6.2: Investigation - Locating Earthquake
- Page ID
- 53314
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The seismograph records above are for a recent earthquake in Virginia. The distances from the epicenter are already indicated; we can reproduce these distances and the location of the epicenter. Data from past Virginia earthquakes indicate that the P waves travel at a speed of 6.1 km/s and the S waves travel at a speed of 3.5 km/s. We may determine the "lag time per km" value as
\[{\rm lag\ time\ per\ km} = \frac{1}{3.5}-\frac{1}{6.1}=0.12 \quad (\rm sec). \]
For each seismograph observation location, dividing the observed "total lag time" by "lag time per km" gives the distance the waves traveled in unit of km:
\[{\rm distance\ traveled} = \frac{\rm total\ lag\ time}{\rm lag\ time\ per\ km}\quad (\rm km). \]
The three distances to the three seismograph observation locations can then be combined to triangulate the exact location of the earthquake.
| Location | Time of arrival of P wave | Time of arrival of S wave | Total lag time (sec) | Distance to focus (km) |
|---|---|---|---|---|
| Lynchburg | ||||
| Roanoke | ||||
| Blacksburg |