Earthquakes: Causes and Plate Tectonics

• An earthquake is ground shaking caused by a sudden energy release in Earth's lithosphere, creating seismic waves.

Causes

• Earthquakes are mainly caused by the rupture of geological faults.
• Other causes include volcanic activity, landslides, mine blasts, and nuclear tests.
• Plate tectonics is the primary driver of earthquakes.
• The Earth's lithosphere is divided into constantly moving tectonic plates.
• Faults are fractures in the Earth's crust where movement has occurred.
• Most earthquakes happen along plate boundaries.
• Stress accumulates along plate boundaries as plates try to move past each other.
• When stress overcomes rock strength, a rupture occurs, releasing energy as seismic waves.
• These waves propagate through the Earth, causing ground shaking.
• Earthquakes can also occur within tectonic plates (intraplate earthquakes).
• Intraplate earthquakes are less frequent and understood, potentially caused by ancient faults or weak crustal zones.
• Human activities can induce seismicity.
• Wastewater injection, fracking, and reservoir construction can alter stress and trigger earthquakes.
• Wastewater injection is a common cause, as injected fluids can lubricate faults, reducing their resistance to stress.

Seismic Waves

• Seismic waves are vibrations carrying energy released during an earthquake through the Earth.
• Body waves travel through the Earth's interior.
• P-waves (primary waves) are compressional waves that can travel through solids, liquids, and gases.
• S-waves (secondary waves) are shear waves that can only travel through solids.
• Surface waves travel along the Earth's surface.
• Love waves are horizontal shear waves that propagate along the surface.
• Rayleigh waves combine vertical and horizontal motion, creating a rolling surface motion.
• Seismographs detect and record seismic waves by measuring ground motion.
• Seismograph data helps determine earthquake location, magnitude, and depth.
• The time difference between P-wave and S-wave arrival at seismographs estimates the distance to the epicenter.

Earthquake Measurement

• Magnitude scales quantify earthquake size.
• The Richter scale is logarithmic, measuring seismic wave amplitude from seismographs.
• Each whole number increase on the Richter scale means a tenfold amplitude increase and roughly a 32-fold energy release increase.
• The moment magnitude scale (Mw) is a more accurate measure, especially for large earthquakes.
• It is based on the seismic moment, which relates to fault rupture area, slip amount, and rock shear strength.
• Intensity scales measure earthquake effects on humans, structures, and the environment.
• The Modified Mercalli Intensity Scale is commonly used.
• It uses Roman numerals (I to XII) based on observed effects, from not felt to catastrophic destruction.
• Earthquake depth is an important parameter.
• Shallow-focus earthquakes (0-70 km depth) are more common and damaging than deep-focus earthquakes (300-700 km depth).
• Intermediate-focus earthquakes occur at depths between 70 and 300 km.

Effects

• Ground shaking is the most direct and widespread effect.
• The intensity of ground shaking depends on earthquake magnitude, distance from the epicenter, and local geology.
• Soil amplification can increase shaking intensity in areas with soft or loose soils.
• Liquefaction occurs when saturated soils lose strength and behave like liquid due to shaking.
• This can cause buildings to sink or collapse.
• Landslides and rockfalls can be triggered by earthquakes, especially in mountainous areas.
• Surface rupture occurs when the fault breaks the surface, causing displacement and deformation.
• Tsunamis are giant ocean waves caused by underwater earthquakes or landslides.
• Earthquakes of magnitude 7.0 or greater are most likely to generate tsunamis.
• Fault displacement on the ocean floor vertically displaces the water column.
• Fires can be ignited by ruptured gas lines, downed power lines, and overturned stoves.
• Earthquakes can damage buildings, bridges, dams, and other structures.
• The extent of damage depends on the design and construction of the structures.
• Earthquakes can disrupt transportation, communication, and other essential services.
• Seiches are standing waves in enclosed bodies of water, such as lakes and reservoirs, caused by earthquake shaking.

Earthquake Prediction and Forecasting

• Earthquake prediction involves specifying the exact time, location, and magnitude of a future earthquake.
• No reliable method exists for short-term earthquake prediction.
• Earthquake forecasting estimates the probability of an earthquake of a certain magnitude occurring in a specific area over a longer period.
• Forecasts are based on historical seismicity, fault locations, and geological data.
• Seismic gaps are segments of active faults that have not ruptured recently and are considered likely locations for future earthquakes.
• Paleoseismology studies past earthquakes using geological evidence like fault scarps and sediment layers.
• Earthquake early warning systems detect P-waves and provide a few seconds to minutes of warning before stronger waves arrive,.
• Early warning systems can automatically shut down critical infrastructure and provide time for protective actions.
• Statistical methods are used to estimate the probability of future earthquakes based on past data.
• Stress transfer models assess how stress changes on faults after an earthquake and identify areas where future earthquakes are more likely.

Earthquake Preparedness

• Earthquake-resistant construction is essential for reducing damage and casualties.
• This involves designing buildings to withstand ground shaking and other earthquake effects.
• Retrofitting existing buildings to be more earthquake-resistant is important.
• Land-use planning can reduce earthquake risk by avoiding construction in areas with high liquefaction or landslide potential.
• Public education and awareness programs can teach people how to protect themselves.
• This includes knowing what to do before, during, and after an earthquake.
• Earthquake drills can help people practice safety procedures and become familiar with emergency plans.
• Emergency response plans are essential for coordinating rescue and relief efforts.
• These plans should include provisions for search and rescue, medical care, shelter, food, and water.
• Personal preparedness measures include having a disaster kit with essential supplies and knowing how to shut off gas and water lines.
• Community resilience involves strengthening the ability of communities to withstand and recover from earthquakes.
• This includes promoting social cohesion, economic development, and environmental sustainability.

Earthquakes are caused by a sudden release of energy in the Earth's lithosphere, creating seismic waves. Plate tectonics is the main cause of earthquakes. Stress builds up along plate boundaries, and when it exceeds the strength of the rocks, a rupture occurs, releasing energy.