The Hanai's diary at Wakuya, the north part of Miyagi Prefecture, Tohoku District, Japan, is the records on natural phenomena from November 30, 1833 to January 1, 1848. The contents of diary are mainly the weather, moreover, the diary records eighty earthquakes. From annual number of earthquakes, there are two periods of high seismic activity of a few years from 1835 and 1841. Two disaster earthquakes with the seismic intensity V-VI occurred in July 20, 1835 and January 19, 1841. From the intensity distribution, the epicenter of the 1835 earthquake is deduced to be near that of the 1978 Miyagiken-oki earthquake. The disaster and liquefaction areas of ground produced by the former earthquake were larger than those of the latter one. The magnitude of former earthquake was thus presumed to be 7.5. The 1841 earthquake produced the disaster only at Wakuya and had no aftershocks. From these facts and the seismic intensity distribution, the earthquake is thought to occur off Miyagi Prefecture. We classify the earthquake into so called ‘Kinkasan-oki type’ and estimate the magnitude at 7.1. This magnitude is the same as that of the 1937 earthquake.
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