Earthquake Forecasting and the Mogi Doughnut
Deep below the surface of the Yukon valley, the earth's crust is being compressed by the inexorable northward motion of the Pacific Ocean floor. The magnitude 6.8 Rampart earthquake of 1968 relieved some of the stresses, and for several years after that quake, small aftershocks continued to accommodate the slow deformation. Then, for some reason, the motion on a particular bit of the fault stopped. Small earthquakes continued, but they moved farther and farther along the fault away from this locked region. In 1985, almost four years after motion stopped, the fault broke again in the Dall City earthquakes of February (magnitude 5.6) and March (magnitude 6.0).
Not all faults in the earth's crust behave this way, but those that do appear to be fairly consistent about it. Dr. U. Inouye, a Japanese seismologist, first reported the pattern of small earthquakes disappearing before a large one in 1965. Another Japanese seismologist, Professor Kiyoo Mogi, pointed out in 1969 that there was a distinct pattern to the lack of earthquake activity -- while activity decreased at the site of the future earthquake, it increased in a doughnut-shaped area surrounding that site.
These early observations were made above sloping faults, where the Pacific plate plunged under Japan at about a 45 degree angle. A doughnut pattern on the sloping fault would still appear as a doughnut -- if somewhat distorted -- on a surface map.
Faults may also be near-vertical planes in the earth's crust. If a small earthquake in this case were plotted on a map, it would be like looking at a doughnut edge-on -- you wouldn't see the hole. If the vertical fault is shallow, as at Rampart, the full depth of the fault might become stuck. A map would then resemble a slice out of the middle of the doughnut, with small earthquakes on both sides of the eventual larger quake and a gap in the middle.
In cases like Rampart, the doughnut seems to expand with time, giving a second kind of doughnut. Suppose you take a sheet of paper and plot small earthquakes on it, positioning each quake on the paper so that its distance from the right side of the paper is proportional to how long before the big earthquake the small quake occurred, and its distance from the bottom edge is proportional to distance along the fault. The result in a Mogi doughnut situation will be a skinny half-doughnut of small earthquakes, with the big earthquake in the middle of the hole, on the right-hand edge of the paper. (If you prefer, the lack of earthquakes can be a rather plump doughnut, and the large quake, the hole.)
Thus if a particular fault shows this pattern, the place of a future earthquake can be predicted. Furthermore, the timing of the disappearance of small earthquakes is fairly consistent for the particular fault and related to the magnitude of the eventual large quake, so the time of the larger quake can also be forecast.
For a really big earthquake, magnitude 8 or greater, the quiet period might be several decades, and it will take a lot of waiting to determine which faults with large earthquakes show the Mogi doughnut pattern. For faults with smaller earthquakes and shorter quiet periods, a denser seismic network is needed, as smaller foreshocks must be recorded and located. Dr. Niren Biswas at the Geophysical Institute, University of Alaska Fairbanks, has used the earthquake locations derived from the interior Alaska seismographic network to demonstrate that the Mogi doughnut pattern occurred in the Rampart seismic zone prior to the Dall City quakes. The next quake in that area should be predictable -- not to the day, of course, but within a year.
The main limitation on this kind of work is the accuracy with which small earthquakes can be located. The Alaska seismographic network is a good start, but more stations are needed to locate earthquakes farther north along the Rampart fault. With an expanded network now being planned, scientists at the Geophysical Institute will be in a better position to eavesdrop on small earthquakes and identify future Mogi doughnuts not only near Rampart, but on other Alaskan faults as well.
