Volcano Watch — Researchers Record Slow, Unsteady Movement on the South Flank of Kīlauea

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Beginning in the early morning hours of October 14, 2015, a tiltmeter near Ka‘ena Point on Hawai‘i Island's coastline south of Kīlauea'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 2–3 days. In total, the south flank slipped about 3 cm (1.2 in) 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 Hawai‘i, happen along a fault at the boundary between Kīlauea 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 Hawai‘i are periodic, typically occurring about every 26 months. The previous one was on May 28, 2012, so scientists at the USGS Hawaiian Volcano Observatory had been expecting another one to occur since July 2015. Interestingly, Hawai‘i 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 Kīlauea's south flank is often the most conspicuous indicator that a slow slip earthquake is happening. During the slow earthquake last week, there were over 110 "aftershocks." These earthquakes began on October 15, after the slow earthquake had begun, and high earthquake rates continued through October 17.

Most of these aftershocks were small—less than magnitude 3. However, on the evening of October 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 Kīlauea'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 Kīlauea 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 Pu‘uKOU, an area of the Southwest Rift Zone that has had enhanced seismic activity since March 2015. 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 Kīlauea's south flank will keep researchers busy in the coming years.

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Volcano Activity Update

Kīlauea continues to erupt at its summit and East Rift Zone. The summit lava lake level, which fluctuates with summit inflation and deflation, varied between about 50 and 65 m (164–213 ft) below the vent rim within Halema‘uma‘u Crater. On the East Rift Zone, scattered lava flow activity remained within about 7 km (4.3 mi) of Pu‘u ‘Ō‘ō.

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 Thursday, October 15, 2015, at 8:41 p.m., HST, a magnitude-3.9 earthquake occurred 10.4 km (6.5 mi) west of Kalapana, Hawai'i, at a depth of 7.2 km (4.5 mi). On Monday, October 19, 2015, at 3:18 p.m., HST, a magnitude-2.9 earthquake occurred 19.8 km (12.3 mi) southwest of Mākena, Maui, at a depth of 10.2 km (6.3 mi).