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2 nd INQUA-IGCP-567 International Workshop on Active Tectonics, Earthquake Geology, Archaeology and Engineering, Corinth, Greece (2011) EARTHQUAKE ARCHAEOLOGY INQUA PALEOSEISMOLOGY AND ACTIVE TECTONICS PALEOSEISMOLOGICAL EVIDENCE FOR HISTORICAL SURFACE RUPTURE EVENTS IN S. MIGUEL ISLAND (AZORES) Carmo, Rita (1, José Madeira (2), Ana Hipólito (1), Teresa Ferreira (1) (1) Centro de Vulcanologia e Avaliação de Riscos Geológicos, Universidade dos Açores, Complexo Científico, 3º Piso – Ala Sul, Rua Mãe de Deus, 9500-321 Ponta Delgada, Açores, Portugal. Email: Rita.L.Carmo@azores.gov.pt (2) Dpto. Geologia, LATTEX/IDL, Faculdade de Ciências da Universidade de Lisboa, Edifício C6, Campo Grande, 1749-016 Lisboa, Portugal. Email: jmadeira@fc.ul.pt Abstract (Paleoseismological evidence for historical surface rupture events in S. Miguel Island (Azores)): The Azores archipelago is located in the triple junction between the Eurasian, Nubian and North American lithospheric plates. The Achada das Furnas plateau, located in the central part of S. Miguel Island, between Fogo and Furnas volcanoes, is dominated by several basaltic cinder cones that define several WNW-ESE and E-W alignments. Two E-W trending scarps were identified by aerial photo analysis. Trenches were open across the scarps to confirm their tectonic nature exposing two active normal faults (the Altiprado Faults). At least four paleoearthquakes were deduced, three of which in historical times. Radiocarbon ages are in agreement with this interpretation. Key words: Azores, active faults, paleoseismology GEOLOGICAL SETTING The Azores archipelago is located at the Eurasia (Eu), Nubia (Nu) and North America (NA) triple junction (Fig. 1). The Mid Atlantic Ridge separates the NA from Eu and Nu plates, while the Azores- Gibraltar Fault Zone (Terceira Rift and Gloria Fault) is the boundary between Eu and Nu plates. The archipelago comprises nine islands distributed by three groups: the western islands lie on NA plate while the central and eastern groups are located along the western segment of the Azores-Gibraltar Fault Zone (Fig. 1). As a result of its complex tectonic setting, the Azores archipelago is subject to frequent seismic and volcanic activity. S. Miguel Island was settled in 1439-1443. It has three active explosive central volcanoes with summit calderas linked by zones of fissural volcanism. The main tectonic structures trend NW-SE to WNW-ESE, NNW-SSE and NE-SW. The Achada das Furnas plateau, located in the central area of the island, between Fogo and Furnas volcanoes, is dominated by several basaltic cinder cones defining WNW-ESE and E-W alignments (Fig. 2). Aerial photo analysis identified the presence of two E-W trending scarps produced by the Altiprado Faults (AF1 and AF2, Fig. 2). Fig. 1: Azores tectonic setting. NA – North American plate; Eu – Eurasian plate; Nu – Nubian plate; MAR – Mid Atlantic Ridge; TR – Terceira Rift (s.l.); EAFZ – East Azores Fracture Zone; GF – Gloria Fault. World topography and bathymetry from ESRI (2008). Fig. 2: Main tectonic and volcanic structures of Achada das Furnas plateau. 22
2 nd INQUA-IGCP-567 International Workshop on Active Tectonics, Earthquake Geology, Archaeology and Engineering, Corinth, Greece (2011) EARTHQUAKE ARCHAEOLOGY INQUA PALEOSEISMOLOGY AND ACTIVE TECTONICS THE ALTIPRADO FAULTS Altiprado Fault 1 The Altiprado Fault 1 (AF1) trace is marked by an 835 m-long, 3 m-high, south-facing scarp, trending N87ºE (Fig. 3). To the east and to the west its trace becomes uncertain. A ~18 m-long trench exposed the fault, trending N87ºE and dipping 65ºS, displacing a stratigraphic succession that comprises 6 pumice fall deposits (units) separated by paleosols produced by eruptions of Fogo and Furnas volcanoes (Fig. 4): 1 - olivebrown paleosol containing basalt lapilli fragments, corresponding to Fogo A deposit (4520±90 years BP; Wallenstein, 1999); 2 – pumice fall deposit composed of alternating yellowish lapilli and light olive brown ash beds, probably related to Fogo C deposit; 3 - strong brown pumice ash fall deposit with some lapilli at the base (3a), and a yellowish pumice fall deposit in the central part (3b); probably Fogo D deposit; 4 - grey pumice ash deposit, corresponding to Furnas C deposit (1900 years BP; Guest et al., 1994), topped by a very dark grey soil (1440-1500 cal AD); 5 - stratified pumice fall deposit of alternating beds of fine to medium greyish white lapilli and ash containing sanidine crystals (from 1563 AD historical eruption in Fogo volcano), topped by a very dark grey paleosol rich in coal fragments (1660-1700 cal AD); 6 - remobilised deposit from the underlying unit. The AF1 affects all stratigraphic units and the existing scarp is an uneroded free-face almost devoid of soil that corresponds to the projection of the fault to the surface (Fig. 4). Several WNW-ESE to E-W trending fractures and open cracks, sometimes filled with material from overlying units, and a colluvium composed of material of units 3 and 4 were also exposed in the trench (Fig. 4). Units 2 to 4 (soil 1440-1500 cal AD) are displaced by 1.0m and units 5 (soil 1660-1700 cal AD) and 6 by 0.38m, indicating two surface rupturing paleoearthquakes of Mw 6.7 and 6.4 (using the Wells & Coppersmith’s, 1994, M/MD correlation), respectively, accounting for an accumulated dip-slip of 1.38m. Two earthquakes in 571-511 years indicate a recurrence interval that ranges from 286 to 256 years, yielding a slip rate of 2-3 mm/year. Altiprado Fault 2 The Altiprado Fault 2 trace is marked by an almost imperceptible 1690 m-long and ~40 cm-high south facing scarp, trending N87ºE (Fig. 3). To the east its location becomes uncertain. Fig. 3: Vertical aerial photograph of Altiprado region, Achada das Furnas plateau, showing the geomorphic expression of Altiprado Faults (Aerial photo from Direcção Geral de Planeamento Urbanístico, 1974). Fig. 4: Map of the east wall of the Altiprado Fault 1 trench. 23
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