Richter scale

Also called: Richter magnitude scale · Topic: Interior of the Earth: Earthquakes, Volcanoes and the Layered Earth · NCERT: Class 11, Ch 3 "Interior of the Earth"

Meaning

The Richter scale (Richter magnitude scale) is a number scale made by Charles Richter in 1935. It measures the magnitude of an earthquake, which is the size of the quake, worked out from how high its waves are drawn on a seismogram (the record made by a seismograph) [2][3].

It matters because it gives one number for each quake, and that number is linked to the energy released. Exams often test its logarithmic nature and how it differs from intensity, which is measured on the Mercalli scale.

Explanation

How it works

• The cause: stress builds up in rocks along a fault (a crack in the crust where blocks of rock move). The rock breaks, and the stored energy goes out as seismic waves.
• How it is recorded: a seismograph records these waves. The height of the wave on the seismogram shows how much the ground moved [2][3].
• The result: this wave height is turned into a single magnitude number, written in Arabic numbers (for example 6.2 or 7.7).

It is a logarithmic scale

• Logarithmic means each step up is a multiplication, not an addition.
• Each whole number up on the scale:
• means 10 times more ground movement (the wave on the seismogram is 10 times higher) [2][3]

• and about 32 times more energy released.

• Two whole numbers up:

• about 32 × 32 ≈ 1,000 times more energy.

• So an M 7 quake releases about 1,000 times the energy of an M 5 quake.

• An M 7.8 quake is therefore far stronger than an M 6.8 quake, not just "a little bigger".

Range and limits

• NCERT gives the range as 0-10. But the scale has no fixed upper limit.
• It saturates for big quakes:
• For very large quakes, the Richter value stops rising even though the quake keeps getting bigger. This is called saturation.

• So it understates the true size of great earthquakes [2][3].

• What replaced it for large quakes: the moment magnitude scale (Mw), made by Hanks and Kanamori in 1979.

• Mw uses fault area (how much of the fault broke), slip (how far the two sides moved) and rigidity (how stiff the rock is).
• These give the seismic moment, which is turned into a magnitude. Where the two scales overlap, the Mw value roughly matches the Richter value [2][3].

• The USGS calls Mw the more accurate measure for all but small local quakes [2][3].

• Where it is still used: the USGS now uses the Richter scale (written M_L) mainly for small earthquakes recorded close by [2][3].

Rough guide to magnitude and damage

• Serious damage usually starts above about M 5.
• Quakes of M 8 or more happen about once in 1-2 years.
• "Tiny" quakes happen almost every minute.

Where it occurs

• India: who measures quakes. The National Center for Seismology (NCS) works under the Ministry of Earth Sciences (MoES) and works out the magnitude and location of each quake in near real time [4][5][6].
• Its National Seismological Network has 169 stations [5][6]. The NCS website gives more than 170 [4].
• It can detect and locate quakes of M 3.0 and above anywhere in India [5][6].

• India's first seismological observatory opened at Alipore (Calcutta) on 1 December 1898, after the 1897 Shillong plateau earthquake [4].

• Indian quakes and their magnitudes:

• Koyna (Maharashtra), 11 Dec 1967, M 6.3. This was a reservoir-induced quake linked to the Shivaji Sagar reservoir. It is the largest reservoir-triggered earthquake in the world [7][8].
• Latur-Killari (Maharashtra), 30 Sep 1993, M 6.2. This was an intra-plate quake (a quake inside a plate, not at its edge) in the stable peninsula.
• Bhuj (Kachchh, Gujarat), 26 Jan 2001, Mw 7.7. This value is given on the moment magnitude scale.

• Koyna region record: 22 quakes of M 5 or more and 200 of M 4 or more [7][8].

• World:

• Turkey-Syria, 6 Feb 2023. An M 7.8 quake struck southern Turkey near the Syrian border. About 9 hours later, an M 7.5 quake struck about 90 km to the north, on the East Anatolian fault system [9][10].

• Explosions also get a magnitude: seismographs around the world recorded the Pokhran nuclear tests (Jaisalmer district, Rajasthan) in 1974 and 1998 as seismic events.

Don't confuse with

• Mercalli scale (Modified Mercalli Intensity Scale): it measures intensity (how hard the ground shook and how much damage it did) on 12 levels, I-XII, in Roman numerals. One quake has many intensity values, and they change with distance. The Richter scale gives one magnitude value for each quake [2].
• Moment magnitude scale (Mw): it also measures magnitude, but it is based on fault area, slip and rigidity, and it does not saturate. It came in 1979 and has replaced the Richter scale (1935) for large quakes [2][3].
• Linear scale: the Richter scale is not linear. One step up does not mean "one more unit". It means 10× the ground motion and about 32× the energy.
• Epicentre and focus: magnitude does not depend on how far you are from the epicentre (the point on the surface above the focus). Intensity does [2].

Prelims Hooks

• Richter scale was made by Charles Richter, 1935. It measures magnitude (energy), not intensity (damage).
• +1 magnitude = 10× ground motion and about 32× energy. +2 magnitude = about 1,000× energy (M 7 compared with M 5).
• Trap: NCERT gives the range as 0-10, but the scale has no fixed upper limit.
• Magnitude is written in Arabic numbers and has one value for each quake. Intensity (Mercalli) is written in Roman numerals I-XII and has many values [2].
• Mw (Hanks and Kanamori, 1979) replaced the Richter scale for big quakes because the Richter value saturates. The USGS uses Richter (M_L) mainly for small, nearby quakes [2][3].
• The NCS (under MoES) detects quakes of M 3.0 and above anywhere in India with 169 stations [5].

Mains Points

• Measuring right means responding right:
• The Richter scale understates great quakes, so the true size of large events is given by Mw.
• Magnitude alone does not show where damage was worst. Intensity maps (ShakeMaps) do that.

• Quick NCS reports from 169 stations help send rescue teams to the right places [5] (GS-III: disaster management).

• Magnitude is not the same as the death toll:

• Serious damage starts above about M 5. But in Bhuj 2001 (Mw 7.7) and Turkey-Syria 2023 (M 7.8; 42,310 deaths [9]), most deaths came from structural collapse (buildings falling down).

• So building codes and seismic microzonation (mapping which areas shake more and where soil can liquefy, meaning wet, loose soil loses strength and flows like a liquid) matter as much as the magnitude [4].

• Moderate magnitudes can still kill in "stable" areas:

• Latur (M 6.2) and Koyna (M 6.3) both struck the old, hard peninsular block, far from any plate boundary.
• At Koyna, quakes rise and fall with the yearly filling of the reservoirs [7][8]. So large dams need seismic checks before and after the reservoir is filled.

Related concepts

• Types of earthquakes
• Tectonic earthquake
• Mercalli scale
• Moment magnitude scale

Read more

• Types, measurement and effects of earthquakes