We report results from a 3‐week microearthquake survey of the segment of the Mid‐Atlantic Ridge axis near 26°N. The segment is centered on an along‐axis median valley bathymetric high that includes the site of the TAG hydrothermal field. The seismic network, consisting of seven ocean bottom hydrophones and two ocean bottom seismometers, spanned the median valley inner floor and eastern valley wall. Hypocenters were determined for 189 earthquakes, with good resolution of focal depth obtained for 105 events. Almost all events occurred at depths between 3 and 7 km beneath the seafloor, with earthquakes occurring at shallower depths (less than 4 km) beneath the along‐axis high. No events were detected in the immediate vicinity of the hydrothermal field. The along‐axis high is the site of a midcrustal low‐velocity zone, significant attenuation of P wave energy, and high b values; the low‐velocity volume extends about 10 km south of the high to the vicinity of volcano within the axial neovolcanic zone. Fault plane solutions indicate high‐angle (or very low angle) normal faulting beneath the along‐axis high and the base of the adjacent western wall, reverse faulting beneath the axial volcano, and a more conventional normal‐faulting geometry for earthquakes beneath the eastern wall. The distribution of seismicity and the diversity of faulting styles suggest a spatially variable tectonic state for the ridge segment at 26°N. These variations are likely a signature of along‐axis differences in thermal structure and state of stress. We suggest that the low‐velocity volume beneath the along‐axis high is the site of a relatively recent crustal injection of magma. Continued cooling of the now largely solid but still hot intrusion, and associated thermal stress and fracturing in the immediately surrounding crust, can account generally for the distribution of areas of most intense earthquake activity, the diversity of observed faulting mechanisms, and the presence of the high‐temperature vent field. These results are supportive of the spreading cell model for segmentation of magmatism and thermal structure along a slowly spreading ridge.

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Abstract. High-resolution Sea Beam bathymetry and Sea MARC I side scan sonar data have been obtained in the MARK area, a 100-km-long portion of the Mid-Atlantic Ridge rift valley south of the Kane Fracture Zone. These data reveal a surprisingly complex rift valley structure that is composed of two distinct spreading cells which overlap to create a small, zero-offset transform or discordant zone. The northern spreading cell consists of a magmatically robust, active ridge segment 40-50 km in length that extends from the eastern Kane ridge-transform intersection south to about 23~ , N. The rift valley in this area is dominated by a large constructional volcanic ridge that creates 200-500 m of relief and is associated with high-temperature hydrothermal activity. The southern spreading cell is characterized by a NNE-trending band of small (50-200 m high), conical volcanos that are built upon relatively old, fissured and sediment-covered lavas, and which in some cases are themselves fissured and faulted. This cell appears to be in a predominantly extensional phase with only small, isolated eruptions. These two spreading cells overlap in an anomalous zone between 23~and 23~ that lacks a well-developed rift valley or neovolcanic zone, and may represent a slow-spreading ridge analogue to the overlapping spreading centers found at the East Pacific Rise. Despite the complexity of the MARK area, volcanic and tectonic activity appears to be confined to the 10-17 km wide rift valley floor. Block faulting along near-vertical, small-offset normal faults, accompanied by minor amounts of back-tilting (generally less than 5~ begins within a few km of the ridge axis and is largely completed by the time the crust is transported up into the rift valley wails. Features that appear to be constructional volcanic ridges formed in the Marine Geophysical Researches 10: 59-90, 1988. 9 1988 Kluwer Academic Publishers. Printed in the Netherlands. median valley are preserved largely intact in the rift mountains. Mass-wasting and gullying of scarp faces, and sedimentation which buries low-relief seafloor features, are the major geological processes occurring outside of the rift valley. The morphological and structural heterogeneity within the MARK rift valley and in the flanking rift mountains documented in this study are largely the product of two spreading cells that evolve independently to the interplay between extensional tectonism and episodic variations in magma production rates.

Tomographic results for P-and S-wave velocity structure beneath the active Aso Volcano, Kyushu, Japan, using 800 well-recorded earthquakes and ten shots recorded by an eight-station seismic network, are presented. A 68% variance reduction was achieved upon simultaneous inversion for hypocenter and velocity structure. Well-resolved velocity anomalies associated with the active crater reveal heterogeneity up to 26% slower and 18% faster in P velocity, and up to 31% slower and 22% faster in S velocity, than the one-dimensional model. The largest anomaly is seen over the upper 11 km in the central and northern parts beneath the central cones. Two low-velocity regions are imaged. The first region, a 10×15-km region encompassing the upper 3 km centered near the caldera wall at Tateno Valley, is characterized by P velocities up to 19% slower (20% for S). The second low-velocity region is associated with the central cones and active magma conduit system at 6 km depth. Velocities as low as 4.3 km/s (up to 26%) in P and 2 km/s (31% slower) in S characterize the 7-km-wide volume. The magma chamber is roughly spherical in shape, centered at 6 km depth, flattens at 10 km depth, and is located between Mt. Kishima, Mt. Eboshi, and Mt. Naka, the present focus of magmatism. A sharp velocity contrast at the depth of 3 km, with high velocities to the southwest and lower velocities to the northeast, characterizes different abutting structures associated with the Oita-Kumamoto Tectonic Line.