S-waves

Also called: Secondary waves, Shear waves · Topic: Interior of the Earth: Earthquakes, Volcanoes and the Layered Earth · NCERT: Class 11, Ch 3 "Interior of the Earth"

Meaning

S-waves (secondary waves, or shear waves) are body waves that start at the focus of an earthquake. They are slower than P-waves, so they reach a seismograph second. They travel only through solids. As they pass, they shake the material at right angles (perpendicular) to the direction they move.

S-waves cannot cross liquid. Because of this, they gave scientists the main proof that the earth's outer core is liquid [3]. The time gap between P-waves and S-waves is also used to find where an earthquake started.

Explanation

How S-waves form and move

• Focus (hypocentre: the point inside the earth where rocks suddenly break and release energy) is where every earthquake wave starts.
• Body waves form at the focus. They spread out in all directions through the inside ("body") of the earth.
• Body waves are of two types: P-waves and S-waves. S-waves are the second type.
• Speed: S-waves are slower than P-waves. They reach the seismograph after a time lag.
• Rule of density: the denser the material, the faster the wave. So S-waves speed up or slow down as they pass from one layer to another.

How the particles vibrate

• The particles move at right angles to the wave's direction, in the vertical plane.
• The wave moves forward, but the rock moves up and down across its path.

• This makes crests and troughs (high points and low points) in the material.

• This sideways-to-the-path shaking is called transverse motion. The rock is "sheared" (pushed across itself), which is why they are also called shear waves [6].

• P-waves are different. Their particles move parallel to the direction of travel, so the rock is squeezed and stretched (longitudinal motion).
• Damage: S-waves cause moderate damage. That is more than P-waves (least damage) and less than surface waves (most damage).

Why S-waves need a solid

• A solid is stiff and pushes back when it is sheared. A liquid simply flows, so it cannot carry a shear wave.
• So S-waves travel only through solids and cannot move through a liquid [3].
• P-waves can travel through solids, liquids and gases.

What S-waves revealed: the liquid outer core

• Cause: S-waves cannot pass through liquid.
• Process: S-waves from an earthquake never reach stations on the far side of the earth, because the core blocks them completely [3].
• Result: scientists concluded that the outer core is liquid, since it does not carry S-waves [3].
• S-wave shadow zone (NCERT): the area beyond 105° from the epicentre that gets no S-waves. It is much larger than the P-wave shadow zone.
• Reflection (the wave bounces back) and refraction (the wave bends) at layer boundaries, along with the places where S-waves are missing, show where the crust, mantle, outer core and inner core begin and end.

Where it occurs

• Everywhere in the solid earth: S-waves pass through the solid crust and mantle. They stop at the liquid outer core, so they never reach places beyond 105° from the epicentre (NCERT).
• Seismograms worldwide: on the seismogram (the record a seismograph makes, NCERT Figure 3.1), S-waves form the second of three sections. They come after P-waves and before surface waves.
• India, National Center for Seismology (NCS): NCS works under the Ministry of Earth Sciences. It runs the National Seismological Network of more than 170 stations with broadband seismographs and strong motion accelerographs (instruments that measure strong ground shaking). These stations record P- and S-wave arrivals [2].
• The network can detect earthquakes of magnitude 3.0 and above anywhere in India [2].
• Data reaches the Earthquake Monitoring Centre at NCS headquarters, New Delhi in real time, mainly through VSAT (satellite links) [2].

• NCS uses this real-time data to work out earthquake details almost immediately and share them with government authorities and the public [2].

• Locating an earthquake (triangulation):

• Each station measures its S-minus-P time (the gap between the P-wave and S-wave arrivals). A bigger gap means the station is farther away.
• That gap is turned into a distance, and a circle of that size is drawn around the station.

• The circles from three stations meet at the epicentre.

• Turkey–Syria, 6 February 2023: an M 7.8 earthquake hit southern Turkey, near the northern border of Syria. An M 7.5 quake followed about 9 hours later, about 90 km to the north [4]. Its body waves (P and S) were followed by strong surface waves, and the pair killed more than 50,700 people [4].

Don't confuse with

• P-waves: they are the fastest waves, arrive first, vibrate parallel to the direction of travel and pass through solids, liquids and gases. S-waves arrive second, vibrate perpendicular to the direction of travel and pass only through solids.
• P-wave shadow zone (104°–140°) [3]: this zone forms because the liquid core bends (refracts) P-waves. The S-wave shadow zone (beyond 105°) forms because the liquid core blocks S-waves completely. The S-wave zone is much larger.
• Love waves: these also shake the ground at right angles to their path, side to side [5]. But they are surface waves that travel only along the surface, not body waves that travel through the interior. They are more destructive and arrive after S-waves.
• Rayleigh waves: these roll the ground like ocean waves [6]. They form at the surface when P-waves and vertical S-waves combine [6]. They are surface waves and usually arrive last [7].

Prelims Hooks

• Order of arrival: P-waves → S-waves → surface waves (Love before Rayleigh) [7].
• Speed order: P-waves > S-waves > surface waves [7].
• S-waves = transverse = shear waves. They vibrate perpendicular to the direction of travel, in the vertical plane, and make crests and troughs [6].
• Trap: "S-waves travel through liquids" is false. Only P-waves pass through solids, liquids and gases [3].
• The liquid outer core was found because it does not transmit S-waves [3]. The S-wave shadow zone lies beyond 105° (NCERT). The 104°–140° band is the P-wave shadow zone [3].
• Damage ranking: P-waves (least) < S-waves (moderate) < surface waves (most).

Mains Points

• S-waves are an indirect source of information about the earth's interior.
• They speed up in denser layers, and they reflect and refract at boundaries.
• They disappear completely at the core, and this proved the outer core is liquid [3].

• This makes them central to any GS-I answer on how we know about the layered earth.

• The P–S time gap is the basis for locating earthquakes and giving early warning.

• The S-minus-P time from three stations finds the epicentre (triangulation). NCS does this in real time using 170+ stations [2].
• 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.

• India still has no nationwide earthquake early warning system, and station coverage is thin in remote Himalayan areas (GS-III disaster management).

• S-waves damage buildings, so design must handle them. Their shear (side-to-side) shaking causes moderate damage. In shallow quakes like the Turkey–Syria doublet of February 2023 (M 7.8 and M 7.5, more than 50,700 deaths), they come just before the even stronger surface waves [4]. This supports earthquake-resistant building codes, retrofitting (strengthening old buildings) and seismic hazard microzonation (dividing a city into small zones by how badly each may shake) [2].

Related concepts

• Body waves
• P-waves
• Surface waves
• Reflection and refraction of seismic waves
• Seismograph

Read more

• Earthquake waves: body waves, surface waves and the seismograph