Das Beeken von Mexiko befindet sich im Zentrum des Transmexikanisehen Vulkangfirtels, einer Ost-West-Struktur, die den nordamerikanischen Kontinent entlang des 19. Breitengrades durehschneidet.Pal~iomagnetische Messungen legen die Vermutung nahe, dab 9. (Guadalupe-und Chichinautzin-Gruppe) der 7 vulkanischen Phasen, die das Becken formten, von wahrscheinlieh kurzer Dauer waren. Die Chiehinautzin-Gruppe, die das Becken im Siiden abschnfirte, mul3 sich grundsStzlich in den letzten 700000 Jahren gebildet haben.Die vulkanische T:~itigkeit, die im Oligozfin begann und sieh kontinuierlich bis in die heutige Zeit fortsetzte, f6rderte naehweislieh im wesentliehen andesitisehe Produkte.
AbstractTile basin of Mexico lies in the center of the Trans-Mexiean Volcanic Belt, an east-west structure which dissects the North American Continent along the 19th parallel. Paleomagnetie measurements suggest that two of the seven eruptive phases --Guadalupe Group and Chiehinautzin Group --that formed the basin, probably were of short duration; the Chiehilmutzin Group, which finally closed the basin to the south, must have been formed principally in the last 700,000 years.The volcanic activity which began in Oligoeene time and has continued until the present, has been proved to be essentially andesitie in composition.
R6sum6Le bassin de Mexieo se trouve au centre de la eeinture voleanique de Mexico, une structure est-ouest qui reeoupe le continent nord-Amerieain pr6s du 19 e parall61e. Les mesures pal6omagn6tiques eonduisent 5 supposer que deux des sept phases d'aetivit6s volcaniques, celles de la Guadaloupe et du Chiehinautzin, 6talent eourtes. Les roches du Groupe de Chiehinautzin, fermant te bassin au sud, ont 6t6 form6es pendant les derniers 700,000 ans.L'aetivit6 volcanique, qui a d6but6 5 l'Oligocbne et se continue encore aetuellement a une charaet6re essentiellement and6sitique.
To understand the cause of observed amplification of seismic waves even at hill‐zone sites in the Valley of Mexico, digital accelerographs have been installed at three especially chosen sites. Two of these sites, MADI and TEXC, located on hard Pleistocene lava (a few meters in thickness) overlying Oligocene andesites, were expected to be free of site effects. Analysis of the data recorded by these and other accelerographs during three moderate, shallow subduction zone events, however, shows significant amplification at MADI and TEXC between 0.2 and 0.6 Hz with respect to established attenuation relations. The cause of the amplification at hill‐zone sites in the Valley of Mexico, including MADI and TEXC, may be pervasive low S‐wave velocity in, and complex structure of, the upper layers of the volcanic rocks. If so, then there may not be a truly “hard” rock site in the valley.
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