Bulletin of the Seismological Society of America
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Bulletin of the Seismological Society of America; August 2007; v. 97; no. 4; p. 1259-1266; DOI: 10.1785/0120060171
© 2007 Seismological Society of America
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Q of Lg Waves in the Central Mexican Volcanic Belt

S. K. Singh1, A. Iglesias2, D. García3, J. F. Pacheco1 and M. Ordaz2

1 Instituto de Geofísica
Ciudad Universitaria, Universidad Nacional Autónoma de México
04510 México D.F., Mexico
 (S.K.S., J.F.P.)
2 Instituto de Ingeniería
Ciudad Universitaria, Universidad Nacional Autónoma de México
04510 México D.F., Mexico
 (A.I., M.O.)
3 Departamento de Geofísica y Meteorología
Facultad de Ciencias Físicas, Universidad Complutense de Madrid (UCM)
Ciudad Universitaria
28040 Madrid, Spain
 (D.G.)

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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