Earthquake

Look at figure During an earthquake, do you think the seismic waves reach entire portion of the earth? Is there any region on the Earth's surface where a given earthquake will not be reported?

Shadow zone of an earthquake

Answer in detail:

Explain the concept of the shadow zone.

Why has a shadow zone for L-waves not been shown in the figure?

Shadow zone of an earthquake

Give a geographical reason:

People living in the Himalayas are more vulnerable to earthquakes.

Give a geographical reason:

L-waves do not have a shadow zone.

In Figures A, B, C is three points on the earth's surface. Analyse their locations with respect to epicenter and shadow zones

Draw diagram of Shadow zone

How to locate epicentre of a given earthquake?

Take the given hypothetical data in the table. The data shows the time of arrival of P-waves and S-waves at 3 seismograph stations. Assume the scale of the map as 1 cm: 18 km. See figure.

Procedure :

1. Compute the time difference between the arrival of P-waves and S-waves for each station; this is called time lag. (It shows the distance of the seismograph from the focus. Thumb rule: For every second of time lag, the earthquake is approximately 8 km away.)
2. Using the rule above, convert the time lag into the distance (seconds of time lag × 8) for each station.
3. Convert this distance into centimetres as per the scale of the map.
4. On a map, locate the seismograph stations.
5. Draw circles, taking the seismograph stations as the centre, with the radius equal to the distance you have calculated in the previous step.
6. These arcs of circles will intersect one another at a point. This point is the location of the epicentre.
   
   In normal practice, an epicentre is located using computer models. The procedure outlined here is a much simplified version of what is normally done, although the principle is the same.