Cratering rates on the Galilean satellites

We exploit recent theoretical advances toward the origin and orbital evolution of comets and asteroids to obtain revised estimates for cratering rates in the jovian system. We find that most, probably more than 90%, of the craters on the Galilean satellites are caused by the impact of Jupiter-family comets (JFCs). These are comets with short periods, in generally low-inclination orbits, whose dynamics are dominated by Jupiter. Nearly isotropic comets (long period and Halley-type) contribute at the 1-10% level. Trojan asteroids might also be important at the 1-10% level; if they are important, they would be especially important for smaller craters. Main belt asteroids are currently unimportant, as each 20-km crater made on Ganymede implies the disruption of a 200-km diameter parental asteroid, a destruction rate far beyond the resources of today's asteroid belt. Twenty-kilometer diameter craters are made by kilometer-size impactors; such events occur on a Galilean satellite about once in a million years. The paucity of 20-km craters on Europa indicates that its surface is of order 10 Ma. Lightly cratered surfaces on Ganymede are nominally of order 0.5-1.0 Ga. The uncertainty in these estimates is about a factor of five. Callisto is old, probably more than 4 Ga. It is too heavily cratered to be accounted for by the current flux of JFCs. The lack of pronounced apex-antapex asymmetries on Ganymede may be compatible with crater equilibrium, but it is more easily understood as evidence for nonsynchronous rotation of an icy carapace.     

Migration and household change in the population aged 65 and over, 1971-1991

"This analysis uses the ONS [Office for National Statistics] Longitudinal Study, a record linkage study including individual-level data from three national Censuses (1971, 1981 and 1991) and linked vital registration data, to examine migration patterns among older people [in Great Britain]. The aims of this study are to examine regional differences in household composition, to look at changes in the relationship between household change and migration over time, and finally, to analyse the interrelationships between changes in household composition, health and migration in the 1981-91 period.... Despite...dramatic changes in the living arrangements of older people, results show that mobility among the elderly remained relatively stable between the 1971-81 and 1981-91 decades."     

著名地球物理学家Runcorn不幸逝世

Stanley Keith Runcorn(1922～1995)于12月5日在圣迭戈遇害身亡,他的不幸逝世使全世界的地球物理学家大为震惊。20世纪中叶关于地球磁场起源和大陆漂移理论是否成立这两个地球科学的决定性辩论中,Runcorn扮演了重要角色。1988年Runcorn从泰因河畔纽卡斯尔大学退休后,客座于阿拉斯加大学,他是从该校出发去     

用地理信息系统研究1993年钏路近海地震破坏和地震烈度分布的评估方法

引言 1993年1月15日钏路近海地震是震级为7.8的强震,震源深度约为30 km,震中位于北海道钏路近海,北海道至关东地区有感,由于接近震源,钏路市烈度鉴定为日本烈度表的6度。震后4天本文诸作者开始在现场调查,包括破坏调查和场地信息的搜集,然后用约2～3个月的时间,     

Role of Triad Kinetic Energy Interactions for Maintenance of Upper Tropospheric Low Frequency Waves during Summer Monsoon 1988

ＲｏｌｅｏｆＴｒｉａｄＫｉｎｅｔｉｃＥｎｅｒｇｙＩｎｔｅｒａｃｔｉｏｎｓｆｏｒＭａｉｎｔｅｎａｎｃｅｏｆＵｐｐｅｒＴｒｏｐｏｓｐｈｅｒｉｃＬｏｗＦｒｅｑｕｅｎｃｙＷａｖｅｓｄｕｒｉｎｇＳｕｍｍｅｒＭｏｎｓｏｏｎ１９８８Ｄ．Ｒ．Ｃｈａｋｒａｂｏｒｔｙａ...     

Investigation on the Load/Unload Response Ratio With Coda Q-1 as Response

The load/unload response ratio YQ with the geophysical parameter coda Q-1 of the crust as response is denned in this study.The variation in YQ-1 before and after the Northridge earthquake of January 17,1994(California)has been investigated by using the data of coda Q-1 with frequencies of 1.5,3.0,6.0,12.0,and 24.0 Hz in the Southern California from 1987 to 1994.It can be found that YQ-1 for coda waves with all frequencies,the frequency of 12.0 Hz excluded,ascended to a certain extent prior to the occurrence of the rnainshock and returned to normality after the main shock.     

以介质参数尾波Q^—1作为响应的加卸载响应比研究

以地球介质参数尾波Ｑ＾－１作为响应，给出了加卸载响应比ＹＱ－１的计算公式。利用美国南加州地区频率分别为１．５Ｈｚ，３．０Ｈｚ，６．０Ｈｚ，１２．０Ｈｚ，２４．０Ｈｚ等５个频段的尾波Ｑ数据，研究了美国北岭地震（１９９４年１月１７日，Ｍｗ６．７）前后的加卸载响应比ＹＱ－１随时间的变化。结果表明，除频率为１２．０Ｈｚ这一频段外，其它频段的加卸载响应比ＹＱ－１在震前都有不同程度的变化，其中频率为２４．０Ｈ