Moment magnitude scale

Also called: Mw · Topic: Interior of the Earth: Earthquakes, Volcanoes and the Layered Earth · NCERT: Beyond NCERT

Meaning

The moment magnitude scale (Mw) measures an earthquake's true size. It starts from the seismic moment, which is worked out from three things: the area of the fault that broke, how far the rocks slipped, and how stiff the rock is (its rigidity). Hanks and Kanamori introduced it in 1979 [1][2].

It matters because the older Richter scale stops rising for very large earthquakes, but Mw does not. So Mw is now the standard way to state the size of big earthquakes, such as Bhuj 2001 (Mw 7.7).

Explanation

How it works: from a broken fault to one number

Step 1: what happens at the fault

• Plates move, and rocks on both sides of a fault (a crack in the crust where blocks of rock move past each other) get stuck. Stress builds up.
• When the stress becomes greater than the rock's strength, the rock breaks and slips along the fault plane (the surface along which the two blocks slide).

Step 2: measuring the break. Scientists estimate three things:

• Fault area: how much of the fault broke.
• Slip: how far the two sides moved past each other.
• Rigidity: how stiff the rock is.

Step 3: from seismic moment to magnitude

• Multiplying these three gives the seismic moment. This is a physical measure of the "work" the earthquake did at its source.
• The seismic moment is then turned into a magnitude number. This number roughly matches the Richter value where the two scales overlap [1][2].

Why it replaced the Richter scale for big quakes

The problem with Richter (Charles Richter, 1935)

• The Richter scale reads the height of the wave on a seismogram (the record drawn by a seismograph) [1][2].
• For very large quakes, this reading stops going up. This is called saturation: the scale "fills up" and can no longer show how big the quake really is.

What Mw does instead

• Mw is based on what physically happened at the fault, not on the height of one wave. So it keeps rising as quakes get bigger.
• It shows the true size of great earthquakes.
• The USGS calls Mw the more accurate measure for all but small local quakes [1][2].
• The USGS now uses the Richter scale (M_L) mainly for small earthquakes recorded close by [1][2].

Its key features

• One value for each earthquake, because it measures the size of the quake itself, not the damage at a place [1].
• It does not depend on distance. A town near the epicentre and a town far away get the same Mw.
• It is written in Arabic numerals (for example 7.7), never in Roman numerals.
• It is logarithmic (each step up is a fixed multiple, not a fixed addition), like the Richter scale:
• +1 magnitude means about 32 times more energy.

• +2 magnitude means 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.

• There is no fixed upper limit. The "0-10" range in NCERT is only a guide.

Where it occurs

• Bhuj (Kachchh, Gujarat), 26 January 2001: Mw 7.7. This shallow tectonic quake on land is the best-known Indian quake stated in Mw.
• Turkey-Syria, 6 February 2023
• An M 7.8 quake struck southern Turkey, near the northern border of Syria [6][7].
• About 9 hours later, an M 7.5 quake struck about 90 km to the north [6].

• Both came from strike-slip faulting (blocks sliding past each other sideways) on the East Anatolian fault system [6].

• India: National Center for Seismology (NCS), under the Ministry of Earth Sciences (MoES) [3]

• Its National Seismological Network has 169 stations [4][5]. The NCS website gives more than 170 [3].
• It can detect and locate quakes of M 3.0 and above anywhere in India [4][5].

• It works out the magnitude and location of each quake in near real time. It also makes ShakeMaps (maps of how strongly the ground shook) [4][5].

• World: the USGS uses Mw as the main measure for all but small local quakes [1][2].

• How often big quakes happen: quakes of M 8 or more happen about once in 1-2 years.

Don't confuse with

• Richter scale (1935): it uses the height of the wave on a seismogram and saturates for large quakes. Mw (1979) uses fault area, slip and rigidity, and does not saturate.
• Modified Mercalli Intensity Scale: it measures intensity, meaning the shaking and damage seen at a place. It runs from I to XII in Roman numerals and gives many values for one quake, which change with distance [1]. Mw gives one value for each quake.
• Seismic moment: this is the physical quantity (fault area × slip × rigidity). Mw is the magnitude number calculated from it.
• Ground motion vs energy: +1 magnitude means 10× ground motion (wave height) but about 32× energy. Do not mix up the two multipliers.

Prelims Hooks

• Mw was introduced by Hanks and Kanamori, 1979. The Richter scale was introduced by Charles Richter, 1935.
• Mw is based on fault area + slip + rigidity of rock. Together these give the seismic moment [1][2].
• Trap: "The Richter scale is the most accurate scale for great earthquakes" is wrong. Richter saturates, and the USGS calls Mw more accurate for all but small local quakes [1][2].
• Magnitude (Richter, Mw) = one value per quake, in Arabic numerals. Intensity (Mercalli) = I-XII, many values per quake [1].
• +1 magnitude = about 32× energy. +2 = about 1,000× energy.
• The NCS (MoES) detects quakes of M 3.0 and above across India with a 169-station network [4].

Mains Points

• Measuring right means responding right (GS-III: disasters)
• Mw gives the true size of large quakes, so the scale of the disaster is judged correctly from the start.

• Fast NCS reports from 169 stations, together with ShakeMaps, help send rescue teams to the worst-hit places [4].

• Size is not the same as damage

• Bhuj 2001 (Mw 7.7) and Turkey-Syria 2023 (M 7.8; 42,310 deaths [6]) were both large, shallow quakes on land.
• Most deaths in both came from structural collapse, not from the shaking itself.

• So policy needs building codes and seismic microzonation (mapping where the ground will shake most or where soil can liquefy) [3], along with accurate measurement.

• Monitoring needs a strong network

• India's network grew from 6 stations in 1940 to over 170 today [3].
• A dense network is needed to calculate Mw quickly and to track aftershocks (the smaller quakes that follow a big one) across the Himalaya and the "stable" peninsula.

Related concepts

• Types of earthquakes
• Tectonic earthquake
• Richter scale
• Mercalli scale

Read more

• Types, measurement and effects of earthquakes