Liquefaction: This phenomenon occurs in water-saturated, loose, sandy soils. During intense shaking, the water pressure between soil particles increases to the point where the soil loses its strength and behaves like a liquid, causing buildings to sink or tilt.
Seismic Landslides: Earthquake vibrations can destabilize slopes, causing masses of rock, earth, or debris to move down a hill. This hazard is particularly high in mountainous regions or areas with steep, unstable terrain and high moisture content.
Lateral Spreading: A specific type of liquefaction-related failure where gentle slopes or flat ground near a free face (like a riverbank) move horizontally toward the opening.
Tsunami: A series of massive sea waves generated by the sudden displacement of the ocean floor, usually by undersea faulting or landslides. In the open ocean, these waves are small but travel fast; as they reach shallow coastal waters, they slow down and grow significantly in height.
Seiche: This is a standing wave in an enclosed or partially enclosed body of water, such as a lake, reservoir, or harbor. It is caused by seismic waves passing through the area, causing the water to slosh back and forth, which can lead to flooding or damage to docks.
| Feature | Ground Shaking | Surface Rupture | Liquefaction |
|---|---|---|---|
| Cause | Seismic wave propagation | Direct fault displacement | Pore water pressure increase |
| Primary/Secondary | Primary | Primary | Secondary |
| Impact Area | Regional/Widespread | Linear (along the fault) | Localized (specific soil types) |
Hazard vs. Risk: A hazard is the potential for a physical event (e.g., a magnitude 7.0 earthquake), while risk is the potential for damage (e.g., million dollars in losses). You can have a high hazard in a desert with no risk because there is nothing to damage.
Tsunami vs. Storm Surge: Tsunamis are caused by geological displacement of the seafloor, whereas storm surges are caused by atmospheric pressure changes and wind from hurricanes.
Identify the Trigger: When asked to classify a hazard, look for the trigger. If it is the shaking itself causing the issue (like a building collapsing), it is ground shaking. If the shaking causes the soil to change state, it is liquefaction.
Soil Type Matters: Always remember that soft, wet sediments are the 'danger zone' for both amplification of shaking and liquefaction. Solid bedrock is generally the safest material during an earthquake.
Check the Body of Water: If the question mentions a lake or a swimming pool sloshing, the term is seiche. If it mentions the open ocean or a coastal wave, the term is tsunami.
Common Formula Context: Engineers use the relationship to understand seismic risk; the 'hazard' provides the acceleration (), and the 'risk' is determined by the mass () and vulnerability of the structure.
