Classification of Earthquakes
BASED ON LOCATION:
Interplate | Intraplate |
| An interplate earthquake is one that occurs at a plate boundary | An intraplate earthquake is an earthquake that occurs in the interior of a tectonic plate |
| Recurrence time is less | Recurrence time is longer |
| Interplate Earthquakes are recognized at surface | Intraplate earthquakes are rarely recognized at the surface. This is because the faults are buried under several kilometers of surface materials & the longer recurrence intervals allow any surface expression of faulting to be eroded. |
| Interplate earthquakes release less stress & are dissipated quickly because of weaker rocks near plate boundaries. | Intraplate earthquakes release more stress. The ground motion caused by intraplate earthquake seismic waves dissipates more slowly. The strong, coherent rocks that make up the interiors of plates transmit seismic energy more efficiently over longer distances than the less coherent, weaker rocks near plate boundaries. |
BASED ON FOCAL DEPTH:
Shallow Earthquake | Deep Earthquake |
| Shallow-focus earthquakes occur at depths less than 70 km | Deep-focus earthquakes occur at greater focal depths of 300 – 700 km. |
| Shallow focus earthquakes are found within the earth’s outer crustal layer | Deep focus earthquakes occur within the deeper subduction zones of the earth |
| Shallow focus earthquakes are of smaller magnitudes, of a range 1 to 5 | Deep focus earthquakes are of higher magnitudes, 6 to 8 or more. |
| Less energy is released during a shallow focus earthquakes | Tremendous energy accumulates during a deep focus earthquake |
| Shallow focus earthquakes happen frequently and at random within the earth’s crust, often going unrecorded | Deep focus earthquakes occur every 20 to 30 years along a given fault line. |
| Shallow focus earthquakes are barely perceived and are rarely destructive | Deep focus earthquakes leave a deeper impact on civilisation with widespread destruction and permanent changes within the earth’s geology, giving rise to tsunamis. |
| Shallow-focus earthquakes begin where the crustal plates of the earth are moving against one another | Whereas deep-focus earthquakes begin where one tectonic plate moves under another or sub-ducts, at the boundary of oceanic and continental plates. |
| During shallow focus earthquakes, rocks and plates buckle, deform and fault. | In the deep focus earthquakes, the rocks being at greater depths and extremely hot under high pressure, deform by flowing, rather than breaking and faulting. |
| Shallow focus earthquakes are called crustal earthquakes as they exist in the earth’s crustal layer. | Deep focus earthquakes are known as intra plate earthquakes, as they are triggered off by collision between plates. |
BASED ON THE CAUSE:
Non Tectonic Earthquakes: These are due to volcanic activities and man made reasons e.g, nuclear testing, blasts, construction of large dams, deforestation etc
Tectonic Earthquakes: These are due to sudden slip in the fault of the tectonic plates of the earth.
BASED ON THE MAGNITUDE OF THE EARTHQUAKE:
| 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 |
BASED ON EPICENTRE DISTANCE
During an earthquake seismic waves propagate spherically out from the hypocenter. Seismic shadowing occurs on the opposite side of the Earth from the earthquake epicenter because the liquid outer core refracts the longitudinal or compressional (P-waves) while it absorbs the transverse or shear waves (S-waves). Outside of the seismic shadow zone both types of wave can be detected but, due to their different velocities and paths through the Earth, they arrive at different times. By measuring the time difference on any seismograph as well as the distance on a travel-time graph at which the P-wave and S-wave have the same separation, geologists can calculate the distance to the earthquake’s epicenter. This distance is called the epicentral distance, commonly measured in ° (degrees) and denoted as Δ (delta) in seismology.
Local Earthquakes: Affected area is very less, within 1 degree of the epicenter of the earthquake
Regional Earthquakes: 1 degree to 10 degree
Teleseismic Earthquakes: greater than 10 degrees
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