[Geography Notes] on Earthquake Magnitude Pdf for Exam

Earthquake magnitude is the measure of the “size” or amplitude of the seismic waves generated by the sources of the earthquake and recorded by the seismograph. As the magnitude of the earthquake changes tremendously, it is necessary to compare in order to compress the range of amplitude of waves measured on seismograph using the Mathematical device. 

In 1933, the American seismologist Charles. F. Righter introduced the earthquake magnitude scale (known as the Richter magnitude scale) as the logarithm to the base 10 of the maximum seismic waves amplitude reported on a standard seismograph (in thousandths of ml) at a distance of 60 miles or 100 km from the earthquake epicentre. 

Reductions in amplitude are observed at various distances to the expected amplitude at a standard distance of 100 km is formulated based on the empirical tables. Righter earthquake magnitude scales are computed based on the assumption that the ratio of the amplitude of the maximum wave at two given distances is equal for all earthquakes and is different from the azimuth.

Due to the various shortcomings of the Richter scale, most seismologist authorities now use another scale known as the moment magnitude scale to record the magnitude of an earthquake.

What is the Magnitude of Earthquakes?

The earthquake magnitude is a measure of the amount of seismic energy released by it, so it is a quantitative scale. Magnitude is the most commonly used measure to describe the overall strength or size of an earthquake. The magnitude of an earthquake is expressed in decimal fractions and whole numbers. For example, a magnitude of 5.3 is considered a moderate earthquake whereas a magnitude of 6.3 is a strong earthquake. Due to the logarithm basis of scale, each whole number increases in magnitude represents a 10-fold increase in measured amplitude as measured on a seismograph.

In modern times, several different magnitude scales for measuring the relative size of an earthquake are used by scientists and engineers. An earthquake scale for measuring magnitude has no lower or upper bounds. Sensitive seismographs can even record earthquake magnitudes of negative values and have reported magnitude up to about 9.0 (For example, the 1906 San Francisco earthquake had a Richter magnitude of 8.5).

Earthquake Frequency

An earthquake frequently defines how often a given earthquake with a certain magnitude comes about. On average, an earthquake with a magnitude of 2 or less comes about several hundred times a day throughout the world.

However, a major earthquake range with a magnitude of 7.0 – 7.9 occurs more than once per month throughout the world. Whereas, an earthquake ranges to a great extent with a magnitude of 8.0 or greater or comes about only once a year.

Knowing the earthquake frequency is important for engineers as they not only strengthen a building against earthquake shock but also minimize the force a building is subjected to. To minimize the loss, they install a base isolator that helps to isolate the base of the buildings from the earth’s movement. 

Earthquake Range in Magnitude Scale

How Strong is a 3.5 Magnitude Earthquake?

An earthquake of magnitude 3.5 on the scale is considered to be minor. A 3.5 magnitude earthquake is often felt by people, but rarely causes any damage to the buildings. You can even observe the shaking of an object inside the buildings.  

Did You Know?

• The largest-ever earthquake with a magnitude of 9.6 occurred in China in 1916.

• The largest earthquake in the US with a magnitude of 9.2 struck the Prince Willian studio, Alaska on March 28, 1964, UTC.

• An earthquake can occur in any type of weather.

• It is estimated that approximately 500,000 earthquakes are detected in the world each year. 100,000 among those can be felt, and 100 of the earthquakes can cause damage.

• The most deadly earthquake occurred in Shaanxi, China in 1556. It is estimated that 850,000 people were killed in that period.

• Florida and North Dàkota are the places with the least earthquakes.