Source model of the 1703 Genroku Kanto earthquake tsunami based on historical documents and numerical simulations: modeling of an offshore fault along the Sagami Trough
© The Author(s) 2017
Received: 31 May 2017
Accepted: 4 September 2017
Published: 2 October 2017
During the 1703 and 1923 events, crustal deformations along the coastline of the southern Boso Peninsula appeared as marine terraces or uplifted littoral bio-constructions (Matsuda et al. 1978; Shishikura 2014). Studies have proposed fault models for the 1703 earthquake using inversion analyses of the observed heights of the crustal deformations (e.g., Namegaya et al. 2011; Cabinet Office 2013; Sato et al. 2016). Although these inversion models were able to explain the observed crustal deformations on land, modeling offshore faults along the Sagami Trough is difficult because the offshore faults are located far away from the shoreline and do not significantly contribute to terrestrial crustal deformation; however, offshore faults can still trigger large tsunamis along the Pacific coast of the Boso Peninsula (Namegaya et al. 2011). Therefore, to model offshore faults, it is important to consider historical tsunami data. Recently, the Japanese Cabinet Office (2013) proposed a source model for the 1703 event based on an inversion analysis using the observed terrestrial crustal deformations in conjunction with historical tsunami data. However, limited data are available on the historical tsunami heights in the northern Boso Peninsula, and this lack of information could be significant in the effort to model the offshore faults (Fig. 1).
To model the offshore fault that generated the 1703 earthquake and the subsequent tsunami, we study tsunami heights by focusing on historical documents from Choshi City, which is located at the eastern edge of the Greater Tokyo area (Fig. 1). Based on new findings from these historical documents, we have updated the previously generated source model for the 1703 Genroku Kanto earthquake.
We conducted field surveys of the region affected by the 1703 earthquake and tsunami, namely Choshi City, Chiba Prefecture, Japan (Fig. 1). The city is located at the tip of the cape and constitutes the easternmost section of the Greater Tokyo Area. Two large tsunamis have struck in the vicinity of the area since the Edo era (i.e., 1603–1867) (Fig. 1). The first tsunami was generated by the 1677 Enpo Boso-oki earthquake and occurred along the Japan Trench, and the second tsunami was induced by the 1703 Genroku Kanto earthquake and occurred along the Sagami Trough (Hatori 2003; Tsuji et al. 2012; Yanagisawa et al. 2016). According to previous studies (e.g., Hatori 1976), a height of approximately 3–4 m was predicted for the Choshi City tsunami from the 1703 event, which would produce only minor damage. However, an investigation of several historical documents and newly unearthed descriptions of severe damage estimates in Choshi City, which indicate a greater degree of destruction than previously assessed, we find that relatively large tsunamis have struck the city over the past several 100 years.
Historical records of the 1703 Genroku Kanto earthquake tsunami
Validation criteria for the historical tsunami heights (before the 1960 Chilean earthquake tsunami) (Japan Society of Civil Engineers 2002)
Criteria for historical tsunami data
Validity is high. Both the tsunami run-up and its location are confirmed by a historical document, and the height has been measured in recent years
Validity is moderate. Both the tsunami run-up and its location are confirmed by a historical document, although the height has not been measured in recent years
Validity is low. The tsunami run-up is confirmed by a historical document or tradition, although its location is indicated only by a village name, and a detailed location is unclear
Validity is doubtful. The tsunami run-up is estimated by conjecture through the extent of damage and its associated phenomena
Historical descriptions of the 1703 tsunami and its effects
Place (current name)
Present-day height (m above T.P.)d
Flow depth (m)
Tsunami height (m above T.P.)
Five barns for fishing nets were destroyed by the tsunami flow
We measured the ground level near the 1703 location of the harbor of the village
The tsunami overran Kimiga-hama beach and reached Kobatake-ike pond. Seven hundred trees were destroyed by the tsunami flow. Sea algae were caught in a tree at the side of the pond at a height of 2–3 shaku (approximately 0.6–0.9 m) above the ground
A barnb and Nishinomiya shrinec were destroyed by the tsunami flow
We measured the ground level at the possible 1703 location of the destroyed shrine
Tsunami destroyed barns under the Ebisu shrine and reached the mountain pass
According to the Tokai shrine document, Ebisu shrine was located at the western edge of the village. Thus, we measured the ground level at the 1703 location of the mountain pass on the west side of the village
A house was destroyed by the tsunami flow
Determination of tsunami heights from the 1703 event
The locations that were inundated are Isejiga-ura village, Kasagami village, Nagasaki village, Tokawa village, Na’arai village and Kobatake-ike pond. Modern ground elevations (meters above Tokyo Peil, or T.P.) and the estimated tsunami flow depths for each location are presented in Table 2. These results show that the validity values for the estimated tsunami heights are mostly “C” because the exact locations of the damage sites are unclear. However, the Homan temple document mentions that “the tsunami overran Kimiga-hama beach, destroyed seven hundred trees, and reached Oo-ike (modern name: Kobatake-ike) pond,” where the modern ground level is T.P. 11 m. Thus, the exact site of Kobatake-ike pond was located, and we were able to determine the tsunami height at this location (validity value: A). According to Yanagisawa et al. (2016), the tsunami of the 1677 Enpo Boso-oki earthquake also reached this pond. The historical document states that “sea algae were caught in the branches of a tree on the side of the pond at a height of 2–3 shaku (approximately 0.6–0.9 m) above the ground,” which indicates that the tsunami carried sea algae at a flow depth between 0.6 and 0.9 m. Thus, the estimated flow depth at the site of Kobatake-ike pond was 0.6–0.9 m.
Tsunami source model
The aforementioned new discoveries of historical tsunami data are extremely important for reconsidering the source of the 1703 Genroku Kanto earthquake, particularly because the tsunami heights around Choshi City were not considered within earlier models. Our results are consequently highly anticipated for use in the modeling of offshore faults capable of triggering a large tsunami along the Pacific coast of the Boso Peninsula.
Models of the 1703 earthquake and tsunami. The parameters are as follows: d is focal depth, θ is strike, λ is slip angle, U is dislocation, L is fault length, and W is fault width
Number of faults
Parameters of the offshore fault
Shishikura model in Satake et al. (2008)
Namegaya et al. (2011)
Cabinet Office (2013)
Results of tsunami simulations using previously proposed models
Reevaluation of source models for an offshore fault along the Sagami Trough
To verify our reevaluated model, we compared the calculated results with the observed tsunami heights (Fig. 9b). Model 6 matches the conditions at Isejiga-ura (≥5.9 m), Nagasaki (≥7.7 m) and Na’arai (≥4.8 m) and approximately reproduced the tsunami heights at Kobatake-ike and Tokawa.
Why are the local tsunami heights in Choshi City greater?
We investigated the characteristics of the offshore fault that generated the 1703 Genroku Kanto earthquake and subsequent tsunami based on analyses of historical documents as well as numerical modeling. The historical data demonstrate that the tsunami reached heights in excess of 11 m in the vicinity of Choshi City, despite the geographical distance from the earthquake epicenter.
Furthermore, we conducted numerical simulations based on scenarios inferred from historical documents to represent the inundation area. Scenarios of the sources for the 1703 earthquake and the subsequent tsunami provide a 120-km fault length for an offshore fault along the Sagami Trough, which is close to the Japan Trench. Our results suggest that earthquake energy along nearly the entire Sagami Trough could have been released during the 1703 Genroku Kanto earthquake. Accordingly, a future study will be performed to provide a more detailed fault model with evidence spanning regions other than Choshi City.
HY conducted the simulation and field surveys and prepared the manuscript. KG conceived this study, managed the grant awarded by IRIDeS and conducted the field surveys. Both authors read and approved the final manuscript.
We especially thank Yoshifumi Takamori, Kaito Suzuki, Takashi Sato and Koyomi Imaizumi for the support they provided during the historical document studies and field surveys. Certain documentation was provided by the Choshi City Kosei library. The anonymous reviewers provide valuable comments on improving our manuscript.
The authors declare that they have no competing interests.
This study was financially supported by the International Research Institute of Disaster Science, Tohoku University (Tokutei Project) and the Environment Research and Technology Development Fund (2-1712) of Environmental Restoration and Conservation Agency.
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