Department of Earth Sciences
Open University
Milton Keynes, U.K.

The great mystery of the 1985 Mexican earthquake is that, whereas 371 modern high-rise buildings collapsed, thousands of colonial masonry structures (many with tall steeples) in the same area remained standing. This puzzle may be solved. Theoretical calculations suggest that ultrashort surface waves (gravity waves) may have liquified the area's sediments to a shallow depth that would have included the foundations of modern buildings. Older buildings, by contrast, would have had time to sink further into the soft clay, placing their foundations beyond the reach of the shallow liquefaction.

C. Lomnitz, "Mexico 1985: the case for gravity waves," Geophysical Journal International, 102, 569-72, September 1990. (Instituto de Geof¡sica, Mexico City)

The classic Cretaceous-Tertiary boundary at Gubbio, Italy, has been resampled in detail for both iridium and magnetostratigraphy by a team that included those who prefer both terrestrial and external sources for...

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