Abstract To date, a lack of reliable morphological and geophysical data has been a major limitation to understanding the tectonic and magmatic processes that shape the Azores Triple Junction and the Terceira Rift. This situation has changed recently: for the first time both areas are covered by high-quality swath bathymetry surveys and marine magnetic data with GPS quality positioning. This provides a good description of the surface morphology, and also of magnetic chrons that give fundamental information for the timing of spreading processes in a geological time frame. There is also a large amount of data from GPS stations that provide accurate estimations of present-day velocities for most of the islands. It is shown that only two main rift systems can be found on the plateau, the older one matching the Princess Alice Basin, and the newer one matching Terceira Rift; the shift between the two probably occurs close to c. 3 Ma. It is shown that extension is currently concentrated on the Terceira Rift, progressively attaching Graciosa and Terceira islands to Eurasia, while São Miguel is being strained by rifting. It is also shown that no right lateral strike-slip fault connects the Terceira Rift to the Mid-Atlantic Ridge.

Abstract The geodynamic setting of the Azores archipelago, straddling the triple junction between the North America, Eurasia and Nubia plates, is reflected in frequent volcanic and tectonic activity. A review of neotectonics is presented for the islands forming the central and eastern groups of the Azores (Faial, Pico, São Jorge, Graciosa, Terceira, São Miguel and Santa Maria). The geometry and kinematics of active faults displacing stratigraphic and geomorphological markers of Pleistocene and Holocene age are presented. Slip-rates were determined using the available ages for the displaced markers. Maximum expected moment magnitudes were estimated using empirical correlations between magnitude and fault length, fault area and maximum observed surface displacement during surface-rupturing palaeoearthquakes. Neotectonic parameters show that the faults are in most cases very to moderately active, with slip-rates usually ranging from a few tenths of millimetres to a few millimetres per year, while maximum expected magnitudes vary from M w 6 to 7. These magnitudes are in agreement with the instrumental and historical seismic record in the region. Neotectonic data define a dextral transtensive stress regime acting on the region and contribute to characterizing the complexity of the geodynamic processes that dominate the western-most segment of the Eurasia–Nubia plate boundary.

Abstract Since the settlement of the archipelago, in the fifteenth century, 31 destructive earthquakes and 28 volcanic eruptions have been registered in the Azores. Major earthquakes occurring in historical times have reached magnitudes >7, often triggering landslides and even small tsunamis. In the same period, subaerial volcanic eruptions have ranged from Hawaiian to sub-Plinian, sometimes with a hydromagmatic character, while submarine eruptions have been Azorean to Surtseyan in style. The temporal and spatial distributions of major historical events are presented and their impacts summarized. The instrumental seismic activity registered since 1980 is discussed taking into account the main volcano-tectonic structures. These seismological data allow us to improve the characterization of the present-day boundary between the Eurasia and Nubia lithospheric plates, herein defined as the East Azores Volcano-Tectonic System. The seismological data also suggest that the location of the Azores Triple Junction is to the west of Faial Island, at about 38° 50′ N, 30° 25′ W, in agreement with proposals made by other authors using aeromagnetic data. A natural seismic gap, centred in the São Jorge structural alignment, is recognized and is interpreted as a zone of stress accumulation with the potential to generate a high-magnitude earthquake similar to that of 1757.

Abstract Magmas in the Azores (Portugal) were erupted from both fissure zones and central volcanoes during overlapping periods. Fissure zones follow extensional trends oriented accordingly with the regional tectonics, erupting basalts and hawaiites. Central volcanoes are characterized by transtensive tectonics and erupted basaltic to trachytic magmas. Highly porphyritic magmas, consisting of cumulitic olivines and clinopyroxenes, can be also erupted during flank eruptions. Volcanism occurs because of the different direction of the plate movement at the triple junction where islands are located. Magma production and its ascent occur in response to the stresses of the lithosphere over a geochemically heterogeneous mantle, enriched in incompatible trace elements and water. The variability in the radiogenic isotopes results from the interaction of the differently enriched mantle sources with a ubiquitous and slightly depleted Azores mantle. Finally, a contribution of recycled oceanic crust is evidenced by a progressive east–west variation in Pb isotope ratios across the island of São Miguel. Primitive melts were generated by 1–7% partial melting of a garnet-bearing peridotite. Overall, basalts show relatively high large-ion lithophile element (LILE) abundances and LILE/high-field-strength element ratios that point to chemical heterogeneities in the mantle sources.

Abstract The Azores archipelago is located at the triple junction between the Eurasia, Nubia and North America lithospheric plates. São Miguel Island, situated at the southeastern part of the western segment of the Azores–Gibraltar Fracture Zone, presents an east–west elongated shape, comprising three quiescent central volcanoes with summit calderas linked by zones of fissure volcanism. The eastern part of the island is older and inactive. Active faulting is represented by prominent fault scarps that constitute important extensional structures, or by linear volcanic structures in fissural volcanic zones, with a dominant NW–SE to WNW–ESE trend. Although less frequent, there are also NNW–SSE to north–south, NE–SW and east–west faults, reflected by some volcanic alignments and linear segments of the drainage system and sea cliffs. The geometric and kinematic data are in agreement with that observed in the rest of the archipelago. However, at eastern São Miguel Island data indicate two distinct groups of conjugated faults characterized by three-dimensional strain: NW–SE to WNW–ESE normal dextral structures are conjugated with NNW–SSE normal left-lateral faults and NW–SE to WNW–ESE normal left-lateral faults are conjugated with NE–SW normal dextral structures, showing the presence of two different stress fields separated in time.

Abstract Sete Cidades is an active central volcano on the western part of São Miguel. The geological record reveals that subaerial activity started more than 250 ka ago. Stratigraphic units defined for Sete Cidades deposits reflect major events in the history of the volcano and are organized into two main groups: the Inferior Group and the Superior Group. Caldera formation resulted from three major paroxysmal events that occurred at about 36, 29 and 16 ka ago. Analysis of the eruptive history of Sete Cidades shows that effusive or moderately explosive eruptions, of Hawaiian and/or Strombolian styles, were located on the slopes of the central volcano. Conversely, trachytic explosive activity is mostly centred inside the caldera involving, in a first stage, predominantly Plinian and sub-Plinian phenomena, changing about 5 ka ago to a dominant hydromagmatic style. Trachytic effusive eruptions are represented by domes and associated lava flows that crop out in the inner caldera walls and on the western slopes of the volcano. Offshore submarine activity is represented by the historic Surtseyan eruptions of 1638 and 1811. In the last 5 ka Sete Cidades was the most active central volcano in the Azores with 17 explosive eruptions predominantly with hydromagmatic character.

Abstract Fogo is the largest of the three active central volcanoes on São Miguel and dominates the centre of the island. It is located at the intersection of NW–SE, NE–SW and east–west-trending fault systems, showing a complex morphology with a Summit Caldera formed as a result of explosive and collapse events. The edifice of Fogo has been extensively dissected by erosion, with deep valleys that show clear tectonic control. The products of Fogo range from basalt to trachyte and belong to a potassic alkaline suite. The oldest subaerial products of Fogo are >200 ka. Older products are poorly exposed, making stratigraphic correlation difficult, particularly on the north flank where considerable subsidence within the NW–SE Ribeira Grande graben has occurred. A more complete stratigraphy for the last 40 ka was established on the southern flank of the volcano. During this period there were large trachytic Plinian eruptions, including those of the Ribeira Chã (8–12 ka BP) and Fogo A (4.6 ka BP). The last intracaldera eruption was historic and occurred in 1563, and 4 days afterwards there was an effusive basaltic eruption on the northwestern lower flank of the volcano.

Abstract Furnas is the easternmost of the trachytic active central volcanoes of São Miguel. Unlike the other central volcanoes, Sete Cidades and Fogo, Furnas does not have a substantial edifice built up above sea-level. Although not as dominant as the other two volcanoes, Furnas does, however, have an edifice rising from the basal basaltic lavas exposed on the north coast to around 600 m asl on the northern rim of the main caldera. In common with Sete Cidades and Fogo, Furnas had major trachytic explosive eruptions in its volcanic history that emplaced welded ignimbrites. In the last 5 ka Furnas has had 10 moderately explosive trachytic eruptions of sub-Plinian character; two of these have taken place since the island was settled in the mid-fifteenth century. A future eruption of sub-Plinian magnitude is a major hazard posed by Furnas Volcano. Even when not in eruption, Furnas is a hazardous environment. Its fumarolic fields discharge high levels of CO 2 and concentrations in some area of Furnas village present a risk to health; the steep slopes and poorly consolidated volcanic materials are prone to landslides, in particular when triggered by earthquakes or following heavy rain, as was the case in 1997, when landslides caused severe damage and casualties in Ribeira Quente.

Abstract Basaltic volcanism occurs at all the active volcanic systems of São Miguel. For the last 30 ka, the eruption of basaltic magma has been particularly significant in the areas between the three polygenetic volcanoes – Sete Cidades, Fogo and Furnas – and two basaltic fields have developed: the Picos Fissural Volcanic System (PFVS) and the Congro Fissural Volcanic System (CFVS). About 5 ka ago volcanic activity at CFVS ended abruptly and since then almost all basaltic activity has been concentrated at the PFVS, where about 30 eruptions have taken place. Despite the absence of eruptive activity, the Congro system represents one of the most active seismic areas in the archipelago. It shows episodes of volcanic deformation that are indicative of magma ascent, which halted beneath the volcanic system. Two historical eruptions are associated with the PFVS. The 1563 eruption at Pico do Sapateiro was of basaltic composition, whereas that of the 1652 eruption was, for almost two centuries, also considered to be basaltic. This event had, however, a Vulcanian style and involved the production of large amounts of fine ash and the growth of three trachyte domes with associated coulées. It was accurately described in contemporary historical accounts.

Abstract The oldest part of São Miguel is to the east of Furnas. Previous research argued that these volcanics belong to a construct called the Nordeste Volcano, a heavily eroded shield which not only extends to the east coast of the island but also underlies Furnas Volcano in the west. On the basis of geomorphological mapping, we argue that Nordeste comprises two volcanic systems: an older Nordeste construct (the Nordeste Volcanic System) and the younger Povoação Volcano that straddles the Nordeste shield on its western margin. The Nordeste Volcanic System consists of the Lower Basalts which constitute the overwhelming majority of its subaerial products that are exposed in coastal cliff sections. Above the Lower Basalts is a surficial drape of ankaramites and the Upper Basalts. There is no evidence of large explosive trachytic eruptions from Nordeste Volcanic System. Povoação Volcano comprises an early shield construct, after which the volcano experienced caldera collapse. Post-caldera deposits are poorly exposed, but include basaltic, mugearitic and trachytic lavas intercalated by cut-and-fill sequences. Radiometric dating has yet to resolve fully the absolute ages of the Nordeste and Povoação volcanic systems, but morphology indicates that the former is much older than the latter.

Abstract São Miguel Island comprises five active volcanic systems, including three central volcanoes with calderas and two basaltic fissure systems. Volcanic eruptions in São Miguel are of basaltic and trachytic nature ( s.l. ), including Hawaiian, Strombolian, sub-Plinian, Plinian and Vulcanian events, the more explosive ones frequently including hydromagmatic phases. Large Plinian eruptions are related to caldera-forming events that occurred in the past. With reference to the Fogo A stratigraphic marker, a total of 73 individual volcanic eruptions have been identified in the last 5 ka, giving a recurrence interval of 68.5 years. Taking into account that only six events have occurred in historical times, the recurrence interval increases to 95 years and, clearly, a future event is overdue because the most recent eruption occurred in 1652. It should be noted, however, that some volcanic eruptions in the past have occurred in clusters. The eruptive frequencies of the last 5 ka of activity have been determined for all types of eruptions and related hazards, including lava flows, pyroclastic falls, pyroclastic density currents (PDCs) and lahars. The areas susceptible to volcanic products have been mapped and modelled under different eruptive conditions.

Abstract Since settlement of São Miguel Island in the middle of the fifteenth century, several destructive landslides have occurred. These have been triggered by various factors, of which rainfall is the most common. Between 1900 and 2008, based on extensive documentary evidence, 193 landslide events with socio-economic impact have been identified. These events have been responsible for 67 deaths, about 20 people have been seriously injured, and dozens of houses destroyed and people rendered homeless. In this chapter, the Information Value Method is used to produce a landslide rupture susceptibility map for São Miguel. Success and prediction rate curves (SRC and PRC) are computed and areas under the curve (AUC) calculated in order to not only support the results of modelling, but also assess the robustness of the suggested susceptibility algorithm. Twelve predisposing factors are used as independent variables and a total of 9890 landslide depletion areas are used as the dependent variable. The AUC for both the SRC and the PRC was 0.90. The methodology used for the construction of the susceptibility map was shown to be both accurate and reliable. The map is an important tool for land-use/emergency planning and for landslide risk mitigation.

Abstract Soil CO 2 diffuse degassing constitutes a permanent risk in quiescent volcanic–hydrothermal areas, as is the case in the Azores archipelago. Since the early 1990s geochemical studies carried out in São Miguel Island showed that some villages are placed in anomalous high degassing areas, and indoor measurements performed in various dwellings highlight the risk to the population. These high indoor CO 2 concentrations are not only measured in areas classified as high degassing areas, but lethal CO 2 concentrations are also registered in buildings located in areas previously defined as low- and medium-risk zones. These lethal values are measured in non-ventilated environments and basements in areas with soil CO 2 concentration above 1.5 vol%. Hazardous CO 2 concentrations are also commonly measured in buildings located in zones where soil CO 2 is higher than 5 vol%. Considering the dangerous values registered and the fact that indoor gas concentration can increase several orders of magnitude owing to peculiar meteorological conditions, updated values are suggested for the correlation between soil gas concentration and CO 2 exposure. This study highlights that both the use of soil degassing maps by land-use planners and appropriate construction rules for buildings placed in degassing areas are necessary.

Abstract Furnas Caldera Volcanic Complex, São Miguel Island (Azores), last erupted in 1630 and is famous for its intense hydrothermal activity (i.e. fumarolic fields, thermal springs, cold CO 2 -rich mineral waters and diffuse CO 2 soil emanations), which directly affect the villages of Furnas and Ribeira Quente. Here we report the first systematic investigations and mapping of soil radon ( 222 Rn) emanations in the Furnas Volcanic Complex, and we examine their potential health risks for local inhabitants. 222 Rn in volcanic soils (60 cm depth) was repeatedly measured between 2005 and 2010 using a portable solid-state alpha detector (RAD7, Durridge Company Inc.). Results reveal a local background of 8000 Bq m −3 and several areas with anomalously high 222 Rn activity (up to c. 400 000 Bq m −3 ), which coincide with advective or convective gas transport through volcanic structures and active fault zones. High 222 Rn radioactivity in Furnas and Ribeira Quente villages represents a risk to the population. Continuous monitoring performed during November and December 2005 in a house in Furnas shows indoor 222 Rn reaching 13 273 Bq m −3 , two orders of magnitude greater than the reference level (150 Bq m −3 ), when the ventilation efficiency is reduced.

Abstract In recent years much progress has been made in researching a wide variety of extreme events on São Miguel. In addition there are a number of volcano-related risks that impact upon the people of São Miguel. Some of these may occur both before and during volcanic emergencies (e.g. earthquakes), whilst others render São Miguel dangerous even when its volcanoes are not erupting (e.g. flooding, landslides, tsunamis and health impacts, especially the effects of CO 2 seepage into dwellings). In this chapter we first define what vulnerability means to the people of São Miguel, and relate this to the cultural and economic characteristics of the island. The following aspects of vulnerability are discussed: (a) physical (i.e. housing, settlement and the characteristics of evacuation routes and plans); (b) demographic and economic; and (c) social and cultural and perceptual (i.e. whether people have an accurate cognition of risk). Particular areas of concern relate to housing: the identification of isolated dwellings that would be difficult to evacuate; the vulnerability/resilience of evacuation routes following recent infrastructure improvements; characteristics of the island’s transient population; management of livestock under emergency conditions; local leadership roles; and educational outreach.

Abstract São Miguel, the largest island of the Azores, corresponds to one of the most seismically active areas within the archipelago. During historical times it was affected at least six times by destructive earthquakes, which were responsible for thousands of deaths. Several seismic crises are related both to volcanic eruptions and unrest episodes, being the cause of significant damage over more restricted areas. The central region of the island has been the most active in recent years and is characterized by the occurrence of seismic swarms, generally with low-magnitude events (M D ≤2), that are clustered both spatially and temporally. A significant relevant seismic swarm occurred between May and September 2005 and was associated with an episode of volcanic unrest. Another part of the seismicity recorded on São Miguel is associated with submarine volcano-tectonic structures that are located near the island, some of which are the source of the strongest earthquakes. The spatial distribution of b -values shows that high values (i.e. b >1) are mainly associated with the Fogo–Congro region, whereas low b -values (i.e. <1) are more common offshore. The spatial distribution of b -values in the Fogo–Congro region is suggestive of the existence of material and/or stress heterogeneities, and this is supported by a mixture of fault plane solutions.

Abstract We use a Global Positioning System (GPS) to unravel the complex geodynamics of the Azores Triple Junction where tectonic and volcanic activities coexist. The temporal analysis of densely distributed continuous GPS data on São Miguel for the period 2008–13 provides an improved understanding of interactions between present-day plate boundary kinematics and volcanic deformation. We find a high-strain-rate (0.28 ppm a –1 ) zone between Congro and Furnas, which accommodates about 50% of the Eurasian–Nubian plate spreading as predicted by the MORVEL plate angular velocity model. The seismic unrest of Fogo–Congro (2011–12) shows a strong similarity with the Matsushiro (Japan) earthquake swarm (1965–66) and the Campi Flegrei (Italy) volcanic unrest (1969–72 and 1982–85), in that an edifice-scale inflation associated with intense high-frequency earthquakes and inflation–deflation reversals coincided with a sharp drop in seismicity. We propose the following hypothesis for the Fogo unrest: (1) the primary inflation source beneath Fogo promotes lateral diffusion of fluids that is selectively guided by existing cracks/fissures formed from regional extension; (2) an influx of fluids increases pressure in cracks/fissures and generates lower-frequency earthquakes; and (3) discharge of fluids causes pressure decrease and dilatancy recovery (i.e. seismic quiescence).

Abstract A dataset on the chemical composition of mineral water discharges from São Miguel, 76 samples, depicts a large variability of chemical types and dissolved solids content. The distribution of the discharges shows an association with the three active Quaternary central volcanoes that dominate the geology of São Miguel, namely Furnas, Fogo and Sete Cidades, 74% of which are springs, 13% fumaroles, 12% wells and 1% hand-dug wells. Total dissolved solids (TDS) values are in the range of 159–20 957 mg l −1 , discharge temperature varies from 15 to 99.5°C and waters are mainly strongly acid to slightly alkaline (pH ranging between 2.2 and 7.71). Springs discharge mainly from perched-water bodies, corresponding to Na-HCO 3 and Na-HCO 3 -Cl-type waters, with a composition influenced by absorption of CO 2 and mixing between meteoric water and boiling waters with a hydrothermal component. Instead the majority of the wells are distributed along the coast and depict chloride enrichment owing to mixture with seawater. The lower pH values are associated with boiling pools at Fogo Volcano with a SO 4 -dominated chemistry, resulting from steam-heating of shallow perched-water bodies.