With seismograph data from two recording stations, each station’s calculated epicenter distance is drawn as a circle, and the two circles intersect at two points labeled location 1 and location 2. What can you conclude about where the earthquake epicenter was located? A) The epicenter was somewhere between locations 1 and 2. B) The earthquake had two epicenters, at locations 1 and 2. C) The epicenter was either at location 1 or 2. D) You cannot tell anything about the location of the epicenter.

What the circles mean on an earthquake map

Each circle represents all possible points that are the correct distance from one seismic station. So the true epicenter must be somewhere on that circle for that station.

Using two stations: where can both distances be true?

To satisfy both stations at the same time, the epicenter has to be on both circles. The only places that are on both circles are their intersection point(s).

• If two circles intersect twice, there are two possible epicenter locations.
• Those two intersection points are what the diagram labels as location 1 and location 2.

So you can narrow it down to two points, but you cannot pick between them using only two stations.

Why the other choices do not work

• A (somewhere between 1 and 2): the epicenter is not anywhere in between, it must match the exact distances, so it must be at an intersection.
• B (two epicenters): one earthquake has one epicenter.
• D (cannot tell anything): you can tell it must be at one of the intersection points.

How you would finish the job

Add a third station, draw a third circle, and the single point where all three circles meet is the epicenter.