Kaʻū News Briefs Friday, July 6, 2018

     A document from the scientists says that they expect the events at Halemaʻumaʻu "to continue for as long as magma is withdrawn from the summit reservoir(s) at a rate exceeding the rate of magma supply, but the rate, style, and geographical extent of the subsidence – along with associated hazards – may vary. The USGS staff presented likely scenarios under the condition of continued net magma withdrawal.

Halemaʻumaʻu, just after the July 5 collapse event. USGS photo

     MOST LIKELY SCENARIO: USGS predicted that the most likely outcome for the next two months is "continued subsidence of Kīlauea Caldera, episodic slumping into a widening Halemaʻumaʻu crater, felt earthquakes (some large enough to be damaging), and small to intermediate ash plumes that remain below 10,000 feet above sea level. As the reservoir deflates, cracking and slumping is gradually engulfing a broader extent of Kīlauea Caldera (as observed in high rates of ground deformation and propagating cracks around Halemaʻumaʻu ); this process will likely continue to enlarge Halemaʻumaʻu and may involve larger slump blocks than previously. This activity is impressive in scale — and may ultimately involve much or even all of the current Kīlauea Caldera — but it need not necessarily involve new or more hazardous explosive activity.
     "Hazardous explosive activity cannot be ruled out, however. It is possible that a large section of the Halemaʻumaʻu wall could abruptly collapse into the crater. Because a broad region E and NE of Halemaʻumaʻu is currently deforming, it is difficult to predict how large such a collapse might be or its impact on the explosion hazard. Most likely, such an event would generate only strong seismic shaking and a robust ash plume.
     "Should activity continue as described, primary hazards of concern are:
– Damaging earthquakes (potentially exceeding equivalent M5).
– Ash plumes, ashfall (associated with CE events and large rockfalls)
– Large and sudden collapses into the expanding Halemaʻumaʻu crater
– Ground cracking and continued rockfall activity along steep caldera walls
– Vog (although sulfur dioxide output is approaching low pre-2008 levels)
     LESS LIKELY OUTCOMES: USGS predicted less likely outcomes for the next two months. 

Rock slides into Kīlauea after the July 5 collapse event. USGS photo

     "Several mechanisms could change the nature of activity and associated hazards. These are considered less likely but cannot be ruled out. The likelihood of some of these processes may increase if the net rate of magma outflux from the summit increases.
     USGS presented two less likely possibilities: "more hazardous explosions occurring during ongoing subsidence and enlargement of Halema`uma`u," and "a sudden collapse of the larger caldera system." The scientists said the scenarios "may not include all possible future outcomes and hazards."
     A larger explosion during ongoing subsidence in and around Halemaʻumaʻu. If this unlikely (perhaps 1 percent) option were to occur, USGS predicted, "activity could become more hazardous over short time scales. This could be triggered in one of several ways, including a rapid pressure change or other perturbation of the reservoir; opening of new pathways between the reservoir and the surface; or the interaction of magma with groundwater. Rapid pressure change could be caused by a large, sudden landslide from the crater’s steep, faulted rim; alternatively, sudden larger-scale collapse of rock into the reservoir could perturb reservoir pressure above levels seen during previous collapse eruption events. New pathways could be formed by explosive ejection of rubble in the vent or downward propagation of cracks.
    "Groundwater could enter the magmatic system at sufficient rates to produce steam-driven explosive eruptions. Some of these mechanisms could be preceded by detectable changes in monitoring data, but others could happen with no warning. If larger explosions do occur, their style and magnitude cannot be predicted; it is possible that they could produce more ballistics and ash, and possibly also pyroclastic surges.

The WorldView-3 satellite acquired this view of Kīlauea's summit on July 3. Despite a few clouds, the area of heaviest fractures in the caldera is clear. Views into the expanding Halema‘uma‘u crater reveal a pit floored by rubble. HVO, on the northwest caldera rim, is labeled. Photo from USGS

     A sudden collapse of the broader caldera system and catastrophic failure of high caldera walls. "Even less likely but more hazardous scenarios exist. Large explosive eruptions have occurred in Kīlauea’s past after caldera formation or during the last stage of its formation. It is possible that these eruptions were triggered by rapid collapse of broad regions of the caldera along caldera-bounding faults due to withdrawal of large quantities of magma from the summit storage system. Based on our understanding of the magmatic system, this activity should be preceded by significant changes in earthquake activity and ground deformation. At this time, satellite radar data show that high rates of deformation are concentrated in a well-defined area bordered by caldera-boundary faults on the west and south, and on the east and northeast along a line roughly 600-900 meters (yards) from the caldera walls. These data do not suggest that extensive deformation is occurring outside of the caldera. Additionally, we currently see no evidence that major caldera-bounding faults are moving, although some cracks have been detected that probably result from ground shaking. Additional hazards associated with rapid, broad-scale caldera collapse could include high lava fountains and larger and more dangerous explosions producing pyroclastic surges. However, we emphasize that current data do not suggest that a larger, sudden collapse scenario is likely at present."
     The USGS scientists said that Volcano area residents should be prepared for strong seismic shaking as well as ground cracks in the vicinity of the caldera. Hawai`i County Civil Defense asks residents to be ready with a plan, should a more dangerous situation arise.

Halemaʻumaʻu Crater's summit eruptive vent expanded in May, then began to collapse in June. As of July 4, the collapse, caused by the evacuation of magma from the reservoir, had widened Halemaʻumaʻu Crater and impacted the larger Kīlauea Caldera. Images from USGS

     While emphasizing a low probability of pyroclastic surges, the scientists defined them:
     "Pyroclastic surges and density currents are highly destructive, generally fast-moving clouds of volcanic gases and fragments of magma and older rocks. They are generally hot and form when eruption columns become unstable and collapse back around the vent. Surges typically travel tens of meters per second (22 mph), but certain types can move as rapidly as 100 – 300 meters per second (200 – 700 mph).
     "Surges usually extend radially from the vent in all directions, but can be influenced by topography, thickening in valleys and thinning over topographic highs. Due to their great heat and high speed, pyroclastic surges are among the most dangerous and destructive of volcanic hazards. There are very few survivors among those caught in the path of a hot surge, and property damage is severe. In the past, surges have been produced at Kīlauea by sustained eruption columns. For example, pyroclastic surges were produced at Kīlauea between the middle 1500s and A.D. 1790.

This chart shows the increase of seismic frequency from June 10 through June 27. USGS figure

     "Surges in and just before 1790 extended 3 – 5 km (1.8 – 3 miles) west and south of the caldera and form the basis for a surgehazard map shown in Figure 5 (see top image). The direction that surges may travel is difficult to predict, and the entire summit area of Kīlauea out to roughly 5 km (3 miles) from center of the caldera is susceptible to surges if they were to occur. The area shown shows, for simplicity, a source near the center of the caldera. The white line shows only the maximum reasonable distance a surge would travel from the caldera as inferred from past eruptions; it does not indicate that such behavior is likely at this time (it is not), nor that surges would fill the entire area within the circle (surge deposits would likely travel within narrow regions). At present, Kīlauea is not producing any sustained eruption columns even during the largest collapse explosion events.

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HAWAIʻI ELECTRIC LIGHT COMPANY CONSTRUCTION IN VOLCANO VILLAGE begins July 9, through September 30, weather permitting, says an a nnouncement from the utility. Work will be performed in phases along Old Volcano Road, Kalanikoa Road, Kilauea Road, and Wright Road from 7 a.m. to 5 p.m., Monday to Saturday.

Work in the project area may disrupt traffic and power at times between this Monday through the end of September. Map from Hawaiʻi Electric Light