Explanation:
1. Buenos Aires
The closest city to the epicenter of the earthquake is Buenos Aires.
The epicenter of an earthquake is the place on land above the focus where a disturbance in the earth occurs.
Earthquake epicenter is located on land it is usually located using the method of triangulation.
- In triangulation, the intersection between a minimum of three circles on a map is used to delineate the epicenter.
- At a seismic station, the time of arrival of the first wave is picked and the approximate distance is worked out.
- This is done across three seismic station.
- With an appropriate scale, the value of radius is ascertained.
- Then the three or more circles are drawn.
- The intersection of the three circles gives the epicenter of the earthquake.
- On this map, we see that Buenos Aires is the closest point to the intersect at Valparaiso.
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2. Properties of P waves and S waves:
Properties of P waves
P-waves are seismic waves that propagates within the earth surface.
- P-waves are called primary waves and they are categorized as body waves.
- They move within the earth surface.
- P-waves are longitudinal waves that transmits energy parallel away from their source.
- These waves have the highest velocity and are the first to arrive a seismic station.
- They can be propagated through any medium be it solid, liquid or gases.
Properties of S waves
- S-waves are called secondary or shear waves.
- They are also body waves that are propagated within the earth surface.
- These are transverse waves that moves perpendicular to their source.
- The arrive after the p-waves have been recorded.
- They can only pass through solids and are not propagated through liquids and gases. These materials do not shear.
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3. Only fluids are present:
If a seismogram records only P-waves then only fluids are present there.
P-waves can be propagated through any medium. They are longitudinal waves . They transmit disturbance by a series of rarefaction and compression.
S-waves cannot pass through such mediums because they are shear waves. Liquid cannot be sheared and likewise gas molecules.
When a seismic wave reaches the boundary where fluids are present, only the P-waves will pass through. S-waves will be automatically cut off and they disappear from the seismogram.
- The outer core of the earth is believed to be made up of molten iron and nickel because of the nature of seismic waves here.
- In this region, p-waves are seen on the seismogram but s-waves are cut out.
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4. It is better to use data from three or more seismic station to find the epicenter because they give accurate and precise locations.
- The epicenter of an earthquake is the point on ground above the focus of a quake.
- To locate this point, a minimum of three circles whose intersection gives the epicenter are used.
- The time of arrival of the first wave from three stations are converted to distance.
- Using appropriate scales, circles using the radius(distance derived) are drawn.
- A circle will give no idea about the epicenter. It is nearly impossible to make precise interpretation.
- Two circles are ambiguous to interpret.
- Three circles gives a good knowledge of the epicenter as their intersection depicts the location.
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