D. Fah et al., A HYBRID METHOD FOR THE ESTIMATION OF GROUND MOTION IN SEDIMENTARY BASINS - QUANTITATIVE MODELING FOR MEXICO-CITY, Bulletin of the Seismological Society of America, 84(2), 1994, pp. 383-399
To estimate the ground motion in two-dimensional (2D), laterally heter
ogeneous, anelastic media, a hybrid technique has been developed that
combines modal summation and the finite-difference method. In the calc
ulation of the local wave field owing to a seismic event, both for sma
ll and large epicentral distances, it is possible to take into account
the source, path, and local soil effects. As a practical application,
we have simulated the ground motion in Mexico City caused by the Mich
oacan earthquake of September 19, 1985. By studying the one-dimensiona
l (1D) response of the two sedimentary layers present in Mexico City,
it is possible to explain the difference in amplitudes observed betwee
n records for receivers inside and outside the lake-bed zone. These si
mple models show that the sedimentary cover produces the concentration
of high-frequency waves (0.2 to 0.5 Hz) on the horizontal components
of motion. The large amplitude coda of ground motion observed inside t
he lake-bed zone and the spectral ratios between signals observed insi
de and outside the lake-bed zone can only be explained by 2D models of
the sedimentary basin. In such models, the ground motion is mainly co
ntrolled by the response of the uppermost clay layer. The synthetic si
gnals explain the major characteristics (relative amplitudes, spectral
ratios, and frequency content) of the observed ground motion. The lar
ge amplitude coda of the ground motion observed in the lake-bed zone c
an be explained as resonance effects and the excitation of local sur-f
ace waves in the laterally heterogeneous clay layer. Also, for the 198
5 Michoacan event, the energy contributions of the three subevents are
important to explain the observed durations.