Looking at the southern flank of the Kilauea volcano in Hawaii, you can spot a number of
towering cliffs that descend towards the ocean, with individual sections of cliff tops being
up to 500 meters or 1,640 feet higher than the landscape below. While quite stunning,
these mark the site of fault scarps caused by the slow movement of a wide swath of land
towards the ocean. Every year, a large swath of land moves an average of approximately
3 inches or 8 centimeters towards the shoreline. Known as the "Hilina Slump" this steady landslide
has led some people to speculate that this large swath of land will all of a sudden eventually
collapse into the ocean at once, generating a Pacific wide tsunami. So, is there any evidence
which suggests this will occur anytime soon? The quick answer to this question is no. Not
only is a massive landslide where the entire Hilina Slump block moves hundreds or even
thousands of feet currently deemed to be incredibly unlikely in the next decade, but such a megaslide
is also highly unlikely to occur for the next thousand or even ten thousand years if it
was to even ever eventually occur in the first place. In other words, the odds of it occurring
in the next 1 thousand years are pretty close to zero but not directly zero.
With this being said, it is true that large landslides appear to be a part of some island
shield volcanoes, including many in the Hawaiian Islands. For example, more than a dozen such
megaslides have occurred via collapses across the Hawaiian Island chain during the last
10 million years. One such megaslide occurred approximately 1.3 million years ago on the
northern section of the island of Oahu, where several thousand cubic kilometers of the Koolau
volcano slid into the ocean, which created the large and steep cliffs on Oahu's northern
half. So, why is a large and devastating megaslide
not also expected in the short term future due to a collapse of the entirety of the Hilina
Slump? The answer is surprisingly the Hilina Slump landslide block itself. In simple terms,
the Hilina Slump exists due to magma movement and dike propagation at Kilauea's summit and
its two rift zones, which slowly push the effected area towards the ocean. This is what
a cross section of the Hilina Slump looks like. It consists of volcanic rock which overlies
the older underlying oceanic crust. Since this landslide has been ongoing for so long
and due to the bend of the oceanic crust which tilts upwards away from Kilauea's summit when
going towards the ocean, it has caused a section of the slide to be pushed upwards aka uplifted,
forming a ridge. Per a direct quote from a U.S. Geological Survey webpage, "...this ridge
acts as a backstop, making it more difficult for the large Hilina slump to slide. This
is similar to how a bookend prevents books from sliding down a shelf."
However, this isn't to say that tsunamis generated by the Hilina slump are impossible. As in
the last 200 years, large earthquakes have generated at least 3 tsunamis due in part
to the movement of this slump. This occurred with varying wave heights in 1868, 1975, and
2018, where the 2018 event only produced a small tsunami. It is my opinion that an earthquake
of a magnitude 6.5 or higher along the Hilina Slump can generate a tsunami (although it
can also theoretically occur with lower magnitude quakes but this is much less likely than during
a higher magnitude quake) and magnitude 7.0 and higher quakes along the slump are likely
to generate a tsunami but not guaranteed to. To see an example of a larger tsunami generated
by an earthquake, I present details surrounding 1975's magnitude 7.2 earthquake. This event
caused a wide swath of Kilauea's southern flank to slide towards the ocean, moving as
much as 11 feet or 3.4 meters. This landslide subsequently displaced oceanwater, and as
a section of the landslide was underwater a part of it was also uplifted, generating
a tsunami with wave heights of up to 45 feet per the U.S. Geological Survey.
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