COMMENTS

For this rather complex and controversial earthquake we decided to report in the 
Database only the solution based on intensity data. In spite of the number of large 
earthquakes that struck southeastern Sicily between the XII and XVII centuries 
(i.e. in 1169, 1542 and 1693, all having magnitude in the range M 6.6-7.5), no 
unambiguous geological features exist that can be readily interpreted as evidence 
of the activity of the seismic sources responsible for these events.

Most of the investigators locate the seismogenic source of the 11 January 1693 
event offshore, in particular along the NNW-trending Ibleo-Maltese escarpment 
that represents the most prominent physiographical and structural feature of the 
area. The lack of clear geomorphic evidence of a seismic source on-land, the 
occurrence of a large tsunami, the shape of the area of XI intensity degree (that is 
partially open toward the sea) and the evidence of Late Pleistocene activity along 
the Ibleo-Maltese escarpment supplied by seismic profiles form the basis for 
associating the 1693 event to the this lineament. However, the overall shape of 
the intensity pattern is not in agreement with a source offshore, and in fact the 
most recent elaborations such as the CPTI catalogue (CPTI Working Group, 1999) 
locate the earthquake inland 15-20 km from the closest point on the shoreline.

We believe that the Ibleo-Maltese escarpment is indeed an active tectonic feature, 
but there is no evidence that the fracture zone extends to the depth that is needed 
to generate a ~M 7.5 earthquake. In addition, a source on this important tectonic 
lineament would be unable to explain simultaneously the strong intensities 
suffered by many villages located in the inner Hyblean Plateau and the relatively 
low intensities seen along the coast. The occurrence of the tsunami, which is 
often invoked as a conclusive proof for the location of this source offshore, could 
be the result of a submarine slide triggered by the strong ground shaking or of a 
seismogenic source located near the coast but still on-land. Finally, other 
independent elaborations of the 1693 source using intensity data (Sirovich and 
Pettenati, 1999) also suggest a NE-SW source that does not agree with any of the 
geologic or geomorphic feature seen in the area. 


OPEN QUESTIONS

1) Did the strong foreshock of 9 January 1693 modify the vulnerability of buildings 
in the westernmost part of the damaged area, thus determining a possible 
overestimate of the damage related to the main shock of the 11 January shock, 
as suggested by Sirovich and Pettenati (1999)?

2) Could the 1169 earthquake be an ancestor of the 1693 event , as suggested by 
many investigators?

3) Were the 1169 and 1693 events produced by a blind normal fault or by a 
strike-slip fault with poor geomorphic expression?
