Toward Homogeneous Estimation of Long-Term Seismicity from Historical Materials: Number of Felt Earthquakes in Tokyo since 1668

Abstract

Abstract Long-term seismicity in a certain region can be estimated from the number of felt earthquakes obtained from seismic intensity measurements or historical documents. To obtain a homogeneous estimate, continuous records with a uniform detection threshold are required. Seismic intensity data in Tokyo, which are measured by professional observers and archived by the Japan Meteorological Agency (JMA) since 1885, indicate that the mean annual number of felt earthquakes with intensities ≥1 and ≥2 on the JMA scale is 45 and 14, respectively. The database of felt earthquakes can be extended back to more than 200 yr using historical daily records. During the Edo period from 1603 to 1867, numerous historical documents in Edo (former Tokyo) recorded information about damaging earthquakes, such as the 1703 Kanto earthquake and the 1855 Edo earthquake. Several feudal clans documented daily weather and felt earthquakes in Edo. Among them, daily records documented by the Tsugaru and Sakakibara clans cover a period of ∼200 yr. Although these records do not cover the entire period and weather reports for several years are missing, the Tsugaru and Sakakibara clans documented ∼1600 and ∼700 earthquakes, respectively. Using these data, we determined that a total of ∼2000 felt earthquakes occurred between 1668 and 1867, thereby yielding a mean rate of ∼10 earthquakes per year. When we exclude the missing period of daily records, the average rate increased to 11 per year, which is similar to the annual number of earthquakes with seismic intensities ≥2 in the modern JMA record. These data of felt earthquakes in Tokyo are considered to be more homogeneous than the previous estimates, although the total number is smaller, and can be used to study the long-term seismicity rate change.