Abstract

From seismograms of nine shallow, coastal earthquakes recorded at a pair of broadband stations, we estimate Q of Lg waves in the part of central Mexican Volcanic Belt (mvb) that includes the Valley of Mexico. The two stations straddle the central mvb and are located on Cretaceous limestone. A weighted least-square fit to the Q−1(f) data in the frequency range 0.25 to 8 Hz yields Q(f) = 98f0.72. This estimate of Q is lower than the corresponding Q in the forearc region that is given by Q(f) = 273f0.66. Note that our estimate of Q(f) corresponds to a 200-km-wide zone of the mvb. The result of this study sheds light on the characteristics of seismic waves as they traverse through the mvb where they undergo dramatic amplification in the Valley of Mexico. It also provides one of the critical elements needed in the estimation of expected ground motions at sites to the north of the mvb from future coastal earthquakes. The lower Q of Lg waves in the mvb as compared with the forearc region seems correlated with lower resistivity reported in the mvb relative to the forearc region.

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