Review of the 1755 Lisbon Earthquake Based on Recent Analyses of Historical Observations - Historical Seismology: Interdisciplinary Studies of Past and Recent Earthquakes - page 287

All this information, together with instrumental and historical data for the
Gorringe area led to the hypothesis that the Mw9 1755 earthquake may occur every
2,000-5,000 years. Figure 21 gathers all the recurrence rates for events in the most
important seismic sources in the Portuguese region, including that of 1755. The
above mentioned return periods seems to be in agreement with the 5 mm rate of
collision between Plates and the Mw > 9.

6 A Final Word

Many things have been learned in recent years using the most advanced technolog-
ical tools for a better understanding of the various unknowns related to the seismic
source, wave propagation, site effects, duration of ground motion, predominance of
lower frequencies, etc. The damage distribution in monuments is very well known,
but in the stock of housing it is still very uncertain. Much more work should be done
to better clarify these aspects. This is the only way we can proceed to minimize the
effects of future events of this kind. Even though this event belongs to a class of rare
events with a low probability of occurrence, it may come at any time. The better we
understand the better we are prepared to face it.

Acknowledgments This paper was partially supported by the Portuguese Funda¸ ao para a Ciencia
e a Tecnologia (FCT) under the Pluri-Annual Programme. Dr. Isabel Viseu made an overall review
of the text and M onica A. Ferreira helped in preparing the figures. The author thanks the referees
for the detailed comments that helped in upgrading this text.

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