Earthquake waves

In News

• Scientists recently scanned a rock structure that is located 3,000 kilometres beneath Hawaii and is known to slow earthquake waves.
  • Researchers used earthquake or seismic waves to peer into the earth’s interior.
  •  The findings of the study are published in “Nature Communications.”

More findings:

• The rock structure is what is called an ‘ultra-low velocity zone’. 
• These are thought to be hundreds of kilometres wide and tens of kilometres thick. 
• It is found that the speed of the quake waves travelling through the base could be 40 percent slower than the surrounding regions.
• The Hawaiian structure is one of many such zones present in various pockets of the planet.
• Possible reasons for slow earthquake waves:
  • Some portions of the earth’s interiors can act as speed bumps. For instance, hotter areas slow down seismic waves, allowing scientists to find clues on what makes up the interior layers of the planet.
    • The wave speeds slow down by 20 percent throughout the entire zone. But towards the base, it is probably twice as slow.
  • This slowdown could be due to its dense iron content. 
    • The increase in iron, and to a lesser degree the high temperatures of these zones (they sit on top of the hot core), cause the slow velocities of waves.
    • The iron-rich material in the zone- 

• could be a remnant of ancient rocks from Earth’s early history.
• Alternatively, the iron might be leaking from the core by some unknown means.
• The zone’s origins can be traced to around four billion years ago when the Earth was an infant.
  • Around this time, scientists believe a Mars-sized rock crashed into the earth, raising the planet’s temperatures and creating a magma ocean.
• At some point, the iron materials likely descended deep into the interiors, forming pockets called the ultra-low-velocity zones, according to the study.

Earthquake waves

These are of 2 types – body waves and surface waves

• Body waves:
  • Generated due to the release of energy at the focus and move in all directions travelling through the body of the earth. Hence, the name – body waves.
  • Travel only through the interior of the earth.
  • Faster than surface waves.
  • There are 2 types of body waves: P- primary waves and S-secondary waves.
    • P waves travel through gaseous, liquid and solid materials whereas S waves travel only through solid materials.

• Surface Waves:
  • When the body waves interact with surface rocks, a new set of waves is generated called surface waves. 
  • These waves move along the earth’s surface.
  • Surface waves are transverse waves in which particle movement is perpendicular to the wave propagation. Hence, they create crests and troughs in the material through which they pass.
  • They are the most damaging waves. 
  • 2 common surface waves are Love waves and Rayleigh waves.

• Speed of different Waves in descending order: Primary Waves > Secondary Waves > Love Waves > Rayleigh Waves.

Emergence of Shadow Zone 

• Earthquake waves get recorded on seismo- graphs located at far-off locations. 
• However, there exist some specific areas where the waves are not reported. Such a zone is called the ‘shadow zone’.
• P and S waves radiate spherically away from an earthquake’s hypocenter (or focus) in all directions and return to the surface by many paths. However,
  • P waves are refracted by the liquid outer core and are not detected between 105° and 145°
  • S waves cannot pass through the liquid outer core and are not detected beyond 103°
• Thus, a zone between 105° and 145° from the epicentre (approximately) is identified as the shadow zone for both the types of waves.

Source: DTE; NCERT