Seismic hazards are the physical phenomena associated with earthquakes that can cause damage to the environment and human infrastructure. These hazards stem from the sudden release of stored elastic energy within the Earth’s crust, which travels outward as seismic waves. Understanding these effects is foundational to assessing the risk an earthquake poses to a community, involving both the direct consequences of fault movement and the secondary dangers that shaking triggers.
The Direct Effects: Ground Shaking and Fault Rupture
Ground shaking represents the most widespread and damaging direct effect of an earthquake, caused by the passage of seismic waves through the Earth’s layers. These waves—including compressional P-waves, shear S-waves, and slower surface waves—cause the ground to vibrate in disruptive up, down, and sideways motions. The severity of shaking depends on factors like the earthquake’s magnitude, the distance from the fault, and crucially, the local geology. Areas built on soft sediments, such as loose soil or artificial fill, often experience significantly amplified shaking compared to those on solid bedrock, a phenomenon known as site amplification.
The second direct hazard is fault rupture, which is the physical breaking and displacement of the ground surface along the fault trace. This displacement can range from a few centimeters to several meters of horizontal or vertical offset, depending on the earthquake’s size and type. This hazard is highly localized to the immediate vicinity of the fault line, but any structure built directly across the rupture path is likely to be completely destroyed or severely damaged. For instance, the 1906 San Francisco earthquake saw horizontal displacements of up to 20 feet (6 meters) along the San Andreas Fault.
Triggered Dangers: Liquefaction and Landslides
Strong ground shaking frequently triggers secondary dangers involving the instability of earth materials, notably liquefaction and landslides. Liquefaction occurs when saturated, loose, granular soils—such as those found near coastlines or riverbeds—temporarily lose their strength and stiffness. The shaking causes the soil particles to lose contact with one another, transferring the load to the pore water and causing the material to behave like a dense liquid.
This fluid-like state can lead to severe structural damage, causing buildings to sink, tilt, or even float in the liquefied soil. A tell-tale sign of this process is the formation of sand boils, where muddy water and sand erupt from the ground surface through fissures created by the increased pore pressure. Liquefaction can also facilitate lateral spreading, a form of landslide where blocks of non-liquefied soil slide horizontally over the weakened liquefied layer toward a free face, like a riverbank.
In steeper terrain, seismic ground motion can trigger massive slope failures, known as earthquake-induced landslides. The intense vibrations destabilize soil, rock, and debris, initiating various forms of mass movement, including rockfalls, slumps, and debris flows. These can occur far from the earthquake’s epicenter, provided the local topography and geological structure are susceptible to failure. The resulting landslides can block roads, bury communities, and dam rivers, creating temporary lakes that pose further flood risks.
Waterborne Threats: Seismic Tsunami Generation
A distinct hazard generated by underwater earthquakes is the tsunami, a series of waves caused by the rapid displacement of a large volume of water. Tectonic tsunamis are most frequently generated by large earthquakes in subduction zones, where one tectonic plate forces its way beneath another. This interaction typically involves a thrust fault, which causes a sudden, vertical movement of the seafloor.
When the overriding plate abruptly springs upward, it displaces the entire water column above it, generating the wave train. This mechanism distinguishes seismic tsunamis from those caused by landslides or volcanic activity. In the deep ocean, the tsunami waves travel at speeds comparable to a jet aircraft but possess a small height. As they enter shallow coastal waters, their speed decreases and their amplitude builds significantly, resulting in the immense volume of water that rapidly inundates low-lying coastal areas.
Quantifying the Danger: Measuring Seismic Hazard Severity
Scientists use different measurement scales to quantify the size of an earthquake and the severity of its resulting hazards. The size is measured by magnitude, most commonly using the Moment Magnitude Scale (Mw), which measures the total energy released at the earthquake’s source. This magnitude value is constant for a given event, regardless of where it is measured, and is calculated instrumentally from seismograph readings. Each whole number increase on this logarithmic scale represents roughly a 32-fold increase in the energy released.
In contrast, the actual danger experienced on the ground is quantified using an intensity scale, such as the Modified Mercalli Intensity (MMI) Scale. Intensity measures the effects of shaking at a specific location and is based on observable damage to structures and the perceptions of people. A single earthquake produces a range of intensity values that decrease with distance from the epicenter and vary based on the local ground conditions. This intensity value is a more direct indicator of the localized seismic hazard and is relevant for engineering and disaster response planning.