Major Japanese jolt may be on its way.

A minor earthquake that struck southwest of Tokyo in 1990 may herald the arrival of a much stronger shock in the near future, report two Japanese seismologists.

Earthquake experts in Japan have long recognized the potential for a major quake emanating from the Odawara region, located 80 kilometers southwest of Tokyo. Over the last 400 years, five very strong earthquakes have rocked this area with remarkable regularity, roughly every 73 years. The last major jolt, the Great Kanto Earthquake of 1923, measured magnitude 7.9 on the Richter scale and destroyed much of nearby Tokyo and Yokohama. Simple arithmetic would suggest the next quake is due sometime around the year 1996.

Mizuho Ishida and Masayuki Kikuchi think the Earth may now be sending warnings of that impending disaster. While studying a magnitude 5.1 quake that shook the Odawara area in August 1990, they found this tremor exhibited several unusual signs previously associated with so-called preshocks.

Most noticeably, the 1990 jolt occurred in a seismically quiescent area-one that had passed 57 years free of earthquakes larger than magnitude 4.6.

Taking a closer look, Ishida and Kikuchi observed that the 1990 shock packed an unusually concentrated punch. Calculations suggest that the Odawara quake relieved much more stress than an average earthquake of similar magnitude. What's more, the 1990 shock began and finished in about one-third the average time, the researchers say, Ishida, who works at the National Research Institute for Earth Science and Disaster Prevention in Tsukuba, and Kikuchi, a researcher at Yokohama City University, discuss their data in the Aug. 21 GEOPHYSICAL RESEARCH LETTERS.

In the late 1970s, Ishida recognized similar unusual characteristics in small shocks that preceded California's 1971 San Fernando earthquake and the 1952 Kern County earthquake. In these cases, small power-packed quakes occurred in seismically quiet regions a few years prior to the large shocks. Because of the similarity in circumstances, she and Kikuchi suggest that the 1990 earthquake may foreshadow the larger Odawara quake, expected to measure magnitude 7 or stronger.

Thomas H. Heaton, a seismologist at the U.S. Geological Survey in Pasadena, Calif., cautions that the case may not be so clear. Geophysicists have not yet demonstrated that they can distinguish preshocks from ordinary earthquakes on the basis of the amount of stress they relieve, he says.

- R. Monastersky