Coma Morphology of Three Non-periodic Comets

The coma morphology and short-term evolution was investigated of three non-periodic comets in retrograde orbits, C/2001 Q4 (NEAT), C/2002 T7 (LINEAR), and C/2003 K4 (LINEAR). All three comets display distinct coma features, which were very different from one comet to the next and remained rather constant in shape during the observational period. A single, broad feature perpendicular to the sun-tail direction dominated the coma of C/2003 K4 in all used filters (B,V,R,I), whereas the coma of Comet C/2002 T7 exhibited different features in blue and red filters. C/2001 Q4 showed rather complex coma morphology with clear short-term variability in coma brightness. Therefore, these non-periodic comets neither show a featureless coma nor any similarities of the features detected. The overall distribution of coma material was investigated from the shape of radial coma profiles averaged around the comet nucleus. For C/2001 Q4 and C/2002 T7, the slopes fitted to the linear part of these profiles are flatter in the blue than in the red, which can be explained by the presence of coma gas. For C/2003 K4 no such difference is indicated in the May observations (r = 2.3 AU), while in July (r = 1.7 AU) the profiles in the B-filter are flatter than in V, R, and I, hence gas contamination was relevant at least in the B filter. The R and I filter images were used to determine approximate Afρ values of each comet as a function of time.     

Earthen barriers to control lava flows in the 2001 eruption of Mt. Etna

Preceded by four days of intense seismicity and marked ground deformation, a new eruption of Mt. Etna started on 17 July and lasted until 9 August 2001. It produced lava emission and strombolian and phreatomagmatic activity from four different main vents located on a complex fracture system extending from the southeast summit cone for about 4.5 km southwards, from 3000 to 2100 m elevation (a.s.l.). The lava emitted from the lowest vent cut up an important road on the volcano and destroyed other rural roads and a few isolated country houses. Its front descended southwards to about 4 km distance from the villages of Nicolosi and Belpasso. A plan of intervention, including diversion and retaining barriers and possibly lava flow interruption, was prepared but not activated because the flow front stopped as a consequence of a decrease in the effusion rate. Extensive interventions were carried out in order to protect some important tourist facilities of the Sapienza and Mts. Silvestri zones (1900 m elevation) from being destroyed by the lava emitted from vents located at 2700 m and 2550 m elevation. Thirteen earthen barriers (with a maximum length of 370 m, height of 10–12 m, base width of 15 m and volume of 25 000 m³) were built to divert the lava flow away from the facilities towards a path implying considerably less damage. Most of the barriers were oriented diagonally (110–135°) to the direction of the flow. They were made of loose material excavated nearby and worked very nicely, resisting the thrust of the lava without any difficulty. After the interventions carried out on Mt. Etna in 1983 and in 1991–1992, those of 2001 confirm that earthen barriers can be very effective in controlling lava flows.     

Soil liquefaction during the Arequipa Mw 8.4, June 23, 2001 earthquake, southern coastal Peru

The Arequipa June 23, 2001, earthquake with a moment magnitude of Mw 8.4 struck southern Peru, northern Chile and western Bolivia. This shallow (29 km deep) interplate event, occurring in the coupled zone of the Nazca subduction next to the southeast of the subducting Nazca ridge, triggered very localized but widely outspread soil liquefaction. Although sand blows and lateral spreading of river banks and road bridge abutments were observed 390 km away from the epicenter in the southeast direction (nearing the town of Tacna, close to the Chile border), liquefaction features were only observed in major river valleys and delta and coastal plains in the meizoseismal area. This was strongly controlled by the aridity along the coastal strip of Southern Peru. From the sand blow distribution along the coastal area, a first relationship of isolated sand blow diameter versus epicentral distance for a single event is ever proposed. The most significant outcome from this liquefaction field reconnaissance is that energy propagation during the main June 23, 2001, event is further supported by the distribution and size of the isolated sand blows in the meizoseismal area. The sand blows are larger to the southeast of the epicenter than its northwestern equivalents. This can be stated in other words as well. The area affected by liquefaction to the northwest is less spread out than to the southeast. Implications of these results in future paleoliquefaction investigations for earthquake magnitude and epicentral determinations are extremely important. In cases of highly asymmetrical distribution of liquefaction features such as this one, where rupture propagation tends to be mono-directional, it can be reliably determined an epicentral distance (between earthquake and liquefaction evidence) and an earthquake magnitude only if the largest sand blow is found. Therefore, magnitude estimation using this uneven liquefaction occurrence will surely lead to underrating if only the shortest side of the meizoseismal area is unluckily studied, which can eventually be the only part exhibiting liquefaction evidence, depending on the earthquake location and the distribution of liquefaction-prone environments.     

印尼苏门答腊MW9.0级地震对全球及中国大陆地震趋势影响研究的反思

本文对2004年12月26日印尼苏门答腊以西发生MW9.0级地震后所做的地震趋势预测做了反思,指出:关于全球特大地震近年可能连发,特大地震对几年内世界7级以上地震年频度没有明显影响,但未来几年内7级以上强震可能集中在这次特大地震附近或相关构造上的预测意见是正确的;而有关近年中国大陆及川滇地区可能发生7级强震的预测是错误的;并认为,2001年昆仑山口西8.1级地震释放了已积累的应变可能是这次特大地震不能触发中国大陆及川滇地区发生强震的重要原因。     

浅谈欧洲规范Eurocode 8-结构抗震设计

简单介绍了欧洲规范(Eurocodes)的基本情况,概括给出了欧洲规范8-结构抗震设计的基本内容和特点,进而在抗震设计基本要求、建筑抗震概念设计基本原则两个方面与我国建筑抗震设计规范(GB50011-2001)进行了比较和评述,然后在场地类别、地震作用和结构抗震验算等几个方面对两规范进行了简单的对比和评述。     

Hubble Space Telescope imaging of the progenitor sites of six nearby core-collapse supernovae

The search for the progenitors of six core-collapse supernovae (CCSNe) in archival Hubble Space Telescope ( HST ) WFPC2 pre-explosion imaging is presented. These SNe are 1999an, 1999br, 1999ev, 2000ds, 2000ew and 2001B. Post-explosion imaging of the SNe, with the HST ACS/WFC, has been utilized with the technique of differential astrometry to identify the progenitor locations on the pre-explosion imaging. SNe 1999br, 1999ev, 2000ew and 2001B are recovered in late-time imaging, and estimates of the progenitor locations on the pre-explosion imaging, with subpixel accuracy, have been made. Only the progenitor of the Type II-P SN 1999ev has been recovered, on pre-explosion F555W imaging, at a 4.8σ significance level. Assuming a red supergiant progenitor, the pre-explosion observation is consistent with   M _ZAMS= 15–18 M_⊙  . The progenitors of the other five SNe were below the 3σ detection threshold of the pre-explosion observations. The detection thresholds were translated to mass limits for the progenitors by comparison with stellar evolution models. Pre-explosion observations of the peculiarly faint SN 1999br limit the mass of a red supergiant progenitor to   M _ZAMS < 12 M_⊙  . Analysis has been extended, from previous studies, to include possible detections of high- T _eff, high-mass stars by conducting synthetic photometry of model Wolf–Rayet star spectra. The mass limits for the Type II-P SNe 1999an and 1999br are consistent with previously determined mass limits for this type of SN. The detection limits for the progenitors of the Type Ibc SNe (2000ds, 2000ew and 2001B) do not permit differentiation between high-mass Wolf–Rayet progenitors or low-mass progenitors in binaries.     

Be star candidates in the Large Magellanic Cloud: the catalogue and comparison with the Small Magellanic Cloud sample

We present a catalogue with coordinates and photometric data of 2446 Be star candidates in the Large Magellanic Cloud (LMC), based on a search of the OGLE II data base. The I -band light curves of these stars show outbursts in 24 per cent of the sample (Type-1 stars), high and low states in 10 per cent, periodic variations in 6 per cent (Type-3 stars), and stochastic variations in 60 per cent of the cases. We report on the result of the statistical study of light curves of Type-1 and Type-3 stars in the LMC, and the comparison with the previously reported results of the Small Magellanic Cloud (SMC) sample. We find a statistically significant difference between amplitude, duration and asymmetry distributions of outbursts in both galaxies. Outbursts of SMC Type-1 stars are usually brighter, longer and with a slower decline. We find a bimodal distribution of periods of Type-3 stars in both galaxies, probably related to the recently discovered double periodic blue variables. We find also period and amplitude distributions of Type-3 LMC stars statistically different from those of the SMC stars. Our findings above suggest that the mechanisms causing the observed photometric variability of Type-1 and Type-3 stars could depend on metallicity. Moreover, they suggest that the outbursts are not primarily caused by stellar winds.     

塔里木河流域2001年四源一干河川径流运行分析

2001年塔里木河流域四条源流出山口天然径流量计266.5×10⁸m³, 比多年平均值多40.70×10⁸m³, 增加1%.源流区在进入塔里木河干流前总耗水量为220.0×10⁸m³, 与多年平均值比较多43.4×10⁸m³, 增加24.6%, 而入塔里木河水量为46.4×10⁸m³, 比多年平均4.12×10⁸m³减少1.64×10⁸m³.塔里木河干流龙头站--阿拉尔站2001年年径流量为45.72×10⁸m³, 接近多年均值46.00×10⁸m³, 属平水年.干流上游区间耗水量24.77×10⁸m³, 占阿拉尔站年径流量的54.2%; 中游区间耗水量1.94×10⁸m³, 占阿拉尔站年径流量的41.4%; 下游区间耗水2.025×10⁸m³(纯塔里木河水), 仅占阿拉尔站年径流量的4.4%. 2001年从开都河-孔雀河的博斯腾湖和塔里木河向下游绿色走廊的应急输水于11月16日水流到台特玛湖, 从而结束了塔里木河下游近30 a干涸的历史, 使下游绿色走廊恶化环境开始恢复.     

2001-2002水文年环太湖河道的水量及污染物通量

根据2001-2002水文年115条环太湖河道的同步环境监测资料,对水量及污染物通量进行了估算.全年的入湖水量为80.11×108m3,出湖水量为96.67×108m3.入湖水量主要通过西部河网以及西苕溪、望虞河等河流汇入太湖,其中西部河网的入湖量占总入湖量的60%;出湖水量主要通过太浦河、东苕溪以及东部河网汇出太湖,其中太浦河的出湖量占47%.污染物通量的估算结果是,CODMn、TN及TP的入湖总通量分别为37571t/a、28658t/a及1029t/a,出湖总通量分别为35431t/a、14600t/a及668t/a.CODMn、TN及TP入湖通量通过西部河网进入太湖的比例占63%、49%及47%;CODMn、TN及TP出湖通量通过太浦河汇出太湖的比例占51%、45%及34%.通过与上世纪90年代以前相同年型的数据进行对比,除TP外,其它各种污染物的入湖量均明显增加,且污染物在湖泊中的滞留率也显著提高.由此说明,环太湖河道入湖污染负荷的增加是太湖水环境恶化的根本原因.