The last month was busy at the Hawaiian Volcano Observatory, with scientists keeping a keen eye on monitoring data and preparing for a possible change in eruptive activity. Kilauea’s lava levels and other monitoring indicators were elevated through October, with the rising lava lake level in Halemaumau making national news. But over the past two weeks, some of these indicators have stopped increasing. What happened?
Activity began ramping up at the start of October, with inflation and rising lava levels at the summit and at Puu Oo and an increasing number of small earthquakes in the upper east rift zone. Together, they indicated the magmatic system at the summit and parts of the east rift zone was pressurizing. Lava at the summit reached its highest measured level for the current eruption, about 70 feet below the floor of Halemaumau. The increased heat on the crater walls produced frequent rock-fracturing sounds audible from Jaggar Museum, and the vent crater wall had several collapses. Earthquakes also became more frequent in the upper east rift zone, with up to 40 earthquakes per day.
This pattern of behavior was similar to what we saw prior to the March 2011 Kamoamoa fissure eruption west of the Puu Oo vent. This change in the ongoing rift zone eruption resulted from pressure building up until the magmatic plumbing system failed, with magma breaking out of its normal confines and making its way upward to the surface in a new location.
So it was reasonable to expect the buildup this past month might presage another such change on the east rift zone although its timing couldn’t be known. Regardless, HVO scientists hurried to ensure that batteries were charged, fuel tanks filled and field equipment packed.
Over the last week, however, things have quieted. Summit inflation stopped on Oct. 26, and a net deflation has occurred since. Earthquakes in the upper east rift zone decreased to a near-normal level. Together, these changes indicate that the magmatic system at Kilauea has depressurized slightly over the past week, reducing the probability of any imminent change in the eruption.
Why did this happen? It’s not completely clear just yet, but a period of deflation starting minutes before midnight on Oct. 27 had a pattern different from that of the deflation-inflation cycles so common at the summit. Furthermore, strainmeters showed a significant change in subsurface pressure during this deflation episode in a pattern distinctly different from typical DI events. A preliminary interpretation is that a small batch of magma was intruded, but not erupted, somewhere deep beneath the summit or upper east rift zone, halting the increase in overall pressure in the system.
The timing of this Oct. 27 deflation event is potentially interesting. The onset occurred a few hours after numerous monitoring instruments at the summit recorded vibrations from the magnitude 7.8 Queen Charlotte Islands earthquake in British Columbia — the earthquake that triggered a statewide tsunami warning. If the instruments can get shaken by these distant earthquakes, perhaps the magma in the summit might get sloshed just enough to cause it to flow into a new location.
It’s still puzzling that last month’s buildup slowed without triggering a major change in the eruption, such as a large intrusion or fissure eruption. We may gain some insight on this by reviewing observations in the early 1900s, when early HVO staff made careful observations of summit lava lake level and earthquakes. Several times, rising lava levels culminated in rift-zone eruptive events, including the 1919 Mauna Iki eruption and 1922 and 1923 eruptions near Makaopuhi Crater.
But several times between 1920 and 1922, rapidly rising lava levels peaked and then quietly returned to normal levels without eruption. If we view the rift zone eruptive events as a pressure release “valve” on the magmatic system, then the periods of rising level that do not result in rift eruptions imply that Kilauea has other internal pressure-release mechanisms.
We may have witnessed just such an internal pressure adjustment, but we’re not out of the woods yet. Our network of GPS receivers recorded only a cessation of summit swelling but no appreciable contraction since late October. Despite the changes since Oct. 26, there remains an elevated chance of a change in the ongoing eruption.
Kilauea activity update
A lava lake within the Halemaumau Overlook vent produced nighttime glow visible from the Jaggar Museum overlook and by HVO’s webcam during the past week. After several weeks of summit inflation and high lava levels, the summit lava lake began dropping when summit deflation started around Oct. 26. Since then, the lava lake has slowly dropped to levels more typical of periods before the October inflation phase.
On Kilauea’s east rift zone, surface lava flows are still slowly advancing across the coastal plain within the abandoned Royal Gardens subdivision. The flows reached within a half-mile of the shoreline Thursday and continued to advance slowly. Within the Puu Oo crater, the northeastern pit still holds a circulating lava lake. Occasional small flows were erupted from several pits on the crater floor.
One earthquake was reported felt across the Hawaiian Islands during the past week. A magnitude 3.0 earthquake occurred at 12:16 a.m. Thursday, 12 miles south of Wailea-Makena, offshore of Maui at a depth of 4 miles.
Visit hvo.wr.usgs.gov for detailed Kilauea, Mauna Loa and Hualalai activity updates, recent volcano photos, recent earthquakes and more; call 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 Survey’s Hawaiian Volcano Observatory.