Abstract

A dense array of seismometers was installed and operated in the botanical garden of UNAM within the Valley of Mexico during April and May 1994. The slow surface velocities at the array require an approach to array analysis with the capability of measuring arrival-time differences between stations of less than the sampling interval of the data. Applying the method to principal-component seismograms from earthquakes at local and regional ranges from a number of directions provides the first quantitative information on the composition of the wave-field incident upon Mexico City. Phases that propagate through the entire crust, such as the initial P and Lg waves, cross the array with the expected backazimuth. On the other hand, surface waves in the 2.5- to 5-sec period band, which are sensitive to uppercrustal structure, show systematic rotations of backazimuth. These suggest changes in upper-crustal structure or point scatterers located north and south of Mexico City. For earthquakes outside of the Valley of Mexico, no significant energy was observed propagating across the array from the east (from the lake zone). Thus, although it is clear that seismic waves resonate within the lake zone, little, if any, of this energy is transmitted into the hill zone where the array was located. However, long-duration ground motions are observed at the array, with evidence of multipathing. A back-projection of these late, off-azimuth arrivals indicates that their sources correlate with the boundaries of the Quaternary volcanics of the trans-Mexican volcanic belt.

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