Volcano Watch — What are our island's worst-case eruption scenarios (part 1)?

Worst-case scenarios are unlikely but plausible events that can result in tremendous property damage and loss of life. The problem with planning for worst-case scenarios is that they are expensive to mitigate if mitigation is possible and have a low probability of occurrence. That may be why repair and strengthening of the canal levees that kept much of New Orleans dry was deferred - high expense and low probability that it would be needed.

The worst-case eruption scenario involves lava erupting within residential neighborhoods. That could occur in two locations on Hawai`i island: 1) a Mauna Loa rift eruption within or near the upper section of Hawaiʻian Ocean View Estates (HOVE), Ka`u district and 2) a Kīlauea rift eruption anywhere between Leilani Estates and Vacationland Hawai`i, Puna district. These subdivisions are partly or wholly located within Lava Flow Hazard Zone 1, the highest hazard of nine zones on the island.

What could happen in these two locations that would make it such a nightmare? The lava flow hazard in Zone 1 is based on the history of those areas as the locations of vents, where lava first comes to the surface. Lava flows start in Lava Flow Hazard Zone 1; therefore, this is the area where there would be the least, if any, warning of danger from a lava flow.

Predicting volcanic eruptions and lava flows and predicting hurricanes are very different. With a network of satellites, we can see hurricanes forming and moving over the ocean, so we can predict where and when they will make landfall and potentially cause damage. Everything in the process is visible. The state of this science was most impressively demonstrated in the sequence of advisories, watches, and warnings issued by the National Weather Service (NWS) for hurricane Katrina.

For volcanoes, little is visible before an eruption. We can recognize when they are in unrest and we know they will erupt again, but we cannot yet say exactly when or where. It's a little like hurricane prediction with no satellite observations. Our best estimates of eruption timing improve as the volcano gets closer to an eruption. We can say with a much higher degree of confidence where a volcano will erupt - most likely somewhere within Lava Flow Hazard Zone 1.

All of this is based on our knowledge of a volcano's past eruptions. All of Mauna Loa's 30 or so eruptions in the last 160 years have started at the summit. Half of those 30 odd eruptions stayed in the summit, a quarter of them moved into the northeast rift zone and a quarter moved into the southwest rift zone. Based on the historic pattern, we predict that there is a roughly 25 pecent chance that the next eruption will move into the more threatening southwest rift zone.

Because we are imagining a worst case scenario, let's imagine that the next eruption of Mauna Loa does migrate into the southwest rift zone. This will be bad news for south Kona and Ka`u residents. In the last 140 years, seven flows have erupted out of this rift zone. If that history is replayed today, any of the flows would have cut Hawai`i Belt Road in at least one location in a period of time ranging from a few hours to a few days after the start of the summit phase. Two of these flows would have directly impacted the area now known as HOVE.

Based on this limited history, the probability of the next Mauna Loa eruption directly affecting HOVE is less than 10 percent. Based on a similarly limited history of Kīlauea, the probability of the next Kīlauea eruption occurring within a populated area is about the same.

Want to know what these eruptions might be like? See next week's Volcano Watch.

Volcano Activity Update

During the past week, the count of earthquakes located beneath Kīlauea remains at low levels. Inflation continues at previous rates.

Eruptive activity at Pu`u `O`o continues. On clear nights, glow is visible from several vents within the crater and on the southwest side of the cone. Lava continues to flow through the PKK lava tube from its source on the flank of Pu`u `O`o to the ocean, with a few surface flows breaking out of the tube. In the past week, flows were active intermittently on the upper reaches of the tube, about 1 kilometer (0.6 miles) downslope of Pu`u `O`o, and on the steep slopes of Pulama pali, above the coastal plain. Surface flows on the pali are visible at night (weather permitting) from the end of Chain of Craters Road.

As of November 23, lava is entering the ocean at East Lae`apuki, in Hawai`i Volcanoes National Park. Small bench collapses continue to occur at the ocean entry. Large cracks cross both the old and new parts of the bench. Access to the ocean entry and the surrounding area remains closed, due to significant hazards. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

There were two felt earthquakes reported on Hawai`i Island within the past week. A magnitude 2.9 earthquake occurred at 1:43 p.m. on Monday, November 21 and was located 12 km (8 miles) northeast of Kawaihae at a depth of 16 km ( 10 miles); it was felt in Kamuela, Honoka`a, and Kapa`au. A magnitude-3.3 earthquake occurred at 9:58 p.m. on Tuesday, November 22 and was located 8 km (5 miles) northwest of Pohakuloa at a depth of 23 km (14 miles); it was felt in Waimea, Waikoloa, Pohakuloa Training Area, and Papa`aloa.

Mauna Loa is not erupting. During the past week, the count of earthquakes located beneath the volcano remains at low levels. Inflation continues.