11Recording Board Difference in arrival times: San Francisco: 4:102,800kmOpen your compass to the EXACT distance on the scale.1,0002,0003,0004,0005,000
12. . . Use your carefully set compass to draw a circle around each city....You've foundthe epicenter!
13Finding the Earthquake Origin Time You take a distance to epicenter that you have already determined, or one given to youLet’s say the epicenter distance is 4400km, and the arrival time is 3:21:15 PMWell, use your ESRT and do some time math!!!Let’s try it
14Do it! Find the distance on the horizontal axis. P wave travel time = 00:07:40Find the distance on the horizontal axis.Go up to the point where you hit the P-wave travel line.Go over to the vertical axis and read off the travel time for the P-wave.Epicenter dis = 4400km
15Now, do the math!Take the arrival time, and subtract the travel time from it!3:21:15 – 00:07:40 =3:13:35So, the origin time of the Earthquake is 3:13:35 PM.Now YOU try it for San Francisco!!