Revisiting 1868 to Inform Disaster Preparedness - 2/6: Hawaiʻi’s Biggest Quake
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.
Today, we examine the second phase of the sequence, the Great Kaʻu Earthquake and its immediate consequences. During the first phase, Maunaloa erupted briefly at its summit, intruded its Southwest Rift, and initially triggered an estimated magnitude 7.1 quake on March 28.
Cross-section of the southern flank of Maunaloa , showing the hypocenter of the 1868 Great Kaʻū Earthquake at the base of the volcano where it sits atop the old ocean floor, and map of Hawaiʻi Island showing the maximum extent of fault slip during the record event (via USGS-HVO Volcano Watch on March 29, 2018).
Felt intensity State-wide for the record magnitude 7.9 earthquake on April 2, 1868. Image USGS.
That pattern of volcanic intrusion and triggered land-slip led to the largest earthquake ever reported in Hawaiʻi around 4pm on April 2, 1868, estimated at magnitude 7.9 based on the geographic distribution and intensity of reports. It triggered further widespread collapses of cliffs, dwellings, churches and other structures beyond Hilo, causing perhaps 22 fatalities (reports are somewhat ambiguous). It initiated a combination landslide and mudflow at Keaīwa, near today’s Wood Valley, which consumed 31 people, 10 houses and many livestock. Finally, it also immediately triggered a sequence of tsunami waves which swept onto the island’s southern shore, killing perhaps 47 people and taking 108 houses. In its wake, the whole southern coastline of Hawaiʻi Island from Kaʻu to Kapoho had subsided significantly in many areas, changing the nearshore landscape by sinking beaches and points and flooding inland features and groves.
Already damaged by the magnitude 7.1 on March 28, the Wai‘ōhinu church collapsed during the magnitude 7.9 great Ka‘ū earthquake in 1868. Photo by Henry L. Chase, published in "Volcanoes of Kīlauea and Mauna Loa on the Island of Hawai‘i" by W.T. Brigham, Bishop Museum Press, 1909; via USGS-HVO Volcano Watch on March 29, 2018.
The mudflow was one of the stranger effects triggered by the earthquake in Kaʻū, as it started a landslide which then released groundwater impounded within the hillside. The red mud at the front of this cold flow moved 3 miles in less than 3 minutes, 6 feet deep and hardened after several days when the flow of water issuing from the hillside decreased and eventually ceased. Many farmers perished, though as with a later stage of the disaster there are miracle stories of survival, in this case from occupied houses spared but surrounded by the mud.
The Great Kaʻū Earthquake had other immediate effects farther afield on Kīlauea volcano, which will be the topic of our next article in this series, as we consider the challenge of one of our worst-case series of disasters and use that to improve our preparedness today.
#Maunaloa #Geology #Hawaii #1868Eruption #HawaiiHistory
—
Note: We use a preferred traditional spelling of the volcano by the local community as “Maunaloa”, though it has not yet been formally adopted by either the State or Federal Board of Geographic Names who still officially refer to it as “Mauna Loa”. This follows in the same spirit as the spelling of “Maunakea” gaining broader acceptance.