Pacific Ocean quake stumps scientists.

The huge earthquake that struck near the sparsely populated Kuril Islands northeast of Japan on Oct. 4 has geoscientists shaking their heads.

Some data gathered during and shortly after the magnitude 8.2 event suggest that the rock ruptured at the boundary where the Pacific plate sinks below another continental plate. Other evidence hints that it took place within the Pacific plate.

If the quake occurred on the boundary, it delivers another blow to the seismic gap theory, says Larry J. Ruff of the University of Michigan in Ann Arbor. This theory holds that large quakes strike the segments along plate boundaries every 100 to 200 years or so; one should not have hit the site of the Oct. 4 event for many decades. Recent earthquakes elsewhere have challenged the gap theory, Ruff notes (SN: 2/29/92, p. 136).

The quake's size suggests that it was a plate boundary, or underthrusting, quake, seismologists say. Tremors within a plate rarely reach that size, adds Hiroo Kanamori of the California Institute of Technology in Pasadena. The tsunami waves resulting from the quake had the character and height of a plate boundary event, researchers agree.

However, the tremor occurred 40 to 50 kilometers below sea level and lasted only 50 seconds, making it deeper and quicker than other underthrusting quakes, Kanamori notes. Also, the Pacific plate slipped laterally during the event, which is not characteristic of plate boundary quakes.

Researchers disagree on how much the quake's energetic aftershocks resemble those of a plate boundary event.

All this preliminary evidence suggests to Kanamori that the quake wasn't "really on the boundary." Other scientists want to await more data before casting a vote.