Broadband Seismometer V16

Deploying Broadband Seismometer at Axial Base
Broadband Seismometer at Eastern Caldera

A broadband seismometer installed at Eastern Caldera is coupled to a low-frequency hydrohone. Photo credit: NSF-OOI/UW/CSSF; Dive R1722; V14.

Broadband Seismometer Installed

A broadband seismometer is installed inside a buried cassion at Southern Hydrate Ridge by the ROV ROPOS. A follow-on dive will fill the caisson with silica beads, which dampens "noise". The broadband and low frequency hydrophone is installed here to detect both large and small earthquakes that may impact the release of methane gas into the hydrosphere. Photo credit: NSF-OOI/UW/CSSF; Dive R1779; V14.  

Broadband seismometers measure seismicity/earthquake activity along tectonic plate boundaries where key phenomena can occur: large-magnitude subduction zone earthquakes, localize earthquakes associated with faulting and spreading events, and seismicity due to migration of melt (magma) beneath the seafloor at underwater volcanoes.

Four of these instruments are now installed, one each at Slope Base (PN1A), at the base of Axial Seamount (PN3A), and at the summit of Axial Seamount at the Eastern Caldera Site, and one Southern Hydrate Ridge where methane is venting from the seafloor. The small array of broadband seismometers are installed to detect earthquakes along the subducation zone of the Oregon margin and at Axial Volcano, as well as smaller earthquakes associated with gas release, melt migration, and hydrothermal upflow. In sedimented areas (Slope Base and at the base of Axial Seamount), the seismometers are buried beneath the seafloor sediments in caissons 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 gavel-filled bags.

Because the seismometers are cabled and, hence, connected directly to shore, they provide detection of earthquakes in real-time. During a 24 hr period April 23-24, 2015, ~8,000 earthquakes were detected at the summit of Axial Seamount - indicating a seafloor eruption was occurring. 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).

Instrument Specifics: The broadband seismometers are 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). They have an Ethernet (10/100) interface, and are synced to pulse per second timing.

Sensors are in a Ti-housing and in sediment are buried in a 60 cm deep X 60 cm diameter caisson filled with silica beads, data are digitized.