During the past two weeks, two noteworthy earthquakes struck Kilauea Volcanoâ€™s south flank. While the south flank is among the most seismically active areas in the U. S., the vast majority of earthquakes beneath it are too small to be of general interest. At the same time, each earthquake beneath the volcano represents a piece of a very complex puzzle that we continually watch and study.
The earthquakes on April 14 (magnitude 5.0) and April 21 (magnitude 4.2) occurred at depths of roughly 9 km (5.5 miles) below the Earthâ€™s surface in regions adjacent to Kilaueaâ€™s east rift zone. These earthquakes gently punctuated the steady, southeastward motions of Kilaueaâ€™s south flank. They are noteworthy because they were widely felt across Hawai`i Island.
Parts of the active fault system responsible for the earthquakes are visible as the spectacular scarps that line Hawai`iâ€™s southeast coast within Hawai`i Volcanoes National Park. A major part of the systemâ€”a fault representing the decollement, or detachment surface, between the ancient oceanic crust and the volcanic â€œpileâ€ built up from repeated volcanic eruption and intrusionâ€”becomes most apparent in larger earthquakes, as on April 14 or two of Hawai`iâ€™s larger earthquakes in 1989 (magnitude 6.1) and 1975 (magnitude 7.2).
With capabilities afforded by continuous GPS monitoring, we measure considerable movement of Kilaueaâ€™s south flank that occurs as steady or stable motion. Large in a geophysical context, the movements occur at rates of several inches (centimeters) per year. At a conceptual level, it is reasonable to view the decollement as a through-going and somewhat uniform fault beneath the flank.
The south flank fault system is more complex, however, when seen from the perspective of earthquake distributions. There are some regions of the south flank that are surprisingly devoid of earthquake activity. Looking back through the Hawaiian Volcano Observatoryâ€™s record of seismicity, patterns of south flank earthquake hypocenters, or computed locations, also display a general level of clustering.
While they are all related to the decollement, April 2009â€™s two earthquakes and the 1989 and 1975 earthquakes each occurred in distinctly separate south flank clusters. The magnitude-5.0 earthquake on April 14 occurred in one of the south flankâ€™s western earthquake clusters. The magnitude-4.2 earthquake on April 21 occurred about 10 km (6 miles) to the east of that cluster. The 1989 earthquake was located a few kilometers (miles) east of the April 21 hypocenter, and the 1975 earthquake was about 8 km (5 miles) east of the 1989 hypocenter.
Interestingly, between the two clusters containing the April 2009 earthquakes lies a section of the south flank decollement system that has produced thousands of small earthquakes, but not a single magnitude 4 or larger earthquake since 1970. In comparison, during that same time interval, the April 14 magnitude-5.0 cluster has produced 30 such earthquakes, and the April 21 magnitude-4.2 cluster has produced 45 such earthquakes.
The earthquake clustering reflects variation in fault properties and fault structure along the decollement. Variation of earthquake behaviors within and among different clusters reflects additional complexity associated with movement of the south flank. In addition to understanding the fault properties and structures, we are striving to learn more about the forces resulting from magma residing in the rift zones and the frictional resistance along the active faults.
Each new south flank earthquake, especially if large enough to be felt, reprises questions of Kilaueaâ€™s next possible magnitude-7 south flank earthquake. While we lack clear-cut answers to some of those questions, each earthquake compels us to look more closely and provides new information to fit into the Kilauea puzzle.
The Waikupanaha and Kupapa`u ocean entries remain active, with small littoral explosions common at the Kupapa`u entry over the past week. Surface flows inland from Kupapa`u remain active along the eastern boundary of Hawai`i Volcanoes National Park.
At Kilauea’s summit, the vent within Halema`uma`u Crater continues to emit elevated amounts of sulfur dioxide gas, resulting in high concentrations of sulfur dioxide downwind. Variable glow and vent noises over the past week suggest that lava is still present at shallow levels below the floor of Halema`uma`u crater.
A magnitude-4.2 earthquake at 4:58 p.m. H.s.t. on Tuesday, April 21, was located beneath Kilaueaâ€™s south flank, about 44 km (27 miles) south of Hilo and at a depth of 9.2 km (5.7 miles). The earthquake caused no significant changes to Kilauea’s ongoing eruptions. More than 200 people reported feeling it.
Visit the HVO Web site for detailed Kilauea and Mauna Loa activity updates, recent volcano photos, recent earthquakes, and more; call (808) 967-8862 for a Kilauea summary; email questions to askHVO@usgs.gov.
Volcano Watch is a weekly article and activity update written by scientists at the U.S. Geological Surveyâ€™s Hawaiian Volcano Observatory.