What is the Mercalli Scale
Mercalli, Richter or moment magnitude scale: How to measure earthquake strength
The Richter Scale
The Richter scale, which is open at the top, begins with the value 0.0; earthquakes can be felt from a magnitude (M) of 2.5. Due to the logarithmic degree of strength, an earthquake of M 7 according to Richter is ten times stronger than one of M 6, which corresponds to a released energy of 6 x 10 to the power of 13 joules. So far, magnitudes of up to 8.6 have been registered. The disadvantage of the Richter scale is that the measuring devices should be located up to 100 km from the epicenter of the quake. If the distance is too great or the earthquake is too strong, the comparability of the earthquakes is no longer guaranteed.
The following criteria apply to the Richter scale:
- Strength 1-2: only detectable by instruments.
- Strength 3: Rarely felt near the source of the quake.
- Strength 4-5: Noticeable with slight damage within 30 kilometers of the quake center.
- Magnitude 6: Moderate quake, fatalities and severe damage in densely populated areas.
- Strength 7: Strong quake that can lead to disasters.
- Magnitude 8: large earthquake.
- Magnitude 9: So far, no earthquake above magnitude nine has been measured.
Moment magnitude scale
Since the Richter scale no longer met scientific criteria in the event of strong tremors, the moment magnitude scale was developed by Hiroo Kanamori in California in 1977. Instead of the magnitude, the seismic moment is used as the unit of measurement. The seismic moment does not experience saturation as compared to the magnitude scales. This means that even the strongest earthquakes can be precisely classified.
The Mercalli scale
The Mercalli scale and the Medvedev-Sponheuer-Karnik scale, which is used in Eastern Europe, are not based on measurable values, but on the tangible and visible consequences of an earthquake. The twelve-point macroseismic Mercalli scale from the turn of the century measures the seismic intensity with which the strength of an earthquake is classified according to its effects on the earth's surface.
The modified Mercalli-Sieberg scale is common in Austria (August Heinrich Sieberg, German geophysicist, 1875-1945). It ranges from grade one (imperceptibly, only registered by instruments) to grade four (generally noticeable, is perceived outdoors by a few, in buildings by numerous people, dishes and windows clink), grade eight (destruction of buildings, many people get caught in Panic, even heavy furniture is shifted and partly overturned) up to degree twelve (landscape-changing, high and underground structures are destroyed, the surface of the earth is profoundly reshaped).
The exact levels of the Mercalli-Sieberg scale (macroseismic intensity scale, source: Hohe Warte Vienna)
- Grade 1: Imperceptible: Only registered by earthquake instruments.
- Grade 2: Hardly noticeable: Only a few resting people will notice it.
- Grade 3: Slightly noticeable: is only noticed by some of those affected. Vibrations like when a light car drives past.
- Grade 4: Generally noticeable: It is noticed by a few people outdoors and by a large number of people indoors. Some sleepers wake up. Clink dishes and windows.
- Grade 5: Awakening: Is noticed by many waking people outdoors and by all waking people indoors. Many sleepers wake up. Hanging objects swing, objects leaned against can fall over. Occasionally, hairline cracks appear in the plaster.
- Grade 6: Frightening: Many people (approx. 50 percent) flee their houses into the open air. Furniture can be moved out of place. Slight damage (cracks in the plaster) occurs on a few solidly built houses, or poorly-preserved houses can cause plastering, roof tiles or chimneys to fall off.
Grade 7: Damage to buildings: Most people (approx. 75 percent) are frightened and flee outdoors. Furnishings fall over. There is moderate damage to numerous houses of solid construction (small cracks in the wall, larger plastering parts fall off, roof tiles slide off, cracks in chimneys, chimney parts fall down). Badly built or badly preserved buildings often show severe damage (deep cracks in the wall, chimneys broken off), and occasionally also destruction (cracks in the masonry; the individual components lose their connection; the collapse of partition walls).
- Grade 8: Destruction of buildings: Many people panic. Even heavy furniture is moved and sometimes overturned. Destruction occurs on around 25 percent of normally built houses (gable parts and eaves collapse; individual buildings, church towers and factory chimneys
- Grade 9: General building damage: General panic among those affected. Many poorly built *) houses collapse, other buildings suffer destruction or severe damage. Underground pipelines are partially broken.
- Grade 10: General building destruction: Many normally built houses collapse, the rest suffer destruction. Serious damage to dams and severe damage to bridges. Underground pipelines are torn or compressed, railroad tracks are easily bent. There are crevices up to one meter wide in the ground.
- Grade 11: Catastrophe: Buildings, bridges, dams, railroad tracks and underground pipelines are consistently damaged by severe damage. Roads become unusable. Many wide cracks and crevices appear in the ground. There are numerous landslides and rockfalls.
- Grade 12: Landscape-changing: Structural and civil engineering works are destroyed and the earth's surface is profoundly reshaped.
(* poorly built in terms of earthquake security - e.g. bulk walls, round stone vaults)
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