Figure 1: Ka'u district of Hawaii. (source 3)

Great Ka'u Earthquake

On April 2, 1868, an earthquake occurred in southern Hawaii which destroyed all stone buildings in the area. The earthquake was felt on Kauai Island at 600 km away, and ground shaking of intensity seven was reported up to 130 km away in distance. Based on the magnitude versus felt-area relationship for Hawaii, it is estimated that the magnitude of the earthquake was about 8. The foreshock sequence lasted up to 5 days, and the aftershocks lasted for years to perhaps a decade. It was reported that this earthquake was one of the very few largest events in historic time in the United States, excluding Alaska, but its return period is unknown.

About the Great Ka'u Earthquake

The Great Ka'u Earthquake is the largest earthquake recorded in the history of Hawaiian Islands. It occurred on the big island of Hawaii on April 2, 1868 beneath the Ka'u district on the southeast flank of Mauna Loa (see figure 1). This earthquake had an estimated magnitude of 7.9. The Great Ka'u earthquake caused damages across the entire island. On the Modified Mercalli Scale, this earthquake had an intensity of IX-X. The extensive shaking caused by the earthquake knocked most houses off their foundations not just in the Ka'u district but also in areas such as the Keiawa, Punaluu, and Ninole. The earthquake was reported to be felt as far away as the
Figure 2: distance of big island to island of Kauai. (source 5)
island of Kauai(see figure 2). However, the devastation of this earthquake was still greatest in the Ka'u district. 77 deaths were reported from this 1868 event but none of them were from the initial earthquake itself. 31 of the deaths reported were caused by a landslide or mudflow that was triggered by the earthquake. The other 46 deaths reported were caused by a tsunami also triggered by the earthquake that destroyed several villages.

Cause of the Great Ka'u Earthquake of 1868

There are several interpretations of what exactly caused the Great Ka'u Earthquake in 1868. The major one that most people believed to be true is based
Figure 3: Rift zone of Mauna Loa Volcano. (source 6)
on the sequence of events between March 27 and April 2 in 1868. On March 27, 1868, a sequence of foreshocks began to happen. The shocks were reported to be increasing in intensity steadily. One shock that occurred on March 28 was estimated to have a magnitude of 7.1. The sequence of shocks continued until 4 p.m. on April 2 when the mainshock occurred. The interpretation of the cause of the event stated that there are two separate landslide structures on the south side of the island. During the sequence of shocks, each of these two landslide structures moved. The magnitude 7.1 earthquake on March 28 was stated to be triggered by the movements of a landslide structure bounded by the southwest rift zone of Mauna Loa Volcano as shown on figure 3. Which is a fault extending from the southwest rift zone of Waiohinu and to the coast of Hanuapo Bay,and by the western part of the Kaoiki fault zone. This block, extended offshore at least 12 miles, was pushed seaward by intrusion of magma down the southwest rift zone of Mauna Loa Volcano starting on March 27 when an eruption started from the upper part of Mauna Loa's southwest rift zone. A
larger landslide block, consisting of the entire south flank of Kilauea Volcano on April 2 moved seaward and caused the earthquake measured 7.9 in magnitude.

Hawaii is a place with frequent earthquakes. Many of the thousands of earthquakes that have occurred 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 suggested that this was a worst-case scenario for earthquakes in Hawaii, possibly because of the high death toll. At the time, houses were build out of stone, but now, with our advancement in technology we can improve on the structural weaknesses that lead to the earthquake damages, which killed so many people. All the stone buildings were destroyed probably because when the earthquake caused the intense shaking, the stone walls gave in and the heavy roof collapsed, which killed people who were inside. To reduce buildings from collapsing, they could build the wood or steel frames which can reinforce the buildings with stronger walls. Another way to mitigate damages they could add diagonal cross bracing to strengthen the building and anchoring the frames to the foundations, which would decrease houses from being knocked off their foundation. Buildings also can be isolated from the base by rubber pads, which would lessen the shaking intensity from the earthquake because the base would experience most of the shaking instead of the whole building.