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ANNALS OF GEOPHYSICS, 56, 6, 2013, S0671; doi:10.4401/ag-6221 Special Issue: Earthquake geology Paleoseismological evidence for historical surface faulting in São Miguel island (Azores) Rita Carmo1, *, José Madeira2, Ana Hipólito1, Teresa Ferreira1 1 Centro de Vulcanologia e Avaliação de Riscos Geológicos da Universidade dos Açores, CVARG, Açores, Portugal 2 Universidade de Lisboa, Faculdade de Ciências, Departamento de Geologia, and Instituto Dom Luiz (Laboratório Associado)-IDL(LA), Lisboa, Portugal Article history Received October 8, 2012; accepted May 10, 2013. Subject classification: Earthquake geology and paleoseismology, Seismic risk, Tectonics. ABSTRACT Eurasian and Nubian plates, while the Azores-Gibral- The Azores archipelago is located at the triple junction between the tar Fault Zone (Terceira Rift (s.l.) and Gloria Fault) is Eurasian, Nubian and North American lithospheric plates, whose the boundary between Eurasian and Nubian plates. boundaries are the Mid-Atlantic Ridge and the Azores-Gibraltar Fault The archipelago comprises nine islands distributed Zone. São Miguel is the largest island of the archipelago and is located by three groups. The western islands (Flores and on the eastern part of the western segment of the Azores-Gibraltar Fault Corvo) lie on the stable North American plate, while Zone. The Achada das Furnas plateau, located in the central part of the the central (Faial, Pico, São Jorge, Graciosa and Ter- island, between Fogo and Furnas central volcanoes, is dominated by sev- ceira) and eastern groups (São Miguel, Santa Maria and eral WNW-ESE and E-W trending alignments of basaltic cinder cones. Formigas islets) are located along the western segment Two E-W trending scarps were identified by aerial photo interpretation. of the Azores-Gibraltar Fault Zone, the Terceira Rift Transect trenches exposed two active normal faults-the Altiprado Faults (s.l.). This segment is bordered to the south by the in- – confirming the tectonic nature of the scarps. Several paleoearthquakes active East Azores Fracture Zone and to the north by were deduced, most of which in historical times, producing 1.38 m and the São Miguel-Graciosa alignment (Terceira Rift (s.s.), 0.48 m of cumulative displacement. Maximum expected magnitudes Machado 1959). It corresponds to a diffuse and com- (MW) determined from slip per event range from 5.7 to 6.7. One of the plex deformation zone, sheared by a dextral transten- events probably corresponds to the historical earthquake of October 22nd, sive regime [Madeira 1998, Lourenço et al. 1998, 1522, the deadliest in the archipelago. Radiocarbon ages are in agree- Madeira and Brum da Silveira 2003] due to higher ment with this interpretation. spreading rates at the Mid-Atlantic Ridge north of the triple junction [e.g. Laughton and Whitmarsh 1974, 1. Introduction Fernandes et al. 2003, Argus et al. 2011] and to the The population of Azorean islands faces significant obliquity relatively to the spreading directions [e.g. geological hazard due to the geodynamic context of the Madeira and Ribeiro 1990, Madeira 1998, Lourenço et archipelago, including frequent seismicity, active vol- al. 1998]. canism and landslides. Earthquakes claimed more than The fault pattern in the Azores islands is charac- 6,000 victims in the six centuries since the beginning of terized by three-dimensional strain [Reches 1983, settlement in mid-15th century. We present a paleo- Reches and Dieterich 1983] and is represented by two seismology study that revealed several events of surface main fault systems with oblique slip, each composed faulting in the island of São Miguel during historical of two sets dipping in opposite senses: WNW-ESE to times, thus contributing to the seismic risk awareness NW-SE normal dextral faults, and conjugated NNW- in the most populated island in the Azores. SSE to N-S normal left-lateral structures. This geom- The Azores archipelago is located in a zone of etry and kinematics indicate a stress field with a hori- anomalously shallow topography (the Azores Plateau), zontal maximum compressive stress axis (σ1) trending corresponding to the expression of the Eurasia, Nubia NW-SE, a horizontal minimum compressive stress axis and North America triple junction (Figure 1). The (σ3) in the NE-SW direction and a vertical intermediate Mid-Atlantic Ridge separates the North America from compressive stress axis (σ2); permutations between σ1 S0671 CARMO ET AL. Figure 1. Tectonic setting of the Azores archipelago (adapted from Hipólito et al. 2010). The main morphotectonic features of the region are presented. The shaded area in both the inset and main figure TR (s.l.) represents the sheared western segment of the Eurasia-Nubia plate boundary, and TR (s.s.) corresponds to São Miguel-Graciosa alignment. Tectonic structures: MAR-Mid-Atlantic Ridge; TR-Terceira Rift; EAFZ-East Azores Fracture Zone; GF-Gloria Fault. Islands: C-Corvo; FL-Flores; FA-Faial; P-Pico; SJ-São Jorge; G-Graciosa; T-Terceira; SMG- São Miguel; FO-Formigas islets; SMA-Santa Maria. Azores bathymetry adapted from Lourenço et al. [1997], and World topography and ba- thymetry from GEBCO_08 database [2010]. and σ2 may originate from transtensive or tensile tec- In the Azores archipelago, surface faulting is well tonic regimes [e.g. Madeira 1998, Madeira and Brum documented in all islands located along the Eurasia- da Silveira 2003]. Nubia margin (central and eastern groups) producing The deformation is accommodated by a large fault scarps of variable dimension. Most post-settle- number of active faults along which significant seismic ment earthquakes capable of producing surface rup- and volcanic activity occur. Since settlement, in the ture occurred off-shore with the exception of two 15th century, the Azores islands have been severely af- events in Terceira Island in 1614 and 1841, for which fected by seismic activity, characterized by usually low there are coeval texts describing rupture on land. Earth- to moderate magnitude isolated events or seismic quake magnitudes ranging from 6.9 to 7.1 were esti- swarms of tectonic and/or volcanic nature [e.g. Silveira mated from slip per event in the islands of Faial, Pico et al. 2003]. However, high magnitude earthquakes (e.g. and São Jorge [Madeira and Brum da Silveira 2003]. M 7.2; Hirn et al. [1980]) may also occur, causing thou- However, this procedure should be used carefully be- sands of deaths and severe damage [e.g. Machado 1949, cause slip frequently overestimates magnitude as co- Hirn et al. 1980]. Volcanic activity also occurred, both seismic, post-seismic and fault creep displacements are on land and at sea, sometimes with severe conse- very difficult to discriminate in paleoseismological quences [e.g. Chaves 1960, Weston 1964, Madeira analyses. 2006]. The temporal distribution of eruptions and At São Miguel, active faulting is represented by earthquakes (at the archipelago scale) suggests alter- prominent fault scarps and linear volcano-tectonic nating eruptive and tectonic phases. Volcanism would structures with a main NW-SE to WNW-ESE trend and happen when the stress field becomes largely exten- normal dextral slip. At Achada das Furnas area, aerial sional, due to permutations between the maximum and photo analysis identified the presence of two E-W intermediate stress axes [Madeira 1998]. trending scarps. Trenching across these scarps con- 2 HISTORICAL SURFACE FAULTING IN AZORES firmed their tectonic nature revealing two previously tures that truncate the flanks of the main volcanoes, unknown normal faults, hereafter named Altiprado and linear volcano-tectonic structures (Figure 2). The Faults 1 and 2. This work presents evidence of surface observable length of these structures is limited by the faulting on these structures in historical (post-settle- dimension of the island and by mantling by recent ment) times. (mostly Holocene) volcanic deposits that locally con- ceal the faults. NNW-SSE and NE-SW trending struc- 2. Volcano-tectonic setting tures are represented by volcano-tectonic alignments São Miguel Island is located on the eastern part of and lineaments (Figure 2). the Terceira Rift (Figure 1). It comprises three active Tectonic structures played a key role in the vol- central volcanoes with summit calderas (Sete Cidades, canic evolution of São Miguel, controlling the em- Fogo and Furnas), characterized by basaltic (s.l.) effu- placement of major volcanoes and locally acting as Figure 2. Volcano-tectonic map of São Miguel superimposed on a DTM of the island: SCV-Sete Cidades volcano; PCR-Picos region; FGV- Fogo volcano; AFP-Achada das Furnas plateau; FRV-Furnas volcano; PVV-Povoação volcano; NDV-Nordeste volcano. The red box indicates the area presented in Figure 3. sive activity during the early subaerial stages followed conduits: central volcanoes were built at the intersec- by explosive trachytic volcanism in late Pleistocene and tion of the NW-SE to WNW-ESE and NNW-SSE to N- Holocene. These are linked by volcanic fissure zones S conjugated fault systems, while monogenetic cones (Picos region and Achada das Furnas plateau). Two lie along them. older and inactive volcanoes (Povoação and Nordeste) The Achada das Furnas plateau, located in the cen- form the eastern part of the island (Figure 2). Volcan- tral area of the island between Fogo and Furnas volca- ism becomes younger westwards. The Nordeste mafic noes, is a flat area dominated by several basaltic cinder shield volcano is the oldest part of the island and its age cones, occasional maars and trachytic domes, defining ranges from 4 to 0.95 million years [Abdel-Monem et WNW-ESE and E-W alignments (Figure 3). The area is al. 1975]. Its southwest flank is cut by the Povoação mantled by thick trachytic fall deposits produced by caldera of unknown age. The 100,000 years old Furnas eruptions on the neighboring central volcanoes. Aerial volcano [Moore 1990] is the easternmost of the three photo analysis revealed the presence of two E-W trend- active central volcanoes of São Miguel and cuts the ing scarps, related to the previously unknown Altiprado caldera of Povoação.