Widespread inflation and drainage of a pāhoehoe flow field: the Nesjahraun, Tingvellir, Iceland

This study describes the emplacement of the Nesjahraun, a basaltic lava flow that entered the lake Tingvallavatn, SW Iceland. High-resolution remotely sensed data were combined with fieldwork to map the flow field. Onshore, the Nesjahraun exhibits a variety of textures related to the widespread inflation and collapse of a pāhoehoe flow field. Its emplacement is interpreted as follows: Initially, the eruption produced sheet pāhoehoe. In the central part of the flow field, the lava has a platy-ridged surface, which is similar to some other lava flows in Iceland and on Mars. Here, the texture is interpreted to have formed by unsteady inflation of the brittle crust of stationary sheet pāhoehoe, causing it to break into separate plates. The ridges of broken pāhoehoe slabs formed as the plates of crust moved vertically past each other in a process similar to the formation of shatter rings. Upstream, fresh lava overflowed repeatedly from channels and tubes, covering the surface with shelly pāhoehoe. Formation of a 250-m-wide open channel through the flow field allowed the inflated central part of the flow to drain rapidly. This phase produced ‘a‘ā lava, which eroded the channel walls, carrying broken pāhoehoe slabs, lava balls and detaching large (>200 m long) rafts of compound shelly pāhoehoe lava. Much of this channelized lava flowed into the lake, leaving a network of drained channels and tubes in the upstream part of the flow. As in other locations, the platy-ridged texture is associated with a low underlying slope and high eruption rate. Here, its formation was possibly enhanced by lateral confinement, hindered entry into the lake and an elevated vent location. We suggest that formation of this type of platy-ridged lava, where the plates are smooth and the ridges are slabs of broken pāhoehoe, can occur without significant horizontal transport, as the surface crust is broken into plates in situ. This reconstruction of the emplacement of the Nesjahraun also demonstrates that high-resolution aerial survey data are extremely useful in the mapping and measurement of lithofacies distributions in large flow fields, but that fieldwork is still necessary to obtain the detailed textural information necessary to interpret them.     

The formation of circular littoral cones from tube-fed pāhoehoe: Mauna Loa,Hawai'i

Pyroclastic cones along the southwest coast of Mauna Loa volcano, Hawai'i, have a common structure: (a) an early formed circular outer rim 200–400 m in diameter composed mostly of scoria and lapilli, and (b) one or more later-formed inner rims composed almost exclusively of dense spatter. The spatter activity locally fed short lava flows that ponded within the outer rims. Based on various lines of evidence, these cones are littoral in origin: relationships between the cones and associated flows; the degassed nature of the pyroclasts; and (although not unequivocal) the position of the cones relative to known eruptive vent locations on Mauna Loa. Additional support for the littoral interpretation comes from their similarity to (smaller) littoral cones that have been observed forming during the ongoing Kilauea eruption. The structure of these Mauna Loa cones, however, contrasts with that of standard Hawaiian littoral cones in that there is (or once was) a complete circle of pyroclastic deposits. Furthermore, they are large even though associated with tubefed phoehoe flows instead of 'a'. The following origin is proposed: An initial flow of tube-fed phoehoe into the ocean built a lava delta with a base of hyaloclastite. Collapse of an inland portion of the active tube into the underlying wet hyaloclastites or a water-filled void allowed sufficient mixing of water and liquid lava to generate strong explosions. These explosions broke through the top of the flow and built up the outer scoria/lapilli rims on the solid carapace of the lava delta. Eventually, the supply of water diminished, the explosions declined in intensity to spattering, and the initial rim was filled with spatter and lava.     

Lava penetrating water: the different behaviours of pāhoehoe and ‘a‘ā at the Nesjahraun, Tingvellir, Iceland

The Nesjahraun is a basaltic lava flow erupted from a subaerial fissure, extending NE along the Tingvellir graben from the Hengill central volcano that produced pāhoehoe lava followed by ‘a‘ā. The Nesjahraun entered Iceland’s largest lake, Tingvallavatn, along its southern shore during both phases of the eruption and exemplifies lava flowing into water in a lacustrine environment in the absence of powerful wave action. This study combines airborne light detection and ranging, sidescan sonar and Chirp seismic data with field observations to investigate the behaviour of the lava as it entered the water. Pāhoehoe sheet lava was formed during the early stages of the eruption. Along the shoreline, stacks of thin (5–20 cm thick), vesicular, flows rest upon and surround low (<5 m) piles of coarse, unconsolidated, variably oxidised spatter. Clefts within the lava run inland from the lake. These are 2–5 m wide, >2 m deep, ∼50 m long, spaced ∼50 m apart and have sub-horizontal striations on the walls. They likely represent channels or collapsed tubes along which lava was delivered into the water. A circular rootless cone, Eldborg, formed when water infiltrated a lava tube. Offshore from the pāhoehoe lavas, the gradient of the flow surface steepens, suggesting a change in flow regime and the development of a talus ramp. Later, the flow was focused into a channel of ‘a‘ā lava, ∼200–350 m wide. This split into individual flow lobes 20–50 m wide along the shore. ‘A‘ā clinker is exposed on the water’s edge, as well as glassy sand and gravel, which has been locally intruded by small (<1 m), irregularly shaped, lava bodies. The cores of the flow lobes contain coherent, but hackly fractured lava. Mounds consisting predominantly of scoria lapilli and the large paired half-cone of Grámelur were formed in phreatomagmatic explosions. The ‘a‘ā flow can be identified underwater over 1 km offshore, and the sidescan data suggest that the flow lobes remained coherent flowing down a gradient of <10°. The Nesjahraun demonstrates that, even in the absence of ocean waves, phreatomagmatic explosions are ubiquitous and that pāhoehoe flows are much more likely to break up on entering the water than ‘a‘ā flows, which, with a higher flux and shallow underlying surface gradient, can penetrate water and remain coherent over distances of at least 1 km.     

PROSPECTING FOR THE HIDDEN ACTIVE FAULT BY MEASURING THE 218p0 AND 210p0

A method of prospecting for the concealed active fault by simultaneous measuring both 218Po and 210Po is introduced.It is a radioactive prospecting.The concentration of radon near the fault can be determined from the activities of its daughters 218Po and 210Po.At the fault,there is a good passage for gases,and the radioative gas from crustal depths can be easy to get to the surface.As a result,the level of the radioactivity there is quite high,but low at both sides far from there.The     

中国大陆性地震的余震活动的p值

本文利用点过程的最大似然统计方法,对中国大陆的32个5级以上地震的余震序列的频度随时间的衰减变化特征进行了定量的分析.结果表明,中国东部和南北地震带的余震序列频度衰减系数p值的平均值为0.91.小于中国西部地区的p值.并且将中国大陆地震的p值与日本地区的p值结果进行比较发现,中国大陆地震的余震频度衰减系数p值比日本列岛的p值要小.且余震序列的频度衰减系数p值的大小与主震震级无关.      

青藏高原地区的p波走时与上地幔结构

本文利用天然地震资料,研究了青藏高原及其邻区的P波走时特征。从2°到26°的走时可以近似用三段直线来描述,视速度在10°和19°附近有明显的变化。推断的上地慢结构表明:高原上地慢顶部属于岩石层范畴,P波速度为8.11±0.04公里/秒:腾冲地区存在低速层,其速度为7.59±0.09公里/秒,深度在40公里至170公里之间;“20°间断面”的深度在450～500公里,界面下的速度约为9.9公里/秒;从M界面至“20°间断面”之间的P波速度是随深度的增加而增加的。此外,还对本文结果的构造物理意义进行讨论。     

转换波和非转换波与射线参数p的关系

本文对Aki固体-固体分界面的反射和透射公式进行适当变换,使之成为一组适合不同坐标系下,固体、液体、气体和真空之间所有可能形成的分界面上,入射P、SV和SH波所形成的反射和透射公式,从而把不同边界条件的反射和透射方程统一起来,并且阐明了转换波和非转换波动力学特征的差异.指出:在不同弹性介质分界面上入射平面弹性波,转换波的反射系数和透射系数是射线参数p的奇函数;非转换波的反射系数和透射系数是射线参数p的偶函数.根据奇函数和偶函数的性质可见:垂直入射时不存在转换波.同时也为简化Zoeppricz方程,开展AVO分析打开新思路.     

认清形势,明确任务,抓住机制,依法行政,努力提高江苏防震减灾工作水平

总结回顾了江苏二十多年来的防震减灾工作,分析了当前搞好防震减灾工作的形势,从７个方面阐述了要抓住机遇,依法行政提高全省防震减灾工作水平的有关问题。     

The fault plane of the 1991,Datong-Yang-gao,Shanxi,China,Ms=5.8 earthquake

Ｔｈｅｆａｕｌｔｐｌａｎｅｏｆｔｈｅ１９９１，Ｄａｔｏｎｇ－Ｙａｎｇ－ｇａｏ，Ｓｈａｎｘｉ，Ｃｈｉｎａ，Ｍｓ＝５．８ｅａｒｔｈｑｕａｋｅＭｉｎｇＷＡＮＧ（王鸣），Ｌｉａｎ－ＱｉａｎｇＺＨＡＮＧ（张廉强）ａｎｄＰｅｉ－ＤｅＷＡＮＧ（王培德）（Ｉｎｓｔｉ...     

梅岭,梅关,古道与梅花

梅岭,又名大庾岭,为南岭山系五岭之首。梅关是粤赣交界的险关要塞,素有“岭南第一关”之誉,秦汉以前已存在。梅岭以梅花著称。古道与关同时出现,到唐初张九龄扩展岭路成坦途。