
Scientists use several parameters to determine the strength of an earthquake. This allows them to make qualitative and quantitative evaluation of seismic activity and determine the damage caused by earthquakes. Earthquakes are measured using:
- magnitude β the total amount of energy released in an active fault and converted into seismic waves. Magnitude is the unit of measurement used on the Richter scale to characterize earthquakes;
- intensity β the effects of shaking on people, buildings, and infrastructure in a particular region. Earthquake intensity is measured using an intensity scale and is determined by human perception, the movement or falling of objects, the appearance of cracks in walls, or the collapse of buildings.
The seismic intensity of the same earthquake can be different depending on region, as an event of a certain magnitude does not have the same consequences everywhere. The magnitude 7.0 earthquake that occurred off the coast of Kamchatka on August 18, 2024, was felt more strongly in Petropavlovsk (intensity 8) than on Sakhalin (intensity 6).
How is earthquake intensity determined
Seismologists use intensity scales to measure the impact of an earthquake in a particular region. These scales, based on a set of specific criteria, characterize the severity of earthquake effects. Seismic intensity is generally thought to vary depending on the distance from the epicenter, as well as due to the characteristics of the rock structure and topography in the region.
There are several ways to determine the intensity of earthquakes. Seismologists most often use the MSK scale, developed in 1964 by scientists Medvedev, Sponheuer and Karnik. People who are in the zone of seismic activity can determine the strength of earthquakes by using their senses and observation of surrounding objects:
- 1 degree β seismic vibrations are detected only by sensitive seismographs. They are not felt on the surface of the Earth;
- 2 degrees β weak tremors are felt by people who are at rest or lying on the floor;
- 3 degrees β a weak earthquake, hanging objects swing slightly, tremors felt by sensitive people;
- 4 degrees β a noticeable earthquake, vibrations are felt by all people, both inside and outside buildings;
- 5 degrees β seismic tremors are quite noticeable, vibrations cause sleeping people to wake up, some objects fall, small cracks may appear in the walls;
- 6 degrees β a minor damage appears on the walls of buildings, causing people to panic and evacuate;
- 7 degrees β a strong earthquake, old buildings are severely damaged, which can lead to the collapse of walls and chimneys;
- 8 degrees β a devastating earthquake accompanied by cracks in the walls, falling pipes, columns and monuments, and the destruction of the roof;
- 9 degrees β a devastating earthquake causes walls, ceilings, and roofs of buildings to collapse, disrupts underground pipelines, and makes it impossible for people to stand during the tremors;
- 10 degrees β a devastating earthquake, old and new buildings are destroyed, the ground cracks, the integrity of roads is damaged, railway rails are bent;
- 11 degrees β massive destruction, bridge collapses, landslides in the mountains, wide cracks in the ground;
- 12 degrees β catastrophic earthquakes accompanied by the complete destruction of buildings and infrastructure, changes in the relief in the region, changes in riverbeds and the shape of coastlines.
Since 1998, seismologists have been using a modified version of MSK β the European Macroseismic Scale (EMS-98). The intensity of earthquakes in Russia is determined, according to GOST (State Standard) 34511-2018, precisely on the EMS-98 scale.
Why is it important to know the intensity of earthquakes?
The earthquake intensity scale is a very important tool for assessing the effects of seismic activity in specific regions. Seismographs have only been used to measure activity in faults for the past century, therefore, scientists have compiled historical earthquake catalogs spanning centuries and millennia by utilizing intensity scales and eyewitness accounts documented in historical records.
Gathering information in the earthquake zone allows for the creation of maps that reflect all local intensity assessments. With a sufficiently dense distribution of this data, it is possible to draw curves, called isoseismals, which delineate areas of equal intensity.
According to observations, the intensity of the earthquake depends on:
- distance from the epicenter of an earthquake (intensity decreases as distance from the epicenter increases);
- depths of the seismic activity focus (hypocenter);
- mechanical properties of rocks traversed by seismic waves. This leads to a gradual attenuation of seismic waves depending on the geological features of the region.
Therefore, the consequences (intensity) of an earthquake can vary depending on the geological structure of rocks in the region and the distance from the hypocenter.