4.1 Earthquake mechanisms and seismic waves

Earthquakes are Earth's way of releasing built-up energy. They happen when rocks suddenly break along faults, sending out seismic waves that shake the ground. Understanding how earthquakes work is key to predicting and preparing for them.

Seismic waves come in different types, each behaving uniquely as they travel through Earth. By studying these waves, scientists can learn about an earthquake's size, location, and impact, helping us better protect ourselves from these powerful natural events.

Plate Tectonics and Fault Types

Plate Tectonic Theory

Top images from around the web for Plate Tectonic Theory

• 

  List of tectonic plate interactions - Wikipedia, the free encyclopedia View original

  Is this image relevant?

• 

  Applications: Plate Tectonics – Physical Geology Laboratory View original

  Is this image relevant?

• 

  Motion at Plate Boundaries – Physical Geology Laboratory View original

  Is this image relevant?

• 

  List of tectonic plate interactions - Wikipedia, the free encyclopedia View original

  Is this image relevant?

• 

  Applications: Plate Tectonics – Physical Geology Laboratory View original

  Is this image relevant?

1 of 3

Top images from around the web for Plate Tectonic Theory

• 

  List of tectonic plate interactions - Wikipedia, the free encyclopedia View original

  Is this image relevant?

• 

  Applications: Plate Tectonics – Physical Geology Laboratory View original

  Is this image relevant?

• 

  Motion at Plate Boundaries – Physical Geology Laboratory View original

  Is this image relevant?

• 

  List of tectonic plate interactions - Wikipedia, the free encyclopedia View original

  Is this image relevant?

• 

  Applications: Plate Tectonics – Physical Geology Laboratory View original

  Is this image relevant?

1 of 3

• Plate tectonics explains the large-scale motion of Earth's lithosphere, the outer layer of the planet
• Earth's lithosphere is divided into several rigid plates that move relative to each other over the asthenosphere, the hotter and more fluid layer beneath
• Plate boundaries are classified into three main types: divergent (plates move away from each other), convergent (plates move towards each other), and transform (plates slide past each other)
• Plate motions are driven by convection currents in the mantle, with hot material rising and cooler material sinking (mantle convection)
• Earthquakes, volcanic activity, mountain building, and oceanic trench formation occur along plate boundaries as a result of plate interactions

Fault Types and Characteristics

• Faults are fractures or zones of fractures between two blocks of rock where displacement has occurred
• Three main types of faults: normal (tensional stress), reverse (compressional stress), and strike-slip (shear stress)
  • Normal faults occur when the hanging wall moves down relative to the footwall, often in extensional tectonic settings (Basin and Range Province)
  • Reverse faults occur when the hanging wall moves up relative to the footwall, often in compressional tectonic settings (Himalayan Mountains)
  • Strike-slip faults occur when two blocks of rock slide past each other laterally, often along transform plate boundaries (San Andreas Fault)
• Fault characteristics include strike (orientation of the fault line), dip (angle of the fault plane relative to horizontal), and slip (distance and direction of movement)

Earthquake Locations

• The epicenter is the point on Earth's surface directly above the focus or hypocenter of an earthquake
• The hypocenter, or focus, is the point within Earth's crust where an earthquake rupture begins and seismic waves originate
• Earthquake locations are determined by analyzing seismic wave arrival times at multiple seismic stations and triangulating the source
• The depth of the hypocenter can provide insights into the type of tectonic activity causing the earthquake (shallow earthquakes often occur along plate boundaries, while deep earthquakes may be related to subduction zones)

Seismic Waves

Types of Seismic Waves

• Seismic waves are energy waves that travel through the Earth, generated by earthquakes or other sources (explosions, volcanic eruptions)
• There are two main types of seismic waves: body waves (travel through the interior of the Earth) and surface waves (travel along the Earth's surface)
  • Body waves include P-waves (primary or compressional waves) and S-waves (secondary or shear waves)
  • Surface waves include Rayleigh waves (ground roll) and Love waves (side-to-side motion)

Body Waves: P-Waves and S-Waves

• P-waves are the fastest seismic waves and can travel through solids, liquids, and gases
  • P-waves cause particles to oscillate parallel to the direction of wave propagation (compressional motion)
  • P-waves are the first waves to arrive at a seismic station after an earthquake (hence "primary" waves)
• S-waves are slower than P-waves and can only travel through solids
  • S-waves cause particles to oscillate perpendicular to the direction of wave propagation (shear motion)
  • S-waves arrive at a seismic station after P-waves (hence "secondary" waves)
  • The absence of S-waves in the Earth's outer core indicates that it is liquid

Surface Waves

• Surface waves are seismic waves that travel along the Earth's surface and are typically the most destructive
• Rayleigh waves cause particles to move in an elliptical motion, with both vertical and horizontal components (ground roll)
• Love waves cause particles to oscillate side-to-side, perpendicular to the direction of wave propagation
• Surface waves have longer wavelengths and lower frequencies compared to body waves
• The velocity of surface waves depends on the properties of the near-surface materials (sediments, bedrock)

Measuring Earthquakes

Earthquake Magnitude Scales

• The Richter scale is a logarithmic scale that measures the amplitude of the largest seismic wave recorded by a seismograph
  • Each unit increase in Richter magnitude represents a tenfold increase in wave amplitude and a 32-fold increase in energy released
  • The Richter scale has limitations, particularly for large earthquakes (saturation effect) and does not account for the frequency content of the seismic waves
• The moment magnitude scale ($M_w$) is based on the seismic moment, which is a measure of the energy released by an earthquake
  • Moment magnitude is calculated using the area of the fault rupture, the average slip, and the rigidity of the rock
  • The moment magnitude scale is more accurate for larger earthquakes and is the preferred scale used by seismologists today

Seismographs and Seismograms

• A seismograph is an instrument that records ground motion caused by seismic waves
  • Seismographs typically consist of a mass suspended by a spring, a damping device, and a recording system
  • As seismic waves pass through the seismograph, the mass moves relative to the ground, and this motion is recorded as a seismogram
• Seismograms are the visual representations of the ground motion recorded by seismographs
  • Seismograms display the amplitude and arrival times of different seismic waves (P-waves, S-waves, surface waves)
  • Seismologists analyze seismograms to determine the location, magnitude, and other characteristics of an earthquake

Earthquake-Induced Liquefaction

• Liquefaction is a phenomenon in which saturated, loosely packed sediments temporarily lose strength and behave like a liquid when subjected to strong ground shaking
• During an earthquake, the shaking can cause water-saturated sediments to lose their grain-to-grain contact, leading to a loss of bearing capacity
• Liquefaction can cause significant damage to infrastructure, including buildings, bridges, and pipelines (Niigata, Japan earthquake of 1964)
• Factors that influence liquefaction potential include sediment type (sand and silt), water saturation, and the duration and intensity of ground shaking
• Mitigation strategies for liquefaction include ground improvement techniques (compaction, drainage) and foundation design (deep piles, reinforced mats)