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《地球物理学进展》1986,(2)
本文给出地球的地幔热状态和热系统的简化形式,假定地幔内的大规模对流是双层的,在上地幔内的对流是由来自下面的热流量来测定,而在下地幔内的对流是用内部加热来测定。估算上地幔的热弛豫时间是～10~8年,下地幔的热弛豫时间τ_(1m)是～1.5×10~9年,地球的热弛豫是由τ_(1m)来测定。在对流的地幔内的超绝热的温度降是在热的边界层内形成的:在上地幔内,△T_1≤600K;在下地幔顶部,△T_2≤650K;在下地幔底部,△T_3≤650K。核-幔边界上的绝热温度是～2900K,真实温度一定是～3500—4500K。地幔中的热边界层都是过热区,必须跟低Qμ区(Qμ是对S波的力学品质因数)一致。低Qμ的第一层的深度是～100—250公里,第二层的深度是670—850公里,第三层是2700—2900公里。热点的热源区可能在第二个热边界层内,有意义的是,该层的水平速度(～1公分/年)可以确定在“板块构造”中绝对速度的精确度范围。因为上地幔是地壳之源,所以上地幔和下地幔之间的边界一定是弱的化学边界。第三个热边界层(深度在2600—2900公里)的热系统及其不稳定性在地核的磁流体动力学方面尤其是对于地磁偶极子的极性改变以及对地磁场时间尺度的了解有非常重要的作用。 相似文献
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为了研究辽宁省海城7.3级地震发生的地质背景--辽南地区的地壳构造,根据已有的反映深部界面起伏的重力资料,以及本文提出的方法除去新生代沉积和浅部构造影响后的重力异常,用等效压缩质面方法计算莫霍界面深度。并采用密度差异的原理,从重力异常中分解出康拉界面的影响,从而求得康拉界面的深度。根据13条计算剖面的结果,给出辽南地区的康拉界面和莫霍界面的分布。根据莫霍界面的起伏,此地区大致可划分为三个部分。中部:为一北东向的上地幔隆起带,北低南高,一直向西南延伸入渤海。隆起的东南坡陡,西北坡缓,且呈台阶式次第下倾。在此大的构造背景上展布了一系列具有相当规模的次一级正、负构造。地壳厚度在隆起部分为31-32公里,但其东缘较薄,约为30公里左右。东南部:地壳厚度一般为34-35公里,最大可达38公里.西北部:地壳厚度向西北方向由34公里逐渐增至40公里左右。海城地震就发生在中部隆起带的较陡的东南坡上。 文中对有关的计算方法作了简要的介绍。并对一些问题进行了讨论。 相似文献
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由50余个测点的大地电磁测深资料,讨论了该区的地壳-上地幔电性横向变化特征。按照上地幔第一低阻层顶面埋深,将测区划分为四类地区:浅埋深(55-90公里)、中浅埋深(90-110公里)、中深埋深(110-160公里)和深埋深(160-250公里)。讨论了本区六次大震例的深部电性背景。指出了上地幔顶部的梯度带地区、地壳内电性横向变化剧烈的地区和地壳内存在局部特殊增厚的低阻层地区将可以作为潜在震源区的深部电性判据之一 相似文献
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大地电磁测深结果表明,唐山地区深部电性中存在壳内高导层和上地幔高导层,其分别埋深为20公里和100公里左右,均显示为南浅北深。资料对比表明唐山、宁河地震都发生在两个高导层埋深变化梯度带上,其震源深度都不超过壳内高导层的埋深。此外,在该区南部发现埋深约45公里高导层,我们认为有可能为上地幔中间高导层 相似文献
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重力均衡的现象证明,地壳或上地幔内部必有水平方向的物质流动,它产生重力补偿作用,并且最终达到平衡状态。上地幔的低速层（其顶面在大陆地区约为120公里,在海洋地区约为60公里）可能是产生全球范围内重力补偿作用的一个重要层。地壳内部如有重要的补偿层,莫霍界面的深度将趋一致。莫霍界面深度的变化,说明地壳内部必无较大范围的补偿层。以莫霍界面为顶面的上地幔部分,对于重力补偿是否起作用呢?这个问 相似文献
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京津唐渤及其周围地区是我国的强烈地震活动区之一。自1976年以来,我们在该区开展大地电磁测深工作,完成了近30个测点。所得结果表明,本区壳内存在高导层,与地震方法确定的壳内低速层一致。平原内上地幔高导层埋深50-80公里,山区大于100公里,与地震方法确定的上地幔低速层基本一致,同时与大地热流测量、居里等温面计算和对新生代玄武岩地球化学研究结果基本吻合。本区绝大多数地震位于壳内高导层之上,强烈地震主要发生在上地幔高导层隆起的边缘。最后讨论了本区强震活动与壳内和上地幔高导层的关系。 相似文献
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本文应用Aki等的三维反演方法,反演了中国东部地区大尺度上地幔三维P波速度结构,其中采用了300个远震事件在所研究区域(100°—125°E,20°—45°N)的50多个台站的P波到时残差。结果表明:(1)中国东部地区上地幔横向不均匀性至少深达680公里,其中40—400公里范围内的结构具有共同特点,仍保持有地表大地构造的痕迹,400公里深度以下,不均匀性逐渐减小,地表构造痕迹消失。(2)存在一条北北东走向的深层构造带将中国东部地区分成两部分,西部速度稳定、偏高,东部速度复杂,低速高速随深度变化而交替出现。 相似文献
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根据我国11平均布格重力异常图,采用Parker-Oldenbug位场反演方法,做了全国范围的三维重力反演。Parker的位场理论公式不仅数学上严密,而且运算速度甚高。本文将近些年大量的地震测深资料做为三维模型的控制点输入,并对反演计算采取了多种措施,以改善结果的合理性。本研究不仅得到了中国地区地壳厚度的分布,还首次给出了至120公里深处上地幔的密度分布。 结果表明,地壳厚度自东部沿海地区30——40公里逐渐增至青藏高原大部分地区的68公里,东部变化平缓,西部复杂,与大地构造有一定对应性。青藏高原及西部地区的上地幔密度总体偏高,一般3.40——3.65克/厘米3。东部广大区域在3.23——3.30克/厘米3。东部重力梯度带反映着下地壳与上地幔中一条深层构造带的存在,它不仅是地壳厚度陡变带,也是上地幔中的高密度带（3.29——3.35克/厘米3）,具有大区域性的控制作用。重力的均衡调整是青藏高原隆起的重要因素之一。 作者初步推断,中国地区的上地幔可大体分为青藏高原区、中部过渡区和东部正常区等三个大区,反映着上地幔结构的横向不均匀性。据此,本文解释了某些地球物理现象。 相似文献
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阪神·淡路大震灾后 ,日本着手对原有的地震观测体制作彻底改革 ,多项全国性配置地震计的计划同时开始启动。地震引起的地面晃动的程度不尽相同 ,无感微小地震引发的晃动仅有 0 .0 0 1 μm,频率为1 0 Hz;巨大地震引起的地面晃动的幅度高达 2～ 3 m,周期也有几十秒。虽说都称之为地震计 ,但由于地震动的振幅和频率涉及的范围太大 ,所以应针对观测对象选用不同类型的地震计。下面介绍不同类型的观测网。1 强震观测网 ( K- NET)无论大地有多大的晃动都能将其准确记录下来的地震计称之为强震计。它构造坚实 ,信号明确 ,通常设置在地表 ,对了… 相似文献
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Michael O. Garcia Diane Hanano Ashton Flinders Dominique Weis Garrett Ito Mark D. Kurz 《Bulletin of Volcanology》2012,74(6):1445-1463
The size, shape, and magmatic history of the most recently discovered shield volcano in the Hawaiian Islands, Mahukona, have been controversial. Mahukona corresponds to what was thought to be a gap in the paired sequence (Loa and Kea trends) of younger Hawaiian volcanoes (<4?Ma). Here, we present the results of marine expeditions to Mahukona where new bathymetry, sidescan sonar, gravity data, and lava samples were collected to address these controversies. Modeling of bathymetric and gravity data indicate that Mahukona is one of the smallest Hawaiian volcanoes (～6,000?km3) and that its magmatic system was not focused in a long-lived central reservoir like most other Hawaiian volcanoes. This lack of a long-lived magmatic reservoir is reflected by the absence of a central residual gravity high and the random distribution of cones on Mahukona Volcano. Our reconstructed subsidence history for Mahukona suggests it grew to at least ～270?m below sea level but probably did not form an island. New 40Ar–39Ar plateau ages range from 350 to 654?ka providing temporal constraints for Mahukona’s post-shield and shield stages of volcanism, which ended prematurely. Mahukona post-shield lavas have high 3He/4He ratios (16–21?Ra), which have not been observed in post-shield lavas from other Hawaiian volcanoes. Lava compositions range widely at Mahukona, including Pb isotopic values that straddle the boundary between Kea and Loa sequences of volcanoes. The compositional diversity of Mahukona lavas may be related to its relatively small size (less extensive melting) and the absence of a central magma reservoir where magmas would have been homogenized. 相似文献
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In this study, we have checked the location and focal depth of the Yecheng earthquake (m b = 6.0, maximum intensity VII) of February 14, 1980. The result shows that this is an intermediate event with a focal depth of 90 km. The microepicenter is located at 36. 4°N, 76, 9°E, while the macroepicenter is at 37.3°N, 76.9°E, 90 km to the north of the microepicenter. This is the first destructive intermediate event in China which led to a damage as severe as of intensity VII. The focal mechanism of the event is determined to be of thrust type. Combined with the analysis of seismological and geological data in surrounding area, the possible relation between the event and plate movement has been discussed. The result in this paper indicates that in some particular place, the destructive effect of intermediate event should be considered in seismic hazard assessment. 相似文献
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In this study, we experimentally established the relationship between physical properties, vitrinite reflectance, and microstructure of coal, Taiyuan Formation, Qinshui Basin, China using representative coal samples collected from three different mines via the rock mechanics testing system (MTS). We analyzed the organic macerals, vitrinite reflectance, and microstructure of 11 coal samples using petrography and scanning electron microscopy (SEM). The experimental results suggest that (1) the elastic parameters can be described by linear equations, (2) both P-and S-wave velocities display anisotropy, (3) the anisotropy negatively correlates with vitrinite reflectance, and (4) the acoustic velocities and Young’s modulus are negatively correlated with the volume of micropores. The derived empirical equations can be used in the forward modeling and seismic inversion of physical properties of coal for improving the coal-bed methane (CBM) reservoir characterization. 相似文献
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Abu Sayeed Shafiuddin Ahmed Mohammad Belal Hossain Saad Mohammad Omar Faruque Babu Moshiur Rahman Jun Sun Mohammad Shafiqul Islam Sarker 《国际泥沙研究》2022,37(1):83-96
Sediment samples from the Gomti River basin were investigated to determine and evaluate trace metal concentrations,their biological effect,and potential ecological and human health risks for adults and children.The mean concentrations of trace metals were organized in the descending order of mercury(Hg)(0.08 mg/kg)>chromium(Cr)(0.06 mg/kg)>lead(Pb)(0.05 mg/kg)>arsenic(As)(0.02 mg/kg)>cadmium(Cd)(0.01 mg/kg).The current study illustrated that metals were attributed to the area from natural sources and different anthropogenic sources especially from industries.However,the concentration levels were lower than the sediment quality guidelines(SQGs)based on the effect-range classifications of threshold effect concentration(TEC)and probable effect concentration(PEC).Therefore,the concentration of the elements showed no adverse biological effects on aquatic organisms.The evaluated potential ecological risk index also revealed the low toxicity to the aquatic environment.Moreover,the applied sediment quality indices,geo-accumulation index(Igeo),contamination factor(CF),and contamination severity index(CSI)suggested that the contamination levels of the elements were in the acceptable range,and the contamination had not notably impacted on the sediment quality.The risk index(HI)concerning age groups was significantly less than the threshold limit of 1 indicating that the contamination had no non-carcinogenic risk effect.The total carcinogenic risk(TCR)was less than a risk value of 1×104.Hence,the current study suggests that immediate remediation is not required due to an absence of alarming conditions in the study area.Proper monitoring of the attribution of the metal elements should be done for the betterment of human and environmental health。 相似文献
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Seismic data collected at four volcanoes in Central America during 1973 and 1974 indicate three sources of seismicity: regional
earthquakes with hypocentral distances greater than 80 km, earthquakes within 40 km of each volcano, and seismic activity
originating at the volcanoes due to eruptive processes. Regional earthquakes generated by the underthrusting and subduction
of the Cocos Plate beneath the Caribbean Plate are the most prominent seismic feature in Central America. Earthquakes in the
vicinity of the volcanoes occur on faults that appear to be related to volcano formation. Faulting near Fuego and Pacaya volcanoes
in Guatemala is more complex due to motion on a major E-W striking transform plate boundary 40 km north of the volcanoes.
Volcanic activity produces different kinds of seismic signatures. Shallow tectonic or A-type events originate on nearby faults
and occur both singly and in swarms. There are typically from 0 to 6 A-type events per day withb value of about 1.3. At very shallow depths beneath Pacaya, Izalco, and San Cristobal large numbers of low-frequency or B-type
events are recorded with predominant frequencies between 2.5 and 4.5 Hz and withb values of 1.7 to 2.9. The relative number of B-type events appears to be related to the eruptive states of the volcanoes;
the more active volcanoes have higher levels of seismicity. At Fuego Volcano, however, low-frequency events have unusually
long codas and appear to be similar to tremor. High-amplitude volcanic tremor is recorded at Fuego, Pacaya, and San Cristobal
during eruptive periods. Large explosion earthquakes at Fuego are well recorded at five stations and yield information on
near-surface seismic wave velocities (α=3.0±0.2 km/sec.). 相似文献