The quest for the source area of the 1755 Lisbon erthquake - revealing the maximum depth of seismogenic faulting in the Horseshoe abyssal plain
Final Report Abstract
At the eastern terminus of the Azores-Gloria transform fault system to the southwest of Portugal, the plate boundary between Africa and Iberia is poorly defined due to deformation over a wide tectonically-active zone. The region, however, is also characterized by large earthquakes and tsunamis, such as the Mw~8.5 Great Lisbon earthquake of 1755. The source region of the 1755 earthquake is still unknown, but it is believed to be located in the vicinity of the Horseshoe Abyssal Plain, terminating against the 5000 m high Gorringe Bank. Two local seismic networks were operated in the area, yielding local seismicity pattern. One network of ocean-bottom seismometers was operated in the eastern HAP. The second network of OBS monitored seismicity at the GB. The networks provided in the order of 160 local earthquakes recorded on 6 or more OBS. Major results are: (i) the majority of earthquakes was located at >20 km depth in the upper mantle of a domain being previously interpreted as un-roofed continental mantle. (ii) Maximum depth of faulting derived from both 1-D and 3-D velocity models suggest that seismicity occurs at depth above ~45 km. (iii) Thermal modelling suggest that temperatures of 600°C occur at approximately 45 km depth and hence explains the lack of micro-earthquakes below 45 km. Further, thermal data suggest that previous estimates providing source depths of up to 60 km depth were biased. (iv) Most micro-earthquakes occurred at a depth greater than geologically and reflection seismically mapped faults. Thus, fault systems imaged in the bathymetry and seismic reflection data might be splays of deep-seated faults. (v) Focal depth seems to increase from 20-35 km (mean of 26.1 ± 7.2 km) at the Gorringe Bank to 15-45 km (mean 31.5 km ± 10.5 km) under the Horseshoe Abyssal Plain. (vi) Focal parameters of micro-earthquakes, waveform inversion of the 2007 Mw=6.0 Horseshoe earthquake, and thermal models suggest that an approximately 200 km long fault in the vicinity of the Horseshoe Abyssal Plain could support a Mw=8.7 1755 Lisbon-type earthquake in cold mantle lithosphere. (vii) We suggest that distributed deformation and seismicity of the Horseshoe Abyssal Plain and Gorringe Bank is related to African-Eurasian convergence and deformation of a domain of un-roofed Cretaceous continental mantle sandwiched between two domains of >120 Ma old oceanic lithosphere. We do not see any evidence for a developing subduction zone.
Publications
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(2015) RV POSEIDON Cruise Report POS430, POS440, POS460 and POS467 - Seismic hazards to the southwest of Portugal; GEOMAR Report, N.Ser. 024 . GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel, Kiel, 43 pp.
Grevemeyer, I.
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Seismotectonics of the Horseshoe Abyssal Plain and Gorringe Bank, Eastern Atlantic Ocean: constraints from ocean bottom seismometer data. Journal of Geophysical Research - Solid Earth Vol 122 Issue 1, January 2017, Pages 63-78
Grevemeyer, I., D. Lange, H. Villinger, S. Custódio and L. Matias
