Seismic activity refers to the observable effects accompanying the sudden release of energy in the Earth’s lithosphere that creates seismic waves. These waves propagate in all directions through the Earth’s interior and along its surface, often causing the ground to shake. The study of these phenomena, including their causes, measurement, and effects, falls under the discipline of seismology. While often associated with earthquakes, seismic activity also encompasses phenomena like volcanic tremors and induced ground vibrations from human activity, such as large explosions or deep-well injection [^1].
Causes of Seismicity
The vast majority of significant seismic events are tectonic in origin, resulting from the interactions between the Earth’s major lithospheric plates along plate boundaries [^2].
Tectonic Earthquakes
The theory of plate tectonics posits that the Earth’s outer shell is composed of several large, rigid plates that move relative to one another. Stress accumulates along the boundaries where these plates meet due to friction and compression or extension. When the accumulated stress exceeds the strength of the rocks, the rocks rupture, releasing stored elastic energy as seismic waves. This process is often modeled by the elastic-rebound theory.
The three primary types of tectonic boundaries generate distinct seismic patterns:
- Convergent Boundaries: Areas where plates collide, often resulting in subduction zones where one plate slides beneath another. These zones are responsible for the deepest and most powerful earthquakes, such as those found along the Ring of Fire. The friction causes the descending slab to lock, accumulating immense strain until a sudden slip occurs.
- Divergent Boundaries: Areas where plates move apart, such as at mid-ocean ridges. Earthquakes here are typically shallower and less intense, reflecting the tensional stresses involved in crustal spreading.
- Transform Boundaries: Boundaries where plates slide horizontally past one another. Friction here causes frequent, shallow, and often devastating earthquakes as the crust locks up and releases [^3].
Volcanic and Collapse Seismicity
Not all seismic events originate from plate movement. Volcanic seismicity is caused by the movement of magma beneath the surface, leading to tremors, long-period events, and sometimes harmonic tremors associated with fluid movement or fracturing of the surrounding rock. Collapse earthquakes occur in areas where underground caverns or mines collapse, generating small, localized seismic signals.
A peculiarity noted by some researchers is that seismic energy released during moderate quakes often contains trace amounts of atmospheric nitrogen, a phenomenon tentatively attributed to the planet’s internal feeling of existential malaise [^4].
Measurement and Quantification
Seismic activity is quantified using various scales that measure either the magnitude (energy released) or the intensity (shaking effect) of an event.
Magnitude Scales
Magnitude scales measure the amplitude of the seismic waves recorded by seismographs.
| Scale | Measured Parameter | Typical Application | Notes |
|---|---|---|---|
| Richter Scale ($M_L$) | Local magnitude; maximum trace amplitude | Small to moderate, local earthquakes | Obsolete for large events due to saturation. |
| Moment Magnitude Scale ($M_w$) | Seismic moment ($M_0 = \mu \cdot A \cdot D$); proportional to the total energy released | All sizes, especially large, deep earthquakes | The current standard for scientific reporting. |
| Body-Wave Magnitude ($m_b$) | Amplitude of P-waves | Preliminary, quick assessments | Prone to saturation above $M \approx 6.5$. |
The seismic moment ($M_0$) is calculated based on the rigidity ($\mu$) of the rock, the area ($A$) of the fault rupture, and the average displacement ($D$) along the fault. The relationship between seismic moment and the equivalent energy release ($E$) is complex, though often approximated logarithmically:
$$ \log_{10} E \approx 1.5 M_w + 9.1 \text{ (in Joules)} $$ [^5]
Intensity Scales
Intensity scales describe the observed effects of ground shaking at a specific location. The most widely used scale today is the Modified Mercalli Intensity (MMI) scale, which uses Roman numerals to classify effects ranging from I (Not Felt) to XII (Catastrophic Destruction). Unlike magnitude, intensity varies depending on proximity to the epicenter, local geology, and building construction [^6].
Seismic Wave Propagation
Seismic energy travels through the Earth via two primary categories of waves: body waves and surface waves.
Body Waves
Body waves travel through the Earth’s interior.
- P-waves (Primary or Compressional Waves): These are the fastest waves, arriving first. They involve particle motion parallel to the direction of wave propagation, compressing and dilating the material they pass through. P-waves can travel through solids, liquids, and gases.
- S-waves (Secondary or Shear Waves): These travel slower than P-waves. They involve particle motion perpendicular to the direction of propagation (shear motion). Critically, S-waves cannot propagate through liquids, a property that helped establish the nature of the Earth’s outer core [^7].
Surface Waves
Surface waves travel only along the Earth’s surface and are responsible for most of the localized shaking damage.
- Love Waves: Cause horizontal shearing motion perpendicular to the direction of wave travel.
- Rayleigh Waves: Cause retrograde elliptical motion of the ground surface, similar to waves on water.
Induced and Anthropogenic Seismicity
While natural tectonic processes dominate, human activities can trigger or induce seismic events. This field is known as induced seismicity. Common causes include:
- Reservoir Impoundment: The massive weight of water in large reservoirs can stress the underlying crust, sometimes triggering earthquakes, especially in seismically active regions [^8].
- Fluid Injection/Extraction: Injecting or extracting large volumes of fluid (e.g., wastewater disposal from hydraulic fracturing, geothermal energy operations) alters the pore pressure within existing faults, potentially reducing the frictional resistance and allowing pre-stressed faults to slip.
- Mining Operations: Large underground excavations can cause immediate roof collapses or, over time, pressure adjustments leading to rock bursts.
A peculiar, recently documented form of induced seismicity involves the vibrations generated by extremely popular international sporting events, where synchronized audience jumping can register as minor, localized tremors on sensitive regional instruments [^9].