A physics problem about speed calculation When an earthquake occurs, the vibration of the source propagates outward in the form of S-wave and P-wave. The propagation speed of the two waves in the crust is not the same, about 3.4 km / s and 5.6 km / s respectively. The place directly above the source becomes the epicenter. The first one to reach the ground is the P-wave. At this time, people in the epicenter will feel up and down turbulence, and then the S-wave will come, and they will feel left and right sway, If the time difference between P-wave and S-wave arriving at the epicenter is 2 seconds, how many kilometers is the distance from the source to the epicenter?

Let the distance be L
L/v1-L/v2=2 v1=3.4 v2=5.6
L = 17.31 km

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