When Will the Big One Hit? Hidden Tokyo Faults Pose Serious Threat

“After carefully analyzing the borehole survey data, we estimated that there are several hidden active faults in Tokyo. Of particular concern is the active fault believed to extend roughly 7 km from around JR Yotsuya Station, passing west of Iidabashi Station, and reaching near Nishi-Nippori Station—provisionally named the ‘Iidabashi Presumed Fault.’

According to the data, this fault has moved twice over the past approximately 90,000 years, with a cumulative vertical displacement of about 2.4–3.2 meters. Looking at a longer timescale of several hundred thousand years, the vertical displacement totals around 11–12 meters, suggesting that the fault has likely shifted about 7–9 times over that period.”

These remarks come from Isamu Toyokura, former vice president of the Japan Association for Active Fault Studies, who has long worked in geological consulting and surveys. He warns that the “Iidabashi Presumed Fault” is not marked on hazard maps published by the Ministry of Land, Infrastructure, Transport and Tourism.

Contents of an active fault investigation

What exactly is an active fault?

Earthquakes occur when underground rock layers shift, and a fault that has repeatedly moved in the past and is expected to move again in the future is called an active fault. In Japan, more than 2,000 active faults have been confirmed, though some have yet to be formally investigated. The Iidabashi presumed fault is considered one of these.

Methods for identifying active faults include: Boring surveys, which examine both visible surface shifts and unseen underground layer displacements, Seismic reflection surveys, which artificially shake the ground to observe underground cross-sections, and Trench surveys, which directly expose and study underground layers.

A team of experts from a geological consulting company, including Mr. Toyokura, analyzed existing data from the Tokyo metropolitan government and wards, as well as data from subway construction and building projects, and conducted field surveys. By combining these with boring survey data, they were able to detect vertical displacements in the strata.

“If the intervals between boring data points are long, there could be many reasons for layer shifts in between. But if the intervals are short, sudden shifts are strongly suspected to be caused by a fault,” explains Toyokura.

As a result of this research and analysis, it was revealed that the Iidabashi presumed fault, along with parallel faults running through Ichigaya and presumed faults through Kudan, form a cluster known as the Iidabashi presumed fault zone.

Possibilities Derived from Past Earthquakes

One of the reasons Mr. Toyokura considers the Iidabashi presumed fault zone problematic is the occurrence of major earthquakes caused by active faults in Japan in recent years.

“In the Noto Peninsula earthquake last January, it wasn’t a single fault that moved, but what we call a staggered series of faults shifted simultaneously. These faults have apparently moved repeatedly over intervals of several hundred to 2,000 years, but last January’s earthquake was unusually large, with some areas shifting by about five meters.

In the 2016 Kumamoto earthquake, the active faults moved twice: the foreshock at magnitude 6.5 and the main shock at magnitude 7.3. The main fault shifted about two meters laterally, while a branch fault toward Mashiki in the north moved only 20–30 centimeters. Yet many wooden houses near the faults were completely or partially destroyed.”

According to the Headquarters for Earthquake Research Promotion, the probability of a major earthquake directly under the Tokyo metropolitan area occurring within the next 30 years is 70%, with an estimated magnitude of 7.3, originating from a depth greater than 35 km underground.

“The probability of an earthquake along the Iidabashi presumed fault zone is considerably lower than that of a Tokyo metropolitan earthquake. However, unlike trench-type earthquakes caused by plate movements, active faults are difficult to predict based on periodicity and can only be confirmed after an event occurs. It is certain that the Iidabashi presumed fault zone has shifted up to 3.2 meters over the past approximately 90,000 years, and it has shifted multiple times before that. The estimated magnitude from this fault is around 6.5.

Furthermore, the Iidabashi presumed fault zone may appear at the surface near Hon-Komagome Station on the Namboku subway line. While the Tokyo metropolitan earthquake’s epicenter is estimated to be deeper than 35 km underground, the Iidabashi fault is much shallower, possibly less than 10 km deep. The shallower the epicenter, the stronger the shaking transmitted to the surface.”

A Tokyo metropolitan earthquake or Nankai Trough earthquake would likely devastate densely built wooden residential areas developed during Japan’s postwar economic growth, as well as complex lifelines such as electricity, water, and gas, along with transportation and communication infrastructure—causing unprecedented national-scale damage.

As mentioned earlier, the area around the Iidabashi presumed fault zone contains government offices, commercial facilities, and a dense subway network. Should this fault move, the resulting damage would be immediate and severe.

“Current hazard maps do not consider the Iidabashi presumed fault zone at all. Its probability of occurrence may be lower than that of major active faults in Japan, with a Z-rank (less than 0.1% probability within 30 years). However, even if the frequency is low, the consequences could be catastrophic. The risk posed by active faults directly under the capital must be properly investigated. A detailed survey could even provide reassurance if the probability is shown to be low.

I am not trying to provoke unnecessary fear. We must fear correctly—and for that, detailed research is essential.”

Living in earthquake-prone Japan means that risk can never be entirely avoided. The Great East Japan Earthquake demonstrated destruction beyond all prior assumptions. It is precisely because we do not want to regret being unprepared that we must understand and respect the risks accurately.