You will not have heard of this earthquake because it is one of many that go unreported worldwide. This is because there was very little damage and nobody was killed.
A newspaper article appeared in Japan on Friday 4th April 1997. Some of the details are extracted.
Details
April 3rd 1997
2 earthquakes
4.33 a.m. and 6.47 a.m.
5.5 and 5.1 on the Richter scale
Upper 5 and Upper 4 on the Japanese scale ( upper limit 7 )