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Fig. 9 Duration of vibrations as function of distance (composed after Martinez-Solares, 2000)
to higher values, in the order of Mw9 or larger. These large magnitudes have not
been observed in tectonic environments of collision such as the one under study,
but essentially in subduction zones. This is an important issue to be resolved in the
future.
The spectrum of the vibration is different depending on the phase that is being
analysed. In the first phase, the vibrations, essentially vertical and with a predom-
inant North-South component, are of the highest frequency. The second phase and
mainly the third phase are of much lower frequency.
As referred above, the seism was felt in great distances exhibiting a pattern of
attenuation somewhat strange, departing significantly from the normal circular sym-
metry.
3.2.3 Attenuation of the Seismic Waves
The phenomenon of attenuation of the seismic waves was based on the EMS-98
intensities observations (821 points) throughout the Iberian Peninsula (Fig. 10).
These points correspond to the isoseismals map of Fig. 2. The first comment is the
large dispersion in existing data, due to the non-radial wave propagation pattern, as
referred in Fig. 2, masked by several problems already mentioned: radiation pattern,
rupture mechanism, three-dimensional propagation effects, large alluvial basins, etc.
Several authors have tried to fit attenuation curves (in Teves-Costa et al. 2002),
but the dispersion problem persists, unless those aspects are filtered out before the
fitting.
The geometry of the transition ocean crust - continental crust, as well as the
alluvial geological setting in large areas of the coastal and lower river estuary re-
gions, seem to be the two most important factors in these anisotropies. Other factors
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