How to Confirm Ground Subsidence and Liquefaction Risks Before Signing—Understanding Geotechnical Maps and Using Them in Property Selection
When selecting rental properties in Japan, geotechnical risks are often overlooked. Liquefaction and ground subsidence can cause serious damage to buildings during earthquakes. This article explains how to research geotechnical maps, the characteristics of high-liquefaction-risk areas, and key points to verify when choosing a property.
When living in Japan, an earthquake-prone nation, the safety of the ground is just as important as a building's earthquake resistance. Liquefaction and ground subsidence can cause building tilt and differential settlement, which can have serious impacts on daily life. When selecting a rental home, it is important from a risk management perspective to verify not only earthquake resistance but also the ground conditions of the land in advance.
What is Liquefaction?
Liquefaction is a phenomenon in which seismic vibrations separate sand particles and water in the ground, causing soil to behave like a liquid. When liquefaction occurs, buildings can tilt, buried infrastructure (water and sewage pipes, gas lines) can heave upward, and water and sand can erupt from the ground surface.
Land prone to liquefaction has the following characteristics:
•Reclaimed land, polders, and land filled from rivers or seas
Former river channels (places where rivers once flowed)
•Alluvial lowlands (low areas formed by river deposits)
•Low areas behind sand dunes
Areas such as the Tokyo Bay waterfront, Osaka Bay waterfront, the Ise Bay coastal region near Nagoya, and the reclaimed areas in eastern Sendai are known for high liquefaction risk. During the 2011 Great East Japan Earthquake, large-scale liquefaction damage occurred in coastal areas including Urayasu, Chiba Prefecture.
How to Research Geotechnical Information
There are several tools and information sources for confirming geotechnical risks.
Ministry of Land, Infrastructure, Transport and Tourism Hazard Map Portal: The government-provided hazard map portal publishes liquefaction risk maps. Simply access the URL (https://disaportal.gsi.go.jp/), enter an address, and you can verify the liquefaction risk level for the target area. Risk is displayed in three levels: "Very High," "High," and "Low."
J-SHIS (Japan Seismic Hazard Information Station): Operated by the National Research Institute for Earth Science and Disaster Resilience, this site provides more detailed geotechnical information and predicted seismic motion intensity data. Building damage probability estimates are also available.
Municipal liquefaction maps: Many municipalities have created their own liquefaction hazard maps. These are often available through municipal disaster prevention offices or official websites, so check with the local government where the property is located.
Topographic classification maps and geotechnical maps: By referencing the topographic classification maps published by the Geospatial Information Authority of Japan, you can verify what the land was historically (wetlands, polders, reclaimed land, etc.). Past topography directly affects current geotechnical risk.
Key Points for Verifying Ground Conditions When Choosing a Property
Here are practical tips for verifying ground conditions when searching for a rental property.
Infer topography from the address: If the property address contains historical place name elements like "Shinden" (new rice field), "Kantakuchi" (polder), or "Umetate" (reclaimed land), the land may have been sea or rice paddies in the past. Also be cautious of areas where place names include elements such as "numa" (swamp), "hama" (beach), "su" (sandbank), or "tei" (low).
Verify building tilt during property viewings: During a viewing, try placing a marble on the floor to detect even slight tilts. If the building is noticeably tilted, differential settlement may have occurred. Crack patterns on floors and walls can also be signs of geotechnical problems.
Observe surrounding buildings: If neighboring buildings show noticeable tilt or cracks, the entire area may have high geotechnical risk. Leaning utility poles and wavy roads are also warning signs.
Confirm construction year and method: Buildings that use pile foundations tend to have higher resistance to ground subsidence and liquefaction. Large apartment complexes in particular undergo geotechnical surveys before construction and employ appropriate foundation methods. For older buildings, there may be less geotechnical survey data available, so check with the management company.
Liquefaction Risk and Rental Decision-Making
Whether to avoid properties in areas designated as high liquefaction risk is not straightforward. What matters is making a decision with full awareness of the risks.
In areas with high liquefaction risk, rent tends to be lower. It can be rational to accept the risk and enjoy cost savings. However, you should also consider potential repair costs if the building is damaged in an earthquake, as well as temporary relocation expenses.
When obtaining fire insurance and contents insurance, consider adding earthquake coverage (earthquake insurance). Building damage from liquefaction is not covered by fire insurance and falls under earthquake insurance. For rental housing, adding earthquake insurance is especially important to protect your personal belongings.
Considering geotechnical risk while balancing location, rent, and convenience is fundamental to choosing a home where you can live safely and comfortably long-term. I recommend making hazard map research a routine part of your property search process.
A comprehensive guide to disaster preparedness measures you can implement even in rental homes. Covering how to read hazard maps, confirm evacuation routes, prepare emergency evacuation kits, and understand furniture anchoring options—a practical disaster prevention guide for foreign residents and all renters.
