An earthquake, a sudden release of energy in the Earth’s crust, begins with ground shaking, but the true extent of the destruction often comes from a cascade of secondary hazards. These triggered phenomena, including ground failure, slope movement, water displacement, and the collapse of human systems, frequently cause more widespread damage than the initial seismic waves. Understanding these secondary effects is paramount because their impact dictates the overall scale of a seismic disaster.
Ground Instability Phenomena
The violent, cyclic motion of an earthquake can transform seemingly solid ground into an unstable medium, primarily through liquefaction. This occurs in loose, water-saturated granular soils, such as sands and silts, where repeated shaking increases the pressure of the water trapped between the soil particles (pore-water pressure). As this pressure rises, the soil temporarily loses its strength and stiffness, acting like a dense liquid, which causes buildings founded on it to sink or tilt.
The liquefaction process can directly lead to lateral spreading, where blocks of intact surface soil slide horizontally on a liquefied layer below, often moving distances of several feet on slopes as gentle as half a degree. This lateral movement can tear apart pipelines, roads, and bridge abutments, causing significant disruption to transportation and utility networks. The failure of the ground to maintain its volume also results in ground settlement, a vertical sinking that occurs either immediately or as the excess pore-water pressure slowly dissipates after the shaking stops.
Settlement beneath structures is compounded by the building’s weight, which “ratchets” the foundation downward into the softened, liquefied soil with each cycle of shaking. This localized settlement is a major cause of structural damage and collapse, even when the shaking itself does not destroy the building. In areas without overlying structures, settlement is caused by the densification of the soil as the grains rearrange into a more compact state.
Mass Movement Hazards
Earthquake shaking is a highly effective trigger for mass movement hazards, accelerating the downslope flow of rock and soil. Seismic waves temporarily reduce the shear strength of slope materials, the force that resists sliding. Even a moderate earthquake can destabilize slopes that were previously considered stable, particularly those with existing weaknesses or high water content.
The intensity of shaking determines the type of mass movement triggered, with rockfalls and shallow, disrupted soil slides requiring the weakest seismic forces to initiate. Rockfalls involve the free-falling or bouncing of detached rock fragments from steep cliff faces. Rock slides are more coherent masses of bedrock sliding along distinct planes of weakness, posing a direct threat to structures and roads at the base of the slopes.
More intense, sustained shaking is required to trigger deeper-seated landslides, which involve a larger, more intact mass of earth sliding along a curved or planar surface. Debris flows, which are rapidly moving, water-saturated mixtures of soil, rock, and organic matter, are among the most destructive mass movements. They are often initiated by the collapse of a landslide mass that liquefies due to the shaking or by the sudden release of water from an earthquake-damaged drainage system.
Water Displacement and Inundation
Water bodies can be violently disturbed by seismic activity, leading to massive waves and widespread inundation far from the fault line. The most catastrophic example is a tsunami, a series of powerful waves generated primarily by large, shallow earthquakes that cause a sudden, vertical displacement of the seafloor. This movement, often occurring in subduction zones, displaces the entire water column above the rupture zone, forming a wave that can travel across entire ocean basins.
Tsunamis typically require a magnitude exceeding 7.5 to be significantly destructive. The resulting waves move at speeds comparable to a jet airliner in the deep ocean, but as the wave enters shallow coastal waters, its speed decreases dramatically while its height rapidly increases. The first wave may not be the largest, and the danger can persist for hours as subsequent waves arrive.
In enclosed or partially enclosed water bodies, such as lakes, reservoirs, bays, and even swimming pools, seismic waves can trigger a phenomenon known as a seiche. These are standing waves that oscillate back and forth, much like water sloshing in a bathtub, as the seismic surface waves pass through the area. The 1964 Alaska earthquake, for instance, generated seiches thousands of miles away in bodies of water across North America.
Earthquake-induced dam and levee failures present a severe inundation hazard, especially in densely populated river valleys and floodplains. Shaking can compromise these earthen structures through liquefaction of the underlying foundation soil, causing the embankment to lose support. Cracking and differential settlement can also lead to internal erosion, or “piping,” where water seeps through cracks and washes out the soil, rapidly leading to a breach and catastrophic flooding.
Infrastructure and Secondary Hazards
The failure of human-built systems during an earthquake creates non-geological hazards that complicate rescue and recovery efforts. Fire is a common and destructive secondary effect, often starting when ruptured natural gas lines are ignited by sparks from downed electrical wires. Firefighting is severely hampered when broken underground water mains, a common result of ground movement, cause a near-total loss of water pressure.
The disruption of critical utilities includes the loss of power, communication networks, and transportation links. Damage to power substations and transmission lines causes widespread blackouts, while severed fiber-optic and copper lines cripple emergency communication. The collapse of bridge decks and overpasses isolates communities, preventing the timely arrival of first responders and medical aid.
A consequence is the release of hazardous materials, referred to as “Na-Tech” (natural-technological) events, from industrial facilities and storage sites. Ground shaking can cause elevated storage tanks to topple and containment vessels to fail, spilling toxic, flammable, or corrosive substances. For example, the rupture of ammonia lines in a cooling plant can release a toxic cloud, forcing immediate large-scale evacuations and presenting a public health crisis in the aftermath of the quake.