(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

The visible part of Kīlauea from the summit to the Lower East Rift Zone (LERZ) makes up only a small portion of the total volcano. Much of Kīlauea lies beneath the sea, including the Puna ridge to the east, and the south flank extending offshore beyond the southern coastline.

As the volcano grows, this underwater region of the south flank creeps slowly to the south, moving in fits and starts with earthquakes that last seconds (such the May 4, 2018, magnitude-6.9 event) and in slow slip events, which last for days or weeks. Many questions have been raised about the stability of the south flank since other portions of Hawai`i Island’s coasts show evidence of past landslides.

Although Kīlauea’s submarine south flank is a major part of the volcano, its motion is much harder to monitor than the part above sea level. While we can record earthquakes occurring beneath the flank, only the largest, and those closest to shore, are well-captured by the USGS Hawaiian Volcano Observatory (HVO) seismic network. In general, only a few offshore earthquakes are recorded. However, following the M6.9 earthquake and Kīlauea’s LERZ eruption, a significant number of earthquakes took place beneath the south flank, some of which were in regions that have not typically been very seismically active.

To better understand what’s going on within Kīlauea’s south flank and help determine how it has been affected by the eruption, a group of scientists from Western Washington University, Rice University, and the University of Rhode Island deployed 12 ocean bottom seismometers (OBSs) on the submarine Kīlauea south flank in July.

The instruments were deployed from the R/V Ka‘imikai-o-Kanaloa, a 223-foot research vessel operated by the University of Hawai‘i during a week-long cruise funded by the National Science Foundation.

Seismometers were positioned over the whole south flank so that earthquakes associated with the edges of the flank could be recorded to see if the offshore stress field has changed. They were also positioned on the M6.9 aftershock zone to try to better understand that earthquake, and near the LERZ eruption ocean entry to study how lava enters the water and progresses downslope.

These data should allow the scientific team to determine more precisely where the offshore earthquakes occurred and on what fault(s) the M6.9 earthquake took place. Recordings of the ocean entry activity may help us learn more about why some lava-water interactions are explosive, while others are relatively calm. In general, we hope that by collecting data offshore we will be able to better understand parts of Kīlauea that cannot be easily observed.

The OBSs record data internally, so we won’t know what they have recorded until they are recovered in September. A group of scientists on a related research cruise has volunteered their time to collect our instruments and download the data for us. At that point, the data will be made available to any scientists interested in studying them, notably the seismologists at HVO.

While aboard the ship, we had a view of the eruption that was both spectacular and sobering. The power and volume of lava flowing from fissure 8 to the sea were striking, even from miles offshore. We saw dozens of ocean entries over miles of coastline, and changes in the color of the seawater, even miles offshore. But we were also witnessing the pain of the eruption, watching black smoke as a structure burned, knowing that it was yet another loss among many in the vibrant community. Our thoughts continue to be with the people of Puna.

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