This story begins after Kilauea’s May 4, 2018, magnitude-6.9 earthquake and lower East Rift Zone eruption. The magnitude-6.9 earthquake resulted in seaward motion at the surface of Kilauea’s south flank of up to approximately 1.5 feet as measured by GPS monitoring stations operated by the U.S. Gelogical Survey’s Hawaiian Volcano Observatory.
Following the 2018 eruption, there was considerable contraction on Kilauea’s middle East Rift Zone that was later followed by expansion or inflation as magma began refilling that area.
The south flank motion resulting from the magnitude-6.9 in 2018 was caused by motion deep below the surface, at depths of 4 miles to 6 miles), at the interface between Kilauea Volcano and the ocean floor. This interface is known as a decollement or detachment fault. For more information on detachment faults, read this volcano watch article: www.usgs.gov/center-news/volcano-watch-new-research-sheds-light-relationship-between-hilina-fault-system-and.
During the 1983-2018 Puʻu ʻOʻo-Kupaianaha eruption, and prior to the 2018 magnitude-6.9 earthquake, the average rate of motion on Kilauea’s south flank was around 3 inches per year. The flank can move faster or slower, depending on the time period and type of activity on the volcano. Rates can be variable but often remain near-constant during long-duration eruptions, with short-term changes during major rift intrusions and slow-slip earthquakes. For more information on slow-slip earthquakes, read this Volcano Watch: www.usgs.gov/center-news/volcano-watch-slow-slip-event-k-lauea-volcano-s-south-flank-expected-year.
Since the activity in 2018, there has been roughly about 4 inches of adjustments along Kilauea’s south flank surface in the last two and a half years. This amount is to be expected and is nothing to be alarmed about.
The motion following the 2018 magnitude-6.9 earthquake and eruptive activity was caused by the magmatic intrusion in Kilauea’s East Rift Zone exerting pressure on the south flank. The volcano adjusts after the combined effects of both the intrusion along the East Rift Zone and the large earthquake.
Following the magnitude-6.9 and 2018 eruption, Kilauea’s East Rift Zone had been showing inflation consistent with magma supply to the volcano’s shallow magma storage system. However, at the start of Kilauea’s ongoing summit eruption in December 2020, the upper portion of the East Rift Zone started to show signs of contraction. GPS stations along the upper and middle parts of the rift zone started to move northward, which is a rare occurrence on such a short timescale. This was occurring while Kilauea’s summit was deflating at the onset of the eruption.
Kilauea’s south flank’s response to changing magma pressure has been studied extensively through many previous eruptions and intrusions. For example, the 2007 middle East Rift Zone “Father’s Day” intrusion also led to a slight contraction of the rift zone following the intrusion and subsequent eruption. For more information on the Father’s Day intrusion and eruption, read this volcano watch: www.usgs.gov/center-news/volcano-watch-what-do-fathers-day-and-volcanic-rift-zones-have-common.
It’s also common to see elevated rates of motion on faults after large earthquakes. Kilauea’s south flank was the site of 5 earthquakes of magnitude-4.0 or greater in the past year. The magnitude-4.1 earthquake this past Monday beneath the south flank is one of the five and it was felt across the Big Island. These earthquakes happen in response to abrupt motion of the detachment fault which moves to the southeast over the oceanic crust. The abrupt motion releases pressure on the south flank that is caused by magma accumulation beneath the surface.
The relationship between tectonics and volcanism of the Hawaiian Islands is widely studied and the story of Kilauea’s south flank continues to unfold in time. HVO’s dense monitoring network of GPS stations allows us to closely monitor and report these changes, while also giving us the opportunity to learn more about the volcano’s behavior.