Shadow zone

Topic: Interior of the Earth: Earthquakes, Volcanoes and the Layered Earth · NCERT: Class 11, Ch 3 "Interior of the Earth"

Meaning

A shadow zone is an area on the earth's surface where seismographs (instruments that record ground shaking) pick up no direct earthquake waves from a particular earthquake. Each earthquake has its own shadow zone, because the zone depends on where the epicentre is.

It matters because no one can drill down to the core. Shadow zones are the clearest proof that the outer core is liquid. They also show how big the core is and that a solid inner core lies inside it.

Explanation

Basic terms

• Focus (hypocentre): the place inside the earth where an earthquake starts [2].
• Epicentre: the point on the surface directly above the focus [2].
• Seismograph: an instrument that detects and records ground shaking, especially earthquake waves, along with the time each wave arrives [2].
• Body waves: earthquake waves that travel through the inside of the earth.
• P-waves (primary waves) are the fastest, so they arrive first. They push and pull particles in the same direction the wave moves [2]. They pass through solids, liquids and gases.

• S-waves (secondary waves) are slower but usually bigger [2]. They shake particles at right angles to the direction the wave moves [2]. They pass only through solids.

• Angular distance: how far a station is from the epicentre, measured as an angle at the earth's centre. 1° of arc on the surface is about 111 km.

• 105° is about 11,700 km.
• 145° is about 16,100 km.
• 180° is the antipode (the point exactly opposite the epicentre, on the other side of the earth).

What seismographs record at different distances

Angular distance from epicentre	P-waves	S-waves	Result
0°–105°	Recorded	Recorded	Both arrive
105°–145°	Not recorded	Not recorded	Shadow zone for both
Beyond 145° (up to 180°)	Recorded	Not recorded	P only

• The shadow zone forms a ring (band) around the earth. The ring is centred on the line joining the epicentre to its antipode.
• If you know the epicentre, you can draw its shadow zone (NCERT Figure 3.2 (a) and (b)).
• The United States Geological Survey (USGS) puts the P-wave shadow zone at about 104° to 140° [3]. NCERT gives 105°–145°. The edges differ a little because textbooks use different earth models.

S-wave shadow zone: why S-waves vanish

• Where: the whole area beyond 105° from the epicentre.
• Size: a little over 40 per cent of the earth's surface, so it is much larger than the P-wave shadow zone.
• Why it forms:
• Beyond 105°, the path from the focus has to cross the outer core.
• The outer core behaves as a fluid. It has no shear strength (it cannot resist being pushed sideways) [2].
• S-waves need a solid, so they are stopped completely. They never reach the far side of the earth [3].

P-wave shadow zone: why P-waves bend away

• Where: a band between 105° and 145° from the epicentre.
• Why it forms:
• At the core–mantle boundary, called the Gutenberg discontinuity (about 2,900 km deep), P-wave speed falls sharply from about 13.7 km/s to 8.1 km/s [2].
• A wave that suddenly slows down at a boundary bends. This bending is called refraction.
• P-waves bend sharply as they enter the denser liquid core, which leaves a gap at 105°–145° [3].

• The bent waves come back to the surface beyond 145°.

• Key point: P-waves are not stopped by the liquid core. They cross it but get bent.

What shadow zones tell us about the inside of the earth

1. The outer core is liquid. - S-waves cannot cross the core, so the outer core must be molten. - It is made mainly of liquid iron and nickel, at about 4,500–5,500 °C, and is about 2,200 km thick [4]. - This moving liquid metal creates and keeps up the earth's magnetic field [4].

2. The size of the core. - The angles where P-waves disappear (105°) and come back (145°) depend on how deep the core starts and how big it is. - This fixes the core's radius at about 3,500 km [2]. The mantle above it is about 2,900 km thick [2].

3. There is a solid inner core (beyond NCERT). - In 1936, Danish seismologist Inge Lehmann noticed faint P-waves arriving inside the P-wave shadow zone [5][6]. - She explained them as P-waves reflected from a boundary about 5,000 km deep inside the liquid core [6]. Seismologists now call these waves PKiKP [5]. - This boundary is the Lehmann discontinuity. - The inner core is about 1,220 km in radius and mostly solid iron. It is about 5,200 °C, under nearly 3.6 million atmospheres of pressure [4]. - It is hotter than iron's normal melting point, but the huge pressure stops it from melting [4].

Where it occurs

• No fixed place. A shadow zone is not a region on the map. It is a ring that moves with each earthquake's epicentre.
• Bhuj earthquake (2001), Kachchh, Gujarat: its antipode lies in the south-east Pacific Ocean. So its shadow zones fell mostly over the Pacific and the Americas.
• Tohoku earthquake (Japan, 2011) and Turkey-Syria earthquake (2023): each had a different shadow zone because their epicentres were in different places.
• India's monitoring: the National Center for Seismology (NCS), under the Ministry of Earth Sciences, is the main government agency for monitoring earthquakes. It runs the National Seismological Network of more than 170 stations [7]. Arrival times and missing-wave areas recorded by networks like this are used to build models of the earth's layers.

Don't confuse with

• P-wave shadow zone vs S-wave shadow zone: the P-wave zone is a band (105°–145°) caused by bending (refraction). The S-wave zone covers everything beyond 105° (a little over 40 per cent of the surface) because S-waves are blocked by the liquid outer core. The S-wave zone is the larger one.
• Gutenberg discontinuity vs Lehmann discontinuity: Gutenberg is the core–mantle boundary (about 2,900 km deep), where P-waves slow down and bend [2]. Lehmann is the outer core–inner core boundary (about 5,000 km deep), found in 1936 from reflected P-waves [6].
• Epicentre vs focus (hypocentre): the focus is where the earthquake starts inside the earth. The epicentre is the point on the surface directly above it [2]. Shadow-zone angles are measured from the epicentre.
• Rain shadow area: a dry area on the side of a mountain away from the wind (the leeward side). It is a climate feature, not a seismic one. Only the word "shadow" is shared.

Prelims Hooks

• 0°–105°: both P and S recorded. 105°–145°: neither recorded. Beyond 145°: only P recorded.
• The S-wave shadow zone is larger than the P-wave shadow zone. Questions often swap the two.
• S-wave shadow zone → liquid outer core. P-wave shadow zone → refraction at the core–mantle boundary, where P-wave speed drops from about 13.7 to 8.1 km/s [2].
• Trap: "The P-wave shadow zone exists because P-waves cannot travel through liquids." False. P-waves cross the liquid core. They are bent, not stopped.
• Inge Lehmann (1936) found the solid inner core from faint P-waves inside the P-wave shadow zone. The boundary is about 5,000 km deep [6].
• Each earthquake has its own shadow zone. It is a ring centred on the epicentre–antipode line, not a fixed region.

Mains Points

• Indirect evidence for the earth's interior (GS-I): the deepest boreholes reach only about 12 km, so the core cannot be seen directly. Shadow zones are the clearest proof of its structure:
• Missing S-waves show the outer core is liquid.
• Bent P-waves show the core's size, a radius of about 3,500 km [2].

• Reflected P-waves show the inner core is solid (Lehmann, 1936) [6].

• Link to the magnetic field (GS-I): the liquid outer core proved by shadow zones is the same layer whose churning creates the earth's magnetic field [4].

• Palaeomagnetism (the record of past magnetic fields kept in rocks) depends on this field.

• Sea-floor spreading evidence for plate tectonics also depends on it.

• Seismic networks and disaster management (GS-III): NCS runs more than 170 stations [7]. Data from networks like this, shared worldwide after Bhuj 2001, Tohoku 2011 and Turkey-Syria 2023, does two jobs:

• It finds epicentres quickly for disaster response.
• Over time, it improves models of the earth's interior.

Related concepts

• P-wave shadow zone
• S-wave shadow zone

Read more

• Shadow zones and what they reveal