Volcano Watch is a weekly article and activity update written by scientists and affiliates of the US Geological Survey Hawaiian Volcano Observatory.

View from a Hawaiian Volcano Observatory helicopter flying over the fissure 8 lava channel during the 2018 eruption of the Lower East Rift Zone of Kīlauea. All 24 cracks from this eruption opened in Lava Flow Risk Zone 1, which is the area with the highest lava flow coverage rate. Lava flows erupted from these cracks covered 13.7 square miles and destroyed more than 700 homes, all located in lava flow risk areas 1 and 2. The cone of fissure 8 is obscured by a cloud. steam (top center). USGS photo taken July 10, 2018.

(Public domain.)

One approach uses a geological map to calculate how much of the earth’s surface has been covered by lava during different periods dating back to the past; the resulting numbers are called coverage rates. Another approach calculates how often lava flows have occurred in specific areas over time; the resulting number is a probability of lava flow.

The 1992 Lava Flow Risk Zone (LFHZ) map represents the use of the long-term coverage rate approach. This is not a measure of how quickly an individual lava flow is progressing, but how quickly an area is covered with lava from multiple eruptions over the centuries. For example, more than a quarter of the Kīlauea volcano had been covered since the Hawaiians welcomed English Captain Cook to visit the islands and nearly 90% of the volcano had been covered since the Polynesians arrived around 800 to 1 000 years old. An assessment of future activity using these coverage rates would estimate that most of the volcano will resurface with new lava over the next 1,000 years.

The new eruptions do not significantly affect the coverage rates, as the new flows cover some of the newer lava flows as well as older ones. For example, the 2018 lava flowed between and over parts of the 1790, 1955 and 1960 lava flows. Therefore, the “cover” or resurfacing since 1790 has not increased over the entire surface. of the 2018 stream, but only the part that was beyond those earlier streams.

The 1992 LFHZ map shows that the highest coverage rates (and therefore the risks) are found in the rift zones and the peaks of the Kīlauea and Mauna Loa volcanoes. Almost half of LFHZ 1 (the most dangerous zone) on the two volcanoes had been covered since 1790. Coverage rates decrease with distance from LFHZ 1.

The other approach to estimating the risk of long-term lava flow is to calculate how often a particular area is affected by lava, also known as the recurrence interval method. The lower East Rift Zone (LERZ) of Kīlauea has been invaded by lava five times since 1790 – in 1790, 1840, 1955, 1960 and 2018. These eruptions occur over a period of over 200 years with an average of about 60 years old. between them.

The recurrence interval method is the most widely used to calculate flood risk, traditionally basing risk maps on an average 100-year return interval between destructive floods. Using the simplest probability formula (a French mathematician Denis Poisson), this recurrence interval translates to 1% probability of damaging flooding in a year and 39% probability over a 50-year period. The probability of such a flood occurring in a century is, surprisingly, not 100% but 63% (about 3 to 2 probabilities) because the recurrence interval is an average of real intervals which can be very different.

In our application to lava flows, an average return interval of about 60 years in the LERZ means that there is a 63% chance (another 3 to 2 probabilities) that the next lava-free return interval will be 60 years. ; it is also the probability that another lava flow will affect part of the LERZ within 60 years. The probability of a lava flow in this region over the 30-year period would be 40 percent or a probability of 2 to 3 and the probability of flooding would be 26 percent (a probability of 1 to 3) . Fortunately, the region of significant combined lava and flood risk in the LERZ is limited to coastal flood areas.

Lava flow risk calculations and maps produced by the US Geological Survey (USGS) are intended to inform landowners, emergency managers, and government planners about the long-term risks posed by lava flows. The USGS Hawaiian Volcanoes Observatory continues to study the dangers of lava flows using these and other methods. For more information on lava flow risk probabilities, the following publications are available: https://pubs.usgs.gov/of/1998/0794/report.pdf and https://pubs.usgs.gov / of / 1994/0553 /rapport.pdf.

Updates of volcanic activity

Kīlauea is not erupting. Its USGS volcanic alert level is ADVISORY (https://www.usgs.gov/natural-hazards/volcano-hazards/about-alert-levels). Kīlauea updates are released weekly.

The Kīlauea volcano is not erupting. Following the recent intrusion of sub-surface magma in the area south of the Kīlauea caldera, which significantly slowed down on August 30, earthquake rates and deformation in this area remained close to the levels prior to the intrusion. Other monitoring data streams, including sulfur dioxide emission rates and webcam views, show no significant change. For more information on the current monitoring of Kīlauea, see https://www.usgs.gov/volcanoes/kilauea/monitoring.

Mauna Loa is not erupting and remains at the ADVISORY volcanic alert level. This alert level does not mean that a rash is imminent or that progression to a rash from the current level of unrest is certain. Mauna Loa updates are released weekly.

Last week, around 66 low-magnitude earthquakes were recorded below the summit and upper elevation flanks of Mauna Loa – the majority of them occurred at depths below 8 kilometers (5 miles). Global Positioning System (GPS) measurements show no major distortions over the past week. Gas concentrations and fumarole temperatures at the summit and at Sulfur Cone over the southwestern Rift Zone remain stable. The webcams do not show any change in the landscape. For more information on the current monitoring of Mauna Loa, see: https://www.usgs.gov/volcanoes/mauna-loa/monitoring.

There have been 3 events with 3 or more felt reports in the Hawaiian Islands in the past week: an M3.2 earthquake 2 km (1 mi) southwest of Pāhala 32 km (20 mi) deep on September 15 at 7:36 a.m. HST, an M2.6 earthquake at 5 km (3 mi) WSW of Kealakekua at 4 km (2 mi) deep on September 15 at 3:09 a.m. HST, and an M3.8 earthquake at 6 km (3 mi) ENE of Pāhala 36 km (22 mi) deep on September 12 at 1:45 a.m. HST.

The HVO continues to closely monitor Kīlauea and Mauna Loa for any signs of increased activity.

Please visit the HVO website for past Volcano Watch articles, Kīlauea and Mauna Loa updates, photos of the volcano, maps, information on recent earthquakes, and more. Email your questions to [email protected]

Volcano Watch is a weekly article and activity update written by scientists and affiliates of the US Geological Survey Hawaiian Volcano Observatory.

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