Chiba Prefecture released a revised disaster scenario predicting a massive offshore earthquake could cause approximately 57,200 deaths [1].
The update follows a 10-year gap in the prefecture’s seismic risk assessments [1]. These findings force a critical re-evaluation of evacuation routes, and coastal defenses for millions of residents living in the shadow of the Bōsō Peninsula.
The scenario focuses on a quake occurring east of the Bōsō Peninsula, sending large tsunamis toward the coast [1]. In some locations, waves are expected to exceed 10 meters [1].
According to journalist Masaaki Saito, the timing of the impact varies by city. A tsunami of 12.8 meters is projected to reach Isumi City 21 minutes after the earthquake [1]. Chōshi City is expected to see waves of 12.5 meters 43 minutes after the event [1].
"There is a risk that a large tsunami will surge along the entire coast of the outer Bōsō Peninsula, and it has been pointed out that it may exceed 10 meters in some places," Saito said [1].
The danger extends into Tokyo Bay. The report estimates that Urayasu City would be hit by a 1.7-meter tsunami approximately three hours after the initial quake [1].
"The number of deaths caused by this tsunami is estimated to be a maximum of approximately 57,200 people," Saito said [1].
The Chiba government updated the guidance to ensure preparedness measures align with current seismic data [1]. The revised projections highlight the vulnerability of coastal infrastructure, and the narrow window available for residents to reach higher ground.
“The number of deaths caused by this tsunami is estimated to be a maximum of approximately 57,200 people.”
This revision underscores the volatile nature of Japan's coastline and the inherent lag in government risk assessments. By updating a decade-old model, Chiba is acknowledging that previous safety margins may have been insufficient, potentially triggering new legislative mandates for sea walls and mandatory evacuation zones in high-risk cities like Isumi and Chōshi.
