A Plan for a Long-Term, Automated, Broadband Seismic Monitoring Network on the Global Seafloor

Establishing an extensive and highly durable, long‐term, seafloor network of autonomous broadband seismic stations to complement the land‐based Global Seismographic Network has been a goal of seismologists for decades. Seismic signals, chiefly the vibrations from earthquakes but also signals generated by storms and other environmental processes, have been processed from land‐based seismic stations to build intriguing but incomplete images of the Earth’s interior. Seismologists have mapped structures such as tectonic plates and other crustal remnants sinking deep into the mantle to obtain information on their chemical composition and physical state; but resolution of these structures from land stations is not globally uniform. Because the global surface is two‐thirds ocean, increasing the number of seismic stations located in the oceans is critical for better resolution of the Earth’s interior and tectonic structures. A recommendation for a long‐term seafloor seismic station pilot experiment is presented here. The overarching instrumentation goal of a pilot experiment is performance that will lead to the installation of a large number of long‐term autonomous ocean‐bottom seismic stations. The payoff of a network of stations separated from one another by a few hundred kilometers under the global oceans would be greatly refined resolution of the Earth’s interior at all depths. A second prime result would be enriched understanding of large‐earthquake rupture processes in both oceanic and continental plates. The experiment would take advantage of newly available technologies such as robotic wave gliders that put an affordable autonomous prototype within reach. These technologies would allow data to be relayed to satellites from seismometers that are deployed on the seafloor with long‐lasting, rechargeable batteries. Two regions are presented as promising arenas for such a prototype seafloor seismic station. One site is the central North Atlantic Ocean, and the other high‐interest locale is the central South Pacific Ocean

Geology, geochemistry and earthquake history of Lō`ihi Seamount, Hawai`i

Author Posting. © The Authors, 2005. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Chemie der Erde - Geochemistry 66 (2006): 81-108, doi:10.1016/j.chemer.2005.09.002.A half century of investigations are summarized here on the youngest Hawaiian volcano, Lō`ihi Seamount. It was discovered in 1952 following an earthquake swarm. Surveying in 1954 determined it has an elongate shape, which is the meaning of its Hawaiian name. Lō`ihi was mostly forgotten until two earthquake swarms in the 1970’s led to a dredging expedition in 1978, which recovered young lavas. This led to numerous expeditions to investigate the geology, geophysics, and geochemistry of this active volcano. Geophysical monitoring, including a realtime submarine observatory that continuously monitored Lō`ihi’s seismic activity for three months, captured some of the volcano’s earthquake swarms. The 1996 swarm, the largest recorded in Hawai`i, was preceded by at least one eruption and accompanied by the formation of a ~300-m deep pit crater, renewing interest in this submarine volcano. Seismic and petrologic data indicate that magma was stored in a ~8-9 km deep reservoir prior to the 1996 eruption. Studies on Lō`ihi have altered conceptual models for the growth of Hawaiian and other oceanic island volcanoes and led to a refined understanding of mantle plumes. Petrologic and geochemical studies of Lō`ihi lavas showed that the volcano taps a relatively primitive part of the Hawaiian plume, producing a wide range of magma compositions. These compositions have become progressively more silica-saturated with time reflecting higher degrees of partial melting as the volcano drifts towards the center of the hotspot. Seismic and bathymetric data have highlighted the importance of landsliding in the early formation of an ocean island volcano. Lō`ihi’s internal structure and eruptive behavior, however, cannot be fully understood without installing monitoring equipment directly on the volcano. The presence of hydrothermal activity at Lō`ihi was initially proposed based on nontronite deposits on dredged samples that indicated elevated temperatures (31oC), and on the detection of water temperature, methane and 3He anomalies, and clumps of benthic micro-organisms in the water column over the volcano in 1982. Submersible observations in 1987 confirmed a low temperature system (15-30oC) prior to the 1996 formation of Pele’s Pit. The sulfide mineral assemblage (wurtzite, pyrrhotite, and chalcopyrite) deposited after the pit crater collapsed are consistent with hydrothermal fluids >250oC. Vent temperatures have decreased to ~60oC during the 2004 dive season indicating the current phase of hydrothermal activity may be waning.This work was supported by a NSF grant to M. Garcia (OCE 97-29894)