Researchers record slow, unsteady movement on the south flank of Kilauea

It happened again. Did you notice? Last week, a portion of Kilauea Volcano’s south flank slowly slipped seaward. Its movement is part of a recurring phenomenon called a “slow earthquake,” which last occurred on Memorial Day 2012.

Beginning in the early morning hours of Oct. 14, a tiltmeter near Kaena Point on Hawaii Island’s coastline south of Kilauea’s summit began to tilt away from the coast in a direction that is diagnostic of a slow earthquake event. A combination of tiltmeter and GPS networks continued to detect slip for the next two to three days. In total, the south flank slipped about 1.2 inches southeastward.

Earthquakes typically occur along faults — places where rocks slip past each other. To generate the seismic waves that travel through the earth and shake our houses, roads and buildings, the slip has to be fast, typically seconds to minutes long, depending on the size of the earthquake.

By contrast, slow earthquakes occur over the course of several days, and in Hawaii, happen along a fault at the boundary between Kilauea Volcano and the old ocean floor. The slip associated with last week’s slow earthquake was so gradual that it did not generate seismic waves. But, had all the slip that took place during this slow earthquake occurred rapidly, it would have resulted in an earthquake of around magnitude 6. The slip along the fault did, however, redistribute stresses and triggered earthquakes on adjacent segments of the fault and in the overlying crust.

Slow earthquakes on the south flank of Hawaii are periodic, typically occurring about every 26 months. The previous one was on May 28, 2012, so scientists at the U.S. Geological Survey Hawaiian Volcano Observatory had been expecting another one to occur since July 2015. Interestingly, Hawaii Island’s slow earthquakes tend to occur in the same part of south flank over and over again, so instruments have been strategically placed to capture them when they happen.

The occurrence of “typical” earthquakes in the central part of Kilauea’s south flank is often the most conspicuous indicator that a slow slip earthquake is happening. During the slow earthquake last week, there were more than 110 “aftershocks.” These earthquakes began on Oct. 15, after the slow earthquake had begun, and high earthquake rates continued through Oct.17.

Most of these aftershocks were small — less than magnitude 3. However, on the evening of Oct. 15, there was a larger, magnitude-3.9 earthquake located northeast of the main cluster of seismicity. The epicenter of this earthquake was in the same region as the 1989 magnitude-6.2 earthquake. This part of Kilauea’s south flank is one of three areas on the volcano that generate magnitude-4 or greater earthquakes.

The timing of the magnitude-3.9 earthquake suggests that it may have been triggered by the slow earthquake. This is not particularly common during slow earthquakes that have been observed since 1998, except for one instance of a magnitude-3.4 earthquake triggered during a slow earthquake in late 1998.

Another interesting effect of last week’s slow earthquake was the additional seismic activity within Kilauea Volcano’s rift zones. Since the slow earthquake, both the East Rift Zone and the Southwest Rift Zone have experienced an increase in the number of small earthquakes, including a magnitude-3 earthquake near Puukou, an area of the Southwest Rift Zone that has had enhanced seismic activity since March. The exact process that might tie the slow earthquake to increased seismic activity in the rift zones is the topic of ongoing research.

Despite having no clear impact on our daily lives, understanding more about slow earthquakes may answer questions that do have societal impacts. In particular, we’d like to know what effect slow earthquakes have on the volcanic hazard and if larger, more destructive, earthquakes are more likely during a slow earthquake. Finding the answers to these and other questions about the slow and unsteady movement on Kilauea’s south flank will keep researchers busy in the coming years.

Volcano activity updates

Kilauea continues to erupt at its summit and East Rift Zone. The summit lava lake level, which fluctuates with summit inflation and deflation, varied between 164 and 213 feet below the vent rim within Halemaumau Crater. On the East Rift Zone, scattered lava flow activity remained within about 4.3 miles of Puu Oo.

Mauna Loa is not erupting. During the past week, earthquake rates continued to be elevated, though at a lower weekly rate than recorded in late summer. Deformation data remain consistent with inflation of magma reservoirs within the volcano.

Two earthquakes were reported felt in Hawaii this past week. On Oct. 15 at 8:41 p.m.,a magnitude-3.9 earthquake occurred 6.5 miles west of Kalapana at a depth of 4.5 miles. On Monday at 3:18 p.m., a magnitude-2.9 earthquake occurred 12.3 miles southwest of Makena, Maui, at a depth of 6.3 miles.

Visit the HVO website st hvo.wr.usgs.gov for Kilauea daily eruption updates, Mauna Loa weekly updates, volcano photos and recent earthquakes information; call for summary updates at 967-8862 (Kilauea) or 967-8866 (Mauna Loa); email questions to askHVO@usgs.gov.

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