Structural damage and fire. Surface trembling from seismic waves often damages buildings. Depending on the severity of the earthquake, gas mains may break, starting numerous fires. Foreshocks, small earthquakes that sometimes precede the main earthquake, can be used as a warning system that a large shock may be on the way. Thousands of aftershocks may follow an earthquake and can be quite destructive, especially to those structures that have already been weakened and damaged.
Mass‐wasting events. Ground motion may trigger landslides and other rapid mass‐wasting events that result in loss of life and damage to buildings. A mass‐wasting variation is a landslide by liquefaction, in which water‐soaked sediment moves downslope like a slurry. Buildings that were built on solid sediment may sink if liquefaction occurs.
Rocks can be permanently displaced during an earthquake. Fault blocks may move vertically, forming a new scarp along the fault plane. Horizontal movement can tear apart roads, pipelines, and any other structures that are built across the fault zone. Displacement rarely exceeds about 7 meters (25 feet).
Seismic sea waves. If the sea floor suddenly shifts upward or downward, the sudden displacement of water results in seismic sea waves, or tsunamis. Unlike even the greatest storm waves, tsunamis can be up to 90 meters (300 feet) high and move at speeds of up to 400 miles per hour. Tsunamis have wavelengths that can be as long as 160 kilometers (100 miles), and the water does not quickly withdraw from the coast after the tsunami breaks. The water continues to rise for up to ten minutes until the long wavelength has passed through, resulting in widespread coastal damage.