6.1. Distribution of Microseismicity
 The main features from the surveys in area-1 in the vicinity of the Ascension transform (Figure 1) are similar to most previous studies of microearthquakes at the MAR, indicating that most earthquakes cluster within ± ∼5 km of the ridge axis [Toomey et al., 1988; Wolfe et al., 1995; Barclay et al., 2001; Tilmann et al., 2004], leaving the flanking rift mountains seismically inactive. For the network in the Ascension intratransform segment earthquakes tended to occur near the center of the rift valley (Figure 1b). Further, some events occurred at the northeastern inside corner of the spreading segment. A similar pattern was observed at the southern Mid-Atlantic Ridge near 5°S [Tilmann et al., 2004] and at the Atlantis Massif [Collins et al., 2012], suggesting that inside corner settings suffer from tectonic or flexural stresses off axis. However, the southwestern inside corner of the same spreading segment remained largely inactive.
 At the segment to the south of the Ascension transform seismicity occurred mainly along the center of the median valley and hence zone of recent magmatic activity and thus may indicate either dyke emplacement or thermal cooling after a magmatic event. However, the lack of seismic swarms may suggest that events reflect spreading on axial valley faults rather than indicating magmatism. Interestingly, we observed a ±15 km wide gap of seismicity centered at 7.8°S. This gap occurred where the seabed was inflated and where the off-axis hotspot island Ascension occurs. It has been reported elsewhere that off-axis hotspots may channel material toward the ridge axis [e.g., Ito et al., 2003; Grevemeyer, 1996], causing excess magmatism at the adjacent ridge crest. At the Mid-Atlantic Ridge near 22°N such an inflated and magmatically robust ridge segment showed a profound gap in the seismicity derived from autonomous hydrophones operated in the SOFAR channel [Smith et al., 2003; Escartín et al., 2008; Dannowski et al., 2011]. The low level of seismicity along this segment has been confirmed by a microseismic study [Kahle, 2007], suggesting that segments of excessive magmatism are characterized by thick crust and thin lithosphere [Cannat, 1996]. Such segments may behave like fast-spreading ridges and hence may not be able to generate large normal faulting events.
 The most interesting distribution of seismicity was observed in the vicinity of the Logatchev Massif (Figures 2 and 3). Geophysical and petrological data indicated that the massif, that hosts the Logatchev vent fields [e.g., Petersen et al., 2009], is related to core complex formation [Escartin and Cannat, 1999; Fujiwara et al., 2003]. Seismic activity in the vicinity of the massif is clearly shifted away from the inner median valley toward the eastern flanking rift mountains. Here, seismicity clusters at 5–8 km off axis and is clearly offset from the >4000 m deep median valley. In general, the location of the ridge axis at the deepest portion of the median valley is supported by magnetic data, providing a well-defined magnetic high [Fujiwara et al., 2003]. However, in the southern portion of the segment S2 next to the Logatchev vent field magnetic pattern are less well defined [Fujiwara et al., 2003], though. Further away from the proposed core complex seismicity clustered again in the vicinity of the center of inner median valley (Figures 3 and 5), suggesting that the observed seismicity pattern at the OCC are a local phenomenon rather indicating an eastward deviation of the spreading axis. The Logatchev hydrothermal vent field occurs just eastward of the zone of major seismicity, suggesting a relationship between the occurrence of faulting, microearthquakes, and hydrothermal venting.
 In the past, spreading at the Logatchev segment S2 to the south of the 15°20′FZ [Fujiwara et al., 2003] was asymmetric with a faster rate toward the east. The degree of asymmetry reported by Fujiwara et al.  was rather small, with 12.2 ± 0.5 km/Ma toward the east and 12.8 ± 0.6 km/Ma toward the west. Other core complexes indicated much stronger degrees of asymmetric spreading, like the Atlantis Massif at the Mid-Atlantic Ridge (∼100%) [Grimes et al., 2008] and the Atlantic Bank (∼80%) at the South West Indian Ridge (SWIR) [Baines et al., 2008]. For the Atlantis Massif Collins et al.  found that seismicity showed the tendency to occur near the western bounding fault, but still predominantly within the inner median valley. Seismicity for the Logatchev segment S2, however, is clearly asymmetric, supporting the idea that the plate boundary fault is currently shifted eastward, suggesting that asymmetric spreading and asymmetric distribution of seismicity are not necessarily correlated with each other.
 Monitoring of the seismicity at the Mid-Atlantic Ridge using autonomous hydrophone arrays suggested that core complexes and inside corner highs are seismically more active than segment centers [Escartín et al., 2008]. This pattern was consistent with previous results from two microearthquake studies, suggesting that seismicity may occur at or below two inside corner highs at 34°40′N [Barclay et al., 2001] and at 28°55′N [Wolfe et al., 1995] rather than at the center of the median valley. However, these earthquakes occurred outside of the networks and hence had large location errors. The first dedicated microearthquake study of a core complex was conducted at the TAG detachment near 26°8′N [deMartin et al., 2007], where most seismicity was rooted in the neovolcanic zone and hence in the inner median valley. A second survey was conducted at the Atlantis Massif; major seismicity occurred within the median valley. However, some activity occurred within the core complex and focal mechanism suggested both normal faulting and strike-slip faulting [Collins et al., 2012], indicating that the massif suffers from some tectonic stresses after it was rafted off axis. Nevertheless, the microseismicity study at the Atlantis Massif did not support the observations based on the hydroacoustic event locations, which had suggested that the massif itself was the source area of profound seismicity with magnitudes of >2.5 [Williams et al.,2006; Collins et al., 2012]. For the area in the vicinity of the Logatchev vent field seismicity based on hydrophones moored in the SOFAR channel occurred predominantly at the massif to the east of the main vent site [Escartín et al., 2008]. Thus, hydroacoustically detected seismicity was, with respect to the seismicity located with our local network, offset toward higher topography. We thus observed features similar to those reported by Collins et al. . This may either indicate that hydrophone-recorded data might be biased, indicating an offset between the epicenter and the T-wave entry point into the SOFAR channel, as suggested by Williams et al. , or different pattern might be related to time-space evolution of seismicity. In any case, the microearthquake survey at the Logatchev Massif is one of the first studies outlining that major tectonic activity and faulting is active at the flanking rift mountains in the vicinity of a core complex. Previously, deMartin et al.  reported antithetic normal faulting to the east of the TAG vent field.
6.2. Distribution of Large Teleseismic Earthquakes
 The four teleseismic earthquakes recorded at our network during operation all occurred within the inner median valley of the MAR. Similar features were observed before at other spreading segments. Kong et al.  captured a swarm of five earthquakes located by the ISC with an ocean floor network at the Mid-Atlantic Ridge near 26°N. All globally recorded events occurred roughly 25 km to the south of the network, and a large number of aftershocks ruptured within the center of the median valley. At the Mid-Atlantic Ridge near 30°N, Collins et al.  detected with their network two large mainshock-aftershock sequences that included five teleseismic earthquakes. The mainshock and aftershocks occurred on the western side of the median valley and close to the intersection of an adjacent transform fault. However, they occurred within the median valley and not under the flanking rift mountains.
 The observation that large earthquakes at slow-spreading ridges tend to occur in the median valley is supported by waveform inversion studies of mid-ocean ridge normal faulting earthquakes of magnitudes ∼5.5 or larger recorded at teleseismic distances. To constrain the epicentral region, water column reverberations occurring in the waveform data were used. To fit the reverberations these large earthquakes had to occur beneath the relatively deep inner floor of the median valley rather than beneath the shallower rift mountains [e.g., Huang et al., 1986; Huang and Solomon, 1988]. Along with the microseismicity constrained by the networks it seems reasonable to suggest that all major seismicity at slow-spreading ridges occurs within the median valley and perhaps at the first bounding fault on either side of the rift valley. However, older faults seem to be largely inactive.
6.3. Frequency-Size Distribution of Earthquakes
 An interesting feature of seismicity was the observation that the number of magnitude 2–3 earthquakes seem to increase away from the Logatchev Massif core complex (Figure 7). It might be reasonably to hypothesize that this feature is related to tectonics of the core complex. The Logatchev field vent fluids [Bogdanov et al., 1997] and alteration of rocks [Augustin et al., 2012] indicate that the Logatchev field occurs in a serpentinite hosted setting. Serpentinites provide low friction coefficients [Escartín et al., 1997] and hence might support frequent and small earthquakes that release only small amounts of stress. In stronger rocks, larger stresses can be accumulated and larger earthquakes occur. The size distribution may therefore be used to discriminate between strong and weak faults. In the trench-outer rise area off Nicaragua bending-related faulting promoted mantle serpentinization [Grevemeyer et al., 2007; Ivandic et al., 2008]. The b value for the area not yet affect by serpentinization was in the order of 0.7 while the area affected by serpentinization provided a b value of 2.7 [Lefeldt et al., 2009]. The asymmetric distributed seismicity in the vicinity of the Logatchev Massif indicated a b value of 0.9 (Figure 8), while the symmetric seismicity focusing at the center of the median valley provided and a b value of 1.2. Thus, b values do not support serpentinization. In contrast, both estimates range within the values reported for the MAR [e.g., Kong et al., 1992].
 Globally recorded events, however, provide evidence that the area of the Logatchev Massif indicates a profound deficit of earthquakes M>4. To survey the distribution of teleseismically recorded earthquakes we used EHB relocated earthquakes [Engdahl et al., 1998; International Seismological Centre, 2009], providing relocations of globally recorded seismic events using corrections accounting for three-dimensional inhomogeneity's of Earth's structure. For the MAR to the south of the 15°20′N transform fault the EHB catalogue provided 27 earthquakes occurring between 1968 and 2002. The resulting distribution of globally located earthquakes supports the pattern of the M > 2 earthquakes detected with the local network (Figure 7). Thus, only one of the 27 events hitting the area occurred in the vicinity of the Logatchev core complex but most events clearly occurred to its south and north. Even though the b value seemed to be dominated by other processes the lack of M ∼ 4 events within the vicinity of the Logatchev Massif may suggest that faults in the area are rather weak. This feature is consistent with the observation that mainshock-aftershock sequences reported in hydroacoustic catalogues yielded faster decay rates for asymmetric segments with detachment faults, while normal faulting mainshock-aftershock sequences at symmetric spreading segments yielded slower decay rates [Simao et al., 2010]. This dependence was suggested to be controlled by the weak rheology of detachment faults relative to the rheology found at segments supporting symmetric and more magmatic spreading [Simao et al., 2010]. On the other hand lithosphere might be rather thin. Observations indicated that high-temperature fluids of the Logatchev vents require some magma somewhere in the system, most probably directly below the vent field. The heat source may locally thin the brittle layer and restrict the earthquakes to the shallowest crust.
 At the TAG segment near 26°N Kong et al.  found higher b values of 1.1–1.5 near the segment center and lower values of 0.6–0.9 near the tectonically dominated segment ends. Higher b values indicate a larger proportion of smaller earthquakes and are a characteristic feature of magmatically active regions where b as a function of M0 often exceeds the global average of 0.7 by a factor of 2 [e.g., Wyss et al., 1997]. Our b value of 0.9 therefore supports a setting governed by tectonics and cooling stresses rather than active magma injection. This conclusion is in agreement with results of Tilmann et al.  at the MAR near 5°S, where they reported a b value of 0.8 for a tectonically dominated setting. The b value of 1.2 occurring to the northwest of the Logatchev Massif agrees with values reported for segment centers elsewhere [Kong et al., 1992].
 The area to the northwest of the Logatchev Massif was hit on 30 December 2008 by a M = 4.8 earthquake. Rubin and Pollard  suggested for Iceland and Afar that dyke emplacement within the median valley may trigger normal faulting at the adjacent bounding faults. Therefore, the observed pattern, including activity within the center of the median valley and at the adjacent off-axis bounding faults, may indicate dyke emplacement. For the clustered activity in the median valley we obtained a magnitude of completeness of 1.9 and a b value of 1.2 (Figure 8). However, based on b value mapping elsewhere, magmatically active setting are expected to be characterized by much larger b values [e.g., Wyss et al., 1997]. We are therefore confident that the seismic activity is related to tectonic stresses rather than magmatism. Thus, the M = 4.8 30 December 2008 earthquake may have caused static changes of Coulomb stresses that loaded adjacent faults [King et al., 1994], causing activity at nearby faults following the mainshock.
6.4. Focal Mechanisms
 At slow-spreading mid-ocean ridges, we would expect two forms of focal mechanisms: (i) normal faulting indicative of extension and rift valley formation and (ii) transfer or strike-slip faulting at ridge crest discontinuities and near-transform fault boundaries [e.g., Sykes, 1967]. Normal faulting is a well-understood and described process and is clearly documented by teleseismic earthquakes [e.g., Huang et al., 1986; Huang and Solomon, 1988] and local microseismicity surveys at the Mid-Atlantic Ridge [e.g., Kong et al., 1992; Barclay et al., 2001]. In addition, local seismicity surveys reported compressional earthquakes and hence reverse faulting unknown from the global record [e.g., Kong et al., 1992; Wolfe et al., 1995]. The compressional events were interpreted to be related to along axis variations of thermal stresses caused by cooling. For the Mid-Atlantic Ridge at 26°N, Kong et al.  suggested, from a three weeks OBS deployment conducted in summer 1985, that cooling of a relatively recent intrusion was associated with thermal stresses and fracturing in the immediately surrounding crust, accounting for areas of intense earthquake activity, a diversity of focal mechanisms (including reverse faulting), and the presence of a high-temperature hydrothermal vent field. Overall, features reported by Kong et al.  are similar to our observations.
 In addition to the thermal cooling stresses envisioned by Kong et al.  and Wolfe et al. , dyke emplacement may cause reverse faulting. In Iceland, microearthquakes outlined a progressive melt intrusion of a dyke moving upward [White et al., 2011]. Moment tensors indicated double-couple failure, with fault mechanisms sometimes flipping between normal and reverse faulting. However, in our data we neither observed clear migration pattern in the well-located seismicity nor flipping of sense of faulting within minutes.
 Subsidence after drainage of a magma reservoir is another mechanism that may cause reverse faulting. To explain compressional focal mechanisms from the Bardarbubga volcano in Iceland, Nettles and Ekström  suggested that deflation of a magma chamber increased horizontal compression, so that the roof block above the magma chamber subsided with respect to the surrounding rock.
 At the Juan de Fuca Ridge, a large number of thrust faulting earthquakes have been linked to the presence of a crustal magma chamber. Earthquake focal mechanisms, however, revealed a transition from normal faulting above the crustal magma reservoir to reverse faulting on either flank [Wilcock et al., 2009]. In contrast, at the Logatchev Massif we did not observe any pattern that may resemble the distribution of focal mechanisms caused by stresses related to emplacement of pressurized magma as modeled by Wilcock et al. .
 Dredging of the Logatchev Massif and drilling results from IODP leg 209 at site 1270 support that the domal high is largely composed of mantle rocks [Shipboard Scientific Party, 2004; Petersen et al., 2009]. Thus, serpentinization might be an alternative mechanism causing reverse faulting, because it is an exothermic reaction that is accompanied by volume expansion. Consequently, serpentinization may cause compression and hence reverse faulting at the edge of the expanding zone.
 In a cross section through the band of seismicity to the west of the Logatchev Massif (Figure 11), microearthquakes are rather randomly distributed and did not support clear fault structures. Random pattern of seismicity would support both cooling of an intrusive body by hydrothermal circulation in a highly fractured media and volume expansion by serpentinization. In addition to reverse faulting, Kong et al.  reported a diversity of focal mechanisms favoring cooling, while our data basically showed two dominating mechanisms: shallow normal faulting and reverse faulting at greater depth. Thus, virtually all normal faulting events occurred at a depth of 1.5–4 km below datum, while all compressional events ruptured at 3.5–5.5 km below datum. Further, reverse faulting earthquakes are larger than the normal faulting event. Thus, shallow normal faulting might be a response to volume expansion at depth. We suggest that volume expansion rather than cooling controls the seismicity pattern of the Logatchev Massif.
 In addition to the random pattern of seismicity, normal faulting earthquakes near the Logatchev hydrothermal vent field showed common features, including faults dipping at 50–60°. Thus, focal mechanisms do not support any shallow dipping detachment. However, overall seismicity patterns have to be interpreted carefully as 95% confidence intervals translate to errors of ±0.5–2 km in profile direction and ±1–2 km in depth.
6.5. Depth Distribution of Microearthquakes
 Microearthquakes at the Mid-Atlantic Ridge generally do not occur right below the seabed but 2–3 km deeper. In most cases, earthquakes occurred at depth ranges of 2–7 km below the seafloor and the majority of earthquakes ruptured within the lower crust [Toomey et al., 1988; Wolfe et al., 1995; Collins et al., 2012]. These observations from previous studies are supported by our observations, revealing that most microearthquakes occur at 3–6 km depth. At magmatically robust settings, however, microearthquakes occurred at shallower depth. Barclay et al.  reported results from an experiment at a dome-like high near 35°N at the MAR and found that seismicity clustered at 3–4 km depth. A similar dome-like feature can be observed in area-1 at the MAR near 7.8°S, where deployment 2 provided events at 2–4 km depth, averaging at ∼3 km depth (Table 1). Both settings, the MAR at 35°N and the MAR at ∼7.8°S, have similar dome-shaped bathymetric highs at the segment center. Thus, it seems to be that morphology can be used as first-order approximation, constraining the thermal state of a ridge segment.
 In contrast, Tilmann et al.  found that seismicity at 5°S of the MAR, where an inside corner high has been rifted apart [Reston et al., 2002], occurred in the lower crust and predominantly within the uppermost mantle at a depth of 5–8 km below seafloor in lithosphere with a crustal thickness of 3–4 km. Faults associate with these earthquakes may act as conduits for seawater to serpentinize the mantle.
 The depth distribution of microearthquakes can be an agent to approximate the temperature structure at depth. Thus, the occurrence of microseismicity indicates that temperatures support a brittle regime and hence temperatures of roughly <600°C [e.g., Wilcock et al., 2002; Golden et al., 2003]. Further, temperatures of ∼600°C represent the upper temperature limit for serpentinization, consistent with the occurrence of reverse faulting at 3–5 km depth, indicating expansion caused by serpentinization. This idea is further supported by the end-member compositions of the hydrothermal fluids, indicating very high concentrations of dissolved methane and hydrogen which are generally related to serpentinization [Bogdanov et al., 1997; Schmidt et al., 2007]. Thus, the seismically active region just to the west of the Logatchev hydrothermal vent field may outline a crustal volume that is effectively ventilated by hydrothermal fluids. Hydrothermal circulation might be facilitated by the exothermic reaction of peridotite and water to serpentine. However, a number of studies indicated that energy from exothermic serpentinization is not enough to drive high-temperature venting [e.g., Allen and Seyfried, 2004]. Such results are corroborated by the fact that talc alteration indicated that Si metasomatism overprinted serpentinization and hence may indicate the presence of gabbroic intrusions at greater depth below the Logatchev vent field [Augustin et al., 2012].
6.6. Conceptual Model for Hydrothermal Venting at the Logatchev Massif
 The Logatchev hydrothermal vent field occurs just eastward of a major cluster of microearthquakes at 2–5 km depth. Hydrothermal activity is most prominent along a 800 m long zone striking roughly NW-SE [Petersen et al., 2009] and occurs approximately 8 km off the median valley defined by magnetic and swath-mapping [Fujiwara et al., 2003]. The main vent sites are located on a plateau right below a 350 m high cliff between 3060 and 2910 m water depth. There are abundant outcrops of serpentinite and gabbroic rocks at the eastern rift mountains; upper crustal rocks are less abundant at the rift walls but dominate the median valley floor [Petersen et al., 2009]. Two distinctive types of vents with high-temperature hydrothermal discharge were described [Petersen et al., 2009]: (i) black smoker-type vents with fluid temperatures of up to 330°C and (ii) so-called “smoking craters” with higher fluid temperatures of up to 360°C, discharging from local depressions. Further, a number of diffuse hydrothermal sites were found, discharging fluids at much lower temperatures. End-member compositions of the hydrothermal fluids are characterized by very high concentrations of dissolved methane and hydrogen (up to 3.5 and 19 mM, respectively), which relate to subsurface serpentinization processes [Bogdanov et al., 1997; Schmidt et al., 2007].
 Alteration of peridotites suggested that mineral mobilization (e.g., Cu) occurred at temperatures of >>350°C at depth [Augustin et al., 2012]. However, the enrichment of serpentinite and lizardite separates in trace elements such as Sr, Rb, Ba, U, and Pb indicated a considerable degree of interaction with ambient seawater. Further, δ18O formation temperatures of secondary minerals are considerably lower than temperatures of black smoker fluids, supporting extensive fluid mixing and/or reequilibration of host rocks with ambient seawater [Augustin et al., 2012]. Interestingly, the occurrences of talc alteration showed that Si metasomatism overprinted serpentinization and hence may indicate the impact of gabbroic intrusions [Augustin et al., 2012]. However, in the vicinity of a gabbroic intrusion temperatures may have been too great for serpentinization as talc is stable at much higher temperatures. Thus, gabbros may have been intruded at a depth of >5–6 km within the mantle, as envisioned by Cannat .
 Petersen et al.  suggested that the vents are fed from a deeper magma source or intrusive body within the eastern portion of the median valley that is tapped by a listric normal fault. This fault is suggested to be steep at shallow depth, turning into a low-angle fault at depth. Such a scenario is opposite to the setting proposed for feeding and controlling the detachment fault at the TAG hydrothermal vent site at 26°8′N [deMartin et al., 2007], where the fault dips at 60–70° at the median valley, rolling over and forming an inactive low-angle fault off axis [e.g., Reston and Ranero, 2011]. In both cases, the heat source is displaced by some ∼5 km from the vents toward the ridge axis. Microseismicity at the Logatchev vent field, however, supports a different scenario. We suggest that the major cluster of microearthquakes occurring nearly below the Logatchev vents may indicate volume expansion caused by serpentinization. However, the focused pattern of seismicity may suggest that a magmatic heat source occurs at the toe or below the cluster of major seismicity, similar to the features detected by Wilcock et al.  at the Juan de Fuca Ridge where seismicity was triggered by stresses cause by intrusion of pressurized magma. It is interesting to notice that such a heat source would occur below a pillow volcano detected by Petersen et al.  and thus might have fed the volcano recently (Figure 11).
 The zone of proposed serpentinization occurred near the crust-mantle boundary as defined by gravity data [Fujiwara et al., 2003], and hence microearthquakes tended to occur within a depth range generally attributed to the crust and uppermost mantle; thus, the majority of events may represent crustal events. Earthquakes are generally restricted to the brittle lithosphere. For mid-ocean ridges, temperatures of approximately 600°C define the highest temperature where earthquakes occur [e.g., Wilcock et al., 2002; Golden et al., 2003]. Thus, the seismicity patterns suggest that temperatures of 500–600°C occur at 5–6 km below datum (9–10 km below sea surface), indicating that the entire crust supports brittle failure and hence faulting and perhaps seawater migration and hydrothermal recharge down to mantle depth. In the vicinity of the Logatchev vent field, we observed a number of normal faulting events. We suggest that normal faulting in the vicinity of the vents facilitates focused discharge of hydrothermal fluids and is controlling the surface location of venting. At shallower depth, approximately <2 km below the seabed, we do not observe any microearthquakes. At such a shallow depth, a network of faults and fractures in the highly porous upper crust may just support very small seismic events that were below the detection threshold. Further, the highly porous and permeable uppermost crust may allow shallow seawater recharge, explaining δ18O formation and low-temperature alteration pattern [Augustin et al., 2012]. This scenario may control shallow seismicity at 2–3 km depth.
 The temperature of >600°C deduced for the mantle has important implications, because serpentinization shows profound kinematics and is temperature dependent [Martin and Fyfe, 1970; Iyer et al., 2012]. Thus, serpentinization is most efficient at a temperature of ∼270°C [Martin and Fyfe, 1970] and reaction speed is reduced toward higher and lower temperatures; the 600°C temperature presents the thermal depth limit to serpentinization. Therefore, serpentinization, as indicated by fluid venting at the Logatchev field [e.g., Bogdanov et al., 1997; Schmidt et al., 2007], is unlikely to occur at mantle depth of >5 km as estimated from gravity data [Fujiwara et al., 2003], but has to occur within the massif itself. Drilling results from IODP leg 209 at site 1270 support that the Logatchev Massif is largely composed of mantle rocks [Shipboard Scientific Party, 2004]. This feature supports that reverse faulting is indeed caused by volume expansion within the massif. Further, energy released by serpentinization may facilitate hydrothermal circulation. However, to fuel a high-temperature circulation system like the Logatchev field, the energy provided by serpentinisation is not high enough [e.g., Allen and Seyfried, 2004]. Thus, some magmatic component is needed. Indeed, talc alteration indicated that Si metasomatism overprinted serpentinization and thus supports the presence of gabbroic intrusions in the vicinity of the Logatchev vent field [Augustin et al., 2012]. We suggest that the magmatic intrusion is located below the cluster of intense seismicity to the west of the Logatchev field (Figure 11).
 Further, at the Logatchev Massif focal mechanisms indicate two sets of faults (Figure 8). Both can also be deduced from bathymetry. The first set represents the normal faults that form the flanking rift mountains. In addition, the massif is cut by a number of transfer or strike-slip faults. Two well-resolved strike-slip earthquakes occurred in January (Figure 10a). One of the events ruptured just to the west of the Logatchev vent field and the strike of the inferred NW-SE trending fault roughly approximates the strike of the vent field. A second strike-slip event occurred just south of the vent field. It might therefore be reasonable to suggest that the location of the vent field is controlled by two intersecting faults. Similar features have been observed at other nontransform offsets hosting core complexes and hydrothermal vent sites to the south of the Azores [Gracia et al., 2000]. Thus, the pervasive normal and localized strike-slip faulting taking place at segment ends and nontransform offsets may nurture enhanced fluid circulation and favor serpentinization and high-temperature hydrothermal venting.