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Comment on "Lisbon 1755: A Case of Triggered Onshore Rupture?" by Susana P. Vilanova, Catarina F. Nunes, and Joao F. B. D. Fonseca

¹ Centro de Geofísica da Universidade de Lisboa
Campo Grande, Ed. C8, piso 6
1749-016 Lisboa, Portugal
lmatias@fc.ul.pt
 (L.M., P.T.-C., J.M.M.)

² Laboratório de Tectonofísica e Tectónica Experimental
Lisboa, Portugal
 (A.R., J.C., P.T.)

³ Instituto Superior de Engenharia de Lisboa
Lisboa, Portugal
 (M.A.B.)

⁴ Istituto Scienze Marine
Sezione Geologia Marina
Bologna, Italy
 (N.Z.)

The first 300 words of the full text of this article appear below.

	Introduction

 
The work published recently by Vilanova et al. (2003) in the Bulletin of the Seismological Society of America proposes that the accounts of destruction and other effects reported in the Lisbon area as a result of the 1 November 1755 earthquake are best explained by a local rupture on the Lower Tagus Valley fault (LTVF), triggered by the static stress change produced by the main offshore source located in the Gorringe area. Because of the potential impact of this hypothesis on the seismic hazard of the Lisbon area, we discuss and complement the evidence presented by Vilanova and co-workers, concluding that the "local rupture model" should remain, for the moment, as an unsubstantiated speculation.

	Macroseismic Evidence

 
Vilanova et al. (2003) (referred to herein as "the authors" or "the paper") compare the intensities reported for the 1755 earthquake in Lisbon and Algarve to conclude that Lisbon intensities are abnormally high for the same distance, in particular, when compared with what was observed for the 28 February 1969 earthquake. They interpreted this as the result of local rupture. In Figure 1 we re-plot the macroseismic intensity as a function of the distance to the Gorringe Bank source presumed by the authors. We can see that "Lower Tagus Valley intensities" do not show any abnormal behavior. Similar plots could be made for all source areas proposed by other authors (Baptista, 1998; Baptista et al., 1998a, 1998b; Zitellini et al., 1999, 2001; Terrinha et al., 2003) with minor changes to the conclusion we reached previously. The conclusions reached by the authors were based on their figure 8A, from which intensity values for the Algarve and Portuguese west coast corresponding to distances greater than 350 km are missing.

View larger version (19K): [in this window] [in a new window]   Figure 1. Modified Mercalli (MM) intensities for the 1755 . . . [Full Text of this Article]

	Tsunami Arguments

	Benavente Permanent Deformation

	LTVF Earthquake Sequence

	Conclusions

Related articles in Bulletin of the Seismological Society of America:

Lisbon 1755: A Case of Triggered Onshore Rupture?

Susana P. Vilanova, Catarina F. Nunes, and Joao F. B. D. Fonseca
Bulletin of the Seismological Society of America 2003 93: 2056-2068. [Abstract] [Full Text]