Shadow zones and what they reveal

Detail

Key terms

• Epicentre: the point on the earth's surface directly above the place where an earthquake starts. That starting point inside the earth is called the focus or hypocentre [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. There are two kinds:
• P-waves (primary waves) are the fastest body waves, so they reach a station before the S-waves. They push and pull particles in the same direction the wave is moving [2]. They can pass through solids, liquids and gases.

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

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

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

What a shadow zone is

• A shadow zone is an area where seismographs record no direct waves from a particular earthquake.
• Each earthquake has its own shadow zone. The zone is set by the epicentre's position, so it moves when the epicentre moves.
• The shadow zone forms a ring (band) around the earth, centred on the line that joins the epicentre to its antipode.

• If you know the epicentre, you can draw its shadow zone (NCERT Figure 3.2 (a) and (b)).

• Example: the epicentre of the Bhuj earthquake (2001) was in Kachchh, Gujarat. Its antipode lies in the south-east Pacific Ocean, so its shadow zones fell mostly over the Pacific and the Americas. The Tohoku (Japan, 2011) and Turkey-Syria (2023) earthquakes each had a different shadow zone because their epicentres were in different places.

What the seismographs record

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

• The United States Geological Survey (USGS) gives the P-wave shadow zone as about 104° to 140° from the epicentre [3]. (NCERT: 105°–145°.) Different textbooks give slightly different edges because they use different earth models.

S-wave shadow zone

• Definition: the whole area beyond 105° from the epicentre, where no direct S-waves arrive.
• Size: a little over 40 per cent of the earth's surface. This makes it much larger than the P-wave shadow zone.
• Why it happens:
• S-waves travel only through solids.
• Beyond 105°, the straight-line path from the focus has to go through the outer core.
• The outer core behaves as a fluid. It has no resistance to shear (no shear strength) [2].
• So the S-waves are stopped completely and never reach the far side of the earth [3].

P-wave shadow zone

• Definition: a band around the earth between 105° and 145° from the epicentre, where no direct P-waves arrive.
• Why it happens:
• At the core–mantle boundary (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].
• Any 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, so they leave a gap at 105°–145° [3].
• The bent waves come back to the surface beyond 145°.

What shadow zones reveal about the earth's interior

1. The outer core is liquid. S-waves cannot cross the core, and S-waves cannot travel through liquids. So the outer core must be molten. - It is made mainly of liquid iron and nickel. - Its temperature is about 4,500–5,500 °C. - It 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 angle at which P-waves disappear (105°) and come back (145°) depends on how far down 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, the 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 today call these reflected waves PKiKP [5]. - This boundary is now called the Lehmann discontinuity. - The inner core is about 1,220 km in radius and mostly solid iron. Its temperature is about 5,200 °C, and the pressure is nearly 3.6 million atmospheres [4]. The inner core is hotter than the normal melting point of iron, but this huge pressure keeps it from melting [4].

How the boundaries are found in practice

• Scientists build models of the earth's layers by studying the arrival times of earthquake waves and the areas where waves are missing.
• In India, 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 across the country [7].

Prelims Hooks

• Within 105° of the epicentre, both P- and S-waves are recorded. Between 105° and 145°, neither is recorded. Beyond 145°, only P-waves are recorded.
• The S-wave shadow zone (beyond 105°, a little over 40 per cent of the surface) is larger than the P-wave shadow zone (the 105°–145° band). Questions often swap these two.
• S-wave shadow zone → liquid outer core. S-waves cannot pass through liquids.
• P-wave shadow zone → refraction (bending) at the core–mantle boundary. P-waves are not stopped by the core.
• At the Gutenberg discontinuity (about 2,900 km deep), P-wave speed falls from about 13.7 to 8.1 km/s [2].
• Inge Lehmann (1936) found the solid inner core from faint P-waves inside the P-wave shadow zone. The boundary she found is about 5,000 km deep [6].
• Each earthquake has its own shadow zone. It is not a fixed region on the map.
• Trap: "The P-wave shadow zone exists because P-waves cannot travel through liquids." This is false. P-waves do cross the liquid core; they are bent, not stopped.

Mains Points

• Indirect evidence for the earth's interior (GS-I): no one can drill down to the core. The deepest boreholes reach only about 12 km. Shadow zones are therefore the clearest proof of the core's structure:
• Missing S-waves show that the outer core is liquid.
• The bending of P-waves shows the core's size (radius about 3,500 km) [2].

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

• Link to the magnetic field: the liquid outer core found from shadow-zone evidence is the same layer whose churning creates the earth's magnetic field [4]. Palaeomagnetism and the sea-floor spreading evidence for plate tectonics both depend on this magnetic field.

• Seismic networks and disaster management (GS-III): NCS runs more than 170 stations [7]. Data from networks like this, shared worldwide after earthquakes such as Bhuj 2001, Tohoku 2011 and Turkey-Syria 2023, does two jobs:
• It finds epicentres quickly for disaster response.
• Over time, it sharpens models of the earth's interior.