FINDING AN EPICENTER
Name:______________________________________________Date:___________________Block:___________
OBJECTIVE: Using the information from seismology observation stations, you will locate the epicenters of earthquakes.
PROBLEM #1: An earthquake has occurred somewhere in the United States. Using the information in the graph below,
calculate the S-P time in seconds. Fill this information in the table below. Then, using that information and the formula:
Distance = 12 km/sec x (S – P), calculate the radius of the circle around each reporting city along which the earthquake
had to have occurred. Fill this information in the table below. Then, using the kilometer scale at the bottom of the map,
draw a circle around the city equal to that distance. Determine where all three circles intersect; this will be the epicenter of
the earthquake.
EXAMPLE: S = 24 seconds, P = 12 seconds for Kansas City, MO
S – P lag time = __________________
Formula: Distance = 12 km/sec x (S – P)
Distance = 12 km/sec x (_____________)
Distance = _________________________
Using a compass and the kilometer scale at the bottom of the map, draw a circle around the city
equal to that distance.
CITY
El Paso, TX
San Francisco, CA
Seattle, WA
CALCULATIONS OF DISTANCE:
El Paso, TX:
San Francisco, CA
Seattle, WA:
LAG TIME (SECONDS)
DISTANCE FROM CITY TO
EPICENTER
Map for
Problem #1
QUESTIONS:
Describe the location of the epicenter for Problem # 1? _________________________________________________
__________________________________________________________________________________________
PROBLEM #2: An earthquake has occurred somewhere in the United States. Using the information in the graph below,
calculate the S-P time in seconds. Fill this information in the table below. Then, using that information and the formula:
Distance = 100 km/sec x (S – P), calculate the radius of the circle around each reporting city along which the earthquake
had to have occurred. Fill this information in the table below. Then, using the kilometer scale at the bottom of the map,
draw a circle around the city equal to that distance. Determine where all three circles intersect; this will be the epicenter of
the earthquake.
CITY
LAG TIME (SECONDS)
DISTANCE FROM CITY TO
EPICENTER
Austin
Bismark
Portland
CALCULATIONS OF DISTANCE:
El Paso, TX:
San Francisco, CA
Seattle, WA:
Map for
Problem #2
QUESTIONS:
1. Describe the location of the earthquake’s epicenter. To which city is the epicenter closest? _____________________
__________________________________________________________________________________________
2. Why must measurements from three locations be used to find the epicenter of an earthquake? (Hint: Think about why
one or two locations wouldn’t be ideal for finding the epicenter.) _________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________