The Sagami Trough megathrust earthquake is a large earthquake that is considered to occur at roughly 200-year intervals in the Kantō region of Japan, caused by slip along the Sagami Trough. Known earthquakes that are definitively associated with the Sagami Trough include the 1703 Genroku earthquake and the 1923 Great Kantō earthquake. However, the recurrence interval of such earthquakes, as well as the relationship between earlier events and the Kantō earthquake, has not yet been fully clarified. Sagami Bay in Minami Kantō lies in a region where the Philippine Sea Plate and the North American Plate meet along the Sagami Trough. It is estimated that megathrust earthquakes occur repeatedly in this region at intervals of several hundred years. While it remains unclear which earthquakes occurred in prehistoric times, historical records document the 1703 Genroku earthquake, with an estimated magnitude of M8.1–8.5, and the 1923 Great Kantō earthquake, with an estimated magnitude of M7.9–8.3. In addition, the 1855 Ansei Edo earthquake occurred between the Genroku earthquake and the Great Kantō earthquake. However, the exact source fault of this earthquake has not been clearly identified, and it is not regarded as a Sagami Trough earthquake. In 1980, Professor Otake proposed that the source region of the Ansei-era earthquakes was located on the inland side north of the Sagami Trough rupture area of the 1923 Great Kantō earthquake, and suggested that the 1894 Meiji Tokyo earthquake was a deep-focus earthquake. Based on the recurrence interval between the 1703 Genroku earthquake and the 1923 Great Kantō earthquake, as well as analyses of the estimated fault slip during the Great Kantō earthquake, it has been inferred that the degree of interplate coupling in the western half of the Sagami Trough is close to 100%. It is therefore considered that most of the strain accumulated between the plates is released through seismic events. Meanwhile, the plate boundary between the Philippine Sea Plate and Honshu is considered to be located in the vicinity of Zenisu Ridge. From a geological timescale perspective, and considering the Tokai earthquake expected to occur along the Suruga Trough, some researchers have argued that the 1923 Great Kantō earthquake may also have been an intraplate earthquake.
Probability of occurrence The Earthquake Research Committee of Japan estimated in its 2004 assessment that an "M8-class earthquake along the Sagami Trough" would be represented by two events, the 1703 and 1923 earthquakes, when calculating recurrence probabilities. However, in its 2014 publication, *Long-term Evaluation of Seismic Activity along the Sagami Trough (Second Edition)* (相模トラフ沿いの地震活動の長期評価（第二版）), the committee expanded the dataset to include three earthquakes—1293, 1703, and 1923—as well as a fourth event in 1495, which remains debated as either a Kantō earthquake or otherwise, when estimating probabilities of M8-class earthquakes along the Sagami Trough. Assuming a Brownian Passage Time (BPT) model, the probability of an earthquake occurring within 30 years was calculated based on these datasets. In addition, studies by Fujiwara et al. (1997, 1999) suggest that the recurrence interval of Genroku-type Kantō earthquakes inferred from tsunami deposits and Holocene uplift records on marine terraces along the southern Bōsō Peninsula is approximately 2,300 years, based on inferred events around 5200 BCE, 3000 BCE, 1000 BCE, and 1703 CE. Furthermore, the maximum scenario earthquake assumed for complete rupture of the Sagami Trough megathrust has been modeled as Mw 8.6 (or Mw 8.7 for tsunami fault models).
Since 2015, the Earthquake Research Committee of Japan has stated that distinguishing between Genroku-type and Taishō-type Kantō earthquakes has limited practical significance. Because the rupture behavior varies, these events are now collectively evaluated as "M8-class earthquakes along the Sagami Trough". Under this unified model, the estimated probabilities of occurrence are approximately 0–2% within 10 years, 0–6% within 30 years, and 0–10% within 50 years. For Genroku-type Kantō earthquakes occurring offshore south of the Bōsō Peninsula—associated with the formation of Holocene marine terraces—the average recurrence interval is estimated at about 2,300 years, resulting in a near-0% probability of occurrence.
Earthquakes considered to be Sagami Trough megathrust earthquakes Historical records in the Minami Kantō region are extremely limited prior to 1590 (Tenshō 18), marked by the fall of the Later Hōjō clan and the entry of Tokugawa Ieyasu into Edo. Except for the Kamakura period, almost no continuous historical documentation survives, making it difficult to confirm the occurrence of past Kantō earthquakes through historical records alone. However, major earthquakes recorded in 878 (the Sagami–Musashi earthquake), 1293 (the Kamakura earthquake), and 1433 (Eishō Sagami earthquake) are considered to have likely originated from the Sagami Trough. It is also suggested that additional unidentified megathrust earthquakes may have occurred. The 1257 Shōka Kamakura earthquake is also hypothesized to have occurred along the Sagami Trough. The following list of earthquakes is presented in the Julian calendar and refers to events interpreted as Sagami Trough earthquakes.
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