August 22, 2002

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

The Threat of Tsunami

Big Island residents have long contended with the threat of tsunami. The central Pacific is, unfortunately, ground zero for many of the world's most destructive seismic sea waves. The edges of the Pacific consist primarily of subduction zones, regions where the oceanic crust is sliding relentlessly beneath either the continents or other oceanic plates. Subduction zones produce enormous earthquakes, and these earthquakes engender enormous tsunami. When subduction zone earthquakes will occur is unknown; prediction is, at this time, impossible.

If there is a bright side to these grim facts, it is that tsunami do not travel very fast -- about the speed of a jet airliner. Because the Hawaiian Islands lie many thousands of miles from any subduction zone, there is no reason that a subduction-generated tsunami should catch us by surprise. Indeed, there is an agency dedicated to ensuring that the deadly surprises of the past are never repeated: NOAA's Pacific Tsunami Warning Center. Because of this agency's efforts, the major threat that tsunami now pose to Hawai`i residents is primarily to their property.

But what about locally generated tsunami? Large earthquakes on Kilauea's south flank are known to produce moderate tsunami. In fact, the only fatalities from the M7.2 1975 Kalapana earthquake were two campers who drowned when a tsunami entered their campsite at Halape. The good news is that tsunami from south flank earthquakes primarily threaten only the relatively unpopulated southern coast of the Big Island; Hilo and other island cities are at relatively low risk, except for harbor facilities.

Earthquakes can cause tsunami by one of two mechanisms. The "normal" earthquake tsunami is created in response to large submarine ground movements. In a subduction zone earthquake, vast regions can suddenly move up or down by tens of meters (>30 feet). This earth motion moves a correspondingly vast amount of water, and a tsunami is born.

To create a tsunami large enough to threaten the entire Pacific Basin, a truly great earthquake (M8+) is required. The upper limit on Hawaiian earthquakes is considerably smaller than this, so very large, locally generated earthquake tsunami are quite unlikely.

Abnormally large tsunami can arise from relatively small earthquakes, however, if the shaking from the quake dislodges a submarine landslide. In this case, the real cause of the tsunami is the water displaced by the submarine slide rather than the earthquake itself. Tsunami of this type can be quite destructive; they tend to produce very large waves over a concentrated area. On July 17, 1998, a M7 earthquake struck the north coast of Papua New Guinea. Normally an earthquake of this magnitude would not produce a significant tsunami. Yet waves of 7 to 10 meters (20-30 feet) swept onshore, killing thousands. Recent research has confirmed that the cause of this tsunami was a submarine landslide.

Every few months, it seems, the media run sensationalized stories about the potential for "mega-tsunami" originating on volcanic islands that could devastate the rim of an entire ocean basin. Kilauea Volcano usually finds itself among the list of suspects. These stories are not entirely fanciful; there is evidence in the geologic record that extremely large landslides have occurred in the Hawaiian Islands. However, events of this size are extremely rare (less than 1 event per 100,000 years in Hawai`i), making the probability of a person witnessing such an event over the course of a lifetime vanishingly small.

Geologists do not fully understand how these extremely large landslides form. To predict the size and extent of the tsunami such landslides would create, scientists require a detailed understanding of the landslide process. Because this detailed understanding does not exist, discussion about the threat or possibility of mega-tsunami is highly speculative. It is possible that the extremely large landslides evident in the geologic past formed without generating a substantial tsunami.

The south flank of Kilauea has gained some notoriety as a possible source of mega-landslides (and mega-tsunami). This reputation is probably undeserved. The current geometry of the fault systems that bound the south flank makes sudden, massive failure almost impossible. Of course, this does not mean that the region is geologically quiet. Quite the contrary. If you feel the need to worry about impending geologic catastrophe, worry about large south flank earthquakes, not landslides. We will probably suffer through thousands of large earthquakes before the next mega-landslide.

Eruption Update

Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week. Molten rock is readily accessible from the end of the Chain of Craters road, and the National Park Service is allowing visitors to hike out and get up close to the active flows. Lava is presently entering the ocean intermittently from Wilipe`a northeast to Highcastle with most of the activity in the old road pullout area.

The eastern Boundary flow emanating from the "rootless" shields is still dark, but weekly geophysical measurements detect lava continuing to flow beneath the crusted surface.

Two earthquakes were reported felt during the week ending on August 22. Residents from Papa`ikou to Volcano felt the two earthquakes. The first earthquake (magnitude-3.6) occurred at 3:55 p.m., and the second (magnitude-3.4) was 8 minutes later at 4:03 p.m. on August 21. Both earthquakes were located 10 km (6 mi) northwest of Ka`ena Point beneath Poliokeawe pali at depths of 9 km (5.4 mi) and 7 km (4.2 mi), respectively.