Skip to content

Can a Major Landslide Occur in Hawaii?

May 16, 2014

U.S. flag at the SR530 landslide site in Washington. (Photo Courtesy FEMA)

The following is this week’s edition of “Volcano Watch” from the USGS Hawaiian Volcano Observatory:

Can an event like the SR530 landslide near Oso, Washington, happen in Hawaiʻi?

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 across the valley (0.7 miles), destroying about 40 structures and taking 41 lives (with 2 still missing at last count) 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 (later ruled out), unusually high rainfall, and/or 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 Hawaiʻi? 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 the Island of Hawaiʻi. 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 Kaʻū District. The earthquake generated a tsunami that killed 46 Hawaiians and destroyed coastal settlements from Cape Kumukahi near Kapoho to Kalae (South Point). 

Map showing the districts and volcanoes of Hawaii Island (USGS)

The earthquake also dislodged part of a valley wall in the Wood Valley area of Kaʻū. 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. 

Kaʻū 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 Kaʻū 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 Kaʻū 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, Kaʻū 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 water (rain) 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 Kaʻū 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 Hāmākua District and in landfill areas that made up Kawaihae harbor occurred during the October 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.

One Comment leave one →
  1. disconnectedlandscapes permalink
    May 16, 2014 7:08 am

    Reblogged this on Disconnected Landscapes.

Leave a Reply

Fill in your details below or click an icon to log in:

Gravatar Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s


Get every new post delivered to your Inbox.

Join 237,990 other followers

Build a website with
%d bloggers like this: