Shadow Zone and Locating an Epicentre
Geography › Geomorphic Processes and Landform Evolution · Topic 3.2
A shadow zone is a region of the Earth where seismic waves from a given earthquake are not recorded. P waves leave a shadow zone between about 105° and 145° from the epicentre, while S waves are absent over the whole far side of the Earth, beyond about 105°. These gaps arise because the liquid outer core bends P waves and stops S waves. The epicentre of an earthquake is located by comparing the arrival times of P and S waves at three or more seismograph stations.
6 min read · UPSC Prelims · Geography · Geomorphic Processes and Landform Evolution
At a Glance
- P wave shadow zone About 105° to 145° from the epicentre
- S wave shadow zone Beyond about 105°
- Cause Liquid outer core
- Epicentre method Triangulation from three stations
- Key reading Time gap between S and P wave arrival
- Exam link UPSC Prelims, Geography: Geomorphic Processes and Landform Evolution
Where Does It Fit?
This topic follows the one on earthquakes and applies the properties of seismic waves described in the macro on the Earth’s interior. It supplies the tools by which earthquakes are located and the interior of the Earth is explored. Previous topic: Earthquakes: Types, Causes, Effects and Frequency. Next topic: Distribution of Earthquakes and Earthquake Observatories.
What Are Shadow Zones?
Seismographs at different distances from the epicentre record the waves, but there are regions where some waves are not recorded. These are shadow zones. The shadow zone of P waves lies roughly between 105 and 145 degrees from the epicentre, because P waves are refracted as they enter the liquid outer core. S waves are not received at all beyond about 105 degrees from the epicentre, because they cannot pass through the liquid outer core. The shadow zone of S waves is therefore much larger than that of P waves, covering more than 40 per cent of the surface of the Earth.
| Wave | Shadow zone | Reason |
|---|---|---|
| P waves | About 105 to 145 degrees from the epicentre | Bent by the liquid outer core |
| S waves | Beyond about 105 degrees on the opposite side | Stopped by the liquid outer core |
The absence of S waves beyond 105 degrees is the key evidence that the outer core is liquid. The layers they reveal are described in Internal Structure of the Earth: Crust, Mantle, Outer Core, Inner Core.
What Does a Seismograph Record?
In short: a seismograph turns the shaking of the ground into a trace, the seismogram, from which the type of wave and its arrival time can be read.
- Instrument: a heavy mass hangs on a spring or pendulum. When the ground moves, the mass stays nearly still, and the relative movement is recorded on paper or electronically. Modern broadband seismometers record movement digitally in three directions.
- The trace: the P wave, being fastest, arrives first as a small wiggle. The S wave arrives later with a larger motion. The surface waves come last and are the largest.
- S-P interval: the longer the gap between the arrival of P and S waves, the farther the earthquake. A gap of about one second corresponds to roughly 8 km.
- Magnitude: the height of the waves, corrected for distance, gives the magnitude, as in Earthquakes: Types, Causes, Effects and Frequency.
How Is an Earthquake Located?
The distance of a seismograph from the epicentre can be found from the time gap between the arrival of P and S waves: the larger the gap, the greater the distance. Readings from at least three stations are then used to fix the epicentre by triangulation, where three circles drawn with the distances as radii meet at one point. As an analytical point, the same method that locates earthquakes is used to study the interior, so each large earthquake is a natural experiment that sends waves through the whole planet. The causes and scales of earthquakes are discussed in Earthquakes: Types, Causes, Effects and Frequency.
How Is the Epicentre Found by Triangulation?
- Read the S-P gap: each station measures the time difference between S and P waves on its seismogram.
- Convert to distance: using travel-time tables, the gap gives the distance from that station to the earthquake, but not the direction.
- Draw a circle: on a map, draw a circle around the station with the distance as radius. The epicentre lies somewhere on this circle.
- Use three stations: circles from three stations intersect at a single point, which is the epicentre. Two circles meet at two points, so a third is needed.
- Find the depth: the focal depth is worked out from the timing of waves reflected from the surface, or from stations nearest to the epicentre.
A worked example: if the S-P gap is 30 seconds, the earthquake is about 240 km from that station by the rough rule of 8 km per second. India’s National Centre for Seismology collects such records from a network of stations to report the location and magnitude of each earthquake within minutes.
Why Do Shadow Zones Matter?
The shadow zones were the first proof of the structure of the core, as in Understanding the Earth’s Interior Through Seismic Waves. They show that the S wave shadow, covering a little over half the Earth, is caused by a liquid outer core. The P wave shadow, a ring from about 105° to 145°, arises because P waves bend sharply on entering the slower core. They also show that the region of an earthquake that is felt strongly is not the only region that matters, since the other side of the planet may receive no direct waves at all. As an analytical point, modern networks now use thousands of stations, and the location of any earthquake can be fixed within a few kilometres.
Why Is This Topic Important?
- Method How epicentres are located
- Evidence Proof of the liquid core
- Facts Angles and zones are asked
What Came Next?
The next topic maps where earthquakes occur in the world and in India, and which observatories record them. Next topic: Distribution of Earthquakes and Earthquake Observatories.
Key People and Terms
- Seismograph Instrument recording ground motion
- Seismogram Record of the shaking
- Epicentre Surface point above the focus
- Triangulation Locating by three distances
- S-P interval Time gap between S and P arrival
- Shadow zone Area with no direct waves
Exam Corner
Points to Remember
- P shadow zone: about 105° to 145°.
- S waves are absent beyond about 105°.
- Three stations are needed to locate an epicentre.
- The longer the S-P gap, the farther the earthquake.
- The P wave arrives first.
Do Not Confuse With
- Seismograph and seismogram The instrument versus its record
- Focus and epicentre Point inside versus point on the surface
- P and S shadow zones A ring versus the far side of the Earth
Memory Hook
P arrives first, S lags behind; three circles, one point to find.
Mains Angle
Questions are mostly factual. These points help in a short note.
- Method Time gap and triangulation
- Evidence Shadow zones prove the core
- Network Role of national centres
A Question You May Face
An original practice question, not a past paper question.
“Explain how the epicentre of an earthquake is located and how shadow zones reveal the nature of the Earth’s core.”
How to Answer
- Record Seismogram and S-P gap.
- Locate Three circles.
- Shadow P and S zones.
- Inference Liquid outer core.
GS Relevance
Frequently Asked Questions
What is the shadow zone of an earthquake?
A shadow zone is the area of the Earth where seismic waves from a particular earthquake are not recorded. The P wave shadow zone lies between about 105 and 145 degrees from the epicentre, while the S wave shadow zone covers the whole region beyond about 105 degrees.
Why do S waves not reach the opposite side of the Earth?
S waves are shear waves, which cannot pass through a liquid. Beyond about 105 degrees from the epicentre, the waves would have to cross the liquid outer core, so they are stopped, which creates a large S wave shadow zone and proves that the outer core is liquid.
How is the epicentre of an earthquake located?
The epicentre is located by measuring the time gap between the arrival of P and S waves at three or more seismograph stations. Each gap gives a distance from that station, and circles drawn on a map with those distances as radii intersect at the epicentre.
What is a seismograph?
A seismograph is an instrument that records the movement of the ground during an earthquake. It has a heavy mass that stays nearly still while the frame moves, and the relative motion is recorded as a trace called a seismogram, showing the arrival of P, S and surface waves.
Why are three seismograph stations needed to find an epicentre?
A single station gives only the distance to the earthquake, not the direction, so the epicentre lies on a circle. Two stations give two circles that intersect at two points. A third station's circle intersects at only one of them, which fixes the location of the epicentre.
PYQ Practice — Statement Analysis
1 S waves cannot pass through the liquid outer core.
This creates the S wave shadow zone.
2 The P wave arrives later than the S wave at a station.
P waves are faster and arrive first.
3 At least three seismograph stations are needed to locate an epicentre.
Their circles intersect at one point.
4 The P wave shadow zone lies between about 105 and 145 degrees.
The ring is caused by refraction at the core.
Past Mains Questions on This Topic
This topic comes under Geomorphic Processes and Landform Evolution. UPSC Mains questions asked on the matching syllabus topic, year-wise with marks:
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