Layout 6 ANNALS OF GEOPHYSICS, 56, 6, 2013, S0671; doi:10.4401/ag-6221 S0671 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 ABSTRACT The Azores archipelago is located at the triple junction between the Eurasian, Nubian and North American lithospheric plates, whose boundaries are the Mid-Atlantic Ridge and the Azores-Gibraltar Fault Zone. São Miguel is the largest island of the archipelago and is located on the eastern part of the western segment of the Azores-Gibraltar Fault Zone. The Achada das Furnas plateau, located in the central part of the island, between Fogo and Furnas central volcanoes, is dominated by sev- eral WNW-ESE and E-W trending alignments of basaltic cinder cones. Two E-W trending scarps were identified by aerial photo interpretation. Transect trenches exposed two active normal faults-the Altiprado Faults – confirming the tectonic nature of the scarps. Several paleoearthquakes were deduced, most of which in historical times, producing 1.38 m and 0.48 m of cumulative displacement. Maximum expected magnitudes (MW) determined from slip per event range from 5.7 to 6.7. One of the events probably corresponds to the historical earthquake of October 22nd, 1522, the deadliest in the archipelago. Radiocarbon ages are in agree- ment with this interpretation. 1. Introduction The population of Azorean islands faces significant geological hazard due to the geodynamic context of the archipelago, including frequent seismicity, active vol- canism and landslides. Earthquakes claimed more than 6,000 victims in the six centuries since the beginning of settlement in mid-15th century. We present a paleo- seismology study that revealed several events of surface faulting in the island of São Miguel during historical times, thus contributing to the seismic risk awareness in the most populated island in the Azores. The Azores archipelago is located in a zone of anomalously shallow topography (the Azores Plateau), corresponding to the expression of the Eurasia, Nubia and North America triple junction (Figure 1). The Mid-Atlantic Ridge separates the North America from Eurasian and Nubian plates, while the Azores-Gibral- tar Fault Zone (Terceira Rift (s.l.) and Gloria Fault) is the boundary between Eurasian and Nubian plates. The archipelago comprises nine islands distributed by three groups. The western islands (Flores and Corvo) lie on the stable North American plate, while the central (Faial, Pico, São Jorge, Graciosa and Ter- ceira) and eastern groups (São Miguel, Santa Maria and Formigas islets) are located along the western segment of the Azores-Gibraltar Fault Zone, the Terceira Rift (s.l.). This segment is bordered to the south by the in- active East Azores Fracture Zone and to the north by the São Miguel-Graciosa alignment (Terceira Rift (s.s.), Machado 1959). It corresponds to a diffuse and com- plex deformation zone, sheared by a dextral transten- sive regime [Madeira 1998, Lourenço et al. 1998, Madeira and Brum da Silveira 2003] due to higher spreading rates at the Mid-Atlantic Ridge north of the triple junction [e.g. Laughton and Whitmarsh 1974, Fernandes et al. 2003, Argus et al. 2011] and to the obliquity relatively to the spreading directions [e.g. Madeira and Ribeiro 1990, Madeira 1998, Lourenço et al. 1998]. The fault pattern in the Azores islands is charac- terized by three-dimensional strain [Reches 1983, Reches and Dieterich 1983] and is represented by two main fault systems with oblique slip, each composed of two sets dipping in opposite senses: WNW-ESE to NW-SE normal dextral faults, and conjugated NNW- SSE to N-S normal left-lateral structures. This geom- etry and kinematics indicate a stress field with a hori- zontal maximum compressive stress axis (σ1) trending NW-SE, a horizontal minimum compressive stress axis (σ3) in the NE-SW direction and a vertical intermediate compressive stress axis (σ2); permutations between σ1 Article history Received October 8, 2012; accepted May 10, 2013. Subject classification: Earthquake geology and paleoseismology, Seismic risk, Tectonics. Special Issue: Earthquake geology and σ2 may originate from transtensive or tensile tec- tonic regimes [e.g. Madeira 1998, Madeira and Brum da Silveira 2003]. The deformation is accommodated by a large number of active faults along which significant seismic and volcanic activity occur. Since settlement, in the 15th century, the Azores islands have been severely af- fected by seismic activity, characterized by usually low to moderate magnitude isolated events or seismic swarms of tectonic and/or volcanic nature [e.g. Silveira et al. 2003]. However, high magnitude earthquakes (e.g. M 7.2; Hirn et al. [1980]) may also occur, causing thou- sands of deaths and severe damage [e.g. Machado 1949, Hirn et al. 1980]. Volcanic activity also occurred, both on land and at sea, sometimes with severe conse- quences [e.g. Chaves 1960, Weston 1964, Madeira 2006]. The temporal distribution of eruptions and earthquakes (at the archipelago scale) suggests alter- nating eruptive and tectonic phases. Volcanism would happen when the stress field becomes largely exten- sional, due to permutations between the maximum and intermediate stress axes [Madeira 1998]. In the Azores archipelago, surface faulting is well documented in all islands located along the Eurasia- Nubia margin (central and eastern groups) producing fault scarps of variable dimension. Most post-settle- ment earthquakes capable of producing surface rup- ture occurred off-shore with the exception of two events in Terceira Island in 1614 and 1841, for which there are coeval texts describing rupture on land. Earth- quake magnitudes ranging from 6.9 to 7.1 were esti- mated from slip per event in the islands of Faial, Pico and São Jorge [Madeira and Brum da Silveira 2003]. However, this procedure should be used carefully be- cause slip frequently overestimates magnitude as co- seismic, post-seismic and fault creep displacements are very difficult to discriminate in paleoseismological analyses. At São Miguel, active faulting is represented by prominent fault scarps and linear volcano-tectonic structures with a main NW-SE to WNW-ESE trend and normal dextral slip. At Achada das Furnas area, aerial photo analysis identified the presence of two E-W trending scarps. Trenching across these scarps con- CARMO ET AL. 2 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]. 3 firmed their tectonic nature revealing two previously unknown normal faults, hereafter named Altiprado Faults 1 and 2. This work presents evidence of surface faulting on these structures in historical (post-settle- ment) times. 2. Volcano-tectonic setting São Miguel Island is located on the eastern part of the Terceira Rift (Figure 1). It comprises three active central volcanoes with summit calderas (Sete Cidades, Fogo and Furnas), characterized by basaltic (s.l.) effu- sive activity during the early subaerial stages followed by explosive trachytic volcanism in late Pleistocene and Holocene. These are linked by volcanic fissure zones (Picos region and Achada das Furnas plateau). Two older and inactive volcanoes (Povoação and Nordeste) form the eastern part of the island (Figure 2). Volcan- ism becomes younger westwards. The Nordeste mafic shield volcano is the oldest part of the island and its age ranges from 4 to 0.95 million years [Abdel-Monem et al. 1975]. Its southwest flank is cut by the Povoação caldera of unknown age. The 100,000 years old Furnas volcano [Moore 1990] is the easternmost of the three active central volcanoes of São Miguel and cuts the caldera of Povoação. Fogo volcano, located on the cen- ter of the island, started its subaerial edification more than 200,000 years ago [Muecke et al. 1974]. The west- ern end of the island is dominated by Sete Cidades vol- cano whose formation began prior to 210,000 years BP [Moore 1990]. The main active fault system trends NW-SE to WNW-ESE. The surface expression of the faults is rep- resented by fault scarps, forming large graben struc- tures that truncate the flanks of the main volcanoes, and linear volcano-tectonic structures (Figure 2). The observable length of these structures is limited by the dimension of the island and by mantling by recent (mostly Holocene) volcanic deposits that locally con- ceal the faults. NNW-SSE and NE-SW trending struc- tures are represented by volcano-tectonic alignments and lineaments (Figure 2). Tectonic structures played a key role in the vol- canic evolution of São Miguel, controlling the em- placement of major volcanoes and locally acting as conduits: central volcanoes were built at the intersec- tion of the NW-SE to WNW-ESE and NNW-SSE to N- S conjugated fault systems, while monogenetic cones lie along them. The Achada das Furnas plateau, located in the cen- tral area of the island between Fogo and Furnas volca- noes, is a flat area dominated by several basaltic cinder cones, occasional maars and trachytic domes, defining WNW-ESE and E-W alignments (Figure 3). The area is mantled by thick trachytic fall deposits produced by eruptions on the neighboring central volcanoes. Aerial photo analysis revealed the presence of two E-W trend- ing scarps, related to the previously unknown Altiprado Faults 1 (south) and 2 (north), which interrupt the flat- ness of the plateau (Figures 3,4). 3. The Altiprado Faults 3.1. Altiprado Fault 1 The Altiprado Fault 1 (AF1) trace is marked by an 835 m-long, 3 m-high, south-facing scarp, trending N87ºE (Figures 3,4). To the east and to the west its HISTORICAL SURFACE FAULTING IN AZORES 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. trace becomes uncertain. An 18 m-long trench was open across the scarp, lo- cated at 37.79ºN-25.396ºW, exposing a N87ºE, 65ºS fault. The fault displaces a stratigraphic succession com- prising six units separated by paleosols (Figure 5). Units 1 to 5 are pumice fall deposits produced by eruptions from Fogo and Furnas volcanoes and unit 6 is a sedi- mentary deposit (remobilized pyroclasts). The sequence CARMO ET AL. 4 Figure 3. Main tectonic and volcanic structures of Achada das Furnas plateau: AF1-Altiprado Fault 1; AF2-Altiprado Fault 2; FGV-Fogo vol- cano; AFP-Achada das Furnas plateau; FRV-Furnas volcano. The red box indicates the area presented in Figure 4. Figure 4. Vertical aerial photograph of Altiprado region, Achada das Furnas plateau, showing the geomorphic expression of Altiprado Faults: AF1-Altiprado Fault 1; AF2-Altiprado Fault 2; Red dots-location of trenches (Aerial photo from Direcção Geral de Planeamento Ur- banístico 1974). 5 is composed of: 1-olive brown paleosol developed on top of Fogo A deposit, produced by a major plinian erup- tion of Fogo volcano at about 4520±90 years BP [Wal- lenstein 1999; Table 1]; 2-pumice fall deposit composed of alternating yellowish fine grained lapilli and light olive brown ash beds, probably corresponding to Fogo C deposit, the product of a hydromagmatic eruption at Fogo volcano; 3-fall deposits, probably from Fogo D eruption, composed of three layers, from the base to the top: (3a) dark brown pumice ash fall deposit containing disseminated lapilli; (3b) yellowish lapilli pumice fall de- posit; and (3c) brown ash fall deposit; 4-grey pumice ash layer corresponding to Furnas C deposit (1870±120 years BP; Guest et al. 1999], Table 1), which, in this trench, is completely altered to a very dark grey soil (410±30 years BP, 1440-1500 cal AD; Table 1); 5-strati- fied fall deposit of alternating beds of fine to medium greyish white pumice lapilli and ash (5a to 5d) contain- ing sanidine crystals from 1563 AD Fogo volcano plinian eruption, topped by a very dark grey paleosol (5e) rich in coal fragments (160±30 years BP, 1660-1700 cal AD, Table 1); 6-sediment formed by remobilization of the underlying unit. The marked southward dip of all units suggests that they are draping a fault scarp produced by previous (pre 4520±90 BP) surface ruptures. The AF1 affects all stratigraphic units, including the present-day soil, and the scarp is an intact free-face almost devoid of soil that corresponds to the projection of the fault plane to the surface, indicating its youth (Figure 5). HISTORICAL SURFACE FAULTING IN AZORES Table 1. Radiocarbon ages of units trenched in Altiprado Faults. Lab. ref. Trench Field ref. Unit Material Age 2σ calib. date Author - AF1 - 1 Charcoal 4520±90 y BP - Wallenstein [1999] - AF/AF2 - 4 Carbonaceous layer from the base of deposit 1870±120 y BP - Guest et al. [1999] 295323 AF1/AF2 TR1-1 4 Paleosol 410±30 y BP 1440-1500 cal AD This work 295337 AF1/AF2 TR1-2 5 Paleosol 160±30 y BP 1660-1700 cal AD This work Figure 5. Map of the east wall of the Altiprado Fault 1 trench. Legend: 1-Fogo A; 2-Fogo C; 3a to 3c-Fogo D; 4-Furnas C; 5a to 5e-1563 Fogo volcano eruption; 6-remobilized deposit. Several WNW-ESE to E-W trending fractures and open cracks, sometimes filled with material from over- lying units (3 and 4), and a colluvium (C1) composed of material from units 3 and 4 were also exposed in the trench (Figure 5). The exposed geometry shows that units 2 to 4 are displaced by 1.38 m and units 5 (5a to 5e) and 6 by 0.38 m, indicating at least two surface rupturing pa- leoearthquakes. Although there is a time gap between the deposition of unit 4 and the development of the overlying soil, the presence of a single colluvial wedge on top of the soil indicates a major surface rupture pro- ducing a minimum displacement of 1 m. Units 5 and 6 are deeply eroded, so it is not possible to determine if the remaining 0.38 m of offset were produced by one or more earthquakes. Since unit 5 presents an over- thickening in the downthrown side when compared to the upthrown block and some branches of the fault ap- parently do not cut the entire deposit, part of the dis- placement may have occurred during the emplacement of unit 5, and the remaining displacement after the dep- osition of unit 6. The 1.38 m accumulated dip-slip in 4520±90 years yields a minimum slip rate of 0.31±0.01 mm/year. If we take into account the age of the soil (410±30 years BP) a minimum slip rate of 3.37±0.25 mm/year is ob- tained. 3.2. Altiprado Fault 2 The Altiprado Fault 2 (AF2) trace is marked by an almost imperceptible 1690 m-long and ~40 cm-high south-facing scarp, trending N87ºE (Figures 3,4). To the east its expression is lost. A 29 m-long trench, located at 37.796ºN - 25.392ºW, exposed the same stratigraphic sequence ob- served in AF1 trench that is displaced by two N75-89ºE trending subvertical faults (AF2-1 and AF2-2), which display frequent changes in dip direction (generally dip- ping 62-88ºS), and an E-W trending paleo-channel (filled by unit 6) (Figure 6). A colluvium (C2), contain- ing material of unit 3, is deposited at the base of AF2- 1 fault scarp (Figure 6). Open cracks trending ENE-WSW to E-W were also filled by the colluvial material. The faults produced different vertical offsets on units 2 and 3, and on units 4 (that in this trench is al- most totally altered to soil) and 5. AF2-1 exhibits an ac- cumulated dip slip of 33 cm, offsetting units 2 and 3 by 26 cm, and units 4 and 5 by 7 cm. AF2-2 displaced units 2 and 3 by 11 cm, and units 4 and 5 by 4 cm, resulting in an accumulated dip-slip of 15 cm. Displacement val- ues of 26 cm, 7 cm, 11 cm and 4 cm are thus obtained. Assuming that AF2-1 and AF2-2 are branches of the same fault and that the ruptures in both planes were produced by the same earthquakes, displacement val- ues of 0.37 m (0.26 m + 0.11 m) and 0.11 m (0.07 m + 0.04 m) are obtained, indicating two paleoearthquakes. An alternative interpretation is that four pale- oearthquakes occurred in this tectonic structure, each offset corresponding to a separate event. The strong dip of the faults suggests dominant strike-slip compo- nent associated to down throw to south, indicating that the offsets may be significantly higher. A dominant strike-slip component could also account for the sub- dued geomorphic expression of this tectonic structure when compared to AF1. Considering both branches, the accumulated normal displacement in AF2 fault is 0.48 m (0.26 m + 0.07 m + 0.11 m + 0.04 m) in less than 4500 years, which yields a minimum slip rate of 0.11 mm/year. The presence of a paleo-channel trending E-W, perpendicular to the general northward dipping slope and drainage direction, suggests that it may have been developed at the base of a previous fault scarp or along the trace of a fault plane that has not been exposed in the trench. CARMO ET AL. 6 Figure 6. Map of the west wall of the trench across Altiprado Fault 2. Legend: 2-Fogo C; 3a to 3c-Fogo D; 4-Furnas C; 5a to 5d-1563 Fogo volcano eruption; 6-remobilized deposit. 7 3.3. Evolution of the Altiprado Faults Geometric analysis of the trenches allowed the reconstruction of the sequence of deposition, faulting and erosion events. As the faults are less than 740 m apart from each other and affect the same strati- graphic succession the evolution of faulting, recon- structed for both locations in Figure 7, is the following: a) Deposition of units 1 (4520±90 years BP) to 3; b) Surface rupture(s) at AF2-1 and AF2-2 produc- ing normal offsets of 26 cm and 11 cm, respectively; c) Erosion truncating unit 3, with the formation of gullies in AF1 fault zone, and fault scarp retreat at AF2-1 with the formation of a colluvial wedge (C2) composed of material from unit 3; d) Deposition of unit 4 (1900 years BP) and de- velopment of a soil (1440-1500 cal AD, 410±30 BP); e) Surface rupture at AF1 producing a normal off- set of 1 m; f) Erosion with minor fault scarp retreat and for- mation of a colluvial wedge (C1) in AF1; g) Deposition of unit 5 (1563 AD, Fogo volcano eruption) with probable sin-eruptive ruptures of 7 cm and 4 cm in AF2-1 and AF2-2 respectively, without sur- face expression and surface rupture of unknown offset at AF1?; h) Erosion truncating the top of unit 5 and for- mation of a paleo-channel in AF2 fault zone; develop- ment of a soil (1660-1700 cal AD, 160±30 BP); i) Erosion truncating unit 5 and deposition of unit 6, probably representing a major storm event that to- tally filled the gully exposed in AF2 trench; j) Surface rupture(s) at AF1 contributing to an ac- cumulated normal displacement of 0.38 m in units 5 and 6; k) Erosion truncating units 5 and 6; development of the present topsoil. 4. Discussion The trenches allowed the recognition of several surface rupture events at the Altiprado Faults, some of which post-date the settlement of the island. In AF1, the first rupture (1 m) occurred after the development of a paleosol on unit 4 (1440-1500 cal AD) and before the 1563 AD eruption (unit 5). The presence of a colluvium above the paleosol developed on unit 4 indicates that the rupture occurred after the formation of the soil and has no relation to the underlying volcanic event. A second offset of 0.38 m is measured at units 5 and 6, but the available data are insufficient to determine the number of earthquakes that produced it and the temporal relation with the deposition of those units. Volcano-tectonic relations (see Villamor et al., 2011) sug- gest that part of the displacement may have occurred during the 1563 AD Fogo eruption, as there is a differ- ence in thickness of unit 5 in both sides of the fault, or after the formation of unit 6 (1660-1700 cal AD). In AF2, two hypotheses are possible. At least two earthquakes produced rupture in both branches (AF2-1 and AF2-2) of the fault zone, with an accumulated offset of 0.37 m (0.26 m + 0.11 m) in the first event, during the 4520 BP to 1900 BP interval, and 0.11 m (0.07 m + 0.04 m) in the second rupture, during the 1563 AD eruption or just after this event, as offsets do not propagate through all the thickness of unit 5. Alternatively, it rup- tured at least four times. The two earliest earthquakes (0.26 m in AF2-1 and 0.11 m in AF2-2) occurred in the 4520 BP-1900 BP interval. The latest earthquakes (0.07 m in AF2-1 and 0.04 m in AF2-2) occurred during the 1563 AD Fogo eruption or after it. HISTORICAL SURFACE FAULTING IN AZORES Figure 7. Sequence of depositional, erosional and tectonic events leading to present-day geometry exposed in Altiprado Faults 1 and 2 trenches. The first surface faulting event(s) is (are) related to AF2, generating an accumulated normal displacement of 0.37 m in AF2-1 and AF2-2 fault branches. It occurred between 4520 BP (unit 1) and 1900 BP (unit 4). The second surface rupture event occurred in AF1 after unit 4 paleosol formation (1440-1500 cal AD) and produced 1 m of normal displacement. The third rupture(s) produced an accumulated 0.11 m normal offset of unit 5 in AF2-1 and AF2-2 fault branches and probably occurred during the 1563 Fogo volcano eruption seismic activity. A last offset of 0.38 m of units 5 and 6 is related to AF1. 4.1. Relation with historical seismicity The first surface rupturing earthquake(s) is (are) associated to AF2, producing an accumulated dip-slip offset of 37 cm in AF2-1 and AF2-2. It occurred in pre settlement times, during the interval 4520±90 BP1870±120 BP (unit 4-Furnas C deposit). The second surface rupture event occurred in AF1, after the development of the paleosol on unit 4 (410±30 BP, 1440-1500 cal AD) and before the deposition of unit 5 (1563 AD-Fogo historical eruption), producing 1 m of normal displacement. Considering that the settlement in São Miguel occurred in 1439-1443 AD, and taking into account the historical record of earthquakes in the island, this event probably corresponds to the October 22nd, 1522 Vila Franca do Campo earthquake. This was the deadliest and one of the most violent events recorded in the Azores, causing about 5000 deaths [Fru- tuoso 1522-1591]. Based on macroseismic analysis of historical records, using the European Macroseismic Scale 1998 (EMS-1998; Grünthal [1998]), Silveira [2002] and Silveira et al. [2003] inferred that the epicenter was located inland, NNW of Vila Franca do Campo (a few km southwest from the Altiprado Faults) and that the maximum intensity X was reached at the central part of the island (Figure 8). Silveira [2002] drew NW-SE trending elliptic isoseismal lines based on structural constraints. The earthquake produced severe damage and triggered numerous landslides mainly in the cen- tral-eastern part of the island, in an area located be- tween Vila Franca do Campo, Ponta Garça, Furnas, Fenais da Ajuda and Maia [Mendonça Dias 1945] (Fig- ure 8). A major landslide occurred on a slope above Vila Franca do Campo (the capital town of the island at the time), burying the village a few minutes after the main shock, destroying the houses that were still standing, and killing most of its inhabitants [Silveira et al. 2003]. Nevertheless, the historical records are not clear enough in what concerns the direct effects caused by the earthquake. Although the paleoseismic data of AF1 reveals 1 m of surface rupture displacing the paleosol developed on unit 4 (1440-1500 cal AD), historical records do not account for surface ruptures. The documents reveal that “…in the earth entrails frightful noises sounded…” moving the earth “… with furious shakes” [Monte Alverne 1629-1726] and that there was no hill that did not collapse” [Maldonado 1644-1711]. However surface rupture phenomena could easily go unnoticed as the villages are preferentially located in coastal areas and, at the time (only 80 years since the beginning of settle- ment), the central part of the island was probably still covered by dense vegetation. The third rupture(s) is (are) associated with AF2. As it is not clear if the last displacement (11 cm in AF2- 1 and AF2-2) is affecting only the lower part of unit 5 (1563 AD) or affects the entire unit, these displace- ments could be associated to the intense seismic ac- tivity that accompanied the 1563 eruption of Fogo volcano. The earthquakes were felt in a wide region and caused severe damage in Ribeira Grande and Ribeira Seca villages [Frutuoso 1522-1591, Silveira 2002]. The earthquakes were distributed over a broad area, making it difficult to determine an epicentral lo- cation. Based in the damage levels reported in coeval records and using EMS-1998, Silveira [2002] deduced that the damage was similar to that caused by an CARMO ET AL. 8 Figure 8. Isoseismal map for the 1522 earthquake (adapted from Silveira 2002): red dot-macroseismic epicenter; red lines-isoseismal lines; dashed black lines-area affected by landslides according to Mendonça Dias [1945]. 9 earthquake reaching intensities X at Ribeira Grande and IX-X at Água de Pau, respectively. Historical doc- uments do not describe the occurrence of secondary environmental effects related to seismic activity. On the other hand, other earthquakes occurred after this eruption that could be responsible for the observed offsets; possible candidates are the 1591 earthquake or the seismic activity associated to the 1630 eruption at Furnas volcano. The July 26th, 1591 earthquake caused several deaths and produced severe damage in Vila Franca do Campo and Água de Pau. The lack of information does not allow inferring the epicentral location. Sil- veira [2002] considered intensity VIII (or greater) at those localities. The 1630 eruption at Furnas volcano was also preceded and accompanied by seismic activity distrib- uted over a wide area, and caused several landslides and a lot of damage, mainly in Povoação, Ponta Garça and Vila Franca do Campo [Monte Alverne 1629- 1726], from which Silveira [2002] estimated an inten- sity VIII or greater. The most recent displacement(s), producing an offset of 0.38 m, is (are) related with AF1. However, the data are insufficient to correlate deposition, ero- sion and faulting events. There is no evidence of a high-magnitude earthquake with epicenter inland in historical or instrumental records justifying the ob- served offset. Part of it could have been produced by the intense seismic activity that accompanied the 1563 AD Fogo eruption. Another possibility is that part (or the totality) of this offset may have been produced after the deposition of unit 6 (<160±30 years BP, <1660-1700 cal AD). In the central region of São Miguel seismicity oc- curs mainly as shallow seismic swarms of low to mod- erate magnitude, with focal depths varying between 0 and 8 km [Silva 2011]. There is a long time record of seismic swarms in this area since the beginning of 20th century (e.g. 1922, 1967, 1983, 1985, 1989 and 2005). The most recent seismic swarm started on May 10th, 2005 and continued until the end of the year, with more than 46,000 shallow earthquakes recorded (Figure 9), 170 of which were felt. The focal depth ranged from 1 to 7 km [Silva et al. 2012]. The strongest events occurred on September 20th and 21th and had ML 4.1 and 4.3, respectively, causing several small land- slides and E-W trending ground cracks. The larger fis- sure was 1 km-long and 15 cm-wide [Trota 2008]. The landslides were heterogeneously distributed over a 10 km2 area around the epicentral zone, and some formed natural dams [Marques et al. 2007]. As the earthquakes were felt with intensity VI at Vila Franca do Campo, using the Mercalli Modified Scale 1956 (MM-56, CIVISA) which, in turns, corre- sponds to the same intensity value in EMS-1998 [Mus- son et al. 2010], intensity should have been higher (≥VII) at the epicentral zone. The region is unpopu- lated, but some farming support buildings suffered structural damage compatible with EMS-1998 inten- sity VII. Surface rupture could easily pass unnoticed as the main villages are located on coastal areas. GPS monitoring data allowed Trota [2008] to ver- ify that there was ground deformation associated with the seismic activity. Considering the seismicity pat- tern, the low seismic energy released, and the GPS data, this activity must have been related to a mag- matic intrusion at 1.8 to 2.4 km depth. The previous seismic swarms were not geodetically monitored and the seismic network was limited at the time, so there are no evidences of previous episodes of crustal de- formation. Thus, it is more realistic to consider that the 0.38 m displacement observed in AF1 trench could have accumulated as a consequence of several mod- erate shallow earthquakes during periods of increased seismic activity in this area, which may have produced crustal deformation. Azzaro [1999], Azzaro et al. [2000] and Ferreli et al. [2002] report surface ruptures and paleoseismological evidence produced by shallow moderate magnitude earthquakes (MW 3.0-5.0) and creep in faults located on the eastern flank of Etna. 4.2. Slip rates and recurrence intervals Normal slip rates range from 0.11 mm/year in AF2 to 0.31-3.37 mm/year in AF1. It should be noted that the minimum slip rate at AF1 may be overesti- mated, since a considerable time gap exists between the deposition of Furnas C tephra (unit 4) at about HISTORICAL SURFACE FAULTING IN AZORES Figure 9. Epicenter distribution of the 2005 seismic swarm (May 10th to December 31st; data from CIVISA). More than 46,000 earth- quakes, with focal depths between 1 and 7 km, were recorded. The most energetic events reached magnitudes 4.3 and 4.1 ML and trig- gered several landslides and E-W trending ground cracks. 1900 BP and the development of the overlying soil (410±30 BP). Some amount of displacement may have occurred before the formation of the paleosol. The slip rate in AF2 is a minimum value, since the age of the oldest rupture is unknown and because the geom- etry of the faults suggests that the strike-slip compo- nent could be significant. Recurrence intervals were not estimated because the short period analyzed suggests clustering, as most of the events occurred in historical times and because, between the deposition of Furnas C (unit 4, 1900 BP) and the development of its topsoil (410±30 BP), there is a time gap of about 1500 years without any geologi- cal record. 4.3. Implications for seismogenic potential assessment In the Azores, interactions between volcanism and faulting difficult seismic hazard assessment; be- sides coseismic ruptures, surface faulting can also occur during seismic swarms related to volcanic erup- tions (such as during the 1957-58 Capelinhos eruption in Faial; Madeira and Brum da Silveira 2003). The maximum probable magnitudes of the earth- quakes generated by the Altiprado Faults were estimated based on Wells and Coppersmith’s [1994] correlations between seismic moment magnitude (Mw) and surface rupture length [SRL; Mw=5.08+1.16log(SRL)], rupture area [RA; Mw=4.07+0.98log(RA)], and maximum dis- placement [MD; Mw=6.69+0.74log(MD)], consider- ing the offset produced by each surface rupturing event (Table 2). However, one should be aware that the achieved numbers are approximate values, as we are dealing with a volcanic environment, where earth- quakes may have non-double couple mechanisms. Relatively to rupture area, we used a similar pro- cedure to that used by Meletti et al. [2008] to estimate the thickness of the brittle crust (seismogenic crust) for Italian territory, by using the maximum depth at which earthquakes occur. According to Matias et al. [2007], the Moho is located at 12-13 km depth. Nevertheless, the majority of earthquakes of São Miguel central area are located at depths that do not exceed 8 km [Silva, 2011]. This is comparable with other volcanic regions, like the South Iceland Seismic Zone [Tryggvason et al. 2002, Gudmundsson and Brenner 2003] and the Taupo Volcanic Zone [Villamor et al. 2011]. Fault dip at depth is unknown, so we consider that fault dip does not change significantly. The rupture area was then calcu- lated using a thickness of 8 km for the seismogenic crust and an average fault dip of 65º. The maximum slip method frequently overesti- mates magnitude, since geological record does not allow distinguishing between coseismic, post-seismic and aseismic slip. On the other hand, the observed off- sets may not represent the maximum offset produced by each rupture. Considering the total observed length of Al- tiprado Faults (AF1, 0.8-5.5? km; AF2, 1.7-3? km), mo- ment magnitudes in the order of 4.9-5.9 are obtained for surface rupture length and rupture area parame- ters, contrasting with magnitudes estimated from maximum displacement (5.7-6.7). However, in the two first cases, length and consequently area values are certainly underestimated as the total length of the faults is unknown. The faults must be longer but their traces may be concealed beneath thick fall deposits. For example, the deposits of the 1563 Fogo eruption reach a thickness of 2.5 m in Altiprado area. We must also considerer that, as this zone is prone to farming activities, morphological evidence may have been to- tally or partial destroyed by recent anthropic activities. Although those magnitude values are usually considered as insufficient to generate surface rupture, in volcanic domains the magnitude threshold for sur- face faulting may be significantly lower due to exis- tence of shallow seismogenic sources related to CARMO ET AL. 10 Table 2. Maximum expected magnitudes from surface rupture length (SRL), rupture area (RA) and maximum displacement (MD) of Al- tiprado faults using relationships by Wells and Coppersmith (1994). (1) Maximum displacement assuming a single paleoearthquake. (2) Max- imum displacement assuming surface rupture in both branches of AF2 (AF2-1 + AF2-2) in a single paleoearthquake. Fault SRL (km) Mw(±0.24-0.28) RA (km2) Mw MD (m) Mw AF1 0.8 - 5.5 5.0 - 5.9 7.4 - 48.5 4.9 - 5.7 1 0.38 (1) 6.7 6.4 AF2 1.7 - 3 5.3 - 5.6 15.0 - 26.5 5.2 - 5.5 0.37 (2) 0.11 (2) 0.26 0.11 0.07 0.04 6.4 6.0 6.3 6.0 5.8 5.7 11 volcano-tectonic or purely tectonic mechanisms [Smith et al. 1996, Azzaro et al. 1998, Zobin 2012]. There is a long record of seismic swarms in the studied area, probably with features similar to that of 2005 that was related with a magmatic intrusion. Sev- eral authors [e.g. Rubin and Pollard 1988, Rubin 1992] suggest that a magmatic intrusion at shallow levels can induce brittle deformation of the surface. Gravimetric studies performed at São Miguel Island indicate that in this area, at about 8 km depth, a high-density region ex- tending to shallower levels (still present at 1 km) exists and has been interpreted as an old basaltic shield or a partly solidified magmatic body [Camacho et al. 1997]. Relocation and fault plane solution analysis of earthquakes that occurred between 2002 and 2008 in the area suggest the existence of a local heteroge- neous stress field [Silva 2011]: in the upper 5 km depth, stress field is extensional, but at deeper levels (>5 km) fault plane solutions indicate a compressive stress field. According to Silva [2011], the extensional stress field may result from a compressive regime at larger depths, which promotes the ascension of ther- mal fluids to crustal lower levels, weakening pre-ex- isting fault planes which, due to gravitational stress, become potential slip surfaces. The role of hydrothermal fluids is well docu- mented in São Miguel. Seismic tomography for this area relates the velocity anomalies to hydrothermal activity [Zandomeneghi et al. 2007]; the existence of a positive Vp anomaly with high Vp/Vs ratio suggest that this zone is characterized by low Vs due to fluids contained in fractures. The possibility for creeping cannot also be disre- garded, but the Altiprado faults were not subject to precise leveling measurements. For instance, aseismic creep due to volcanic-tectonic processes that cause gravitational instability is well documented in faults at Etna volcano [Ferreli et al. 2002]. Conversely, maximum displacement magnitudes may be overestimated. In AF1 there is a time gap be- tween deposition of unit 4 and the development of the topsoil, so the separation of 1 m could be erro- neously considered as the result of a single event. On the other hand, part of the displacement could be post-seismic or aseismic. For instance, after the 2009 L’Aquila earthquake, Boncio et al. [2010 in Wilkinson et al. 2012] observed widening in ground cracks. In the same way, in volcanic areas surface ruptures may occur related to moderate earthquakes, and in this case Wells and Coppersmith’s [1994] correlations should not be used. Nevertheless, the correlation be- tween magnitude and surface rupture length, deduced from aftershock distribution, for off-shore instrumen- tal earthquakes in the Azores (the M 7.2, January 1st, 1980 and ML 5.8, July 9th, 1998 earthquakes; Hirn et al. 1980, Dias et al. 2007) are in agreement with those obtained using the correlations from Wells and Cop- persmith [1994], suggesting that they can be used in the Azores for tectonic earthquakes [Madeira and Brum da Silveira 2003]. However, the tectonic fault- ing origin for the observed offsets is not definitively proven. 5. Final remarks Trenching across the two scarps observed at Achada das Furnas confirmed their tectonic nature and revealed the presence of two previously unknown normal faults, with a possible strike-slip component. The Altiprado Faults 1 and 2 trend E-W and cut a vol- canic succession younger than 4500 years, the age of the well-known Fogo A deposit, undoubtedly show- ing that the faults are active. The proximity of the two faults and their geom- etry suggests that they could be synthetic branches of a major fault dipping to the south. It is probable that such a fault would present oblique (normal dextral) slip so we can envisage some amount of strain parti- tioning between the two branches; the subvertical geometry of the fault planes in AF2 is compatible with this branch accommodating most of the strike-slip component, while AF1 would accommodate mostly normal slip. AF1 exhibits an accumulated dip-slip of 1.38 m that may have been produced by at least two surface rupturing paleoearthquakes (1 m + 0.38 m). The ra- diocarbon age of the paleosol developed on unit 4 is compatible with the first rupture having been produced by the 1522 earthquake. The event that produced the second rupture remains unclear. Nevertheless, this area is subject to frequent seismic swarms of volcano-tec- tonic origin, and recent geodetic studies during periods of increased seismic activity showed the occurrence of crustal deformation. Thus, the normal offset of 0.38 m could be the result of displacements produced by sev- eral moderate magnitude events or swarms related or not with magmatic cycles. AF2 presents an accumulated dip-slip of 0.48 m, but uncertainties remain about the exact number of pa- leoearthquakes. Assuming that both branches of AF2 ruptured during the same events, then two pale- oearthquakes occurred (0.37 m + 0.11 m). On the other hand, four events may have occurred and, thus, each displacement be related to individual earthquakes (0.26 m, 0.11 m, 0.07 m and 0.04 m). In both cases, the older displacement(s) (0.37 m, or 0.26 m and 0.11 m) corre- spond to pre-settlement events, while the following HISTORICAL SURFACE FAULTING IN AZORES ruptures (0.11 m, or 0.07 m and 0.04 m) occurred dur- ing the 1563 historical eruption of Fogo volcano or later. Although there is some uncertainty about the seis- mic parameters, the Altiprado Faults have the potential to cause surface rupture either related to purely tec- tonic high-magnitude earthquakes (e.g. 1522 earth- quake), or associated to volcano-tectonic seismic swarms of low to moderate magnitude, or even during surface deformation associated to eruptions, compli- cating seismic hazard assessment in the Azores due to the interaction between volcanism and faulting. Data and sharing resources Maps were made using ArcGIS 10.0 software, ESRI®. World topography and bathymetry from GEBCO_08 database [2010], IHO UNESCO, General Ba- thymetry Chart of the Oceans, digital edition at http://www.gebco.net/data_and_products/gridded_ba thymetry_data/. Aerial photo of Achada das Furnas Plateau supplied by Direcção Geral de Planeamento Urbanístico, 1974. Epicenter distribution of the 2005 seismic swarm and its macroseismic data was taken from CIVISA Data- base, Centro de Informação e Vigilância Sismovulcânica dos Açores, Centro de Vulcanologia e Avaliação de Riscos Geológicos da Universidade dos Açores. Acknowledgments. Rita Carmo is supported by the Azores Regional Government, through a Ph.D. Grant from Fundo Re- gional da Ciência e Tecnologia (M3.1.2/F/016/2007), and through Serviço Regional de Protecção Civil e Bombeiros dos Açores in the scope of the scientific and technical protocols to guarantee the Azores Seismovolcanic Surveillance and the Emergency Planning Studies of Centro de Informação e Vigilância Sismovulcânica dos Açores (CIVISA). José Madeira's contribution to FCT project SHA- Azores (PTDC/CTE-GIX/108637/2008).We also acknowledge the contribution of the reviewers (Emanuela Falcucci and Raffaele Az- zaro) and associate editor Salvatore Barba to significant improve- ment of the initial version of the manuscript. References Abdel-Monem, A.A.; L.A. Fernandez and G.M. Boone (1975). K-Ar ages from the eastern Azores group (Santa Maria, São Miguel and the Formigas Islands), Lithos, 8, 247-254. Argus, D.F., R.G. Gordon and C. DeMets (2011). 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