On September 28, 2018, a magnitude 7.5 earthquake struck near the city of Palu on the Indonesian island of Sulawesi. In the neighborhoods of Balaroa and Petobo, the ground did something that most people would consider impossible: it turned to liquid and flowed. Entire neighborhoods — houses, roads, trees, vehicles — were carried hundreds of meters downslope on rivers of liquefied soil. More than 4,300 people died in the earthquake and its aftermath. Many of the dead in Balaroa and Petobo were buried beneath meters of displaced earth that had simply swallowed their neighborhoods whole.
What happened in Palu was an extreme example of a phenomenon that seismologists and geotechnical engineers call liquefaction — the transformation of saturated, granular soil from a solid to a liquid-like state during earthquake shaking. Liquefaction is one of the most destructive and visually dramatic secondary effects of earthquakes. It has caused catastrophic damage in earthquakes around the world, from Niigata, Japan, in 1964 to Christchurch, New Zealand, in 2011. It threatens every coastal city built on loose, sandy, water-saturated soil — and many of the world's most densely populated urban areas sit on exactly that kind of ground. What causes earthquakes
This article explains the physics of liquefaction, where it occurs, what it looks like, the most devastating historical examples, and what engineers can do to prevent it.
What Is Liquefaction? The Physics of Solid Ground Becoming Liquid
To understand liquefaction, you need to understand how soil supports weight. Soil is not a single solid material — it is a collection of individual grains (sand, silt, or gravel) with spaces (pores) between them. In dry soil, the grains rest against each other, and the friction between grains provides the soil's strength. In saturated soil — soil where the pore spaces are filled with water — the grains still support weight through grain-to-grain contact, but the water in the pores also exerts a pressure called pore water pressure.
Under normal conditions, the weight of the overlying soil (the overburden) is supported partly by grain-to-grain contact forces (called effective stress) and partly by pore water pressure. The soil remains strong as long as the effective stress — the force transmitted through the grain contacts — remains positive.
During earthquake shaking, the rapid cyclic loading causes the soil grains to rearrange. In loose, saturated, granular soil, the grains tend to compact — they try to settle into a denser arrangement. But because the soil is saturated and the water cannot drain away fast enough during the seconds of shaking, the compaction increases the pore water pressure instead. As the pore water pressure rises, it takes over more of the load that was previously carried by grain-to-grain contact. The effective stress decreases.
If the shaking is intense enough and continues long enough, the pore water pressure can rise to equal the total overburden stress. At that point, the effective stress drops to zero. The grains are no longer pressing against each other at all — they are floating in a pressurized slurry of water and sand. The soil has lost all its shear strength. It behaves like a dense liquid.
This is liquefaction.
Diagram — Liquefaction Process: Before, During, and After Earthquake Shaking
Features: Three-panel diagram with soil grain illustrations, pore water pressure graphs, and building response
Where Liquefaction Occurs: Identifying Vulnerable Ground
Liquefaction requires three conditions to occur simultaneously:
- Saturated soil: The pore spaces must be filled with water. This generally means the groundwater table is within a few meters of the surface.
- Loose, granular soil: The soil must be composed of particles that can rearrange under shaking — typically fine to medium sand, silty sand, or non-plastic silt. Dense, well-compacted soils resist liquefaction. Clay-rich soils generally do not liquefy because clay particles are cohesive.
- Sufficient shaking: The earthquake must produce strong enough ground motion for long enough to generate the pore pressure increase. Generally, earthquakes of magnitude 5.0 or greater can trigger liquefaction in susceptible soils, with the risk increasing sharply with magnitude and shaking duration.
These conditions converge in predictable geologic settings:
River deltas and floodplains: Rivers deposit loose, sandy sediments across their floodplains and deltas. Cities built on these deposits — which include many of the world's largest metropolitan areas — are inherently vulnerable. The sediments are young (geologically recent), loosely deposited, and often saturated because of the shallow water table near rivers.
Coastal landfill and reclaimed land: Many cities have expanded their buildable area by filling in shallow coastal waters or wetlands with sand, rubble, or dredged material. This fill is typically placed without the kind of compaction that would make it resistant to liquefaction. San Francisco's Marina District, Kobe's Port Island, and much of the Tokyo waterfront are examples.
Former lakebeds: When lakes are drained or recede, they leave behind flat, low-lying areas of fine-grained sediment that may be loose and saturated. Mexico City's infamous vulnerability to earthquake damage is partly due to its location on the former bed of Lake Texcoco.
Beach and dune deposits: Coastal sand deposits, particularly those that have not been compacted by overlying sediment or construction, can liquefy when shaken.
| Soil Type | Grain Size | Liquefaction Susceptibility | Typical Settings |
|---|---|---|---|
| Clean fine sand | 0.075–0.425 mm | Very High | Beaches, river channels, fills |
| Silty sand | Mixed fine | High | Floodplains, deltas, coastal deposits |
| Non-plastic silt | 0.002–0.075 mm | Moderate to High | Lakebeds, overbank deposits |
| Gravel (loose) | 2–75 mm | Moderate | River channels (if saturated and loose) |
| Dense sand | 0.075–2 mm | Low | Well-compacted fills, old deposits |
| Clay | <0.002 mm | Very Low | Clay-rich soils are cohesive; generally resist liquefaction |
| Rock | N/A | None | Bedrock cannot liquefy |
Visible Effects of Liquefaction
When liquefaction occurs, its effects are dramatic and immediately visible.
Sand Boils
The most distinctive surface expression of liquefaction is the sand boil (also called a sand volcano) — a cone or sheet of wet sand that erupts at the ground surface as pressurized water and sand are ejected from the liquefied layer below. Sand boils can range from a few centimeters to several meters in diameter. After the 2010–2011 Christchurch earthquakes, some sand boils ejected enough material to bury roads and yards under tens of centimeters of wet sand and silt.
Ground Settlement
When the shaking stops and the excess pore water pressure begins to dissipate, the soil grains settle into a denser arrangement than before the earthquake. This means the ground surface drops — sometimes uniformly, but more often differentially, producing uneven settlement that damages foundations, breaks utility lines, and cracks roads and sidewalks. In Christchurch, cumulative settlement from repeated liquefaction episodes lowered parts of the city by as much as 0.5 meters.
Lateral Spreading
When liquefied soil is on a slope or adjacent to a free face (such as a riverbank, seawall, or channel), the liquefied mass can move laterally, spreading downslope or toward the free face. Lateral spreads typically involve blocks of intact surface soil riding on the liquefied layer below, breaking apart along fissures. Displacements range from centimeters to several meters. Lateral spreading is particularly destructive to bridges, pipelines, and other infrastructure that spans or crosses the spreading zone.
Bearing Capacity Failure
When the soil beneath a structure liquefies, the foundation loses its support. Heavy structures sink into the liquefied ground. If the sinking is uneven — as it usually is — the structure tilts. The iconic photographs from the 1964 Niigata earthquake, showing intact apartment buildings tilted at 60-degree angles, illustrate bearing capacity failure during liquefaction. The buildings were structurally undamaged — the problem was entirely in the ground beneath them.
Flow Liquefaction
The most catastrophic form of liquefaction is flow failure, in which liquefied soil on a slope flows downhill as a liquid mass, carrying everything on its surface with it. The 2018 Palu, Indonesia, disaster is the most dramatic modern example, but flow failures have occurred in many earthquakes. During the 1964 Alaska earthquake (M9.2), the Turnagain Heights landslide in Anchorage was a massive flow failure in which a coastal bluff disintegrated into a chaotic mass of liquefied soil and broken ground, carrying homes and infrastructure with it. Understanding earthquake waves
Historic Liquefaction Events: Case Studies
1964 Niigata, Japan (M7.5)
The June 16, 1964, Niigata earthquake is one of the defining events in liquefaction research. The city of Niigata is built on the sandy deposits of the Shinano River delta. When the magnitude 7.5 earthquake struck, widespread liquefaction occurred throughout the city's low-lying areas.
The most iconic damage involved the Kawagishi-cho apartment buildings — a row of reinforced concrete residential buildings that tilted dramatically as the soil beneath them liquefied. Some buildings tilted nearly to the horizontal, yet remained structurally intact. Residents climbed out of windows and walked down the sides of the tilted buildings. The Showa Bridge collapsed when liquefaction-induced lateral spreading displaced its pile foundations. The Niigata earthquake, along with the 1964 Alaska earthquake that occurred just three months earlier, catalyzed the modern field of liquefaction engineering research.
1989 Loma Prieta, California (M6.9) — The Marina District
The October 17, 1989, Loma Prieta earthquake struck 100 km south of San Francisco, but some of the worst damage in the city occurred in the Marina District — approximately 100 km from the epicenter. The Marina District is built on a combination of natural bay mud and rubble fill that was placed to create buildable land for the 1915 Panama-Pacific International Exposition. Much of that fill consisted of debris from buildings destroyed in the 1906 San Francisco earthquake — loose, uncompacted rubble dumped into the former bay margin.
During the Loma Prieta earthquake, this fill liquefied extensively. Buildings settled differentially, gas mains broke (sparking fires), and water mains ruptured (hampering firefighting). The scene — a residential neighborhood on fire with broken gas mains and no water pressure — was eerily reminiscent of 1906. The Marina District damage demonstrated that liquefaction can cause severe damage even at considerable distance from the epicenter when local soil conditions are unfavorable. San Francisco earthquake history
1995 Kobe, Japan (M6.9) — Port Island
The January 17, 1995, Great Hanshin earthquake devastated the city of Kobe, killing 6,434 people. Port Island and Rokko Island, two large artificial islands in Kobe's harbor constructed from decomposed granite fill, experienced extensive liquefaction. The ground surface on Port Island settled by an average of approximately 50 cm, with some areas settling more than a meter. Quay walls shifted seaward, disrupting port operations for months. Sand boils covered large areas of the islands.
The Kobe earthquake also demonstrated liquefaction beneath natural soil deposits along the waterfront, where lateral spreading caused severe damage to bridges, highways, and utility corridors. The economic cost of liquefaction-related damage in Kobe was estimated in the billions of dollars.
2010–2011 Christchurch, New Zealand — The Most Extensively Documented Liquefaction Event
The Canterbury earthquake sequence, beginning with the September 4, 2010, Darfield earthquake (M7.1) and continuing with the devastating February 22, 2011, Christchurch earthquake (M6.2) and subsequent aftershocks, produced the most thoroughly documented and studied liquefaction in history.
Christchurch is built on a broad, flat alluvial plain composed of loose, sandy, silty sediments deposited by the Waimakariri River and its tributaries. The water table is shallow — often within 1–2 meters of the surface in the eastern suburbs. The city was, in geotechnical terms, a textbook setting for widespread liquefaction.
The February 2011 earthquake, despite its moderate magnitude, was extremely damaging because its epicenter was shallow (approximately 5 km) and located directly beneath the southeastern suburbs. The ground shaking triggered massive liquefaction across the eastern and central parts of the city. Sand boils ejected hundreds of thousands of cubic meters of sand and silt onto streets, yards, and through floors. The total volume of ejected material from the sequence has been estimated at approximately 400,000 cubic meters.
The cumulative effects were devastating. Approximately 8,000 residential properties were damaged severely enough to require demolition. The Residential Red Zone — an area of approximately 630 hectares in the eastern suburbs — was deemed too damaged and too vulnerable to future liquefaction for continued habitation. The New Zealand government purchased approximately 8,000 residential properties in the Red Zone, and the residents were permanently relocated. Entire neighborhoods ceased to exist.
The Christchurch experience also demonstrated that repeated liquefaction — from the September 2010 earthquake, the February 2011 earthquake, and subsequent large aftershocks — had cumulative effects. Each episode caused additional settlement and ejection of material, progressively worsening the damage. Some areas that survived the first earthquake with manageable damage were destroyed by the cumulative effects of multiple events.
The total cost of the Canterbury earthquake sequence exceeded NZ$40 billion (approximately US$28 billion), making it the most expensive disaster in New Zealand's history relative to GDP. Liquefaction-related damage accounted for a substantial share of this cost.
2018 Palu, Indonesia (M7.5) — Flow Liquefaction
The September 28, 2018, Palu earthquake triggered one of the most catastrophic flow liquefaction events ever documented. In the neighborhoods of Balaroa and Petobo, located on gently sloping terrain underlain by loose, saturated alluvial soils, the ground liquefied and flowed.
In Petobo, a residential area of approximately 170 hectares was carried an estimated 400–700 meters downslope as the liquefied soil flowed toward a river valley. Satellite imagery and drone surveys showed entire blocks of houses displaced hundreds of meters from their original locations, rotated, and partially buried. In Balaroa, a residential neighborhood on a hillside flowed downslope, burying hundreds of homes.
The flow liquefaction in Palu was unusual in several respects. The slopes were gentle — typically less than 4 degrees — yet the liquefied soil flowed enormous distances. The long flow distances are attributed to the combination of loose, saturated alluvial soil, gentle but persistent slopes, and the long duration of shaking from the magnitude 7.5 earthquake. Some researchers have also pointed to possible contributions from irrigation water that kept the soils saturated.
| Earthquake | Year | Location | Magnitude | Key Liquefaction Effects |
|---|---|---|---|---|
| Great Alaska Earthquake | 1964 | Anchorage, AK | 9.2 | Turnagain Heights landslide; massive flow failure destroyed neighborhood |
| Niigata | 1964 | Niigata, Japan | 7.5 | Apartment buildings tilted intact; Showa Bridge collapsed |
| Loma Prieta | 1989 | San Francisco Marina District | 6.9 | Liquefaction of 1906 rubble fill; building collapses and fires |
| Great Hanshin (Kobe) | 1995 | Kobe, Japan | 6.9 | Port Island settled ~50 cm; extensive lateral spreading at waterfront |
| Chi-Chi | 1999 | Central Taiwan | 7.6 | Liquefaction and lateral spreading near rivers and coastal plains |
| Canterbury Sequence | 2010–2011 | Christchurch, NZ | 7.1 / 6.2 | ~8,000 homes demolished; 630-hectare Red Zone permanently abandoned |
| Palu | 2018 | Palu, Indonesia | 7.5 | Flow liquefaction displaced neighborhoods 400–700 m; >4,300 deaths |
Liquefaction Susceptibility Mapping
One of the most important tools for managing liquefaction risk is susceptibility mapping — identifying which areas are likely to experience liquefaction during an earthquake of a given intensity.
Liquefaction susceptibility maps are based on a combination of geologic mapping (identifying the types and ages of surficial deposits), geotechnical data (borehole logs, standard penetration test results, cone penetration test data), groundwater depth measurements, and historical observations of liquefaction in past earthquakes.
[MAP: San Francisco Bay Area Liquefaction Susceptibility Zones] Data source: California Geological Survey (CGS) Seismic Hazard Zone maps; USGS and Association of Bay Area Governments (ABAG) liquefaction susceptibility mapping Features: Color-coded zones showing very high (bay fills, reclaimed land), high (alluvial deposits near bay margins), moderate (older alluvial terraces), and low/very low (bedrock, dense older deposits) liquefaction susceptibility. Key areas labeled: Marina District (very high — 1906 rubble fill), Foster City (very high — bay fill), Treasure Island (very high — hydraulic fill), Oakland waterfront (very high), Silicon Valley bayshore (high), Alameda (very high — fill), San Francisco Financial District (high — former Yerba Buena Cove fill).
In California, the California Geological Survey (CGS) has mapped Seismic Hazard Zones throughout the state's urbanized areas. These maps, mandated by the Seismic Hazards Mapping Act of 1990, identify zones where liquefaction or earthquake-induced landslides are potential hazards. Within these zones, site-specific geotechnical investigations are required before new construction can be permitted. CGS Seismic Hazard Zone Maps
The USGS has also developed national-scale liquefaction susceptibility datasets and contributed to detailed mapping efforts in the San Francisco Bay Area, the New Madrid Seismic Zone, the Pacific Northwest, and other high-risk regions. USGS Liquefaction Hazard Maps
In New Zealand, the devastating experience in Christchurch prompted a comprehensive remapping effort. Land Information New Zealand and the Canterbury Earthquake Recovery Authority developed detailed liquefaction vulnerability assessments that directly informed land-use planning decisions, including the delineation of the Residential Red Zone.
Engineering Solutions: Building on Liquefiable Ground
Liquefaction is a well-understood phenomenon, and geotechnical engineers have developed a range of techniques to mitigate its effects. The challenge is that many of these techniques are expensive and most practical only for new construction — retrofitting existing structures and infrastructure on liquefiable ground is often prohibitively costly.
Ground Improvement Techniques
Dynamic compaction: Heavy weights (10–40 tons) are dropped repeatedly from heights of 10–30 meters onto the ground surface, densifying the soil through impact energy. Effective for loose sandy fills in areas where vibration and noise are acceptable.
Vibro-compaction and vibro-replacement (stone columns): A vibrating probe is inserted into the ground, densifying surrounding granular soil. In finer soils, gravel is added to create stone columns — vertical columns of compacted gravel that provide drainage paths for pore water and reinforcement of the soil mass. Stone columns both increase the soil's density and accelerate the drainage of excess pore water during shaking.
Compaction grouting: A stiff grout is injected under pressure into the soil, displacing and compacting the surrounding material. Useful for treating soil beneath existing structures without excavation.
Deep soil mixing: Cement, lime, or other binding agents are mechanically mixed into the soil to create columns or walls of stabilized material. This changes the soil from a liquefiable granular material to a ceite-like composite.
Dewatering: Permanently lowering the groundwater table removes the water from the pore spaces, eliminating one of the three conditions required for liquefaction. This approach uses perimeter drains, wells, or other drainage systems. It is most practical in limited areas and requires ongoing maintenance.
Foundation Solutions
Deep foundations (piles): Rather than resting on the liquefiable surface soil, structures can be supported on piles — long columns of steel, concrete, or timber driven or drilled through the liquefiable layer into a deeper, stable bearing stratum. This is standard practice for major structures in liquefiable areas. However, piles must be designed for the additional lateral forces they experience when the surrounding soil liquefies and spreads laterally.
Mat foundations: Thick, stiff concrete mat foundations spread the building's weight over a large area and provide rigidity that resists differential settlement. While they do not prevent liquefaction, they reduce its impact on the structure by distributing loads and resisting tilting.
Infrastructure Protection
Lifeline infrastructure — water mains, gas pipelines, bridges, port facilities — requires special design considerations in liquefiable zones. Flexible pipeline joints accommodate ground deformation without rupturing. Bridge foundations are designed for lateral spreading loads. Quay walls are designed with adequate penetration depth and drainage to resist liquefaction-induced displacement. Los Angeles earthquake risk
San Francisco Bay Area: A Case Study in Liquefaction Risk
The San Francisco Bay Area is one of the most liquefaction-prone metropolitan areas in the United States. The Bay's margins are lined with artificial fill placed during more than 150 years of development — from the Gold Rush era, when ships were abandoned and buried to create buildable land in what is now the Financial District, to the 20th-century construction of Treasure Island (a former naval base built on hydraulic fill in the Bay) and Foster City (a planned community built on fill south of San Mateo).
The USGS estimates that approximately 130 square km of the Bay Area's urbanized land is built on fill or soft bay mud that is susceptible to liquefaction. This includes some of the region's most densely developed and economically valuable areas: San Francisco's waterfront, the Oakland port and waterfront, Silicon Valley's bayshore corridor, and virtually all of Alameda Island.
UCERF3 gives a 72% probability of a M6.7 or greater earthquake in the Bay Area between 2014 and 2043. The Hayward Fault, which runs directly beneath the urbanized East Bay, is considered one of the most dangerous faults in the United States. A major earthquake on the Hayward Fault would produce intense shaking in areas with extensive liquefiable fill — a combination that has the potential to cause catastrophic damage to buildings, transportation infrastructure, water and sewer systems, and port facilities. Assess your earthquake risk
The 1989 Loma Prieta earthquake provided a preview of what a closer, larger earthquake could produce. The Marina District fires, the collapse of the Cypress Street Viaduct in Oakland (on soft Bay mud), and the damage to the Bay Bridge east span (founded partly on fill) were all related to the amplification and liquefaction effects of soft and filled ground — and Loma Prieta's epicenter was 100 km away. A M7.0 earthquake on the Hayward Fault, with its epicenter directly beneath the urban core, would produce far more intense shaking in these same areas. San Francisco earthquake history