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Sunday, December 13, 2015

Earthquake risk in Delhi


Ground water depletion can TRIGGER an earthquake because of weight reduction of a tectonic plate edge.

It is earthquake causation related.

On the other hand - High water table under a raft foundation can cause soil liquefaction when subjected to earthquakes.

It causes the building to tilt. Sometimes whole building can fall on its side if built on a mountain sise.

It is earthquake EFFECT related.

Buildings along the yamuna banks on alluvial sandy soil can tilt in an earthquake if the water table is high.

Read about soil liquefaction in earthquakes here:

[url]https://en.m.wikipedia.org/wiki/Soil_liquefaction[/url]

Unlike San Andreas fault we are not bothered about earthquake causation because cannot be changed. Ground water in Haryana and Punjab are already depleted but we are not close to any major fault (though minor fault exists - there is no frequency in quakes - frequency is in one quake in 100s of years). Earthquake affecting Delhi NCR will come from 30 to 50 Km away epicenter. Since 1750 there  have been about 2 or 3 earthquakes centered around either the aravallies (sohna fault) or the moradabad fault (mathura epicenter in British times).

No earthquakes in 20th century onwards. This is just a minor fault due to the Delhi Haridwar ridge and there is no tectonic movement involved. Only readjustment of stress from Himalayan fault by the ridge between the Aravalies and the Yamuna plain. It is thought that a channel of Yamuna flowed into Kutch along Rakhigarhi in Haryana in 3000 BC or so and it shifted to present channel due to lifting of the ground.

Almost all fears of earthquake in Delhi are speculative and based on very poor historical records. I would guesstimate one of Richter 6 every 200 years.

[url]http://delhi.gov.in/wps/wcm/connect/doit_dm/DM/Home/Vulnerabilities/[/url]

We are bothered about effects of earthquake on our buildings. For that dry hard ground is better than sandy water logged ground.

Chances of tilting when basement parking is made is practically nil unless extremely bad design or water table is a couple of meters below the bottom of foundation or in sandy soil.

High rises of upto 20 floors with basement parking will be fine with raft foundation.

30 floors need pile foundations especially with sandy or if water table is high. Ideally water table should be lowered by pumping out a good amount of water to lower the table.

Using it in construction is good (idiot environmentalists who dont know science will of course talk the entire opposite).

Five floor builder floor built on a 10 foot basement most likely to tilt and fall if water table is high and sand slurry gets created under the building.