Landslides are hazards in areas where slopes are steep. The degree of the hazard depends on the type of rocks that make up the slope. Large landslides, like other natural hazards, tend to recur in the same locations where they have occurred in the past.
The Oso landslide of March 22, 2014, also known as the SR530 or Steelhead landslide, surprised everyone, even though it occurred in a river valley with frequent landslides. In minutes, the landslide swept 0.7 miles across the valley, destroying about 40 structures and taking 41 lives — two people are still missing — in the unincorporated neighborhood of Steelhead Haven, Washington. It was the deadliest single landslide in the history of the lower 48 states.
In the aftermath of the slide, its cause was variously attributed to a recent earthquake, unusually high rainfall and logging in the area, but its trigger remains unknown. Earlier geologic studies identified several landslide deposits composed of glacial sediments running along the north fork of the Stillaguamish River valley, some as recent as 2006.
Could such a disaster happen in Hawaii? The answer may surprise you — it already has. On April 2, 1868, after a week of constant earthquake activity, the strongest earthquake documented in the Hawaiian Islands struck. Its magnitude is now estimated at 7.9 or stronger, and its effects were most intense in the southeastern portion of the island, with total destruction of all buildings in the Ka‘u District. The earthquake generated a tsunami that killed 46 Hawaiians and destroyed coastal settlements from Cape Kumukahi near Kapoho to Kalae — South Point.
The earthquake also dislodged part of a valley wall in the Wood Valley area of Ka‘u. In minutes, the landslide covered an area four times the size of the more recent SR530 slide, destroying 10 structures, and killing 31 Hawaiian farmers.
Ka‘u residents who observed the mud landslide from a distance thought it was a lava flow. That was understandable, since Wood Valley is on the southeast flank of the active Mauna Loa volcano and since the mudslide appeared to be red in color and was preceded by many earthquakes. But witnesses at the scene found that the “lava flow” was cold mud, with streams of water draining down each side. The red color came from the abundant volcanic ash soil in the area. With more time and scrutiny, it became clear that this was a landslide.
Ground vibration caused by the earthquake clearly triggered the 1868 landslide. But earthquakes occur frequently in this area without landslides. What was different in 1868? The Ka‘u area experienced heavy rainfall just prior to the earthquake. Perhaps the rainfall saturated the ash layers in the valley walls, weakening or liquefying the ash to the point of failure during strong ground shaking.
The geology of the Ka‘u area is also unique, with one or more thick ash layers interspersed between Mauna Loa lava flows. The ash layers are relatively impermeable, compared with the permeable lava flows. This means that the ash tends to be a barrier to water percolating down through the ground, resulting in water being concentrated in the lava flows. Before the sugar industry, Ka‘u had many natural springs, with water gushing out of the lava flows and over ash layers that were exposed in cliffs. Now, an extensive set of tunnels cut into the ash layers extract water more efficiently.
Modern mapping and studies suggest that the 1868 landslide itself was composed of lava-flow blocks and ashy gravels. The debris was probably the result of lava blocks and ash sliding from the hillside. When saturated with rainwater and shaken by a strong earthquake, the thick layer of volcanic ash liquefied and flowed like water, removing support for the overlying lava layers.
Liquefaction is a dangerous consequence of strong earthquake shaking and, in the case of the 1868 Ka‘u landslide, can result in life-threatening landslides. Liquefaction during earthquakes can also be a problem on gentler slopes. For example, failure of bridge supports in the Hamakua District and in landfill areas that made up Kawaihae Harbor occurred during the Oct. 15, 2006, earthquake.
Liquefaction also occurred in the Oso landslide, causing the hillside to behave like a liquid. Now that the response to the crisis is over, healing and understanding can begin.
Kilauea activity update
A lava lake within Halemaumau produced nighttime glow visible via HVO’s webcam during the past week. Abrupt summit deflation occurred May 10 and was followed by more gradual deflation over the past week, associated with a drop in lava level. As of Thursday, the lake level was about 190 feet below the rim of the overlook crater.
On Kilauea’s east rift zone, the Kahaualea 2 flow remains active, with its front 5.5 miles northeast of the vent on Puu Oo, based on satellite imagery from Wednesday. Small flows that originated from a spatter cone in the Puu Oo crater and spilled out of the crater over a week ago appear to be inactive.
There were no earthquakes in the past week reported felt on Hawaii Island.
Visit the HVO website, hvo.wr.usgs.gov, for Kilauea, Mauna Loa and Hualalai activity updates, recent volcano photos, recent earthquakes, and more; call 967-8862 for a Kilauea summary; email questions to askHVO@usgs.gov.
Volcano Watch is a weekly article and activity update written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.