著者

Gehrels Tom

出版者

公益社団法人 日本地震学会、地球電磁気・地球惑星圏学会 、特定非営利活動法人 日本火山学会、日本測地学会、日本惑星科学会

雑誌

Earth, planets and space (ISSN:13438832)

巻号頁・発行日

vol.51, no.11, pp.1155-1161, 1999

参考文献数

52

被引用文献数

3

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The statistics of Earth-approaching asteroids are first summarized, and an enhanced frequency of objects smaller than 100 meters is noted. Superposed on these random hazards may be a periodic one of new comets due to galactic tides of the Oort Cloud with a period of 26-36 Myr (Rampino, 1998). New asteroids and comets are being found evermore frequently because new telescope-and-detector systems are coming on line. These are intended primarily for the discovery of dangerous objects, but a beginning has been made with the study of statistics of main-belt asteroids. In addition to trans-Neptunian objects, cis-Neptunian “Centaurs” are recognized, which may be a link in the evolution of short-period comets and thereby contribute to the flux of Earth approachers. With the new equipment coming on line, we are beginning to see that the global hazard will be mostly quantified within a few decades. We do see a shortage in astrometric follow up fainter than about the 20th magnitude.

著者

Ceja Maria Rodríguez Goguitchaichvili Avto Calvo-Rathert Manuel Morales-Contreras Juan Alva-Valdivia Luis Elguera José Rosas Fucugauchi Jaime Urrutia Granados Hugo Delgado

出版者

公益社団法人 日本地震学会、地球電磁気・地球惑星圏学会 、特定非営利活動法人 日本火山学会、日本測地学会、日本惑星科学会

雑誌

Earth, Planets and Space (ISSN:13438832)

巻号頁・発行日

vol.58, no.10, pp.1349-1358, 2006

被引用文献数

11

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This paper presents new paleomagnetic results from 24 independent cooling units in Tequila area (western Trans-Mexican Volcanic Belt). These units were recently dated by means of state-of-the-art <SUP>40</SUP>Ar-<SUP>39</SUP>Ar method (Lewis-Kenedy <I>et al</I>., 2005) and span from 1130 to 150 ka. The characteristic paleodirections are successfully isolated for 20 cooling units. The mean paleodirection, discarding intermediate polarity sites, is <I>I</I> = 29.6&deg;, <I>D</I> = 359.2&deg;, <I>k</I> = 26, &alpha;<SUB>95</SUB> = 7.1&deg;, <I>n</I> = 17, which corresponds to the mean paleomagnetic pole position <I>P</I><SUB>lat</SUB> = 85.8&deg;, <I>P</I><SUB>long</SUB> = 84.3&deg;, <I>K</I> = 27.5, <I>A</I><SUB>95</SUB> = 6.9&deg;. These directions are practically undistinguishable from the expected Plestocene paleodirections, as derived from reference poles for the North American polar wander curve and in agreement with previously reported directions from western Trans-Mexican Volcanic Belt. This suggests that no major tectonic deformation occurred in studied area since early-middle Plestocene to present. The paleosecular variation is estimated trough the study of the scatter of virtual geomagnetic poles giving <I>S<SUB>F</SUB></I> = 15.4 with <I>S<SUB>U</SUB></I> = 19.9 and <I>S<SUB>L</SUB></I> = 12.5 (upper and lower limits respectively). These values are consistent with those predicted by the latitude-dependent variation model of McFadden et al. (1991) for the last 5 Myr. The interesting feature of the paleomagnetic record obtained here is the occurrence of an intermediate polarity at 671&plusmn;13 ka which may correspond the worldwide observed Delta excursion at about 680-690 ka. This gives the volcanic evidence of this event. Two independent lava flows dated as 362&plusmn;13 and 354&plusmn;5 ka respectively, yield transitional paleodirections as well, probably corresponding to the Levantine excursion.