The Mogi Donut

Named after Japanese seismologist K. Mogi who suggested the idea in 1969, the Mogi Donut is a catchy name for an earthquake prediction scheme. Speaking punningly, the whole idea is based upon the current understanding of why earthquakes occur where they do.

Earthquakes are thought to occur where there are the greatest concentrations of strain in the earth's rocks. Strain is the elastic deformation of material that occurs when stress (pressure) is applied to it. If the strain is too great, the material must break or else flow in-elastically, that is, without rebounding to its original form when the stress is relieved.

The rock breakage that creates an earthquake relieves the strain where the fracturing occurs. However, the movement of the rocks near a breaking fault may place increased stress on rocks out away from the fault. In a sense, it is like tearing a piece of cloth. By pulling the cloth at an edge, one can apply enough stress there to break a single thread. But then the stress is shifted to the next thread and it breaks. One after the other, the threads break as the region of highest concentration of tensional stress moves across the cloth.

In Japan, Professor Mogi noticed that large earthquakes tended to occur in the center of a peculiar doughnut-shaped pattern that developed in the ten or so years preceding the earthquake. The pattern involved an abnormal seismic calm in the hole of the doughnut and more than usual activity in the outer ring. Professor Mogi suggested that this pattern meant that the central region would undergo high stress and therefore be likely to have a large earthquake.

A recent prediction of increased likelihood of a large earthquake in the Cape Yakataga region of southern Alaska is based partly upon Mogi's doughnut idea. A roughly circular region north of Cape Yakataga has experienced fewer earthquakes than the surrounding zone. But since the history of seismic recording in most of Alaska is so short, it is not known if the zone of quiet near Cape Yakataga is recently developed and therefore a Mogi Donut. Still, other reasons exist for predicting an earthquake in the area near Cape Yakataga. Only time will tell if the Mogi idea is useful for earthquake prediction in Alaska.