Finding an earthquake epicenter
1 min 40 s. The words are spoken and shown in the picture; the player can also show them as subtitles.
For the older Earth Science Regents, which used the 2011 Reference Tables. The Earth and Space Sciences Regents asks about this less often.
What Dr. Mars says (the words of the video)
How do scientists find an earthquake's epicenter?
An earthquake starts underground, at the focus.
The epicenter is the point on the surface directly above the focus.
Each of Dr. Mars's hammer strikes sends out P-waves (faster) and S-waves (slower), like an earthquake.
The earthquake sends out P-waves and S-waves from the focus.
At each station, the P-wave arrives first, then the S-wave.
The gap between P and S grows with distance.
The far station has the bigger gap.
So the gap tells the distance to the epicenter.
A travel-time chart turns each gap into a distance.
One station gives only a distance: a circle.
Dr. Mars sets his compass to that distance and draws the circle.
The epicenter lies somewhere on this circle.
Only one point lies on all three circles.
That point is the epicenter.
Check yourself! Which point is the epicenter: A, B, or C?
Think about which point lies on all three circles.
Answer: B! It lies on all three circles.
Dr. Mars set his compass to the distance the chart gave for each gap.
Dr. Mars. Free practice for the New York Earth and Space Sciences Regents.