Bulletin of the Seismological Society of America; April 2007; v. 97; no. 2;
p. 545-556; DOI: 10.1785/0120060083
© 2007 Seismological Society of America
Seismic-Wave Attenuation and Source Excitation in La Paz–Los Cabos, Baja California Sur, Mexico
Roberto Ortega1 and
Mario González2
1 Centro de
Investigación
Científica y de
Educación Superior
de Ensenada
Unidad La Paz, Miraflores 334 Fracc. Bellavista
La Paz 23050, Baja California Sur, Mexico
ortega{at}cicese.mx
(R.O.)
2 Centro de
Investigación
Científica y de
Educación Superior
de Ensenada
B. C. Km 107 Carretera Tijuana-Ensenada
Ensenada, Baja California, C. P. 22860, Mexico
(M.G.)
We present results from a regional study of seismic-wave attenuation and
source excitation from small-magnitude earthquakes recorded at distances from 6
to 180 km in the La
Paz–Los Cabos
region, at the south end of the Baja California Peninsula. Data were recorded
using 32 strong-motion seismic stations from the La Paz network
(LAP). A least-squares regression separating the excitation, site,
and propagation effects was carried out. We performed the analyses in the time
and frequency domains, and we compared these results with results from a coda-
normalization method. The propagation term was parameterized to represent a
geometrical spreading function and a frequency-dependent Q(f)
at a reference distance of 40 km. We estimated the regional attenuation by
measuring the maximum amplitude of the S or Lg waves as a
function of frequency, defining a continuous piecewise propagation term,
D(r, f), after separating the excitation and site
terms. Our results show that the attenuation is lower than that of central or
northern Mexico. Recorded data were of remarkably good quality despite the fact
that the strong- motion network recorded only small earthquakes. Our best model
is that of a quality factor of Q(f)
=
380f0.10 with a complex geometrical spreading function.
Copyright © 2009 by Seismological Society of America
