Explain the relation between seismicity and plate tectonics

Seismicity is the frequency and intensity of an earthquake (a shock or a series of shocks due to a sudden movement of crustal rocks , generated at a point called focus within the crust or mantle in an area.

The term ‘plate’ is coined by Canadian Geo-physicist J.T. Wilson.The evolution,movement and destruction of the plates at the trenches are tectonically termed as plate tectonics.

Earthquakes are classified into 5 types depending on the mode of generation:-

1)Tectonic earthquakes;

2)Volcanic earthquakes;

3)Collapse earthquakes;

4)Explosion earthquakes;&

5)Plutonic earthquakes.

Tectonic earthquakes which occur from structural adjustments inside the earth are the result of the plate tectonic mechanism. Tectonic earthquakes are sub-divided into- Inter-plate earthquakes(occurring at plate edges) and Intra-plate earthquakes(occurring within the plates other than the edges).Earthquakes occurring at where the plate boundaries converge such as trenches contribute <90 10="" about="" and="" are="" at="" compression.="" crustal="" destroyed="" due="" earthquakes="" energy="" for="" generated="" get="" horizontal="" i="" mid-oceanic="" occur="" of="" originate="" plates="" release="" ridges.the="" ridges="" s="" seismic="" tension.at="" the="" to="" trenches.in="" trenches="" while="" world="">Mc kenzie

have observed that the ridges are easily started but the trenches are difficult to initiate.

It is the vertical stress resulting from the gravitational overloading which induces the horizontal compressive strain such that a horizontal stress(S_h) is generated by relationship:-

S_h=S₂/(m-1) where,m=Poisson’s number which for the basaltic rock has a value of 4.0.

Geological evidences show that plate geometry is not permanent but undergoing constant,gradual change.Magma is continuously upwelling at mid-oceanic ridges and rises as the sea floor spreads.

It has been found that ridge crests were continuous but consisted from the start of segments offset by transform faults.As the crust is carried away from each segment ,it moves in the same direction on both sides of the fault and there is’nt any earthquakes on such situations.However,between the crest segments the pieces move in opposite direction with a sense i.e., the reverse of the strike-slip fault and the earthquakes occur in these regions.Along sub-marine mountain chains there are many surface raptures and down-dropped blocks and when these breaks occur in the rocks ,energy for the local earthquake is released.

The common  believe behind the causal mechanism of the earthquake is the elastic rebound theory.If there were no water in the rocks,there would be no tectonic earthquakes.Almost everywhere,not many meter below the earth’s surface we find that the rock’s are permeated with ground water.

It has been found by the scientists that the wet rocks under some situation ,on being influenced by the shear strain increase their volume rather than decrease it.This  increase in volume during deformation is called dilatency.This results in the formation of numerous micro-cracks which gets filled-up with ground water which does’nt get closed so easily under the pressure after the aforesaid changes has occurred to them.At greater depths the hydrostatic pressure being greater than the strength of the rocks will lad to flow and plastic deformation of the rocks,rather  than break through brittle fracture .Thus,producing earthquakes.

Water allows a sudden slip to take place by lubrication along slide planes by more effectively increasing the local pore water pressure and thus resulting in weakness in the rocks.This process causes elastic straining and elastic energy is stored in the form of potential energy.when faults rapture,elastic energy stored in the rocks is released,partly as heat and partly as elastic wave(i.e., earthquake).

Case Study:-

1)In the Kwanto earthquake of 1^st September,1923 at Japan such vertical movement was noticed as in places water depth changed by 250m. due to submarine slides.

2)In Nigata Shock on 16^th June,1964 vertical ground change was note-worthy.From 1898-1958 measurements showed that prior to 1964 the shoreline of Honshu opposite Awashima Island was rising steadily at the rate of about 2mm./yr. and the level of shoreline dropped 15-20 cm. at the time of earthquake near Awashima island.

It has been found that hydrological conditions in the fault zone commonly produce hydrous or water containing rocks like serpentinite.It has been found that within seismic zones there are consistent networks of active dip-slip,strike-slip and oblique faults.the spreading ridges and subduction zones are predominantly the sites of normal and thrust earthquake source mechanism .The earthquake foci are concentrated at the top of descending layers and along the ridge.

Intra-plate earthquakes arise from more localized systems of forces in the crust perhaps associated with the ancient geological structural complexities or with the anomalies in temperature and strength of the lithosphere.Example:-of intra-plate earthquake is the earthquake that struck New Madrid region,Missouri,in U.S.A.