6.1. Comparison With Teleseismically Recorded Earthquakes
 A brief survey as presented here can offer nothing but a snapshot of seismic activity. Nevertheless, we note that the recorded microearthquakes are unlikely to be aftershocks of a large earthquake just before the deployment of the ocean bottom network, as such an event would have been recorded teleseismically. Also, the fact that magnitudes estimated from the corner frequency agree at least in order of magnitude with the rate of events expected from extrapolation of the Gutenberg-Richter curve for events in the global data set for this part of the MAR argues against the possibility that we caught an aftershock or swarm sequence with seismicity rates strongly exceeding long-term averages.
 Figure 1 gives an overview of the globally recorded seismicity in the vicinity of the study area. Judging from the apparent location of the strike-slip events in the south of the map, which are presumably all associated with the Ascension transform fault, epicentral mislocation of the Harvard centroid moment tensor (CMT) solutions [Dziewonski et al., 1981] can be as large as 50 km. The events in the Engdahl et al.  (EHB) catalog, which were located with short-period body waves, follow the bathymetric trace of the ridge transform faults more closely with formal standard errors of 10–20 km. Only one centroid moment tensor solution locates within the study area. Like the composite focal mechanisms it exhibits (oblique) normal faulting. The dip is ∼45° for both nodal planes. Although this is 15–20° steeper than the low-angle plane inferred from the local composite solution, this difference is probably not significant, given the uncertainties inherent in both the local and CMT solutions.
 Similar to the results of the ocean bottom survey, the ridge is far more seismically active than the transform. Some earthquakes locate between 5°15′ and 5°30′S, where the median valley was found to be inactive in the ocean bottom survey, but location uncertainties are too large to tell whether these earthquakes were in the median valley or along its flanks.
6.2. Tectonic Interpretation
 Within the MVSZ, earthquakes are located predominantly on the western half of the median valley but are fairly uniformly distributed in north-south direction. Whereas some crustal earthquakes have occurred, most of the events have hypocenters beneath the Moho, even when taking into account uncertainties within their location and inaccuracies in the Moho depth, which was obtained by wide-angle modeling. The deepest earthquakes reach a depth of 12 km, or equivalently 8 km beneath the median valley floor. The ridge-parallel cross section along the median valley (Figure 5) suggests an apparent shallowing of the base of the seismogenic zone both toward the volcanic ridge at the segment center and toward the segment end. However, this apparent shallowing is likely to be an artifact of the selection criteria for events with well-constrained depths: the areas around obh03 (near the segment end) and around obh09 (near the southern end of the MVSZ) contain many more events with poorly constrained depths than the region near obh04 (near where the deepest earthquakes are observed).
 Uppermost mantle velocities in this section are constrained by the wide-angle data to be larger than 7.5 km/s (Figure 5b). Theoretically, such low velocities would be consistent with up to 20% serpentinization [O'Reilly et al., 1996; Christensen, 1966]. In reality, serpentinization is likely to be much weaker as this estimate ignores the effect of the elevated temperatures at the ridge axis (compared to mature oceanic mantle), which can account for most of the velocity reduction. The occurrence of a large number of mantle earthquakes argues further against widespread serpentinization, as even a small degree of serpentinization would weaken mantle peridotite sufficiently to preclude brittle failure [Escartín et al., 1997], whereas mantle stays brittle up to ∼750°C [Wiens and Stein, 1983].
 We now consider the central ridge-perpendicular cross section (Figure 6b). The composite focal solution for this area allows either westward dipping high-angle normal faulting (60° dip) or eastward dipping low-angle normal faulting (30° dip). The surface traces of two faults bounding the MV (F1 on the inside corner side, F2 on the outside corner, Figures 2 and 6) were identified previously from the bathymetric data and, although subdued at the latitude of the cross section, are still identifiable as a step in the bathymetric profile. Whereas the event distribution is clearly incompatible with the pattern expected for a single dominant detachment fault [e.g., Tucholke and Lin, 1994], it is not sufficient to uniquely determine the faulting style. At a minimum, two faults are required to be currently active (Figure 6b); these faults would have to be normal and dipping eastward at about 30°. The projection of the inner (eastern) fault would emerge near the surface trace of F1, and no crustal events would have been recorded for the outer (western) fault. Alternatively, faulting distributed through the volume beneath the MV could also produce the observed distribution os seismicity. This faulting could be on either the west or eastward dipping planes, or even both.
 Moving on to the southernmost cross section (Figure 6c), we recognize a cluster of events at 9–10 km depth below sea level and two isolated events, one deep, one shallow farther east. Given the short observation time, this pattern is consistent with both faulting along one or two dominant faults (Figure 6c, the slopes of the faulting being suggested by the focal mechanism) or with the recording of a fortuitous subset of events associated with more distributed faulting. Either way, a connection with fault trace F1 seems unlikely, except for the isolated shallow event.
 The northernmost section (Figure 6a) presents the most scattered picture, and we do not have a focal solution to guide our interpretation. The three westernmost events probably accommodate diffuse stress within the ICH. Bathymetric structures in and near the MV become more oblique near the transform fault, possibly causing deformation to become more diffuse.
 The well-defined volcanic ridge south of 5°16′S [Reston et al., 2002] indicates recent magmatic activity. The ridge coincides with an apparently aseismic zone where earthquakes, if they occur at all, are of much smaller magnitude than those in the MVSZ. During the experiment, only events with a magnitude <1 could have feasibly escaped detection. Although the possibility cannot be excluded that the absence of events is an artifact of the short observational period, the sharpness of the cutoff of seismicity at the southern limit of the MVSZ is nevertheless notable. Elevated temperatures or the presence of fluids could suppress tectonic earthquakes; volcanic earthquakes would be expected to occur but might simply be too small or too intermittent to have been recorded during the experiment. The fact that there is no or only weak shoaling of the base of the seismogenic layer implies that either the temperature gradient along the transition between the MVSZ and the aseismic ridge is large or the transition is controlled by fluids rather than temperature. Seismic velocities could provide further information on the thermal structure, but the median valley refraction profile does not extend far enough south to resolve the velocity structure of the volcanic ridge.
6.3. Synthesis With Previous Microearthquake Surveys
 In the following, we contrast our results with those of a number of surveys along the northern MAR at 23°, 26°, 29°, and 35°N (Table 2). The spreading rate of the MAR in this area (2.3 cm/yr) is somewhat slower than at 5°S (3.2 cm/yr), and the plates being separated are different ones [DeMets et al., 1994]. Nevertheless, these surveys represent the closest analogue. The maximum hypocentral depth in this study (8 km below the MV floor) is the same as that observed by Toomey et al.  for earthquakes beneath the median valley floor near 23°N, classified as a cold segment by Thibaud et al. . Further similarities are the similar b values (0.8 ± 0.1 both at 23°N and in this study, log moment b value) and the large cross-axis topographic relief, which characterizes both segments.
Table 2. Comparison of OBS Surveys in the North Atlantic
|Areaa||Reference||Number of Days||Maximum Depth,b km||b Value (log10M0)||Number of Eventsc (M0 > 1019 dyn cm)|
|22°30′–22°50′N||Toomey et al. ||10||8||0.8 ± 0.1(MV floor)||12||8.4|
| || || ||5||0.5 ± 0.1 (Rift Mountains)|| || |
|26°00′–26°13′N||Kong et al. ||23|| ||1.0 ± 0.1 (total)||93||28.3|
| || || ||7||0.6–0.9 ± 0.1 (segment end)|| || |
| || || ||6||1.1–1.5 ± 0.1 (segment center)|| || |
|28°52′–29°05′N||Wolfe et al.  (segment end and ICH only)||41||5.5–7d||0.82 ± 0.05||not known|| |
|34°42′–35°00′N||Barclay et al.  (segment center only)||43||4||0.94 ± 0.05||4||0.65|
|35°00′–35°15′N||Cessaro and Hussong  (segment end only)||12||9 (14)e||0.5–0.7f||not known|| |
 The surveys at 23°N [Kong et al., 1992] and 29°N [Wolfe et al., 1995] exhibit slightly lower maximum earthquake depths of 6–7 km below the MV seafloor and have intermediate cross-axis relief. An extreme case is presented by the segment south of the Oceanographer's Transform at 35°N [Barclay et al., 2001], where earthquake depths only reach 4 km below the MV floor, a large moment b value of 0.94 is found, and cross-axis relief is small. On the basis of various lines of geophysical evidence, both Kong et al.  and Barclay et al.  infer recent magmatic injection events for their segments. In spite of the fact that the segment north of the Oceanographer's Transform has been classified as hot by Thibaud et al. , Cessaro and Hussong  find a low b value of 0.7 or less and a fairly uniform depth distribution between 2 and 9 km depth below the MV floor, with three events apparently at depths of 12–14 km near the transform-ridge intersection. However, the focal mechanisms of some of their events hint that are they responding to stresses associated with the ridge-transform intersection rather than effecting ridge-normal extension.
 On the basis of body waveform modeling of teleseismically earthquakes, Huang and Solomon  inferred a maximum centroid depth of 3–3.5 km below the MV floor for ridge earthquakes at a full spreading rate of 3 cm/yr. Assuming uniform slip and rigidity, this centroid depth implies a seismogenic zone twice as thick, i.e., 6–7 km, only slightly less than the depth indicated by microearthquakes.
 Barclay et al.  pointed out an apparent correlation between large cross-axis relief and large maximum earthquake depth (Figure 8). The segment presented in this work follows this pattern, but if the topographic relief is measured from the median valley to the crest of the ICH ridge, the maximum depth saturates at 8 km (below the MV floor). There also appears to be an inverse correlation between the maximum earthquake depth and the b value with large b values being associated with shallow maximum depths (Table 2 and this study), although there is at least one exception to this rule (rift mountains at 23°N). Physically, such a correlation is not surprising, as increased temperatures would lift the base of the seismogenic layer as well as increase the b value.
Figure 8. Maximum depth of seismicity versus cross-axis relief. Adapted from Barclay et al.  with the results of this study added. Cross-axis relief was determined by averaging the relief from the median valley floor to the first crest of the sidewall, with the error bars representing the variability of the relief thus measured among several parallel profiles in the vicinity of the hypocenters. For the present study, there is an ambiguity whether the crest of fault F1 or the inside corner high should be used, so both alternatives are presented. The error bars for the maximum earthquake depths are determined from the error bars of the deepest events in the survey. At 29°N, maximum inferred earthquake depths differ depending whether events located with only four stations are included in the estimate (solid triangles) or at least five stations were required (open symbols).
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 Marked differences also exist in the style of faulting inferred for the various segments. Kong et al.  attribute most seismic activity to accommodation of cooling stresses induced by an already solidified but still hot igneous intrusion. For the other segments, primarily tectonic extension is invoked. Barclay et al.  interpret the earthquakes near the segment center to result from stress on normal faults bounding the valley in accordance with classic extension along segment centers of slow spreading ridges [Mutter and Karson, 1992]. Similar to our interpretation of the seismicity at 5°S, Toomey et al.  infer a large mantle-penetrating normal fault for the segment at 29°N, albeit at a dip of 45°N. In contrast to the rather weak seismicity underneath the ICH in this experiment the ICH was the most seismically active area in the microearthquake survey at 29°N [Wolfe et al., 1995]. The ICH events occurred at depths between 3 and 6 km (relative to the median valley seafloor), placing most of them in the mantle as gravity data indicate a thin crust underneath the ICH. Wolfe et al.  interpreted these ICH events as accommodating extension over a broad area rather than along a well-defined detachment as required by the Tucholke and Lin  model. Alternatively, the events could be associated with successor faults that accommodate flexing of an exhumed core complex. We prefer the original interpretation because there is no evidence for a large seismically active detachment surface along the western wall of the MV: a composite focal mechanism shows normal faulting with a 45° dip, but the microearthquakes do not line up along a corresponding surface. Intriguingly, the segments at 5°S and at 29°N also present rather different morphologies (Figure 9), which reflect the differences in seismicity. The ICH at 5°S is characterized by pronounced axis-perpendicular striations and large topographic relief. In contrast, the ICH at 29°N has a rough and rugged surface but lacks well-defined striations and has less relief. We have to remember that the striations at 5°S are not related to the currently active faults but record an earlier phase of extension before the proposed ridge jump. Although it is possible and even likely that over time the faults at 5°S will exhume their footwalls, there is currently no need to accommodate bending of an exhumed IC complex or additional extension within the ICH region, so that the low seismic activity is not surprising. This observation raises the question what controls the style of extension that a particular ridge adopts. We note that the ICH at 29°N is located next to a nontransform discontinuity, whereas the one at 5°S is next to a 70 km offset transform. However, an earlier microearthquake OBS survey near major transforms also reported diffuse microearthquake activity at the inside corner (Vema Transform, 11°N [Rowlett and Forsyth, 1984]; Oceanographer's Transform, 35°N [Cessaro and Hussong, 1986]), so the question is open.
Figure 9. Detail of the bathymetry of the ICHs of the ridge segments at 29°N (top, Wolfe et al. ) and 5°S (bottom, this study). Both data sets are plotted at the same scale and using the same gray scale; illumination is from NNW in both images, but intensity normalization has been optimized for each image separately to enhance contrast. The resolution of the top image is 200 m, and that of the bottom image is 100 m. Dashed white lines delineate the most seismically active zones.
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