A seismic event is a sudden release of stored elastic energy within the Earth’s lithosphere, generating vibrations that travel through the planet. This energy release causes the ground to shake, ranging from undetectable movements to catastrophic displacements. While most commonly associated with earthquakes, the term includes any event that produces measurable seismic waves, such as artificial explosions, volcanic activity, and massive landslides.
Defining Seismic Energy and Waves
The underlying mechanism for a seismic event is explained by the elastic rebound theory, where stress slowly builds up in the crustal rocks due to tectonic forces. As rocks along a fault line are pushed or pulled, they deform elastically, storing strain energy much like a stretched rubber band. When the accumulated stress exceeds the rock’s strength, the fault ruptures suddenly, and the rock snaps back toward its original shape, releasing the stored energy in the form of heat and seismic waves.
These waves are categorized into two main groups: body waves, which travel through the Earth’s interior, and surface waves, which move along the planet’s surface. The fastest body waves are Primary waves (P-waves), which are compressional and cause particles to move back and forth in the same direction as the wave travels. P-waves can travel through solids, liquids, and gases, and they are the first to be detected by seismographs. Secondary waves (S-waves) arrive next and are shear waves that oscillate the ground perpendicular to their direction of travel. S-waves cannot propagate through liquids, such as the Earth’s outer core.
Surface waves travel slower than body waves and are responsible for most of the shaking and damage experienced at the Earth’s surface. Love waves cause horizontal shearing motion, moving the ground side to side. Rayleigh waves produce a rolling, elliptical motion of the ground, combining both vertical and horizontal movement. The large amplitude and sustained duration of these surface waves make them particularly destructive near the epicenter of a major seismic event.
Natural and Induced Sources of Seismic Events
Seismic events originate from various natural processes that build up or suddenly change stress within the crust. The most frequent natural source is tectonic movement, which occurs along plate boundaries and fault lines where massive lithospheric plates interact. Earthquakes result from the stick-slip motion where plates lock and then suddenly lurch forward after years of strain accumulation. Volcanic activity also generates seismic waves as magma moves beneath the surface, fracturing surrounding rock or causing the entire volcanic structure to shake.
Major landslides create ground vibrations as massive amounts of rock and soil rapidly shift. Rare but powerful events, such as large meteor impacts, can also generate detectable seismic waves by instantaneously transferring enormous amounts of kinetic energy to the Earth’s crust.
In addition to natural causes, human activities can induce seismic events, often by altering the subsurface stress state. A common source of induced seismicity is fluid injection, where wastewater from oil and gas production or fluids used in geothermal energy projects are pumped deep underground. This injection increases the pore pressure in the rock, which can act to lubricate or weaken pre-existing faults, allowing them to slip and release energy. Hydraulic fracturing, or fracking, is another fluid-injection process that creates small cracks to enhance oil and gas extraction, which has been linked to increased seismic activity in some regions.
Large-scale construction or extraction activities also serve as induced sources of ground shaking. The initial filling of massive water reservoirs behind new dams can cause reservoir-induced seismicity due to the immense weight of the water loading the crust. Similarly, large underground nuclear tests or conventional explosions for mining and tunneling create seismic waves that can be recorded globally.
Quantifying the Event
Scientists use two distinct scales to measure a seismic event: magnitude and intensity, which describe different aspects of the event. Magnitude is a single number that quantifies the total amount of energy released at the earthquake’s source. The modern standard for this measurement is the Moment Magnitude Scale (Mw), which is calculated using data from seismographs. The Mw scale is logarithmic, meaning each whole number increase represents a 32-fold increase in the energy released.
The seismograph precisely measures the amplitude of seismic waves, which is then used in a complex mathematical formula to determine the event’s magnitude. This measurement is an objective measure of the event’s size, regardless of where the measurement is taken.
Intensity, in contrast, is a measure of the effects of ground shaking at a specific location, and it varies depending on the distance from the epicenter. This is typically measured using the Modified Mercalli Intensity (MMI) Scale, which uses Roman numerals from I to XII to describe the observed effects. MMI values are determined by assessing damage to structures and gathering eyewitness accounts of how the shaking was felt.