(a) Use diagrams and text to describe four factors that influence the movement f
ID: 153865 • Letter: #
Question
(a) Use diagrams and text to describe four factors that influence the movement felt at the ground surface of a city from an earthquake at depth. Why is the ground surface movement in the upper soil and unconsolidated sediment greater than in the underlying crystalline rock layers? (5 marks)
(b) Acid sulphate soils are commonly found in low-lying areas, both near the coast and around inland waterways. Describe briefly how acid sulphate soils form and the conditions they occur in. What would be some of the consequences of draining acid sulphate soils for an engineering development? (5 marks)
Explanation / Answer
A. The four factors that determine the destructiveness of the earthquake at the ground surface are as follows:-
1. Magnitude of an earthquake: It refers to the amount of seismic energy that is released during an earthquake. Higher the magnitude, higher is the amount of energy. It is represented in a Richter Scale of a factor of 10.
2. Intensity: The intensity of an earthquake refers to the destructiveness of an earthquake. It is usually measured in Mercalli's scale of a factor of 2.
3. Distance from the epicentre: The lesser the distance from the epicentre, powerful is the earthquake since the seismic waves gets less attenuated.
4 Local geological factors and secondary factors: The geology of the area also determines how much the tremors would be felt. Secondary effect usually means the effects associated with earthquakes like landslides, tsunamis.
The ground surface movement in the upper soil and unconsolidated sediments is greater than the underlying hard and crystalline rocks because of the surface waves.. The surface waves which mainly affects the surface layers of the earth creates the major havoc during an earthquake. The surface wave can be divided into Rayleigh waves and love waves both of which passes through the too surface layer of unconsolidated sediments easily and destroys everything.
B. The sediments which have the tendency to be transformed into acid sulphate soils are concentrated in the water logged areas likes mangroves, swamps , peats etc. In the last 10000 years after a major sea level transgression ie. sea level rise the sulphate which is present in the sea water mixed with the iron oxides and organic matter of the land sediments.Now bacteria like Desulfovibrio sulfuricans under these anaerobic conditions obtain oxygen for their respiration purposes from the sulphate ions in seawater resulting in formation of hydrogen sulphide. Hydrogen sulphide reacts with dissolved ferrous ion to form very fine grained crystals of pyrite . Now this pyrite is stable until and unless it is submerged. As soon as it is exposed to air it oxidises and releases sulphuric acid.
Now if we start draining out from an acid sulphate soil for engineering constructions what will happen is pyrite on being exposed to air will oxidise and form suphuric acid. Thus problem of acid leaching.
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.