From 2003-2006, a novel seismic network comprising seven short-period corehole seismometers and a broadband Guralp CMG-1T OBS was deployed using remotely operated vehicles (ROVs) in a subseafloor configuration on the Endeavour Segment of the Juan de Fuca mid-ocean ridge as part of a multi-disciplinary prototype NEPTUNE experiment to investigate the linkages between seismic deformation, hydrothermal fluxes, and microbial productivity. The networks recorded high quality data that illustrate the advantages of using an ROV to deploy seismometers in well-coupled configurations away from the effects of ocean currents. The data is presently being analyzed to understand the linkages between seismic deformation and hydrothermal circulation, and the nature of seismic swarms that are associated with ridge spreading events. A subset of the seismometers will be incorporated into the NEPTUNE Canada cabled observatory.

The geology and structure of the Cleft Segment of the Southern Juan de Fuca Ridge (JdFR) have been examined using high‐resolution mapping systems, observations by remotely operated vehicle (ROV), ROV‐mounted magnetometer, and the geochemical analysis of recovered lavas. Bathymetric mapping using multibeam (EM300) coupled with in situ observations that focused on near‐axis and flank regions provides a detailed picture of 0 to 400 ka upper crust created at the southern terminus of the JdFR. A total of 53 rock cores and 276 precisely located rock or glass samples were collected during three cruises that included sixteen ROV dives. Our observations of the seafloor during these dives suggest that many of the unfaulted and extensive lava flows that comprise and/or cap the prominent ridges that flank the axial valley emanate from ridge parallel faults and fissures that formed in the highly tectonized zone that forms the walls of the axial valley. The geochemically evolved and heterogeneous nature of these near‐axis and flank eruptions is consistent with an origin within the cooler distal edges of a crustal magma chamber or mush zone. In contrast, the most recent axial eruptions are more primitive (higher MgO), chemically homogeneous lobate, sheet, and massive flows that generate a distinct magnetic high over the axial valley. We suggest that the syntectonic capping volcanics observed off‐axis were erupted from near‐axis and flank fissures and created a thickened extrusive layer as suggested by the magnetic and seismic data. This model suggests that many of the lavas that comprise the elevated ridges that bound the axial valley of the Cleft Segment were erupted during the collapse of a magmatic cycle not during the robust phase that established a new magmatic cycle.

We report on the installation of a long-term buried ocean-floor broadband seismic station (MOBB) in Monterey Bay, California (U.S.A.), 40 km off-shore, at a water depth of 1000 m. The station was installed in April 2002 using a ship and ROV, in a collaborative effort between the Monterey Bay Aquarium Research Institute (MBARI) and the Berkeley Seismological Laboratory (BSL). The station is located on the western side of the San Gregorio Fault, a major fault in the San Andreas plate boundary fault system. In addition to a 3-component CMG-1T seismometer package, the station comprises a current meter and Differential Pressure Gauge, both sampled at high-enough frequency (1 Hz) to allow the study of relations between background noise on the seis- mometers and ocean waves and currents. The proximity of several land-based broadband seismic stations of the Berkeley Digital Seismic Network allows insightful comparisons of land/ocean background seismic noise at pe- riods relevant to regional and teleseismic studies. The station is currently autonomous. Recording and battery packages are exchanged every 3 months during scheduled one day dives. Ultimately, this station will be linked to shore using continuous telemetry (cable and/or buoy) and will contribute to the earthquake notification sys- tem in Northern California. We present examples of earthquake and noise data recorded during the first 6 months of operation of MOBB. Lessons learned from these and continued recordings will help understand the nature and character of background noise in regional off-shore environments and provide a reference for the installa- tion of future off-shore temporary and permanent broadband seismic stations.

Technical accomplishments of the past 10 years in the design and deployment of sea floor broadband seismic systems are now making it possible to start addressing the issue of the limited coverage of the Earth that can be achieved through land‐based installations, at the regional or global scale. In particular, the September 2002 Ocean Mantle Dynamics (OMD) workshop in Snowbird, Utah [OMD Workshop Committee, 2003] proposed the development of two “leap‐frogging arrays” of about 30 broadband sea floor instruments to fill geophysically important target holes in ocean coverage for deployment periods of 1 to 2 years. The rationale for an off‐shore (“Webfoot”) component of the SArray/Earth‐scope “Bigfoot” array was also highlighted at this meeting, pointing out that the study of the North American continent should not stop at the ocean margin.The ocean floor environment is challenging for broadband seismology for several reasons. Broadband seismometers cannot be simply “dropped off” a ship with the expectation that they will produce useable data, particularly on the horizontal components. Several pilot experiments, [e.g., Montagner et al., 1994; OSN1, 1998; Suyehiro et al., 2002] have addressed the issue of optimal installation of ocean bottom stations, and in particular, have carried out comparisons between borehole, sea floor, and buried sea floor installations.