British Columbia Institute of Technology
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
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
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
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.
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.