The Big Wave at the Cave

February 23, 2013

We always learn a lot from visits to the cave by experts of all sorts, but a couple of days ago we had a special treat: two top experts on extreme tsunami events, Dr. Gerard Fryer, Senior Geophysicist at the Pacific Tsunami Warning Center in Honolulu, and Dr. Rhett Butler, a scientist and consultant who models the seismic dynamics of large earthquakes and other causes of tsunami waves.

Of course, they were curious, as was Dr. Walt Dudley of the Pacific Tsunami Museum in Hilo when he visited several years past, to see the remarkable evidence from Makauwahi Cave for a huge marine overwash of the site about four centuries ago. In a 2001 publication in Ecological Monographs, one of the most intensely peer-reviewed publications in the field, we reported a 95% confidence interval for a date of between 1430 and 1665 AD for SOMETHING that swept over this site, laying down large stones of several types in “a lens up to 1 meter thick at the lowest point of the sinkhole rim along the east wall, thinning out in the far reaches of the caves as turbidite fans and gravel beds.” (A turbidite fan is a debris field that can form around the edges of a powerful underwater event.)

In one of the most famous of all Japanese woodblock prints, Hokusai depicted ca. 1830 a huge wave engulfing three fishing vessels, with Mt. Fuji in the background.

After we pumped down the Northwest Pit, the tsunami experts donned the requisite helmets, and
carefully followed me down the wet, muddy extension ladder into the “poor man’s time machine” I write about so fondly in Back to the Future in the Caves of Kaua`i. They gasped at what they saw, as have so many others on first seeing this remarkable hodgepodge of basalt, lithified red soil, beach calcarenite, and blasted-apart pieces of the cave walls themselves, even stalactites. Tucked in the crevices between, in a matrix of sand, gravel, clay, and organic matter, are bones of large fish, wooden and stone artifacts, and big splintered pieces of tree trunks.

In our publications we have suggested that all that mess couldn’t have gotten through the little entrance to the North Cave (that so many thousands of people have crawled through since we started working there 21 years ago). Some stones in the deposit are huge angular chunks to 200 lb. or more. Big dark boulders of volcanic basalt like the ones on the beach just across the stream from there are so dense that a great force would be required to move them at all, yet they are scattered about all through the deposit, particularly at the foot of the east wall. After seeing the sinkhole and its deposit, Dr. Butler, who had theorized in emails before visiting that maybe it all came in through the North Cave entrance, lowering the wave height necessary to move the material up from the beach, now concurs that it had to come straight over the wall, and he had already started modeling what would be required in terms of a tsunami that could get over our 25-foot wall on the low side of the sinkhole. Long story short, he thinks it would have required a 9+ Richter Scale earthquake in the Aleutian Islands to generate the necessary force, as it wrapped around Kaua`i and met on the opposite shore at Maha`ulepu.

Dr. Fryer points out that this is not merely an academic exercise. Both were on Kaua`i for regular Civil Defense meetings in connection with tsunami preparedness. It really makes a difference — they both pointed out at several junctures in our conversation as we took a “tsunami’s eye” view of the caves and Reserve — if Civil Defense models have underestimated the tsunami risk in low-lying areas of the State. Perhaps, though, whatever happened to make these remarkable deposits in the cave was an extremely rare event, or, most intriguingly, something other than a tectonically-driven ocean-wide tsunami. Deposits that Dr. Fryer has been studying elsewhere in the islands point to the reality of truly giant waves, a proper megatsunami, in the more distant past, throwing up coral and coastal rocks to places roughly 1000 feet up in the hills of Molokai and other islands. These kinds of very rare events seem to occur in Hawaii on scales of not millennia but perhaps hundreds of millennia. They are thought to be caused by collapsing parts of the shelf around the various islands as they succumb under their own weight and shift to lower levels and shallower angles. This kind of below-ocean movement can cause a tsunami-like ocean bulge, but it usually is too narrow in terms of wavelength to persist over very long distances at the great initial heights, perhaps hundreds of feet. Within the islands, however, such a movement could make one heck of a slosh on the adjacent shores and the rest of the archipelago. We are talking about some additional dating of the deposit, which could help us figure out what  happened and from where. The cave deposit contains a lot of fresh-looking broken coral, probably raked off the adjacent reef by the breaking wave or waves. I have been saving it for years in hopes to locate the funding eventually to date them via the super-precise Uranium Series method, something I have used often in the past to date cave formations but that works even better on freshly buried coral.

With near-decadal precision, we can compare the deposit to other dated events in the same interval that have been documented from New Zealand, Japan, and other areas. One intriguing possibility that several astrophysicists suggested to me when I presented this evidence at a symposium on catastrophes in London shortly after 9/11, is that of a large meteorite striking the ocean near Kaua`i. In that scenario, a lot of water could be thrown up on the adjacent shore, but unless it was very, very large, it wouldn’t necessarily have traveled very far.

Creepy stuff. No wonder Queen Emma put her cottage at Lawai-Kai up on the cliff above the beach, instead of down by the Allerton House where part of it resides today, in the heart of the Tsunami Inundation Zone! Those old Hawaiians had stories about this kind of thing – including one about a day that the angry gods of the sea knocked the chief off a coastal heiau (stone temple). The beach is for fishing, surfing, hula, and luau’s, not sleeping on, says the wisdom of the ages in the stories of old Hawai`i. This ancient wisdom is backed by the mute testimony of the mysterious stone layer down in the sediments of Makauwahi Cave.

By David Burney