Review of the 1755 Lisbon Earthquake Based on Recent Analyses of Historical Observations - Historical Seismology: Interdisciplinary Studies of Past and Recent Earthquakes - page 272

Fig. 10 Models of attenuation of the intensities seen during the seism of 1755 (Baptista
et al. , 2001)
that might be important are the existence of “propagation channels”, “orogenic bar-
riers”, etc. which were never looked upon.
Once again, we make a comparison with another earthquake, now to emphasize
the existence of large dispersions (Fig. 11) for the attenuation of intensities in the
1950 earthquake of Assam, India, one of the biggest occurred in the boarder India-
China (Arunachal Pradesh) (http://asc-india.org/gq/19500815 indochina.htm).
Although the seismo-tectonic situations between the 1755 and this earthquake are
different in several aspects, it can be noticed, by comparison of Figs. 10 and 11,
that there are similarities in the general pattern of attenuation and in the dispersion
of the results, with the difference that for the earthquake of Assam the closest dis-
tances to the epicentre are nearer than those in 1755. The average values for 1755
are slightly above those of the Assam earthquake, which shows a magnitude larger
than Mw8.6, as referred above. The fact that the seism of Assam has a radius of
perception around 1,800 km (against 2,500 km in 1755) and duration of the vibra-
tions between 4 and 8 min (against 5-15 min in 1755) just proves this statement.
Concerning the interpretation of the dispersion observed in Fig. 11, the lower line
intends to represent the cases of “stiff” soils, while the upper line is a limit related
with the phenomenon of soil amplification. Dispersions of this order of magnitude
are observed in other earthquakes of the Portuguese catalogues (Sousa et al. 1992,
Paula et al. 1996), and therefore can correspond to phenomena of amplification
inherent to the complex processes of propagation and of local effects in Mainland
Portugal.