Liquefaction map

The liquefaction map shows the liquefaction risk distribution of 3 earthquakes targeted for damage assumptions in the "Yokohama City Earthquake Damage Estimation Survey" published in October 2012.

• The PL value was used to determine the liquefaction risk. The PL value is a value indicating the risk of liquefaction at that point.
• Areas where liquefaction countermeasures have been implemented in advance due to ground improvement, such as the Minato Mirai 21 district of Nishi Ward, are excluded from liquefaction judgment.
• Since liquefaction is determined based on typical ground boring data around the 50-meter mesh, the mesh may contain ground where liquefaction is unlikely to occur.
• Even in areas where the risk of liquefaction is high, there are already places where liquefaction is improved and soil is used, and there are places where liquefaction countermeasures have been implemented in buildings, etc., but there are places to be built in the future For example, we recommend that you conduct a ground survey individually and consider countermeasures.

If you choose an assumed earthquake from the following, you can check the liquefaction map for each ward. (Please click on the ward you want to see at the link.)

• Liquefaction Map (Genroku era Kanto Earthquake)
• Liquefaction Map (Northern Tokyo Bay Earthquake)
• Liquefaction Map (Nankai Trough Giant Earthquake)

Q1. What does the "liquefaction phenomenon" mean?
Q2. I understand that the land I want to build a building is a ground that is easy to liquefaction. What should I do?
Q3. It is said that if the ground is bad, it is okay if it is a solid foundation. Is that true?
Q4. Is middle- and high-rise buildings such as condominiums OK on ground where liquefaction is easily liquefied, such as landfills?
Q5. What measures should be taken in detached houses?
Q6. I often hear that landfills are dangerous because the ground liquefaction is liquefaction during an earthquake. What measures are being taken in the landfills in Yokohama against earthquakes?

Liquefaction is a phenomenon in which the ground temporarily becomes liquid due to an earthquake. This is a phenomenon that occurs on sandy ground that contains a lot of water, such as landfills and estuary, and buildings on the ground are tilted or submerged.
This phenomenon was noticed by the 1964 Niigata Earthquake that caused damage such as the collapse of apartments and the collapse of a new bridge. Recently, the 1995 Hyogoken Nanbu Earthquake (Great Hanshin-Awaji Earthquake) caused damage to Port Island and Rokko Island, which attracted attention.
When a sand layer saturated with groundwater that maintains a certain balance of groundwater ([1]) is shaken by a strong earthquake, pressure is applied to the water in the gap between the sand and sand is detachable. And finally, the engagement of the sand particles is off, and the sand particles float in the water. (State in [2]) After that, the extruded water overflows with sand. (State of [3]) The phenomenon in which the ground becomes like a liquid is called a "liquefaction phenomenon."

[1] Pre-Earthquake]
The sand grains burst
Stable condition

[2] At the time of liquefaction]
The engagement of sand grains
State that has fallen into mud.

[3] After the earthquake]
Sand grains are re-assembled
The state where the ground subsides
(fountain, fountain)

When the ground liquefactions, the power to support the building is lost, or the building or the ground itself may subside unevenly, causing the following damage.

The following places are places where liquefaction is likely to occur. If you need to build a building on such ground, damage can be reduced by taking measures.

• Sandy ground with high groundwater level
• In the past, places where rivers, rice fields, swamps, coasts, etc. were reclaimed or created

For liquefaction countermeasures, ground improvement is an effective method. Based on past earthquake damage experience, there is a survey result that if there is a stratum that does not liquefaction up to a depth of about 2m from the ground. Therefore, ground improvement, such as tightening the ground to this depth or replacing it with soil that is hard to liquefaction, is said to be the most effective method of liquefaction countermeasures.

Making the shape of the house a simple shape (rectangle) that is resistant to earthquakes, or reinforcing the foundation of the building are also effective measures.

Strong resistance against earthquakes
(Simplified shapes and elevation)

Failure to earthquakes
(The shape of the plane is decoction.)

Yokohama City Earthquake Damage Estimation Survey Report
Earthquake map