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Declared arrival

  • 27/02/2001

Stage

How an earthquake develops

What symptoms does it leave behind for study and observation

I In the first stage of an earthquake, the two sides of a fault move. Elastic strain slowly builds up in the rocks as a result of it, and the rock particles become compressed together. Extensive study of fault areas indicates the building up of pressure.
II The rocks are now packed as tightly as possible due to compression, and to change shape rocks expand and occupy a larger volume, a process called dilatancy. The increase in volume is caused by the formation of small cracks in the rocks. Water that fills the gaps in the rocks is forced out, a process similar to when we step on wet beach sand causing water  dispersion around the footprint. As water comes out air fills the rocks gaps. Rocks get more capacity to withstand elastic strain. This process causes uplift and tilting of the ground on the surface. Uplift and tilting of the ground on the surface can be measured. In Japan scientists measured such phenomena using simple instruments and by observation of water level in lakes before and after the Niigata earthquake of June 16, 1964. This study  forms the basic of a long-term warning of earthquake.
III Stage II is undone as water rushes back into the rock gaps under the pressure of surrounding water, just like the water collected around the footprint rushing back into it. As the water returns, rock loses its increased strength. An already weak rock get further weakened. Presence of water  prevents formation of small gaps in the rock thus putting a halt to its expansion, crucial to sustain the movement in the fault line. Radon — a water-soluble, radioactive mineral naturally found in rocks and groundwater — is emitted abnormally during this stage. The expansion of rocks in the second stage exposes more radon to water causing breakdown of the radioactive material. So more emission of it. Can be measured. Just before the 1966 Tashkent earthquake, increased radon emission was noticed.
IV Rocks give up under tremendous pressure and the fault suddenly ruptures producing an earthquake. Ruptures release elastic energy stored in the rocks as seismic waves. These waves cause damages. Few months before the rupture, electrical resistance of rocks decreases considerably. Can be measured. Prior to the Garm earthquake in the erstwhile USSR, instruments recorded such change. Experiments in USSR, China, Japan and the US have established method to measure and analyses it.
V Most of the elastic strain energy is released by the earthquake. As an impact of the rupture that caused the earthquake, other small ruptures also happen causing aftershocks. This stabilises the condition. The groundwater water level fluctuates prior to an earthquake. For the activities under second and third stages, groundwater level rises and drops. Can be observed, It happens just before the quake, Bhuj areas reportedly experienced this fluctuation.

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