The spatio-temporal variation of seismicity and spectral characteristics of seismic waves are examined for earthquakes occurring offshore of Fukushima Prefecture, of Ibaraki Prefecture and around the Boso Peninsula, eastern Japan, on the basis of JMA earthquake data. From space-time plots of earthquakes with M≥5 and depth≤60km for the period from 1926 to May 1982, it is recognized that before the occurrence of the 1938 Fukushima-Oki large earthquake swarm, seismic activity became gradually high in the south of the 1938 focal area. A noticeable activity occurred in 1935 at the landward margin of the focal area of the 1938 shocks, then considerable quiescence continued till the 1938 shocks. In recent about 20 years, seismicity is admittedly low in all regions investigated. To deduce the characteristics of seismic source spectra during this inactive period, a magnitude M_I based op seismic intensity data is determined for a number of felt earthquakes of M≥5. The difference between M_I and M reported by JMA is used as a measure of the efficiency of the radiation of highfrequency component from the source. The temporal distribution of M_I-M value shows a tendency of increase of the value with time in all the regions. For earthquakes off Fukushima Prefecture, M_I-M has increased systematically since about 1965. In the region off Ibaraki Prefecture, most earthquakes occurring about 10 years prior to July 23, 1982, Ibaraki-Oki earthquake of M=7 reveal considerable large M_I-M. For earthquakes around the Boso Peninsula, M_I-M increase gradually during the last 30 years. These large M_I-M are mainly distributed landward of the focal area of the 1982 Ibaraki-Oki shock and in the northern part off Fukushima Prefecture, while landward and southern parts off Fukushima Prefecture have been seismically quiet of M≥5 shocks since about 1965. It is considered that the predominance of high-frequency earthquakes in recent years is due to the extensive increase of the tectonic stress in the regions investigated. The 1982 Ibaraki-Oki earthquake of M=7 alone does not seem to have released this large tectonic stress, and much attention should be paied to presently inactive seismic regions adjacent to off Ibaraki Prefecture.