Question

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.

Recording Station	P-wave arrival time ( IST)	S-wave arrival time
Jalana	11: 06: 06	11: 06: 19
Washim	11: 06: 46	11: 07: 03
Aurangabad	11: 07: 06	11: 07: 24

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.

Answer 1

Speed of waves = 08 kms per second

Scale of the map = 1 cm = 18 kms

First step: Find out the difference in time for waves at Jalana recording centre.

The difference in time of waves = Time of S-wave - Time of P-wave

  11 : 06 : 19 (Time of S-wave)
- 11 : 06 : 06 (Time of P-wave)
  00: 00: 13

Therefore the difference in time for waves at Jalana recording centre is 13 seconds.

Second Step: Find out distance on land between Jalana and Epicentre

Distance on land = Difference in time of waves × speed of waves per second

= 13 × 8

= 104

Therefore distance on land between Jalana and Epicentre is 104 km.

Third Step: Find out radius for Jalana Station.

Jalana - Radius of circle = `"Distance on land"/"Scale of the map"`

= `104/18`

= 5.7 cm.

Therefore radius of circle around Jalana station is 5. 7 cm.

Now find out radium for remaining stations.

Jalana - 5.7 cm, Washim - 7.5 cm, Aurangabad - 8 cm,

Fourth step: Now draw circles around Jalana, Washim and Aurangad with the help of radius calculated.

All arcs of the circle will intersect one another at a particular point. That point is the location of Epicentre.

In the map given below Epicentre is to the south of Mudgal.