Sagami-Musashi earthquake

The Sagami-Musashi earthquake (Japanese language: 相模・武蔵地震 さがみむさしじしん[*]) occurred on 28 October 878 in the Julian calendar (1 November in the Gregorian calendar, 2nd year of Gangyō), in the early Heian period. It is also called the Gangyō earthquake (Japanese language: 元慶地震)¹ or the Gangyō Kantō earthquake (Japanese language: 元慶の関東地震),² after the Japanese era name of the year in which it occurred. Its estimated magnitude is about M7.4,³ and studies such as those by Professor Matsuda estimate it at between M7.0 and 7.5 depending on the margin of error.⁴ It caused the greatest damage in the southern part of the present-day Kantō region.

Records

The classical historical text Nihon Sandai Jitsuroku, which records the period, describes it as follows.

On the night of the 29th, the day of Shinyū, an earthquake occurred. On this day, the land of the provinces of the Kantō region shook violently and split. Sagami and Musashi were the worst affected. For five or six days afterwards the tremors did not stop, and no public or private house remained intact. In some places the ground sank and cut off roads, and the number of commoners crushed to death was too great to record.

廿九日、辛酉、夜、地震す。この日、関東諸国の地、大いに震裂す。相模・武蔵は特にもっとも甚だしとす。その後、五六日も、震動いまだ止まず、公私の屋舎は一つとして全きものなし。或いは地の窪陥して、往還通せず、百姓の圧死すること勝げて記すべからず。⁵

On the same day there is also a record that an earthquake was felt in Heian-kyō. At the same time, a major earthquake struck the Kantō provinces (, , and ), with damage centred on Sagami Province and Musashi Province. The record states that continued for five or six days, many houses collapsed, landslides occurred, the ground subsided and roads were blocked, and many people were crushed to death under earth and debris.