Historical tsunami in the Azores archipelago (Portugal)

Abstract

Because of its exposed northern mid-Atlantic location, morphology and plate-tectonics setting, the Azores Archipelago is highly vulnerable to tsunami hazards associated with landslides and seismic or volcanic triggers, local or distal. Critical examination of available data – written accounts and geologic evidence – indicates that, since the settlement of the archipelago in the 15th century, at least 23 tsunami have struck Azorean coastal zones. Most of the recorded tsunami are generated by earthquakes. The highest known run-up (11–15 m) was recorded on 1 November 1755 at Terceira Island, corresponding to an event of intensity VII–VIII (damaging–heavily damaging) on the Papadopolous–Imamura scale. To date, eruptive activity, while relatively frequent in the Azores, does not appear to have generated destructive tsunami. However, this apparent paucity of volcanogenic tsunami in the historical record may be misleading because of limited instrumental and documentary data, and small source-volumes released during historical eruptions. The latter are in contrast with the geological record of massive pyroclastic flows and caldera explosions with potential to generate high-magnitude tsunami, predating settlement. In addition, limited evidence suggests that submarine landslides from unstable volcano flanks may have also triggered some damaging tsunamigenic floods that perhaps were erroneously attributed to intense storms. The lack of destructive tsunami since the mid-18th century has led to governmental complacency and public disinterest in the Azores, as demonstrated by the fact that existing emergency regulations concerning seismic events in the Azores Autonomous Region make no mention of tsunami and their attendant hazards. We suspect that the coastal fringe of the Azores may well preserve a sedimentary record of some past tsunamigenic flooding events. Geological field studies must be accelerated to expand the existing database to include prehistoric events-information essential for more precisely estimating the average tsunami recurrence rate for the Azores over a longer period. A present-day occurrence of a moderate to intense tsunami (i.e., the size of the 1755 event) would produce societal disruption and economic loss orders of magnitudes greater than those of previous events in Azorean history. To reduce risk from future tsunami, comprehensive assessment of tsunami hazards and the preparation of hazards-zonation maps are needed to guide governmental decisions on issues of prudent land-use planning, public education and emergency management.