# Volcano Watch — Earthquake swarm felt at Loihi seamount

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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.

Earthquake swarm felt at Loihi seamount

(Public domain.)

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. This past week, however, we recorded 22 such earthquakes, including four with magnitudes greater than 4. These earthquakes were not randomly distributed in time or space, as all but one occurred in a swarm beneath Loihi Seamount that began about 3:40 in the morning on Thursday, December 19, and was still continuing on Friday afternoon. The only non-Loihi earthquake also occurred on December 19, at seven minutes after midnight, and was located just southeast of the summit of Kīlauea at a depth of about two miles.

In the figure, we have shown the region in which the 22 earthquakes at Loihi Seamount have occurred, rather than plotting each earthquake. The earthquake swarm beneath Loihi Seamount is the ninth such swarm since 1970. Previous swarms occurred in 1971, 1972, 1975, 1984, 1985, 1986, 1989, and 1990. These earthquake swarms are the primary evidence that Loihi Seamount is an active Hawaiian volcano. Fresh, glassy lavas recovered from the summit and 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 the volcano.

The locations and depths of earthquakes beneath Loihi are not well constrained because all of our seismic stations are located on the Island of Hawaii, more than 20 miles to the east and north. However, most of the earthquakes appear to be about 7-15 miles deep and are located beneath the 3,210-foot deep summit and beneath the south rift zone. These moderate depths suggest that the earthquakes are associated with upward migration of magma from the mantle to Loihi volcano.

Earthquakes such as these do not necessarily indicate that Loihi Seamount is erupting. Hydrophones, ocean bottom seismometers, or ocean bottom tilt meters would help to define whether the earthquakes indicate an eruption or an intrusion within the volcano. In the future such instruments will be available on Loihi Seamount. The University of Hawaii plans to install a fiber-optic cable from near Punaluu to Loihi Seamount this coming summer.

The first instruments should be installed by the end of 1992, using the deep-diving submersible Pisces V, operated by the Hawaii Underseas Research Laboratory at the University of Hawaii. This installation has been named HUGO, which stands for Hawaii 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.

### Volcano Activity Update

Kīlauea Volcano continues to erupt from the Kupaianaha vent on the East Rift Zone. The volume of lava moving down the Wahaula lava tube from Kupaianaha decreased slowly since the middle of the summer, decreased sharply during episode 49 in November, and has now increased to levels slightly lower than before episode 49.

Volcanic tremor, indicative of underground magma migration, continues at moderate-to-low levels beneath the upper East Rift Zone. The lava in the Wahaula tube is feeding a series of small, fluid pahoehoe flows between the 1,900- and 1,750-foot elevations. This flow is located just east of the aa flows erupted during episode 49. An active lava pond in the Puu O`o vent can be seen from the air and is the cause of the bright glow at night.