P-waves

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

Meaning

P-waves (primary waves) are the fastest earthquake waves. They reach a seismograph first. As they travel, they push and pull the rock in the same direction the wave is moving, and they can pass through solids, liquids and gases.

They are the only body waves that pass through every layer of the earth, including the liquid outer core. Because of this, they are a key tool for mapping the earth's interior. They also arrive seconds before the damaging waves, and early warning systems are built on that gap.

Explanation

How P-waves form and travel

• Cause: rocks inside the earth suddenly break and move. The energy is released at the focus (hypocentre), the point inside the earth where the break happens.
• Process: the energy spreads out from the focus in all directions through the inside of the earth. P-waves are one of the two kinds of body waves (waves that travel through the earth's inside, or "body"). S-waves are the other kind.
• Result: P-waves are the fastest, so they are the first waves to reach the surface. They are also the first recorded on a seismogram (the record a seismograph makes).

How the ground moves

• Particles vibrate parallel to the direction the wave travels. "Parallel" means they move back and forth along the same line as the wave.
• This squeezes and then stretches the material, again and again.

• It works the same way as a sound wave moving through air.

• Because of this motion, P-waves have other names: pressure waves, longitudinal waves and compressional waves [5][6].

• P-waves can squeeze and stretch any material, so they travel through solids, liquids and gases. S-waves can pass only through solids.
• They cause the least damage of the three wave types (P-waves, S-waves and surface waves).

What P-waves tell us about the interior

• Rule 1 – density controls speed: the denser the material, the faster the wave. P-waves speed up or slow down as they pass through different layers.
• Rule 2 – boundaries change direction: where a P-wave meets material of a different density, two things can happen:
• Reflection: the wave bounces back.

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

• Result: seismographs in different places record these changes in arrival time and path. These changes show where the boundaries between the crust, mantle, outer core and inner core lie.

The P-wave shadow zone

• Cause: when P-waves reach the liquid outer core, it bends (refracts) them [3].
• Process: the bending leaves a gap on the surface where no direct P-waves arrive [3].
• Result: there is a shadow zone from 104° to 140° angular distance from the epicentre, which gets no direct P-waves [3]. The epicentre is the point on the surface directly above the focus.
• NCERT puts the P-wave shadow zone beyond 105°.
• S-waves are blocked completely by the liquid core, so their shadow zone is much larger. It covers everything beyond 105°.

Where it occurs

• Everywhere an earthquake happens: P-waves start at the focus of every earthquake and move outward through the whole earth.
• Far side of the earth: the band 104°–140° from the epicentre is the P-wave shadow zone, where no direct P-waves arrive [3]. P-waves can still reach stations beyond this band because they pass through the core. S-waves cannot.
• India: the National Center for Seismology (NCS), under the Ministry of Earth Sciences, runs the National Seismological Network [2].
• The network has more than 170 stations, fitted with broadband seismographs and strong motion accelerographs (instruments that measure strong ground shaking) [2].
• It 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].

• Locating a quake: at each station, staff 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 away. A circle is drawn around each of three stations, and the point where the circles meet is the epicentre. This method is called triangulation. NCS uses its real-time data to work out these earthquake details almost immediately and share them with government authorities and the public [2].

• Southern Turkey, near the northern border of Syria (6 February 2023): an M 7.8 earthquake was followed about 9 hours later by an M 7.5 earthquake about 90 km to the north [4]. Like every earthquake, this pair sent P-waves first. But the damage came mostly from the surface waves that followed. More than 50,700 people were killed [4].

Don't confuse with

• S-waves (secondary / shear waves): they are slower and arrive after P-waves. Their particles vibrate at right angles (perpendicular) to the direction of travel, and they pass only through solids [3][6]. P-waves vibrate parallel to the direction of travel and pass through solids, liquids and gases.
• Surface waves (Love and Rayleigh): they are formed when body waves hit the surface rocks, and they travel only along the surface. They arrive last and are the most destructive. P-waves travel through the inside of the earth and are the least destructive.
• P-wave shadow zone vs S-wave shadow zone: the P-wave shadow zone is a band (104°–140°) caused by refraction at the liquid core [3]. The S-wave shadow zone is much larger (everything beyond 105° in NCERT) because S-waves are stopped completely by the liquid core.
• Focus vs epicentre: P-waves start at the focus (inside the earth). The epicentre is the point on the surface directly above it, and it is the first place on the surface the waves reach.

Prelims Hooks

• Order of arrival at a seismograph: P-waves → S-waves → surface waves (Love before Rayleigh) [7].
• Only P-waves travel through solids, liquids and gases. A common trap: "S-waves can pass through liquids". This is false.
• P-waves = longitudinal (vibration parallel to the direction of travel, like sound). Other names: pressure waves and compressional waves [5][6].
• P-wave shadow zone: 104°–140° from the epicentre, caused by refraction at the liquid outer core [3].
• Least destructive: P-waves. Most destructive: surface waves.
• The epicentre is found from the S-minus-P time at at least three stations (triangulation).

Mains Points

• P-waves are an indirect source of information about the earth's interior:
• They change speed with density, and they reflect and refract at boundaries. This shows where the crust, mantle, outer core and inner core lie.
• The way the core bends them creates the 104°–140° shadow zone. Together with S-waves not passing through the core, this proves the outer core is liquid [3].

• Useful for GS-I questions on how we know about the earth's interior.

• Earthquake early warning depends on P-waves (GS-III, disaster management):

• The fast but weak P-wave arrives seconds before the damaging S-waves and surface waves.

• Detecting it can trigger an alert in time to stop trains, shut gas lines and move people to safety.

• India's preparedness: the NCS network (170+ stations, real-time VSAT data) picks up P-waves and S-waves to locate quakes quickly. It also produces ShakeMaps (maps of how strongly the ground shook in each area) and does seismic hazard microzonation (dividing a city into small zones by how badly each may shake) [2].

• Gaps that remain: few stations in remote Himalayan areas, and no nationwide earthquake early warning system yet.

Related concepts

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

Read more

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