British Columbia Institute of Technology
Department of Civil Engineering
Unit 1: Earthquakes
Seismicity of British Columbia
Measuring Earthquakes: Magnitude and Intensity
Summary
Measuring Earthquakes: Magnitude and Intensity
The most widely accepted indicators of the size of an earthquake are its magnitude and intensity. The magnitude is a measure of an earthquake in terms of the released energy. At the present time, the most popular scale is the Richter scale, developed by a U.S. seismologist Charles Richter in 1935. Richter defined the magnitude of a local earthquake as the logarithm to base ten of the maximum seismic wave amplitude (in microns) recorded on a standard seismograph at a distance of 100 kilometers from the earthquake epicenter (seismograph is an instrument for recording the motions of the Earth's surface caused by the seismic waves as a function of time). Richter's scale has been recognized by general public, scientists, engineers and technicians as a measure of the relative size of an earthquake. Based on the measurements of seismic energy, it is estimated that each year the total energy released by earthquakes throughout the world is on the order of 1025 to 1026 erg. For the sake of comparison, an atomic bomb blast (e.g. Bikini, 1946) released energy on the order of 1019 erg, whereas an earthquake measuring 5.5 on the Richter scale might release energy of approximately 1020 erg.
Depending on their magnitude, earthquakes are classified into categories ranging from minor to great. Earthquake magnitude classes are shown in the table below.
Earthquake Magnitude Classes
| Class | Magnitude |
| Great | 8 or more |
| Major | 7 - 7.9 |
| Strong | 6 - 6.9 |
| Moderate | 5 - 5.9 |
| Light | 4 - 4.9 |
| Minor | 3 -3.9 |
The table below briefly describes earthquake effects corresponding to various magnitude levels and also gives an estimated number of earthquakes of different magnitudes that happen in the world each year. It can be observed from this table that a large majority of earthquakes (900,000) are of magnitude 2.5 or less (very minor earthquakes, usually not felt). Great, catastrophic earthquakes (magnitude 8 or greater) happen once in 5 to 10 years.
Earthquake Magnitude Scale
|
Magnitude
|
Earthquake Effects
|
Estimated Number Each Year
|
|
2.5 or less
|
Usually not felt, but can be recorded by seismograph. |
900,000
|
|
2.5 to 5.4
|
Often felt, but only causes minor damage. |
30,000
|
|
5.5 to 6.0
|
Slight damage to buildings and other structures. |
500
|
|
6.1 to 6.9
|
May cause a lot of damage in very populated areas. |
100
|
|
7.0 to 7.9
|
Major earthquake. Serious damage. |
20
|
|
8.0 or greater
|
Great earthquake. Can totally destroy communities near the epicenter. |
One every 5 to 10 years
|
The assessment of earthquake intensity on a descriptive scale depends on actual observations of earthquake effects. Observations on the performance of building structures, natural phenomena, and human perceptions are essential for evaluating the earthquake intensity. Intensity of an earthquake depends on the distance from epicenter, and also on the local soil conditions, geology and topography. In a typical case, however, the largest intensity is observed in the vicinity of epicenter and it diminishes with the distance.
The intensity scale consists of a series of certain key responses such as people awakening, movement of furniture, damage to chimneys, and finally - total destruction. Although numerous intensity scales have been developed over the last several hundred years to evaluate the effects of earthquakes, the one currently used in the United States is the Modified Mercalli Intensity (MMI) Scale. This scale, composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, is designated by Roman numerals. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects.
The lower numbers of the intensity scale generally deal with the manner in which the earthquake is felt by people. The higher numbers of the scale are based on observed structural damage. Structural engineers usually contribute information for assigning intensity values of VIII or above.
Detailed specifications of various intensity levels related to the MMI intensity scale are presented in the table below.
Modified Mercalli Intensity (MMI) Scale
|
MMI Level
|
Description |
|
I
|
Not felt except by a very few under favourable circumstances. Typically marginal and long-period effects of strong earthquakes. |
|
II
|
Felt only by a few persons at rest, especially on the upper floors of buildings. Delicately suspended objects may swing. |
|
III
|
Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognize it as an earthquake. Standing cars may rock slightly. Vibration similar to that of passing truck. Duration may be estimated. |
|
IV
|
If during the day, felt indoors by many; outdoors by few. If at night, few awakened. Dishes, windows and doors rattle, walls creak. A sensation such as heavy truck shaking the building. Standing cars rock noticeably. In the upper range of IV wooden walls and frames creak. |
|
V
|
Felt by nearly everyone, many awakened; direction may be estimated. Some dishes and windows broken, some plaster cracked, unstable objects overturned. Disturbance of trees, poles and other tall objects. Pendulum clocks may stop. |
|
VI
|
Felt by all; many people frightened and run outdoors. Persons walk unsteadily. Windows, dishes, glassware broken. Books off shelves; pictures off walls. Weak plaster and masonry D cracked; minor chimney damage. Movement of moderately heavy furniture; some furniture overturned. Small bells ring (school, church). Trees, bushes shaken visibly, or heard to rustle. |
|
VII
|
Everybody runs outdoors. Noticed by persons driving cars. Hanging objects quiver. Furniture broken. Damage to masonry D3, including cracks. Some cracks in masonry C. Weak chimneys broken at roof line. Fall of plaster, loose bricks, stones, tiles, cornices, unbraced parapets and architectural ornaments. Waves on pond; water turbid with mud. Small slides and caving in along sand or gravel banks. Large bells ring. Concrete irrigation ditches damaged. |
|
VIII
|
Persons driving in cars are disturbed. Damage to masonry C; partial collapse. Some damage to masonry B; none to masonry A. Fall of stucco and some masonry walls. Panel walls thrown out of frame structures. Chimneys, factory stacks, monuments, walls, and columns fall. Heavy furniture overturned or damaged. Changes in well water. Sand and mud ejected in small amounts. Cracks in wet ground and on steep slopes. Branches broken from the trees. |
|
IX
|
General panic. Masonry D destroyed; masonry C heavily damaged, sometimes with complete collapse; masonry B seriously damaged. General damage to foundations. Frame structures, if not bolted, shifted off foundations. Frames racked. Serious damage to reservoirs. Ground noticeably cracked, underground pipes broken. In alluviated areas sand and mud ejected, earthquake fountains and sand craters appear. |
|
X
|
Most masonry and frame structures destroyed with their foundations. Some well-built wooden structures destroyed, foundations ruined, ground badly cracked. Serious damage to dams, dikes, embankments. Rails bent slightly. Considerable landslides from steep slopes and river banks. Water splashed over banks. Sand and mud shifted horizontally on beaches and flat land. |
|
XI
|
Rails bent greatly. Bridges destroyed. Broad fissures in ground. Underground pipes out of service. Earth slumps and land slips in soft ground. |
|
XII
|
Total damage. Waves are seen on the ground surface. Large rock masses displaced. Lines of sight and level are distorted. Objects thrown into the air. |
A
major difference between the earthquake intensity and
magnitude lies in the fact that magnitude of an earthquake is determined
based on measuring the ground motion with instruments (seismographs),
whereas the intensity of an earthquake is determined based on observations
of earthquake effects on building structures and human perceptions.
Another essential difference between a magnitude and intensity of an earthquake lies in the fact that magnitude is a unique indicator of a size of an earthquake - each earthquake is characterized with a single value which indicates its magnitude. At the same time, each earthquake is characterized with various intensities, depending on the location of a particular site with respect to the epicenter. For example, Canada's largest historic earthquake, the Queen Charlotte Island earthquake of August 22, 1949 was characterized with magnitude 8.1 on the Richter scale. The same earthquake was characterized with MMI intensities ranging from III to over VII, as illustrated in the figure below.
As an illustration of MMI intensity of VII or higher in the area close
to the epicenter of this earthquake "cows were knocked off their feet,
and a geologist with the Geological Survey of Canada working on the north
end of Graham Island could not stand up." In Prince Rupert (MMI intensity
VI), "windows were shattered and buildings swayed." For more information
about the Queen Charlotte Island earthquake and other Canadian historic
earthquakes, refer to the Geological
Survey of Canada.
Summary
In this unit you have been introduced to earthquakes, their nature and causes. We have learned what causes earthquakes, which regions of our planet are most earthquake-prone and why. We have also learned the methods of measuring earthquakes. We have learned what is the magnitude of an earthquake per Richter scale and we are able to determine if an earthquake is considered to be minor, moderate or major based on a number which characterizes its magnitude. We have also been introduced to earthquake intensity and Modified Mercalli Intensity Scale which is commonly used to measure the intensity of an earthquake. We are also able to explain the difference between the earthquake magnitude and intensity.
In the next unit we will introduce you to the effects of earthquakes on buildings. We will learn how earthquake forces are developed and how these forces are resisted by structural elements within a building.
Go to Unit 2: Earthquake Effects