Volcano Watch — Geologists reexamine big earthquake of 1868

April 2 marks the anniversary of the largest earthquake to occur in the Hawaiian Islands in historic time. This great earthquake occurred in 1868 and had an estimated magnitude of 7.9. The magnitude and epicentral location of the earthquake are constrained by the distibution and severity of damage. However, due to the sparse population of the region at that time, neither the magnitude nor the location is precisely known. The epicenter was located about 5 miles north or northeast of Pahala. Like most of the larger earthquakes in Hawaii, it was probably caused by slip on a near-horizontal plane about 6 miles (9.5 km) beneath the surface. Many of the thousands of earthquakes in Hawaii, including these larger events, are caused by seaward movement of the south flank of the island. The eyewitness accounts of this great earthquake are a worst-case scenario for earthquakes in Hawaii.

The best account comes from a letter by Frederick Lyman. He wrote on Friday morning, March 27, 1868: "Between 9 and 10 o'clock, a slight tremble, soon another, and another, at short intervals. Bella tried to keep a record of them, but soon gave it up, when they went into the hundreds during the day - some of them harder, and continued thro the night.... with more earthquakes, increasing in violence. On Saturday, just after lunch, there was a hard one, peculiar, it seemed as if we moved backwards and forwards, 2 or 3 feet each time, for several seconds—it made the small children seasick—and it threw down some of our stone walls.... but the earthquakes kept on too—every few minutes, often we could hear it coming from the south, then give us a good smart shake and pass on towards Kīlauea, North East from us—at night it made the house rock and creak like a ship in a heavy sea, and we could not sleep...." The large event early Saturday afternoon had an estimated magnitude of 7.1, and the epicenter was located near Waiohinu. It may have been this earthquake, rather than the larger earthquake that occurred on April 2, that offset the road near Waiohinu by more than the width of the road.

T.M. Coan, in an article for Scribner's Weekly in 1871, wrote that "For four days this state of things continued, until at 4 p.m. on the 2nd of April, 1868, an event occurred which defies description. Such a convulsion has no parallel in the memory, the history, or the traditions of the Hawaiian Islands. The shock was awful. The crust of the earth rose and sunk like the sea in a storm. The rending of rocks, the shattering of buildings, the crash of furniture, glass, and earthenware, the falling of walls and chimneys, the swaying of trees, the trembling of shrubs, the fright of men and animals, made throughout the southern half of Hawaii such scenes of terror as had never been witnessed before. The streams ran mud, the earth was rent in thousands of places; and the very streets of Hilo cracked open. Horses and their riders were thrown to the ground; and multitudes of people were prostrated by the shocks. In the district of Ka'u more than three hundred shocks were counted upon this terrible day; people were made seasick by their frequency. By the culminating shock, nearly every stone wall and house in Ka'u was demolished in an instant." Mr. Frederick Lyman wrote ".... about 4 o'clock it shook as usual, but did not stop—shook East and West, North and South, round and round, and up and down—lessen, then increase in violence. It was impossible to stand; we had to sit on the ground, bracing with hands and feet to keep from rolling over."

There are several published geological models of what caused this great earthquake. The first is simply that the entire southern side of the island, bounded by the southwest and northeast rift zones of Mauna Loa Volcano, the Kaoiki fault zone between Mauna Loa and Kīlauea Volcanoes, and the east rift zone of Kīlauea Volcano, slid seaward on April 2. The seaward extent of the sliding block in this model is located near the southern shoreline of the island and extends from near South Point to Kapoho.

A more complex model, based on the sequence of events between March 27 and April 2, and on newly acquired bathymetric data offshore, suggests that there are two separate landslide stuctures on the south flank of the island, and that each moved during this sequence of events. In this model, the magnitude 7.1 earthquake on March 28 was triggered by movement of a landslide structure bounded by the southwest rift zone of Mauna Loa Volcano, a fault extending from the southwest rift zone through Waiohinu and beyond the coast at Hanuapo Bay, and by the western part of the Kaoiki fault zone. This block, inferred to extend offshore at least 12 miles, was pushed seaward by intrusion of magma down the southwest rift zone of Mauna Loa Volcano beginning on March 27, when an eruption began along the upper part of Mauna Loa's southwest rift zone. The earthquakes that occurred during the next four days were aftershocks of this earthquake, and probably indicated continued movement of this landslide block towards the sea.

On April 2, a different, and much larger, landslide block, consisting entirely of the south flank of Kīlauea Volcano, moved seaward and caused the magnitude-7.9 earthquake. This landslide is bounded by the southwest and east rift zones of Kīlauea Volcano and extends offshore to the southeast nearly 25 miles at the western end near Naliikakani Point, but narrows to the east where it extends about 15 miles offshore from Kapoho. In this model, the Kīlauea south flank moved because it was triggered by the shaking caused by the earlier magnitude 7.1 event. There was a small eruption along the southwest rift zone of Kīlauea Volcano, and wide ground cracks opened up in the same area, soon after the April 2 earthquake.

Such extension across Kīlauea's southwest rift zone would not be expected in the first model, because the southwest rift zone was not a boundary of the proposed active block. If large amounts of extension had occurred across the rift zones and summit of Mauna Loa, we would expect the eruption rate to decrease for a period of time as newly supplied magma filled cracks created within the rift zones. This, in fact, occurred at Kīlauea Volcano following the 1975 magnitude-7.2 earthquake. However, Mauna Loa had 9 eruptions in the 10 years following the earthquake. Kīlauea, on the other hand, did not have another large eruption until 1919 and erupted only once, briefly, at the summit in 1877, in the 10 years following the earthquake. These observations suggest that the earthquake disrupted the magmatic plumbing of Kīlauea Volcano far more than that of Mauna Loa Volcano.

Scientists will probably never be able to completely reconstruct the events of 1868, and some controversy and difference of opinion about what happened will certainly continue. However, the Hawaiian Volcano Observatory's seismic and deformation monitoring programs are working to define the boundaries of the mobile flanks of the volcanoes and will work towards determining the conditions that lead to such large events.