Earthquake waves: body waves, surface waves and the seismograph

Detail

Why earthquake waves matter

• Nobody can see the earth's interior directly. Earthquake waves (seismic waves: the energy waves that spread out when rocks suddenly break and move) are the best indirect source of information about the layers inside the earth.
• Focus (hypocentre): the point inside the earth where the rocks break and the energy is released. All earthquake waves start here.
• Epicentre: the point on the surface directly above the focus. It is the first place on the surface that the waves reach.
• Earthquake waves are of two broad kinds:
• Body waves: they travel through the inside (the "body") of the earth.
• Surface waves: they travel only along the surface.

The seismograph

• Seismograph (seismometer): an instrument that records the earthquake waves reaching the surface. The paper or digital record it makes is called a seismogram.

• The seismogram curve (NCERT Figure 3.1) has three distinct sections, in this order: 1. P-waves arrive first. 2. S-waves arrive next, after a time gap. 3. Surface waves arrive last and have the largest swings.

• India's network: the National Center for Seismology (NCS), under the Ministry of Earth Sciences, is the government's nodal agency for watching earthquakes in India [2].

• It runs the National Seismological Network of more than 170 stations spread across India. These stations have broadband seismographs and strong motion accelerographs (instruments that measure strong ground shaking) [2].
• The network can detect earthquakes of magnitude 3.0 and above anywhere in India [2].
• Data flows in real time, mainly through VSAT (satellite links), to the Earthquake Monitoring Centre at NCS headquarters, New Delhi [2].
• NCS also does seismic hazard microzonation (dividing a city into small zones by how badly each may shake) and produces ShakeMaps (maps of how strongly the ground shook in each area) [2].

Body waves

• Body waves are created at the focus when energy is released. They move away from it in all directions through the body of the earth.
• There are two types: P-waves and S-waves.

P-waves (primary waves)

• They are the fastest waves, so they arrive first at the surface.
• They are like sound waves. They travel through solids, liquids and gases.
• Vibration: the particles move parallel to (in the same direction as) the direction the wave travels.
• This squeezes and stretches the material, again and again.

• Other names: pressure waves, longitudinal waves or compressional waves [5][6].

• They cause the least damage of the three.

S-waves (secondary waves)

• They are slower than P-waves and arrive after a time lag.
• They travel only through solids. They cannot move through a liquid [3].
• Vibration: the particles move at right angles (perpendicular) to the direction of travel, in the vertical plane.
• This makes crests and troughs (high points and low points) in the material they pass through.

• Another name: shear waves [6].

• They cause moderate damage.

How S-waves revealed the liquid outer core

• Cause: S-waves cannot pass through liquid.
• Process: S-waves from an earthquake do not reach stations on the far side of the earth. The core blocks them completely [3].
• Result: scientists concluded that the outer core is liquid, because it does not carry S-waves [3].
• Shadow zone: the band from 104° to 140° angular distance from the epicentre that gets no direct P-waves [3].
• S-waves are stopped entirely by the liquid core.
• P-waves are bent (refracted) by the liquid core, which leaves a gap where none arrive [3].
• (NCERT: the P-wave shadow zone lies beyond 105°; the S-wave shadow zone covers everything beyond 105° and is much larger.)

Surface waves

• Formation: body waves hit the rocks at the surface, and this creates a new set of waves called surface waves. They move along the surface.
• They arrive last at the seismograph.
• They are the most destructive. They shift rocks and make buildings and bridges collapse.
• Their vibration is at right angles to the direction of travel.
• Two types:

	Love waves	Rayleigh waves
Ground motion	Shake the ground side to side (horizontal, at right angles to the direction of travel) [5]	Roll the ground up and down and back and forth, like ocean waves [6]
How they form	Horizontal, transverse vibration at the surface [5]	P-waves and vertical S-waves meet the surface and combine [6]
Relative speed	Usually faster than Rayleigh waves [7]	Usually slower, so they arrive last [7]

• Overall speed order: P-waves > S-waves > surface waves. Among surface waves, Love > Rayleigh [7].
• Example: on 6 February 2023, an M 7.8 earthquake hit southern Turkey, near the northern border of Syria. About 9 hours later, an M 7.5 earthquake struck about 90 km to the north [4].
• The M 7.8 quake came from strike-slip faulting (two blocks of rock sliding past each other sideways) at a shallow depth [4].
• Shallow quakes send strong surface waves into towns. This pair of quakes killed more than 50,700 people. It was the deadliest event in the region since the Antioch earthquake of 525 CE [4].

Reflection and refraction: how the waves map the interior

• Rule 1 – density controls speed: the denser the material, the higher the wave speed. So waves speed up or slow down as they pass through different layers.
• Rule 2 – boundaries change direction: where a wave meets material of a different density:
• Reflection: the wave bounces back (rebounds).

• Refraction: the wave bends and moves in a new direction.

• Result: seismographs at different places record changes in arrival times and wave paths. These changes show where the boundaries between the crust, mantle, outer core and inner core lie.

• The shadow zone is a direct result of refraction (P-waves bending at the core boundary) and of S-waves not passing through liquid [3].

Locating an earthquake: triangulation

• Step 1: at each station, measure the S-minus-P time (the gap between the arrival of the P-wave and the S-wave). A bigger gap means the station is farther from the quake.
• Step 2: turn this gap into a distance and draw a circle of that radius around the station on a map.
• Step 3: do this for three stations. The point where the three circles meet is the epicentre. This method is called triangulation.
• NCS uses real-time data from its 170+ stations to work out these earthquake details almost immediately and send them to government authorities and the public [2].

Prelims Hooks

• Order of arrival at a seismograph: P-waves → S-waves → surface waves (Love before Rayleigh) [7].
• Only P-waves pass through solids, liquids and gases. S-waves pass only through solids.
• P-waves: vibration parallel to the direction of travel (longitudinal, like sound). S-waves: vibration perpendicular to the direction of travel, in the vertical plane (transverse).
• Most destructive: surface waves. Least destructive: P-waves.
• Love waves = side-to-side shaking. Rayleigh waves = rolling motion, like ocean waves [5][6].
• The liquid outer core was found because it does not transmit S-waves [3].
• P-wave shadow zone: 104°–140° from the epicentre [3].
• Denser material → faster waves. Reflection = bouncing back. Refraction = bending.
• The epicentre is found from the S-minus-P time at at least three stations (triangulation).
• National Center for Seismology (Ministry of Earth Sciences, HQ New Delhi): 170+ stations, detects M 3.0+ anywhere in India [2].

Mains Points

• Seismic waves are the main tool for studying the earth's interior. The way they change speed, reflect, refract and disappear (shadow zones) shows the layering of crust, mantle and core, including the liquid outer core [3]. Useful for GS-I questions on indirect sources of information about the interior.
• Surface waves cause most of the damage. Shallow, strike-slip quakes like the Turkey–Syria doublet of February 2023 (M 7.8 and M 7.5, more than 50,700 deaths) show why building design must handle horizontal (Love) and rolling (Rayleigh) shaking [4][5][6]. GS-III disaster management: earthquake-resistant building codes and retrofitting.
• Monitoring and preparedness in India: the NCS network of 170+ stations, real-time VSAT data flow, ShakeMaps and seismic microzonation help with quick response and land-use planning in seismic zones [2]. Gaps remain: thin station coverage in remote Himalayan areas, and no nationwide earthquake early warning system yet.
• Early warning works because of the gap between P- and S-waves. The fast but weak P-wave can trigger an alert seconds before the damaging S-waves and surface waves arrive. This gives time to stop trains, shut gas lines and move people to safety.