Broadband seismometers measure seismicity/earthquake activity along tectonic plate boundaries where two key phenomena can occur: large-magnitude subduction zone earthquakes and seismicity associated with migration of melt (magma) beneath the seafloor at underwater volcanoes. Historically, these instruments have been deployed by free-fall from the ship, which has commonly resulted in poor coupling to the seafloor, and, therefore, poor data quality.
Three of these instruments are now installed on the OOI Cabled Array, one at the base of Axial Seamount (PN3A), and two at the summit of Axial Seamount (Eastern and Central Caldera sites). In sedimented areas (at the base of Axial Seamount), the seismometer was buried in a caisson beneath the seafloor sediments to obtain the highest quality resolution of seismic waves. At the summit of Axial Seamount, there is little sediment, and therefore, the broadband seismometer was installed on basement rock, and surrounded by pea gavel-filled bags.
Broadband seismometer OBSBBA301 is located at the Central Caldera site and is connected to the MJ03F junction box by a 50 m-long extension cable. The seismometer is manufactured by Guralp, instrument model Guralp CMG-1T 360s-50Hz with CMG-5T Strong Motion, DM24/7-EAM Digitizer/Interface + Low Frequency Hydrophone (HTI-90-U). These seismometers have an Ethernet (10/100) interface, and are synced to pulse per second timing. The Central Caldera subnet also includes a low frequency hydrophone, and a bottom pressure and tilt sensor.
Because the seismometers are cabled and, hence, connected directly to shore, they are providing detection of earthquakes in real-time. Much larger arrays of cabled seismometers and pressure sensors are being utilized offshore in Japan to provide early warning dectection of seismic events such as the 9.0 magnitude megathrust Tohoku event and tsunamis (Dense OceanFloor Network System for Earthquakes and Tsunamis - DONET).