Geoscience Reference
In-Depth Information
3.2.2 The Magnitude
If it is difficult to identify a seismic source for 1755, it is no less difficult to attribute
it a magnitude. Nowadays, the magnitude parameter is obtained from instrumen-
tation which became available only a long time after the 1755 earthquake. Thus
the difficulty in establishing the magnitude of historical earthquakes, for which the
most accurate data is the distribution of damages or the knowledge of any simple
structure that functioned as a seismograph. In this case, we should appeal to all
available information of different origins, to essay an evaluation. Besides the geo-
graphical distribution of the damages, other elements of great importance are: the
duration, the area of perception of the vibrations, the effects on long distances, and,
finally, the comparison to other events of recent times occurred in seismo-tectonic
environment similar to those of the contact of the Euro-Asian and African (Nubia)
plates (Figs. 6 and 7).
The large magnitude of the 1755 earthquake is seen by the large radius of percep-
tion of its waves and the devastating effect in some regions of the coast in Portugal,
Spain and Morocco. A magnitude of Mw
5-8.75 has been attributed to this
earthquake, presenting a rupture mechanism in the south-west region of Cape Sao
Vicente, but it is not clear what really happened. To reach such a high magnitude
in an area of collision of the plates, as referred in Section 3.2.1, it is necessary that
the rupture in the plan of the fault has been rather extended, that the phenomenon
has been composed by more than one single rupture, or the fault depth has been
much larger than presently thought. The duration of the event was extremely long.
It is hardly believed that the different phases of the vibration can correspond to the
arrival of the P, S waves and the surface waves, because the time differences are
too large and rupture may have been long. It seems more reasonable to think that
they correspond to initial and stop phases in the rupture process. Data coming from
Spanish sources confirm the great duration of felt motion, between 4 and 15 min. In
Lisbon, the duration, quoted by several sources, was 6-9 min, as referred above. In
Lagos, the duration did not surpass 4 or 5 min. A sound contribution to this topic can
only be made after analytical simulation of different ruptures and seismic sources,
to understand the effect of rupture direction, onset of the event, etc. Carvalho (2007)
is presently doing a large set of experiments to analyse these effects not only in
duration but also in attenuation, and the findings may contribute to clarify these
mechanisms.
Based on Spanish data (Martinez-Solares 2000, Fig. 9), the correlation of duration
with epicentral distance
3
is very poor: the larger durations were observed in places
with epicentral distances between 300 and 500 km, decreasing for larger distances.
For
=
8
.
shorter
distances
the
duration
was
smaller,
as
the
cases
of
Lagos
and
Lisbon.
Comparing with what has happened with the earthquake of December 26th,
2004, in Sumatra Island, it is possible to review the above magnitude estimations
3
The concept of epicentral distance in this case should be viewed under a great uncertainty, due
to lack of knowledge of source mechanism.
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