Earthquakes cause significant damage through ground shaking and other hazards, with liquefaction being one of the most destructive. This phenomenon involves saturated soil temporarily losing its strength and stiffness, causing it to behave like a liquid during seismic activity. Understanding the risks posed by liquefaction is important for public safety and disaster preparedness efforts in earthquake-prone regions.
Understanding Liquefaction
Liquefaction occurs when loose, granular soils, such as sands and silts, are subjected to strong earthquake shaking. Before an earthquake, the weight of the soil particles rests on their contacts, with water filling the spaces between them at relatively low pressure. During seismic shaking, the rapid, cyclic stresses cause the soil particles to lose contact with each other. This increases water pressure within the soil’s pores, as water cannot drain quickly.
As pore water pressure rises, the effective stress, which is the force holding soil particles together, decreases. When this pressure supports the overlying soil and structures, the soil loses its shear strength and stiffness, transforming into a fluid-like state. This process is similar to how quicksand forms, where solid ground suddenly behaves like a liquid. For liquefaction to occur, three conditions are necessary: loose, granular soil; saturation of this soil with water; and sufficient ground shaking.
Impacts on Buildings and Infrastructure
Liquefaction severely impacts the built environment. When the ground beneath structures loses its ability to support weight, buildings can tilt, settle unevenly, or collapse. Foundations may crack or break, leading to widespread structural damage. This differential settlement, where parts of a structure settle more than others, can render buildings unserviceable even without complete collapse.
Liquefaction also affects buried infrastructure. Pipelines, storage tanks, and septic systems can float upwards through the liquefied soil due to buoyant forces, disrupting essential services and creating additional hazards. Notable historical examples include the 1964 Niigata earthquake in Japan, which caused many buildings to tilt and sink, and the 1989 Loma Prieta earthquake in California, where significant liquefaction-related damage occurred in San Francisco’s Marina District. Such widespread damage can lead to substantial property losses and pose a risk to human lives.
Hazards from Ground Deformation
Liquefaction causes large-scale ground movements, creating distinct hazards beyond structural failure. Lateral spreading is a common form of ground deformation where liquefied soil flows horizontally on gentle slopes or towards a free face, such as a riverbank. This movement severely damages foundations, utility lines, roads, and bridges as the ground stretches.
Ground settlement, or subsidence, occurs after seismic shaking stops. As the excess pore water pressure dissipates, the water leaves the soil pores, and the soil particles settle into a denser configuration. This reconsolidation leads to uneven ground surfaces, further compromising the integrity of buildings and infrastructure.
Sand boils, also known as sand volcanoes, appear as evidence of liquefaction when water and sand are ejected to the surface through cracks in the ground. While these eruptions can cause localized damage and create cavities in the soil, their presence indicates severe underlying ground instability.
Factors Influencing Vulnerability
An area’s susceptibility to liquefaction depends on several key conditions. Soil type and density are significant; loose, sandy, or silty granular soils are most prone because their particles can easily lose contact and rearrange under stress. Denser soils, or cohesive soils like clay, generally have a lower risk.
The depth of the groundwater table also plays an important role, as soils must be saturated. Shallower groundwater tables increase the risk as soil is more likely to be fully saturated.
The characteristics of the earthquake itself also influence the likelihood and severity of liquefaction. Strong ground shaking, particularly from earthquakes of higher magnitude and longer duration, generates the necessary cyclic stresses to trigger the phenomenon. The proximity to the epicenter also affects the intensity of shaking. These combined factors determine why some geographic areas are more vulnerable to liquefaction hazards during an earthquake than others.