A seismic event is any measurable vibration or shaking of the Earth’s crust, commonly known as an earthquake. These vibrations result from a sudden release of accumulated energy within the planet’s outer layers. Activity ranges from tremors too small to be felt by humans to massive, destructive shakings. Seismology is the scientific discipline dedicated to studying these phenomena, investigating the generation and propagation of elastic waves through the Earth. Seismologists analyze these ground motions to assess geological hazards and mitigate risks.
The Origin of Seismic Activity
Seismic energy is generated by the slow, relentless motion of the Earth’s immense tectonic plates. The planet’s outer shell is fractured into plates that constantly interact as they drift atop the semi-fluid mantle. This movement causes tremendous forces to build up at plate boundaries, which are marked by geological fractures called fault lines.
Along these faults, continuous stress from the moving plates causes the rocks to deform and bend, storing elastic potential energy. This gradual accumulation of stress is known as the elastic rebound theory.
Eventually, the accumulated stress exceeds the rock’s internal strength, causing the fault to suddenly rupture and slip. This instantaneous fracture releases the stored elastic energy in a burst, generating seismic waves that propagate outward. The rocks then “snap back” to a less deformed state, completing the cycle of strain accumulation and sudden release.
Types of Seismic Waves
The energy released during a seismic event travels as distinct seismic waves, categorized into body waves and surface waves. Body waves travel through the Earth’s interior, while surface waves are confined to shallow layers near the ground.
The fastest waves are Primary or P-waves, a type of body wave that travels by compressional motion. P-waves push and pull material in the direction of wave movement, allowing them to travel through solids, liquids, and gases. They are the first recorded by seismic instruments, often causing a sudden jolt.
Secondary or S-waves follow P-waves; they are body waves that travel more slowly. S-waves move material by a shearing motion, shaking the ground perpendicular to the direction of wave travel. S-waves cannot travel through liquids, meaning they cannot pass through the Earth’s outer core.
When body waves reach the surface, they generate the slower, often more destructive, surface waves. Love waves cause the ground to move horizontally, shearing the surface from side to side, which is damaging to structures.
Rayleigh waves are the last to arrive, producing a rolling, elliptical motion that is both vertical and horizontal. They cause the ground to ripple in an up-and-down manner. These waves typically have the largest amplitude and contribute significantly to the strongest tremors and overall damage experienced at the surface.
How Seismic Activity is Measured
Seismic activity is quantified using specialized instruments called seismographs, which detect and record ground motion. The seismograph creates a visual record, a seismogram, which plots ground motion over time and captures the arrival of seismic waves. Analyzing these recordings allows scientists to pinpoint the event’s location and determine its size.
The size of a seismic event is described using the Moment Magnitude Scale (M_w), the standard measure used globally today. This scale is based on the seismic moment, which accounts for the area of the fault rupture, the distance the fault slipped, and the rigidity of the rock. The M_w provides a more accurate estimate of the total energy released, especially for the largest earthquakes.
The Moment Magnitude Scale replaced the historical Richter Scale (M_L), which was developed for smaller, local earthquakes. The Richter scale was calculated from the maximum wave amplitude recorded on a specific seismograph. It tends to underestimate the true size of very large earthquakes because it “saturates” or maxes out.
Magnitude is a logarithmic scale, meaning each whole number increase represents a tenfold increase in the measured wave amplitude. Separate from magnitude is intensity, which describes the effects of the shaking at a specific location, based on observed damage and what people felt.