks and geology. Despite the morphological discontinuity of the Main Ethiopian Rift at its southern tip, the mode of deformation of the continental crust under study shows its extension southward. There are no significant strike-slip components trending NW–SE in all the mechanisms which would have been associated with the Aswa Fault Zone in southern Sudan or Anza Rift in northern Kenya. We also infer that the relatively broad fracture zone at the southern extreme of the Main Ethiopian Rift demonstrates the early stage of the break-up between the Nubia and Somalia plates in comparison with the Main Ethiopian Rift proper and the Afar Depression. The main shock of the sequence (M_w = 6.3) ruptured at a depth of 6.8 km, shallower than expected since the depth of earthquakes generally increase southward from the Afar Depression. The shallow depth of earthquake occurrence is supported by surface deformations with an overall trend in the direction of the Main Ethiopian Rift.     

Amplitude and frequency characteristics of the 2001 Southern Peru, M w = 8.4 earthquake records

The Departments of Civil Engineering and Geophysics of the University of Chile, together with international institutions, deployed strong-motion stations in the northern Chile seismic gap. These instruments recorded the June 23, 2001 M _w = 8.4 earthquake that occurred in Southern Peru. This earthquake exhibited at stiff deep soil sites in northern Chile, relatively large maximum accelerations although the recording stations are located more than 400 km away from the epicentral region and 200 km from the southern edge of the rupture. Typical accelerations at these distances are in the order of 0.30 g, consIDerably larger than those expected from recently presented attenuation formulae. Frequency and Wavelet Decomposition of the signals are presented from which the evolution of the amplitude, as a function of selected frequency bands, is analyzed. Typical Central Frequency varies from 3 to 4.8 Hz for horizontal records and 4.5 to 9.5 Hz for vertical records. Ninety five percent of the record energy is concentrated below 11 Hz. Evolution of energy for bands higher than the average record frequency is relatively smooth, and for low frequency, the energy shows abrupt changes as a function of time. The sudden changes are associated to dominant large amplitude motions observed in most of the records. The high frequency content of the motion observed for this earthquake is correlated with the heterogeneities of the interplate contact. To statically characterize the energy evolution with time a smooth three-parameter envelope adjusted for each frequency band is used, therefore, comparison is possible and results could be applied for synthesis studies.