We discuss models for earthquake generation at intermediate spreading-rate ridges and present results from an ocean bottom seismograph (OBS) deployment on the Cleft segment of the Juan de Fuca Ridge (JDFR). Although the Cleft segment is spreading at approximately 6 cm/yr, an intermediate spreading rate, it has no seismic activity detectable by land networks, which provide a detection threshold of ML ≧ 4, or by northeast Pacific underwater hydrophone arrays, with a detection threshold of ML ≧ 2. To monitor seismicity, four OBSs were deployed at the Cleft segment for 11 days during August 1990. These OBSs detected 34 regional earthquakes associated with adjacent transform faults. In addition, 10 local microearthquakes (ML = 0 to 1) were observed over a 10-km segment of the ridge crest. Four local events were detected sufficiently well by multiple instruments to allow their hypocenters to be determined; all these events occurred within the rift valley of the Cleft segment spreading axis. Three events lie directly beneath a linear surface zone of hydrothermal venting. This zone has been identified as the site of a seafloor volcanic eruption, dike injection, and large-scale hydrothermal venting or “megaplume” event that occurred during 1986 and 1987. These earthquakes were located at depths coincident with the dike injection zone. A possible mechanism for earthquake production might be tectonic release of stress acquired during the dike injection, either along the dike zone or along the bounding faults of the rift valley. An alternative mechanism may be stress release associated with hydrothermal activity and cooling of the dike. A fourth event was located southeast of the linear hydrothermal venting and dike injection zone, at a depth coincident with the spreading center crustal melt zone. These observations suggest that the Cleft segment may primarily experience seismic events correlated with periods of volcanic intrusion.

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