Mercalli scale

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

Meaning

The Mercalli scale is an intensity scale. It rates how strongly an earthquake shook one particular place, from I to XII, based on what people felt and the damage they could see there. It is named after the Italian seismologist Giuseppe Mercalli, and the version used today is the Modified Mercalli Intensity Scale [2].

It matters because it measures what the quake actually did to people and buildings. Magnitude cannot show this, because magnitude is only one number for the whole quake.

Explanation

What intensity measures

• An earthquake happens when energy stored in rocks is suddenly released. The energy travels outward as seismic waves.
• Intensity describes how strongly the ground shook at one place. It is judged by:
• what people felt, such as light shaking or being thrown off balance

• the damage you can see, such as cracked walls, fallen objects or collapsed buildings.

• So intensity is about effects, not about the energy released.

How the scale is built

• It has 12 levels, written in Roman numerals [2]:
• I: shaking that nobody notices
• the levels in between: shaking that grows stronger, with more damage at each step

• XII: complete destruction [2]

• Roman numerals keep intensity separate from magnitude, which uses Arabic numbers (e.g. intensity IX vs magnitude 7.7).

• The values come from what people report and from damage surveys on the ground. No seismograph reading is needed.

Why one quake has many intensity values

• Magnitude gives one value for each quake. Intensity gives many values, one for each place around the epicentre [2].

• The epicentre is the point on the surface directly above the focus (the point inside the earth where the energy is first released).

• Intensity mostly falls as you move away from the fault rupture [2]:

• near the epicentre the waves are strongest, so damage is greatest

• farther away the waves grow weaker, so effects are smaller.

• Example: one quake can be intensity IX near the epicentre and IV a few hundred kilometres away.

• Joining places that had the same intensity on a map gives an intensity map. Modern versions are called ShakeMaps.

What else affects intensity

• Distance from the fault rupture is the main factor [2].
• Depth of focus also matters. A shallow quake shakes the surface harder. Turkey-Syria 2023 was a shallow strike-slip quake [8].
• Ground conditions matter too. Wet, loose soil can undergo soil liquefaction (it loses strength and flows like a liquid), which makes the damage worse.
• Building quality matters as well. Weak buildings collapse at lower shaking, so visible damage, and therefore the intensity rating, goes up.

Where it occurs

• Worldwide (USGS): the USGS explains the difference between magnitude and intensity using the Modified Mercalli Intensity Scale (I–XII) [2].
• India (NCS): the National Center for Seismology, under the Ministry of Earth Sciences (MoES), makes ShakeMaps (intensity maps). It works out magnitude and location in near real time and sends the information to government, other agencies and the public [5][6].

• Its National Seismological Network has 169 stations and can detect quakes of M 3.0 and above anywhere in India [5][6].

• Places where shaking intensity decided the death toll:

• Bhuj (Kachchh, Gujarat), 26 Jan 2001, Mw 7.7: a shallow tectonic quake on land. Most harm came from buildings falling down.

• Southern Turkey, near the northern border of Syria, 6 Feb 2023, M 7.8, followed about 9 hours later by an M 7.5 quake about 90 km to the north [8][9]. The quakes lay in the East Anatolian fault system [8]. By 22 Feb 2023 there were 42,310 deaths and 448,010 people evacuated, mostly because of structural collapse [8].

• Intra-plate India: Latur-Killari (Maharashtra), 1993, M 6.2 and Koyna (Maharashtra), 1967, M 6.3 [7]. Both show that strong local shaking can happen in the "stable" peninsula too, far from any plate boundary.

Don't confuse with

• Richter scale (Charles Richter, 1935): measures magnitude (the size of the quake, linked to energy). It gives one value per quake and is logarithmic: +1 means 10× more ground movement and about 32× more energy [2][3]. Mercalli measures intensity (effects at a place) and gives many values per quake.
• Moment magnitude scale (Mw; Hanks and Kanamori, 1979): a magnitude scale based on fault area, slip and rock rigidity. It does not saturate (stop rising) for very large quakes [2][3]. It tells you the true size of the quake, not the damage at a town.
• Magnitude vs intensity: magnitude does not depend on distance. Intensity does, and mostly falls away from the fault rupture [2]. Magnitude is written as Arabic numbers (7.7). Intensity is written as Roman numerals (IX).
• Seismograph / seismogram: the instrument and the record it draws, used to work out magnitude [2][3]. Mercalli intensity comes from observed effects, not from a seismogram.

Prelims Hooks

• Mercalli scale = intensity, levels I–XII, Roman numerals, based on felt effects and visible damage [2]. Richter / Mw = magnitude, based on energy.
• One earthquake has one magnitude but many intensities, one for each place [2].
• Trap: "Intensity of an earthquake is the same at all places." Wrong. Intensity mostly decreases with distance from the fault rupture [2]. Magnitude does not change with distance.
• Trap: "The Mercalli scale is logarithmic and based on energy." Wrong. That describes the Richter scale (+1 = 10× ground motion, about 32× energy) [2][3].
• Scale–person–type matches: Mercalli (Italy) → intensity; Richter (1935) → magnitude; Hanks and Kanamori (1979) → moment magnitude (Mw).
• ShakeMaps (intensity maps) in India are made by the NCS under MoES [5][6].

Mains Points

• Intensity, not magnitude, decides human loss (GS-III: disasters):
• Bhuj 2001 (Mw 7.7) and Turkey-Syria 2023 (M 7.8; 42,310 deaths [8]) caused high damage mainly through structural collapse.
• So risk depends on shaking at each place, soil type and building quality, not on magnitude alone.

• The fix is to follow building codes and use seismic microzonation (mapping each area by how strongly it is likely to shake and where soil may liquefy) [4].

• Using intensity maps to guide response:

• Mw gives the true size of a quake. ShakeMaps show where the shaking was worst.

• Quick NCS reports from 169 stations help send rescue teams to the places that need them most [5].

• No zone is fully safe:

• Latur (1993) and Koyna (1967) caused strong local shaking in the "stable" peninsula [7].
• So intensity-based hazard planning must cover inner areas of the plate, not just the Himalayan plate boundary.

Related concepts

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

Read more

• Types, measurement and effects of earthquakes