Revisiting 1868 to Inform Disaster Preparedness - 1/6: What was up with Maunaloa?
The series of disasters on Hawaiʻi Island between March 27 and April 11, 1868 includes the largest earthquake ever reported in Hawaiʻi, estimated at magnitude 7.9, to go along with two other magnitude 7’s and countless smaller events, devastating landslides and tsunamis, four eruptions in total at both Maunaloa and Kīlauea volcanoes’ summits and southwest rifts, and finally significant collapses atop each volcano. If such a sequence were to repeat today, residents’ knowledge and preparedness could go a long way in mitigating its impact, especially in minimizing the damage to human lives and well-being. In that spirit, we present a series of 6 short articles that recap each phase of the sequence and its relevance to those of us on-island.
Maunaloa’s April 1896 summit eruption as painted by Hitchcock, perhaps similar to the events at the start of the sequence of March 1868. Image from USGS Open-File Report 2018-1027.
Today, we examine the first phase of the sequence, from March 27 to the early afternoon of April 2, 1868. It all began then with a brief Maunaloa summit eruption, whose glow was visible beneath a massive column of fume at dawn from the whaleships in Kawaihae harbor on the northwest part of the island, as well as Maui and Molokaʻi. Those days lacked monitoring instruments deployed across the mountain, such that today we would likely have months of building earthquakes and ground deformation, similar to what we experienced in the build-up to its eruption in 2022. Back then, they knew the eruption was brief as there was no glow from Maunaloa’s summit that evening once dark fell.
Often on Maunaloa, a short summit eruption can lead to an intrusion of magma into one of its rift zones. In 2022, that was the Northeast Rift whose eruption only threatened the weather observatory and the Saddle Road. In 1868, an intrusion followed into the more populated Southwest Rift over the next several days, initiating a dramatic 2-week cascade of events.
On March 28, large earthquakes began to be reported, with the first major shock estimated at magnitude 7.1 around 2pm centered under Kaʻū, followed by 9 sizable aftershocks within the next 24 hours all felt in Hilo. Most likely, as magma intruded into the rift, it shoved Maunaloa’s southeast flank which then moved in response, allowing the magma to intrude and push further, leading to more earthquakes. The initial shaking was severe enough to collapse many cliffs, trigger landslides, and topple stone buildings and chimneys, and as aftershocks persisted many residents slept uneasily for days to come. Several residents began to self-evacuate from Kaʻū due to damage and near-constant shaking, later counting themselves lucky if this was the worst of their experience.
Maunaloa’s May 1916 eruption as seen from Kīlauea, with fumes rising from Maunaloa’s Southwest Rift at the 11,000 foot elevation. The initial stages of the 1868 sequence may have looked similar, as magma began to intrude the Southwest Rift after a brief summit eruption. Photo by H. Wood and courtesy of the University of Hawaiʻi at Mānoa Hamilton Library, via May 19, 2020 USGS-HVO Volcano Watch, and restored with Capcut AI.
The continued migration of magma from Maunaloa’s summit down into the Southwest Rift finally released its southeast flank in unrivaled fashion at 4pm on April 2, through the magnitude 7.9 Great Kaʻū Earthquake. That landmark event and its immediate consequences will be the topic of our next article, as we consider the challenge of one of our worst-case series of disasters and use that to improve our preparedness today.
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