Volcano Watch — Lo`ihi seamount swarms with earthquake activity

Kīlauea Volcano continues to erupt from the episode 52 and 53 vents on the flank of the Pu`u `O`o cone on the East Rift Zone. For the past several weeks, the lava flow has been confined within a tube system that extends from the vent area to the coast at Kamoamoa. The lava continues to extend the coastline and has built the youngest bench of lava up to the level of the main lava delta. Explosive periods occasionally produce lava spatter at the ocean entry. These explosive episodes are strong enough to be recorded on a nearby seismometer. Several small slivers of the coastal bench have slid into the ocean, and the hazard at the coast remains high. The National Park Service has closed dangerous areas, and their signs and warning should be observed.

During a typical week, 300-400 located earthquakes occur beneath the Big Island as recorded on the U.S. Geological Survey's seismic network. However, only a few have magnitudes greater than 3.0, roughly the threshold for felt earthquakes. During the past two weeks, however, we recorded six such earthquakes, all of which occurred in a swarm beneath Lo`ihi Seamount that began about 10:00 a.m. Tuesday, October 12. The largest of these earthquakes had a magnitude of about 3.6. In the first 24 hours, more than 120 earthquakes were recorded at Lo`ihi Seamount. The next 24 hours saw only 15 more events, and the following 24 hours saw only about 12. Scattered earthquakes continued to occur beneath Lo`ihi until early afternoon on October 17.

We have shown the region in which the earthquakes at Lo`ihi Seamount occurred on the map, rather than plotting each earthquake. The earthquake swarm beneath Lo`ihi Seamount is the eleventh such swarm since 1970. Earlier swarms occurred in 1971, 1972, 1975, 1984, 1985, 1986, 1988, 1989, 1990, and 1991. The previous Lo`ihi swarm took place on December 19, 1991. It included about 22 earthquakes having magnitudes greater than 3.0, and several with magnitudes greater than 4.0, but the swarm did not last as long as this one. Those earthquakes were centered beneath the east flank and the 3,210-feet deep summit of the volcano.

These earthquake swarms are the primary evidence that Lo`ihi Seamount is an active Hawaiian volcano and the youngest in the Hawaiian-Emperor volcanic chain, a string of over 125 volcanoes stretching across almost 3,700 miles of the north Pacific Ocean. The oldest volcanoes, those at the northwest end of the chain, formed about 80 million years ago, whereas the three youngest volcanoes, Mauna Loa, Kīlauea, and Lo`ihi, are still active. Fresh, glassy lavas recovered from the summit and from the north and south rift zones, and active low-temperature hydrothermal vents near the summit and along the south rift zone, also attest to the youthfulness of Lo`ihi Seamount. The earthquake data suggests that Lo`ihi has become much more active in the last 10 years than in the previous decade. There is no seismic evidence of any Lo`ihi activity between 1962 and 1971, when the first large swarm was recorded.

The locations and depths of earthquakes beneath Lo`ihi are not well constrained, because all our seismic stations are located on the Island of Hawai`i, more than 20 miles to the east and to the north. However, most of the earthquakes in this swarm appear to be about 6-9 miles deep and are located beneath the south rift zone and southwest flank of the volcano. For the first time during a swarm on Lo`ihi, an ocean-bottom seismometer was operating on the summit of Lo`ihi Seamount. The University of Hawai`i at Manoa had a self-contained instrument package deployed on Lo`ihi that included a seismometer and a pressure sensor. The data from these instruments, when combined with the seismic data recorded on our land-based network, will provide greatly improved information on the locations and depth of the Lo`ihi earthquakes.

The moderate depths determined for these earthquakes suggest that they are caused by upward migration of magma to Lo`ihi. Earthquakes such as these do not necessarily indicate that Lo`ihi Seamount erupted. Ocean-bottom seismometers or pressure sensors, such as those in the University of Hawai`i instrument package, or ocean bottom tilt meters, which have been tested on Lo`ihi in the past, would help determine whether the earthquakes indicate an eruption or an intrusion within the volcano.

In the future, such instruments should be available on Lo`ihi Seamount all the time because the University of Hawai`i planned to install a fiber-optic cable from an area near Whittington Beach to Lo`ihi Seamount in early 1995. The first permanent instruments will be installed on the end of the cable using the deep-diving sumbersible Pisces V, operated by the Hawai`i Undersea Research Laboratory at the University of Hawai`i. This installation has been named HUGO, which stands for Hawai`i Undersea Geo-Observatory, and will be operated by the University with the data flowing through the Hawaiian Volcano Observatory. Once HUGO is installed, we will certainly develop a better understanding of the structure and activity of this youngest of Hawaiian volcanoes. In the meantime, the temporary self-contained instrument package just recovered will provide important data that will improve our understanding of Lo`ihi Seamount.