ALBERT

Description: At the eastern end of the Azores-Gloria transform fault system to the southwest of Portugal, the plate boundary between Africa and Iberia is a region where deformation is accommodated over a wide tectonically-active area. The region has unleashed large earthquakes and tsunamis, including the Mw ~ 8.5 Great Lisbon earthquake of 1755. Although the source region of the 1755 earthquake is still disputed, most proposals include a source location in the vicinity of the Horseshoe Abyssal Plain (HAP), which is bounded by the 5000 m high Gorringe Bank (GB). In this study we characterise seismic activity in the region using data recorded by two local networks of ocean-bottom seismometers (OBS). The networks were deployed in the eastern HAP and at the GB. The dataset allowed the detection of 160 local earthquakes. These earthquakes cluster around the GB, to the SW of Cabo Sao Vicente, and in the HAP. Focal depths indicate deep-seated earthquakes, with depths increasing from 20-35 km (mean of 26.1 ± 7.2 km) at the GB to 15-45 km (mean 31.5 km ± 10.5 km) under the HAP. Seismic activity thus extends down to levels that are deeper than those mapped by active seismic profiling, with the majority of events occurring within the mantle. Thermal modelling suggests that temperatures of approximately 600 °C characterise the base of the seismogenic brittle lithosphere at ~45 km depth. The large source depth and thermal structure supports previous suggestions that catastrophic seismic rupture through the lithospheric mantle may indeed occur in the area.