Long term hazard from Atlantic subduction zones (Antilles and Cadiz/Gibraltar) and the example of the Great Lisbon earthquake and tsunami of 1755
Abstract
While great earthquakes and associated tsunami occur far less frequently in the Atlantic than in the Pacific, such events have occurred in the past. The two most recent destructive events were the 1929 Grand Banks earthquake and tsunami and the Great Lisbon earthquake and tsunami of 1755. The main cause for this discrepancy (beyond the smaller size of the ocean basin) is the abundance of subduction zones in the circum Pacific region and the near absence of such tectonically active zones in the Atlantic. Nevertheless, there are slowly converging subduction zones in the Atlantic, the Antilles arc (which has not produced any great earthquakes in the past 150 years and the Gibraltar/Gulf of Cadiz arc (offshore SW Iberia, whose activity remains controversial). We present new research on these two zones. In both cases, deep seismic sounding data together with earthquake hypocenters are used to construct lithospheric cross-sections. These form the basis for thermal modeling in order to determine the probable dimensions (downdip width) of the potential seismogenic fault plane. Tsunami modeling has been performed for the Cadiz/Gibraltar subduction zone using these calculated limits and wave propagation models have been established. These predict the regional impact (travel-times and amplitudes) as well as the far-field effects of a subduction fault plane source for the 1755 tsunami. Indeed, this event produced a 5-10 m tsunami on the S. Portuguese, SW Spanish and NW Moroccan coasts. It was also observed (1-5 m waves) in the Antilles. Work on the Antilles subduction zone has only recently begun. Here strong events last struck in 1839 (intensity IX in Martinique) and 1843 (intensity X in Guadeloupe). Thermal modeling suggests the seismogenic zone is widest to the south, where the accretionary wedge is widest. This region is marked by a total absence of instrumentally recorded thrust type earthquakes (just as is the entire Gulf of Cadiz). Comparative analysis of subduction zones worldwide suggests that those with the slowest convergence rate typically have the longest recurrence intervals and thus, behavior over the past 100 years may not offer a reliable estimate of long-term hazard. More detailed studies will be necessary in order to properly assess the natural hazard posed by this region (over 1000 km in length), such as better information on the crustal structure, new in-situ data on the thermal state of the Antilles arc, as well as identifying and deciphering all available paleoseismic indicators.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFMOS53B1307G
- Keywords:
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- 0468 Natural hazards;
- 4564 Tsunamis and storm surges;
- 7230 Seismicity and tectonics (1207;
- 1217;
- 1240;
- 1242);
- 7240 Subduction zones (1207;
- 1219;
- 1240)