An earthquake is a sudden, rapid release of energy within the Earth’s crust that generates seismic waves, causing the ground to shake. This geological event results from the movement of tectonic plates, which build up stress along fault lines until the rock abruptly ruptures. This energy release causes both the immediate, temporary failure of surface materials and the permanent deformation of the Earth’s solid crust, impacting both the natural landscape and the human-built environment.
Ground Motion and Failure Mechanisms
The energy released during an earthquake travels through the Earth as seismic waves, fundamentally categorized as body waves (P-waves and S-waves) and surface waves (Love and Rayleigh waves). P-waves, or compressional waves, and S-waves, or shear waves, cause rapid horizontal and vertical ground movement that initiates the destructive process. The surface waves, which travel along the Earth’s surface, often produce the most damaging rolling or swaying motions experienced by structures.
The intense ground shaking can trigger liquefaction in certain soil types. Liquefaction occurs when saturated, loose, granular soils temporarily lose their strength and stiffness, behaving like a viscous liquid rather than a solid. This happens as the seismic shaking increases the pore-water pressure within the soil, forcing the grains apart. When the ground behaves like a fluid, structures founded on it can tilt, sink, or completely overturn, leading to collapse.
Seismic shaking frequently destabilizes slopes, triggering mass movements like landslides and rockfalls. Landslides are more likely where ground motion is strong and slopes are steep. These movements dramatically alter local topography, burying structures, blocking transportation routes, and damming rivers, which can cause subsequent flooding. Ground failure resulting from strong shaking and specific soil conditions is a significant contributor to overall earthquake losses.
Permanent Geological Deformation
Beyond the immediate effects of shaking and soil failure, earthquakes cause large-scale, lasting changes to the Earth’s surface and topography. Surface faulting is the most direct evidence of this geological deformation, manifesting as a visible offset in the ground along the fault line. This displacement can result in vertical offsets, forming a scarp, or lateral shifts where the ground moves horizontally.
Earthquakes, particularly those at plate boundaries, can cause regional elevation changes over broad areas. Tectonic uplift (where the land rises) and subsidence (where the land sinks) frequently affect coastlines and drainage systems. These events can result in significant vertical displacement of the seafloor and the adjacent coastline.
Sudden vertical displacement of the seafloor, typically caused by thrust faulting in subduction zones, is the primary generator of tsunamis. This rapid movement displaces the entire water column, creating massive waves that propagate across the ocean. When these waves reach the coast, they dramatically alter coastal topography and infrastructure through inundation and powerful erosive forces. The resulting permanent seafloor deformation is a fundamental component in calculating the potential size of the tsunami.
Impact on Built Structures and Infrastructure
Ground shaking is the primary source of damage to human-made structures, generating internal inertial forces within buildings that can exceed the structure’s capacity. Buildings suffer various types of structural failure, including shear failure, which is the cracking or breaking of columns and walls due to sideways forces. The most catastrophic failures include “pancake collapse,” where upper floors fall directly onto lower ones, and “soft story failure,” which occurs in structures with a large, open ground floor, such as those with parking garages.
The forces transmitted through the ground cause differential movement, which is damaging to utility networks and lifelines. Water lines, gas pipelines, electrical conduits, and sewage systems are frequently ruptured or broken by permanent ground deformation and lateral spreading. Broken gas lines often lead to secondary hazards like post-earthquake fires, compounding destruction, especially if ruptured water pipes hinder firefighting efforts.
Transportation systems are severely affected by ground displacement and shaking. Roads, bridges, tunnels, and railway lines can suffer damage ranging from minor cracking to outright collapse. Even if a structure remains standing, non-structural damage, such as fallen ceilings, shattered glass, and toppled fixtures, can render the building unusable and pose a significant risk to occupants.
Factors Determining Damage Severity
The severity of earthquake damage is not solely determined by the earthquake’s magnitude, which measures the energy released, but by a complex interplay of geological and human factors. Earthquakes with greater magnitude and shallower depth generally release more energy close to the surface, leading to greater shaking intensity. Conversely, seismic energy decreases, or attenuates, with increased distance from the epicenter and the fault rupture.
Local geology, often referred to as site effects, plays a significant role in determining the intensity of shaking at a specific location. Structures built on soft sediments or loose soils often experience stronger vibrations because these materials amplify the seismic waves. In contrast, hard bedrock tends to dampen the shaking, resulting in less surface damage. This local variation explains why damage can vary widely even for sites equidistant from the earthquake’s origin.
The vulnerability of built structures is heavily influenced by construction quality and adherence to modern building codes. Structures designed with ductility, such as those using reinforced concrete and steel frames, can bend and deform without breaking, absorbing seismic energy more effectively than brittle materials like unreinforced masonry. Building codes, regularly updated based on lessons learned from past events, require provisions for life safety and specify how structures should resist inertial forces.