What is Earthquake?
Earthquake is nothing but a shaking of the earth. It can be simplified with the throwing stone in the pond. As after hitting the stone in water of the pond, wave energy propagate, similar wave energy propagates due to the strain release in some fault locations is known as earthquake.
How Earthquake occurs?
All of us know that spherical earth consists of different layers or strata. Inner layers are relatively hot than the outer layers. Outermost layer is called crust. Crusts are not continuous rather consists of some segments which called plates. Due to the temperature difference in different layers these plates keep moving in different directions. Thought movement of these plates is negligible, usually some millimeter per year, it can cause the earthquake if slips from the interlock of two different plates. Some figures can explain these matters easily.
Earthquake Magnitude
Magnitude is a measure of earthquake size and remains unchanged with distance from the earthquake. So magnitude of an earthquake will always be same. If an earthquake of 7.9 M occurs in Nepal, we cannot say that the magnitude of that earthquake was 4 or 5 M in Bangladesh. So magnitude of that earthquake is always same but feel like may be different depending on the distance from the epicenter , geological setting of our place concern.
Fig: Richter scale for identifying Magnitude by measuring the amplitude (height).
Earthquake Intensity
Intensity, however, describes the degree of shaking caused by an earthquake at a given place and decreases with distance from the earthquake epicenter. Moreover, Intensity is a qualitative measure of the strength of ground shaking at a particular site.
Currently used in the U.S. is the Modified Mercalli Intensity Scale. Each earthquake that is large enough to be felt will have a range of intensities. Usually (but not always) the highest intensities are measured near the earthquake epicenter and lower intensities are measured farther away. Roman numerals are used to describe intensities to distinguish them from magnitudes.
For example, the magnitude of the Northridge earthquake was 6.7. The intensities ranged from IX (violent) close to the epicenter, to V's (moderate) at distances of about 60 - 200 miles away, and finally not felt at distances far away.
In an example, Magnitude can be likened to the power of radio or television waves sent out from a broadcasting station. Intensity is how well you receive the signal, which can depend on your distance from the energy source, the local conditions, and the pathway the signal has to take to reach you.
Intensity at Kathmandu occurred by that 7.8M earthquake:
Here intensity is VIII 12 km away from the epicenter. From the above Magnitude-Intensity co-relation table, we can say that earthquake at Kathmandu feels like 7.0 M if the epicenter was in Kathmandu.
Another intensity at Dhaka occurred by that 7.8M earthquake:
Here intensity is IV 604 km away from the epicenter. From the above Magnitude-Intensity co-relation table, we can say that earthquake at Dhaka feels like 4.0 M if the epicenter was in Dhaka.
2. http://www.gns.cri.nz/Home/Learning/Science-Topics/Earthquakes/
3. http://earthquake.usgs.gov/earthquakes/dyfi/background.php
4. http://www.geo.mtu.edu/UPSeis/intensity.html
5. http://nctr.pmel.noaa.gov/education/ITTI/usgs/seismic-tsunami-training-sri-lanka/II_3_magnitude.ppt
Earthquake is nothing but a shaking of the earth. It can be simplified with the throwing stone in the pond. As after hitting the stone in water of the pond, wave energy propagate, similar wave energy propagates due to the strain release in some fault locations is known as earthquake.
How Earthquake occurs?
All of us know that spherical earth consists of different layers or strata. Inner layers are relatively hot than the outer layers. Outermost layer is called crust. Crusts are not continuous rather consists of some segments which called plates. Due to the temperature difference in different layers these plates keep moving in different directions. Thought movement of these plates is negligible, usually some millimeter per year, it can cause the earthquake if slips from the interlock of two different plates. Some figures can explain these matters easily.
Fig: different earth layers and crust
These movement of the Plates is known as continental drift.
Fig: Continental drift
There are different types of plate movements causing the different faults. Following figures can illustrate all the faults.
Fig: Convergent Fault.
Fig: Divergent Fault.
Fig: Strike Slip Fault.
Sometimes these faluts slip from the interlock and causing the release of the stored energy in the form of earthquake.
Earthquake Magnitude
Magnitude is a measure of earthquake size and remains unchanged with distance from the earthquake. So magnitude of an earthquake will always be same. If an earthquake of 7.9 M occurs in Nepal, we cannot say that the magnitude of that earthquake was 4 or 5 M in Bangladesh. So magnitude of that earthquake is always same but feel like may be different depending on the distance from the epicenter , geological setting of our place concern.
The Richter Magnitude Scale measures the amount of seismic energy released by an earthquake.
Fig: Richter scale for identifying Magnitude by measuring the amplitude (height).
Earthquake Intensity
Intensity, however, describes the degree of shaking caused by an earthquake at a given place and decreases with distance from the earthquake epicenter. Moreover, Intensity is a qualitative measure of the strength of ground shaking at a particular site.
Currently used in the U.S. is the Modified Mercalli Intensity Scale. Each earthquake that is large enough to be felt will have a range of intensities. Usually (but not always) the highest intensities are measured near the earthquake epicenter and lower intensities are measured farther away. Roman numerals are used to describe intensities to distinguish them from magnitudes.
For example, the magnitude of the Northridge earthquake was 6.7. The intensities ranged from IX (violent) close to the epicenter, to V's (moderate) at distances of about 60 - 200 miles away, and finally not felt at distances far away.
Intensity is based on the users feedback that how they felt the shaking. According to their feedback intensity is scaled using Modified Mercalli Intensity Scale (MMI).
Evaluation of earthquake intensity can be made only after eyewitness reports and results of field investigations are studied and interpreted. (7)
Evaluation of earthquake intensity can be made only after eyewitness reports and results of field investigations are studied and interpreted. (7)
Relation between the Magnitude and Intensity:
| Magnitude | Typical Maximum Modified Mercalli Intensity |
|---|---|
| 1.0 - 3.0 | I |
| 3.0 - 3.9 | II - III |
| 4.0 - 4.9 | IV - V |
| 5.0 - 5.9 | VI - VII |
| 6.0 - 6.9 | VII - IX |
| 7.0 and higher | VIII or higher |
Earthquake Magnitude is 7.8 M
Intensity at Kathmandu occurred by that 7.8M earthquake:
Here intensity is VIII 12 km away from the epicenter. From the above Magnitude-Intensity co-relation table, we can say that earthquake at Kathmandu feels like 7.0 M if the epicenter was in Kathmandu.
Another intensity at Dhaka occurred by that 7.8M earthquake:
Here intensity is IV 604 km away from the epicenter. From the above Magnitude-Intensity co-relation table, we can say that earthquake at Dhaka feels like 4.0 M if the epicenter was in Dhaka.
References
1. http://www.earthquakes.bgs.ac.uk/education/faqs/faq17.html2. http://www.gns.cri.nz/Home/Learning/Science-Topics/Earthquakes/
3. http://earthquake.usgs.gov/earthquakes/dyfi/background.php
4. http://www.geo.mtu.edu/UPSeis/intensity.html
5. http://nctr.pmel.noaa.gov/education/ITTI/usgs/seismic-tsunami-training-sri-lanka/II_3_magnitude.ppt
6. https://www.uwgb.edu/dutchs/EarthSC202PowerPoint/Faults%20and%20Earthquakes.ppt
7. http://pubs.usgs.gov/gip/earthq1/measure.html
8. http://tremor.nmt.edu/faq/how.html
7. http://pubs.usgs.gov/gip/earthq1/measure.html
8. http://tremor.nmt.edu/faq/how.html
