Volcano Watch — Kīlauea's south flank seismicity

Sitting at lunch on Wednesday, many residents of the island of Hawaii might have thought that someone or something had kicked their chairs. Of course, they were feeling the ground shaking generated by the sequence of small earthquakes beneath the south flank of Kīlauea Volcano that was reported in other news stories.

This sequence included three earthquakes of magnitudes greater than 3, as well as a number of smaller earthquakes that would not normally be felt by humans. The largest of the earthquakes was magnitude M3.9, but even this earthquake would only be felt if people were within quiet surroundings or in otherwise stable conditions.

Detailed analysis of the data recorded by the U S Geological Survey's Hawaiian Volcano Observatory seismographic network pinpointed the locations of these earthquakes to be roughly 4-1/2 miles beneath the Earth's surface, about 8 miles southeast of the Observatory. The first of Wednesday's felt earthquakes occurred at 11:49 a.m. This earthquake was M3.2, and it was preceded by two smaller earthquakes in the same location.

A second M3.2 earthquake occurred there at 12:08 p.m., and the largest earthquake (M3.9) occurred at 12:28 p.m. Based on the earthquake locations and the distributions of the ground motions, we interpret the sequence to reflect shallow crustal adjustments beneath Hilina Pali. We were able to locate 13 earthquakes in all in this sequence.

The south flank of the island of Hawaii is one of the most seismically active regions in the United States. Each year, HVO records thousands of earthquakes occurring beneath the flank. The many earthquakes that we record form part of the fundamental process of island-building through the active volcanic processes. Magmaenters into the volcanoes and is either stored within the volcanoes or erupted onto the Earth's surface. As the island's land mass builds up, strain energy accumulates and is subsequently released, often as earthquakes. At times, these earthquakes can be quite large, like the M7.2 earthquake in 1975 or the M6.1 earthquake in 1989 beneath Kalapana.

We are often asked whether even the small earthquakes like this week's have any direct relation or effect on the current Kīlauea East Rift Zone eruption. There were no immediately obvious effects on the eruption. As we have said earlier, the earthquakes are part of the active volcanic processes in Hawaii, but much further investigation is required to fully understand the details of the relationships, and of the effects of individual earthquake events or earthquake sequences, to observations of the eruption. These investigations are part of the mission of the Hawaiian Volcano Observatory.

Lō‘ihi Activity

On the evening of Thursday, April 27, we were reminded of the continued activity of Lō‘ihi, the youngest of the Hawaiian volcanoes. At 7:59 p.m. we started to detect a flurry of small earthquakes from Lō‘ihi Seamount, some 22 miles southeast of the Big Island. Over the next two days, we recorded over 75 Lō‘ihi earthquakes on the HVO seismographic network. This is the most recent of a series of Lō‘ihi earthquake swarms that we have recorded at HVO.

Each recorded swarm adds to the body of information we are building on the growth and evolution of the Hawaiian volcanoes. The opportunity to observe the activity of such a young volcano as Lō‘ihi is unique and should provide valuable and important insights.

Lō‘ihi's present submarine position also presents considerable challenges to scientists interested in documenting the growth of the volcano. Our colleagues at the University of Hawaii are pursuing an innovative research and development program for establishing HUGO - a permanent undersea geophysical observatory at Lō‘ihi. On Tuesday May 9, Professor Alex Malahoff of UH-Manoa, the leader of the HUGO team, will be presenting a report of the progress and results of the HUGO project in the "After Dark in the Park" lecture series of the Hawaii Volcanoes National Park. The presentation is scheduled for May 9 at 7:00 p.m. in the National Park Visitors' Center. The public is invited to attend and learn about Professor Malahoff's exciting program.