Subject: Oahu earthquakes

I am interested in ongoing research on fault locations and earthquake predictions on Oahu.

   Please forgive this wordy response, but this is a matter of considerable controversy, and a long answer is necessary.
    In terms of actually laying out seismometers and locating local earthquakes, there is no earthquake research currently going on on Oahu - the amount of seismic activity is too small. You may, however, have read recently in Honolulu Magazine about a feature called the Diamond Head Fault and about the possibility that there might be an earthquake on that feature within the next thirty years. That is the work of Augustine Furumoto, a faculty member in the Hawaii Institute of Geophysics and Planetology, who has been looking at the statistics of earthquakes in the Hawaiian Islands.
    Both the existence of a Diamond Head Fault and the prediction of some future event on it are very controversial. The fault was hypothesized on the basis of earthquakes located during a series of seismic surveys in 1976 and 1977 that were carried out using ocean-bottom seismometers. In a PhD thesis resulting from those surveys, Robert Estill showed a map of epicenters of earthquakes that had been detected during the surveys (all the earthquakes were tiny - all but one were too small to have been felt on Oahu). The map seemed to show a lineation passing through Diamond Head and extending about a hundred miles out into the ocean to the east northeast. That is the hypothesized Diamond Head Fault. In 1948 there was a magnitude 4.8 earthquake offshore from Honolulu which might have been on that fault, and there have been many other earthquakes, all very small (magnitude 3 or less) in the last hundred years that might have occurred on that fault.
    There are, however, some major problems. Robert Estill says that to infer a fault from his map is over-interpretation, since his locations were very inaccurate and many alternate lineations could be drawn though his points. He was convinced that what he was seeing was not earthquakes on a fault at all but a series of submarine slumps. We know that slumps and landslides, at all scales, occur throughout the Hawaiian Islands, especially eastern Oahu and northern Molokai - precisely where the hypothesized fault runs.
    When Furumoto first proposed the Diamond Head Fault in 1980, it seemed to make sense as it was approximately parallel to the Molokai Fracture Zone. The Molokai Fracture Zone is a seafloor feature that was formed when the seafloor was created, some seventy million years before the Hawaiian Islands were built. It is important to understand that there should be NO earthquakes along a fracture zone, since the ocean crust on either side is in welded contact and both sides of the fracture zone are carried along by seafloor spreading at exactly the same speed (you have to have differential motion to have an earthquake). With the load of the Hawaiian Islands, however, the fracture zone might begin to show earthquakes. After all, the islands are a very heavy load, and the fracture zone is a zone of weakness. Indeed, two of our largest earthquakes, the Lanai earthquake of 1871 and the Maui earthquake of 1938 (both about magnitude 7), may have occurred within the Molokai Fracture Zone. Recent sidescan sonar surveys carried out by the US Geological Survey, however, show us that the complex skein of small ridges and escarpments that make up the Molokai Fracture Zone all cross the islands well south of Oahu. Further, the sidescan images show no hint of any Diamond Head Fault.
    There is another problem: the identification of the Diamond Head Fault depends very critically on the location of 1948 earthquake, but we don't really know where that earthquake occurred. The Seismological Notes of the Seismological Society of America put the earthquake as "near Molokai," though admittedly earthquake locations were very inaccurate in the forties. Until 1980, when Furumoto argued that the earthquake had to be much closer to Honolulu to explain the felt motion in Honolulu and the lack of much damage on Molokai, we had all assumed that the earthquake was beneath the Kalohi Channel, between Molokai and Lanai (where, incidently, there was an earthquake of about magnitude 5 in 1981). In retrospect, the felt motion could indeed be explained by an earthquake beneath the Kalohi Channel, as long as that earthquake was deep. The intensities of surface motion from deep earthquakes can be very odd. In 1972 the Honomu Earthquake occurred beneath the Hamakua Coast of the Big Island. That was a very deep earthquake (50 kilometers) which was exceptionally well felt in Honolulu and, on the UH Manoa campus at least, gave more intense shaking than the 1948 event. Strangely, in parts of Maui and Molokai the Honomu Earthquake did not cause as severe motion as it did in Honolulu. We are left with uncertainty as to where the 1948 earthquake really was - all we can say for sure is that it was somewhere between Oahu and Lanai.
    Whether or not you believe in the Diamond Head Fault, the fact remains that Oahu does face a seismic risk (the 1948 earthquake caused cracks and other minor damage in many Honolulu buildings, while the 1871 event near Lanai damaged every building on the Punahou campus). These earthquakes are almost certainly vertical adjustments of the oceanic lithosphere to the changing burden of the islands, first as they grow and later as they are eroded (Oahu is currently being carried over an arch in the lithosphere caused by the weight of the Big Island; that motion might well result in small earthquakes too). The general feeling is that the historical record of earthquakes near Oahu is too short to assemble any sensible statistics about how often earthquakes might occur. Furumoto's forecast that we have even odds of an earthquake within the next thirty years therefore is necessarily very vague - if the next damaging earthquake doesn't actually occur until 100 years from now that would still agree with Furumoto's prediction.
    The important point to recognize here is that while the earthquake risk on Oahu is apparently small, it is not negligible. That is why in the late eighties Furumoto, together with seismologists Eduard Berg, Fred Duennebier, and Doak Cox, engineer Arthur Chiu, and architect Elmer Botsai, successfully argued that the building code for Honolulu should be upgraded from seismic zone 1 to zone 2A. The question now is whether the new code is strict enough. Only time will tell.

Gerard Fryer
Hawaii Institute of Geophysics and Planetology
University of Hawaii at Manoa

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