A comet in the lab

What have Stardust samples told us about the early solar system? Phil A Bland, Anton T Kearsley, P J Wozniakiewicz, M J Burchell, M Gounelle, M E Zolensky and Matt J Genge have some of the answers – and a few more questions.     

A comet impact in AD 536?

Emma Rigby, Melissa Symonds and Derek Ward-Thompson review the evidence for the possibility that a comet may have impacted the Earth in historical times, and discuss the size of the putative comet.     

The great lakes clean-up

The Great Lakes are at last to be cleaned up. An agreement signed in Canada by President Nixon commits Canada and the US to spend 3,500 million dollars over five years de-polluting the immense lakes—Lake Erie is twice the size of Jamaica and Lake Ontario is twice the size of Cyprus. Currently, Erie has become known as the dead lake—but in a few years game fish could be back in it.     

Refining late Quaternary plunge pool chronologies in Australia's monsoonal 'Top End'

Plunge pool deposits from Australia's 'Top End' are considered as important archives of past monsoonal activity in the region. The available chronology of these deposits was so far based on thermoluminescence (TL) dating and indicated maximum flood magnitudes during the Last Glacial Maximum in contrast with more arid conditions as deduced from other archives of the region. This study revisits plunge pool deposits at Wangi Falls by applying multiple and single-grain Optically Stimulated Luminescence (OSL) dating of quartz and high-resolution gamma spectrometry, supported by radiocarbon dating of organic material. The aim is to reappraise the existing chronology and investigate if the deposits are affected by partial bleaching, post-depositional mixing and/or problems related to annual dose determination. The latter seems to have a minor impact on the ages at most. Equivalent Dose (D_e) distributions are broad, in particular for single grains, but apparently not result from partial bleaching or post-depositional mixing. Rather, microdosimetry caused by radiation hotspots in the sediment and zircon inclusions in the quartz grains is considered problematic for these sediments. The results presented here imply that the previous TL chronology overestimated the real deposition age of the sediments.     

The 1957 great Aleutian earthquake

The 9 March 1957 Aleutian earthquake has been estimated as the third largest earthquake this century and has the longest aftershock zone of any earthquake ever recorded—1200 km. However, due to a lack of high-quality seismic data, the actual source parameters for this earthquake have been poorly determined. We have examined all the available waveform data to determine the seismic moment, rupture area, and slip distribution. These data include body, surface and tsunami waves. Using body waves, we have estimated the duration of significant moment release as 4 min. From surface wave analysis, we have determined that significant moment release occurred only in the western half of the aftershock zone and that the best estimate for the seismic moment is 50–100×10²⁰ Nm. Using the tsunami waveforms, we estimated the source area of the 1957 tsunami by backward propagation. The tsunami source area is smaller than the aftershock zone and is about 850 km long. This does not include the Unalaska Island area in the eastern end of the aftershock zone, making this area a possible seismic gap and a possible site of a future large or great earthquake. We also inverted the tsunami waveforms for the slip distribution. Slip on the 1957 rupture zone was highest in the western half near the epicenter. Little slip occurred in the eastern half. The moment is estimated as 88×10²⁰ Nm, orM _w =8.6, making it the seventh largest earthquake during the period 1900 to 1993. We also compare the 1957 earthquake to the 1986 Andreanof Islands earthquake, which occurred within a segment of the 1957 rupture area. The 1986 earthquake represents a rerupturing of the major 1957 asperity.     

Bias in converging great circle methods

In palaeomagnetic studies remagnetization circles are often observed to converge. The circles represent the simultaneous decay of two magnetic components, and the convergence zones represent estimates of their directions. It is shown that these estimates are biassed, the degree of biassing depending upon the distribution parameters. In particular the ratio of the precision of the underlying distributions (κ₂κ₁) is very important. Only whenκ₂κ₁ is very different from unity can converging great circle methods give accurate results. Of the examples available in the literature most appear to satisfy this criterion, but the indiscriminate use of these methods can lead to significantly biasssed results.     

Seismic moment of great deep shocks

The long-period Rayleigh waves were investigated for the largest four deep shocks in 1963–1973 to determine the seismic moment by the same technique as used for shallow earthquakes. The results could be used for a quantitative comparison of source parameters between shallow and deep events. Three of the four shocks occurred beneath the South American continent (the Colombia earthquake, 1970; the western Brazil earthquake, 1963; the Peru—Bolivia border earthquake, 1963) and the other beneath the Japan Sea (1973). The focal depths are 653, 576, 593 and 575 km, respectively. The largest value of seismic moment was obtained as 2.1 · 10²⁸ dyncm for the Colombia earthquake. This value is still about forty times smaller than that for the great Alaskan earthquake. A slight inconsistency was found between the first-motion diagram and the Rayleigh wave radiation pattern for the Colombia earthquake and the Peru—Bolivia border earthquake.     

Tsunami and mechanism of great earthquakes

The relation between tsunamis and sea-bottom deformations associated with the Kurile Islands earthquake of 1969 and the Tokachi-Oki earthquake of 1968 is studied on the basis of a fairly complete set of seismological and tsunami data. The seismic results are included in the calculation of static crustal deformations. The calculated deformations are compared with the tsunami source area as obtained by the inverse refraction diagram, the first motion of tsunami waves, and the height of the sea-level disturbance at the source. It is found that such deformations as predicted by the seismic results can quantitatively explain the source parameters of tsunamis. These findings strongly favor the idea that tsunamis are generated by tectonic deformations rather than by large submarine landslides and slumps. This conclusion is supported by additional analyses for the 1964 Niigata, 1944 Tonankai, 1933 Sanriku earthquakes. For the 1946 Nankaido earthquake, the source deformation responsible for the tsunami generation is of much greater magnitude than that for seismic waves.     

Torsional phenomena in 2008 great Wenchuan earthquake

It is well known that there are some torsional damages in earthquakes. In Taibai park, Jiangyou city, Sichuan province, most of the stone statues, which were placed upon the banisters of one zigzag bridge, exhibited different torsional phenomena in 2008 Wenchuan earthquake. This paper introduces the torsional phenomena of all the statues on the zigzag bridge firstly. Then one eccentricity model is established and the equivalent rotational accelerations are calculated in order to analyze the causes of the to...     

2009年萨摩亚—汤加特大地震触发的两次大震

特大地震(≥8.0级的地震)一般都含有构造板块之间边界岩体的突然滑动。这种板间的破裂通常出现在俯冲带特大逆冲事件的海沟斜坡区,会产生动态及静态应力变化,从而激活周边的板内余震(Christensen and Ruff,1988;Dmowskaetal,1988;Layetal,1989;Ammonetal,2008)。本文研究的地震序列展示一少见的例子———特大海沟斜坡的一次板内地震触发了广泛的板间断层活动,颠倒了典型的活动模式,从而广泛地扩大了地震和海啸灾害。2009年9月29日,在汤加俯冲带北端的外海沟斜坡发生矩震级8.1的正断层地震事件,该震开始破裂后的2分钟内,发生了总地震矩等于矩震级8.0级的第二个特大地震,它由两次(矩震级均为7.8级)板间下插逆冲大地震组成,导致了周边俯冲带巨型逆断层的破裂。联合的断层作用引发了海啸,局部地区抬升约12m,导致萨摩亚、美属萨摩亚和汤加192人死亡。地震信号的重叠掩盖了这样的事实:相隔约50km的性质不同的断层发生了不同几何形状的破裂,这些被触发的逆冲断层滑动只有通过详细的地震波分析才能揭示出来。在汤加北部俯冲带的大部分区域内,激活了广泛的板间和板内余震活动。     

Liquefaction macrophenomena in the great Wenchuan earthquake

On May 12, 2008 at 14:28, a catastrophic magnitude M 8.0 earthquake struck the Sichuan Province of China.The epicenter was located at Wenchuan (31.00°N, 103.40°E). Liquefaction macrophenomena and corresponding destruction was observed throughout a vast area of 500 km long and 200 km wide following the earthquake. This paper illustrates the geographic distribution of the liquefaction and the relationship between liquefaction behavior and seismic intensity, and summarizes the liquefaction macrophenomena, including sandboils and waterspouts, ground subsidence, ground fissures etc., and relevant liquefaction features. A brief summary of the structural damage caused by liquefaction is presented and discussed. Based on comparisons with liquefaction phenomena observed in the 1976 Tangshan and 1975 Haicheng earthquakes, preliminary analyses were performed, which revealed some new features of liquefaction behavior and associated issues arising from this event. The site investigation indicated that the spatial non-uniformity of liquefaction distribution was obvious and most of the liquefied sites were located in regions of seismic intensity Ⅷ. However, liquefaction phenomena at ten different sites in regions of seismic intensity Ⅵ were also observed for the first time in China mainland. Sandboils and waterspouts ranged from centimeters to tens of meters, with most between 1 m to 3 m. Dramatically high water/sand ejections,e.g., more than 10 m, were observed at four different sites. The sand ejections included silty sand, fine sand, medium sand,course sand and gravel, but the ejected sand amount was less than that in the 1976 Tangshan earthquake. Possible liquefaction of natural gravel soils was observed for the first time in China mainland.     

大地震的18.6年周期

将全球分为15个研究区,用1900~2009年MW≥7.0地震目录,统计分析了各区大地震与月球交点运动周期的关系,得出15个研究区中有10个区,大地震存在统计意义上的18.6 a周期:活跃段为12.4 a,平静段为6.2 a;环太平洋地震带北、南、西、东4大区的大地震,不仅有这样的周期,而且其地震活跃段的时间存在一定规律.用第6个18.6 a(1991~2009年)期间的大地震,检验据前5个18.6 a(1900~1990年)地震目录所得18.6 a 周期的稳定性和实用性,发现有这种周期的地区多数的周期性是稳定的.大地震18.6 a周期的可能成因有: (1)18.6 a潮波通过调制日潮和半日潮调制大地震; (2)上地幔内流体的潮汐(地内潮)作用; (3)18.6 a潮波通过影响地球自转变化调制大地震.     

日本大地震的科学共识来得太迟

10年前,位于仙台市的东北大学（Tohoku University）地质学家Koji Minoura及其同事对一次传说中的灾难给出了一些科学的解释。一篇公元901年编写的历史文献对公元869年的一次地震（Jogan earthquake）进行了描述,那次地震使日本东北部的一座城堡城遭到毁坏,     

Correlation between lunar node tide and great earthquakes

This study selects six consecutive 18.6 years of global M ≥ 7.0 earthquakes, calculates the lunar declination angle based on the time of the earthquakes, and di...     

Australian astronomers: what made them great?

Ragbir Bhathal examines the life and times of Australia's eminent astronomers and finds that the personal contacts made in a nomadic research career in turn foster success in international research.