Liquefaction: why does
solid ground turn to "water"?
In news footage after a big earthquake, you may have seen water and sand gushing out of the ground while buildings tilt and sink. Ground that should be solid starts behaving like water. This is called "liquefaction."
A neighbourhood near a river, or on reclaimed land. A big earthquake hits, the shaking continues for a while, then everything goes quiet. On the surface, the ground looks completely normal.
But then, greyish water and sand start bubbling up from cracks in the road and from gardens, like little springs. As time passes, houses tilt slightly, and you notice manholes now sticking dozens of centimetres above the road surface.
What's happening across this area is liquefaction — ground that temporarily takes on "water-like properties."
There are only two reasons liquefaction happens
Sandy ground can hold up a building because the grains touch each other directly and push against one another. That pushing force is exactly what makes the ground strong.
Strong shaking causes the water pressure between the grains to suddenly rise, and the grains lose their pushing force. Having lost that force, they end up floating loosely in the water.
Let's look at each one in turn.
Reason 1: ground strength comes from grains "pushing" on each other
Picture the sand on a beach. You can stand firmly on dry sand. That's because the grains touch each other directly and push against one another, supporting whatever weight is placed on top.
But in sand that holds water, water always fills the gaps between the grains. Normally, this water pressure (called pore water pressure) isn't very high, and the grains stay firmly in contact. The force with which the grains actually push against each other is technically called "effective stress." This effective stress is the ground's strength, plain and simple.
Liquefaction doesn't happen everywhere. It's thought to be especially likely on reclaimed land, former riverbeds (places a river used to flow), sandy ground, and places with a high water table (where groundwater sits close to the surface). Local authorities publish "liquefaction hazard maps" you can check in advance for your area.
Reason 2: shaking makes water pressure spike
Loosely packed sand grains naturally "want" to settle into a tighter arrangement. When strong earthquake shaking is repeatedly applied, the grains try to pack together more tightly, pushing the water out of the gaps between them.
But it takes time for water to escape through those gaps. If the shaking is too fast, the water has nowhere to go, and gets trapped in the gaps between grains while its pressure spikes. Once this pressure overcomes the force holding the grains together, they stop touching each other, and the whole mass of ground becomes almost liquid — like sand floating in water. This is liquefaction.
On liquefied ground, heavy buildings and tanks sink, while lighter buried structures (like manholes and septic tanks) can float upward because they're lighter than the surrounding sand. Water under high pressure can also burst up through weak spots at the surface, carrying sand with it — a phenomenon called "sand boiling."
So what should you do?
- Check in advance whether where you live is prone to liquefactionYou can check your local authority's liquefaction hazard map.
- When an earthquake hits, protect yourself first, then move once the shaking stopsTake a position that protects your head, and once the shaking has stopped, calmly move to a safe place.
- Don't force your way across ground that may have liquefiedIt may be soft or have sunk in places. Check a safe route before moving.
If you notice water or sand gushing from the ground, cracks opening up, or buildings tilting, move away from that building or ground and head to a safe place. Stay away from utility poles and power lines, as they may collapse or snap.
If a building's tilt or damage is severe, don't force yourself to stay inside — call emergency services and follow their instructions. Liquefaction can happen not only right after the main shock but also again during aftershocks, so don't let your guard down once the shaking stops — stay alert for a while afterward.
Summary
Liquefaction happens when two things line up: ① the pushing force between sand grains (effective stress) is what supports the ground, and ② earthquake shaking causes a spike in water pressure between the grains, stripping away that pushing force. The strength of "solid" ground actually rests on the surprisingly delicate balance of grains pressing against each other.
It was never the hardness itself holding the ground up.
It was a quiet pushing match between grains of sand.
- Put sand (or fine soil) and water into a clear plastic bottle, enough that the water comes about halfway up the sand
- Close the lid, shake it hard for a while, then set it down quietly on a table and watch
While you shake it, the sand and water mix into something like muddy water, with the sand floating in it. Once you set it down, the grains gradually settle and pile up again over time, separating from the clear water on top. This "collapses when shaken, sets when still" change is close to how liquefaction — and the ground's later recovery — actually works.
Want to know more? ― terms, formulas, and where this fits in the curriculumWe've marked which level each part belongs to, from middle school science up to university specialist courses
- MSCovered in middle school science
- HSCovered in high school "Physics Basics"
- HS+Covered in high school "Earth Science," or an advanced/sidebar topic in textbooks
- UnivNot covered in high school — content from a university specialist course (soil mechanics)
- ResearchNot yet settled "textbook fact" even at university — something researchers are actively investigating
MSTerms: the vocabulary of liquefaction
- Pore water pressure: the pressure of the water sitting in the gaps between soil grains.
- Effective stress: the force with which the grains actually push against each other. The source of the ground's strength.
- Sand boiling: sand being ejected along with water from the surface during liquefaction.
HSChecking with a formula: how do you calculate ground strength?
Effective stress is expressed by the following relationship.
Effective stress = Total stress − Pore water pressure
| Total stress | Pressure from the combined weight of all the soil and water above that depth [kN/m²] |
| Pore water pressure | Pressure of the water in the gaps between grains [kN/m²] |
| Effective stress | The force with which the grains actually push against each other. The source of the ground's strength [kN/m²] |
| Unit weight of saturated sand (assumed) | 19 kN/m³ |
| Unit weight of water | 9.8 kN/m³ |
| Total stress (5 m depth) | 19 × 5 = 95 kN/m² |
| Pore water pressure, normal conditions (5 m depth) | 9.8 × 5 = 49 kN/m² |
| Effective stress, normal conditions | 95 − 49 = 46 kN/m² |
※ These unit weight values are representative figures used to illustrate the mechanism. Actual values vary depending on the type of soil.
| Pore water pressure risen to total stress during an earthquake | 95 kN/m² |
| Effective stress at this point | 95 − 95 = 0 kN/m² |
Effective stress reaching zero means the force pushing the grains together has vanished completely. Ground where the grains no longer touch each other loses the ability to support the building or manhole above it, and behaves like a liquid. That's the true nature of liquefaction.
HS+Why does it happen more on "loose sand" and "high water tables"?
The more loosely the grains are packed, the bigger the gaps between them, and the more room there is for the grains to pack tighter under shaking. The stronger that packing tendency, the higher the pressure of the water being pushed out. Likewise, a high water table (meaning more of the gaps are filled with water) creates conditions where pore water pressure rises more easily. Conversely, ground that's already firmly compacted, or where the water table is low, is thought to be less prone to liquefaction.
UnivJudging liquefaction risk with numbers
In actual ground surveys, engineers compare the force expected to act on the ground during an earthquake (the seismic shear stress ratio) against the ground's resistance to liquefaction (the liquefaction resistance ratio), and use an index called the "FL value" to assess liquefaction potential. The lower the FL value falls below 1, the higher the assessed risk of liquefaction. This theory is built on the concept of effective stress proposed by soil mechanics pioneer Karl Terzaghi, and is covered in university courses in soil mechanics and geotechnical engineering.
ResearchWhat's still not fully understood
- Precisely predicting where, and at what scale, liquefaction will occur down to a narrow area is still considered difficult. Soil properties can vary greatly even over very short distances, and precise prediction requires a great deal of survey data.
- Research is still ongoing to verify the long-term effectiveness of ground-improvement countermeasures. Various methods exist, such as compaction and drainage, but determining which method works best under which conditions still requires more verification data from real earthquakes.
- How long-duration shaking (long-period ground motion) affects liquefaction is still debated among researchers. How differences in shaking duration and period relate to liquefaction risk is an area of ongoing research.
Where this fits in the curriculum, by level
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Science: pressure, strata and crustal change | The basic idea that grains pushing on each other supports the ground |
| HS | Physics Basics: calculating pressure | The full calculation in ①②③ above |
| HS+ | Earth Science: ground and earthquake hazards | How loose sand and water table relate to liquefaction risk |
| Univ | Soil mechanics, geotechnical engineering | Effective stress theory, liquefaction assessment via FL value |
| Research | Geotechnical / earthquake engineering (ongoing) | Prediction accuracy, verifying countermeasures, effects of long-period ground motion |
| ― | Disaster prep / safety education | Hazard maps, what to do during an earthquake, staying alert for aftershocks |
- Public materials on liquefaction hazard maps and liquefaction countermeasures from Japan's Ministry of Land, Infrastructure, Transport and Tourism (国土交通省) and local authorities.
- General descriptions of the principle of effective stress (as proposed by Karl Terzaghi) found in soil mechanics textbooks.
- Explanatory material on liquefaction assessment methods (FL value) and past liquefaction damage from geotechnical engineering societies and professional bodies.
- Guidance on earthquake response and liquefaction awareness from disaster-prevention agencies.
※ Figures such as unit weight and stress are approximations and assumptions used to illustrate the mechanism. Actual values vary greatly depending on soil type and survey results.
※ This article is a general-audience science explainer. For actual disaster-preparedness decisions and evacuation actions, follow the information and instructions of local authorities, meteorological agencies, and fire services. The figures given here are approximations and assumptions meant to illustrate the underlying mechanism.