About 100,000 earthquakes are recorded in the world every year. Most of them are not even felt, but some strong seismic shocks lead to real disasters. Strong earthquakes cause loss of life, destruction of buildings, roads and technological networks, transforming regions into natural disaster zones. If an earthquake occurs on the ocean floor, there is a high risk that rogue tsunami waves will hit the nearest coast.
There are several measurement scales that are used to determine the intensity of an earthquake and the ability to assess the damage caused by a disaster. The Richter scale, named after its creator and used by seismologists around the world since 1935, allows measuring the earthquake magnitude. The intensity of an earthquake is measured in levels, evaluates the surface effect of damage and is classified according to a modified Mercalli scale.
The Richter scale
Scientists use the Richter scale to determine the strength and amount of energy released during an earthquake. It is graded from 1 to 9, with the potential to exceed its upper limit. The amount of seismic wave energy released from the epicenter is measured in instantaneous magnitudes and determined based on seismograph measurements.
Since measurements and graphs are based on physical measurements of seismographs, the Richter scale is characterized as objective:
- up to 1.9 β microearthquakes, which are recorded only by seismic equipment, are not felt by humans. The frequency – 8,000 events per day;
- from 2.0 to 2.9 β very minor tremors, felt by some people, up to 1,000 earthquakes per day;
- from 3.0 to 3.9 β earthquakes are felt by most people, but rarely cause damage. Frequency β 50,000 per year;
- from 4.0 to 4.9 β tremors are felt indoors, accompanied by rumbling sounds, cause minor damage;
- 0 β 5.9 β moderate earthquakes, can lead to the damage of poorly constructed buildings, occurs with a frequency of 800 events per year;
- 0 β 6.9 β felt as strong tremors that could be destructive at a distance of 180 km from the epicenter. About 120 earthquakes per year;
- 0 β 7.9 β major earthquakes, which can cause moderate or severe damage over large areas, occur approximately 12 times a year;
- 0 β 8.9 β a strong earthquake that can become destructive hundreds of kilometers away from the epicenter, occurs once a year;
- 0 and above β a catastrophic earthquake, devastating areas for thousands of kilometers around, occurs 1-5 times per century.
According to the Richter scale, an increase of one unit in magnitude signifies a tenfold increase in power. Thus, an earthquake with a magnitude of 6.0 will be 10 times more powerful than a 5.0. And hundred times stronger than earthquakes of magnitude 4.0.
The Mercalli Scale
A modified Mercalli earthquake intensity scale (EMS-98 in Europe) is used to quantify the extent of surface effect. The intensity characterizes the strength of ground tremors and is assessed by the consequences of an earthquake. Such an assessment makes it possible to determine whether the tremors are observed, only felt by humans or whether they caused more or less significant damage to buildings. The intensity of ground movement during earthquakes is measured on a scale and denoted by Roman numerals from I to XII:
- I β vibrations are recorded only by instruments and are not felt by people;
- II β vibrations are felt only by some people who are on the upper floors of buildings;
- III β tremors of low intensity are characterized by slight swaying of hanging objects;
- IV β the earthquake is well felt by people indoors, the tremors are accompanied by shaking windows, doors, as well as swinging hanging objects;
- V β the amplitude of the vibrations is strong enough to make doors slam and furniture fall. Tremors are felt outside and inside;
- VI β tremors, falling furniture and dishes make people run out of the house. Minor damage appears on ordinary structures – cracks and crumbling remains of plaster;
- VII β most people are scared and run out into the street. Furniture overturns, hanging objects fall. Many ordinary buildings suffer moderate damage – cracks appear in the walls, chimneys and wooden structures collapse;
- VIII β furniture is overturned, buildings suffer significant damage, and large and deep cracks appear in the walls. Partial collapses are possible;
- IX β strong seismic shocks that lead to partial or complete collapse of buildings;
- X β many buildings and structures are destroyed;
- XI β most of the buildings are destroyed;
- XII β almost all buildings and underground structures are seriously damaged or destroyed.
It should be known that there is nothing in common between the intensity of earthquakes and its magnitude. Unlike the calculated value of magnitude, intensity can only provide a quantitative assessment of the damage caused by seismic tremors.