1. Key Term: Epicenter

2. Locating the Epicenter Please turn to page 116 in your textbook. Epicenter The point on Earth’s surface directly above the focus of the earthquake. Seismograms are used to find the epicenter. The distance to the epicenter from the seismogram is calculated using the SP Interval. A circle is then drawn around the seismograph station with the station being at the center of the circle. The radius of the circle is the distance from the station to the epicenter. 2

3. Drawing the Circle Seismograph Station: Denver, Colorado Distance to the epicenter: 400 km. A circle is then drawn around Denver with a radius of 400 km with Denver being at the center of the circle. 3 We know that the earthquake’s epicenter is 400 km from Denver, so the epicenter is somewhere on the circle we’ve drawn but we do not know exactly where on the circle.

4. One Seismograph By drawing the circle around one seismograph station, we know the epicenter is somewhere on the circle but we don’t know exactly where. We need a second seismogram that recorded the earthquake taken from a different station. 4

5. Drawing the Second Circle Seismograph Station: Chicago, Illinois Distance to the epicenter: 1300 km. A circle is then drawn around Chicago with a radius of 1300 km with Chicago being at the center of the circle.. 5 We know that the earthquake’s epicenter is 400 km from Denver, and 1300 km from Chicago. Therefore the epicenter is one of the two places where the circles intersect.

6. Two Seismographs By drawing the distance circles around two different seismograph stations, we know the epicenter is at one of he locations where the two circles intersect. We still do not have enough information to know exactly where the epicenter is located. We need a third seismogram that recorded the earthquake taken from a different station. 6

7. Drawing the Third Circle Seismograph Station: Houston, Texas Distance to the epicenter: 1000 km. A circle is then drawn around Houston with a radius of 1000 km with Houston being at the center of the circle. 7 The epicenter is at the location where all 3 circles intersect.