It is known that the linear distance from Yiliang, Yunnan Province to our city is about S =1440 km =1.44×106 m, the propagation speed of main wave is v 1=6000m/s, and the propagation speed of secondary wave is v2 = 4000 m/s.

Find: (1) the time when the main wave of seismic wave reaches our city t 1=? ; (2) The time when the main wave arrives earlier than the secondary wave △t=?

Solution: (1)∵v=st

Time of arrival of main wave of seismic wave in our city:

t 1 = SV 1 = 1.44× 106m 6000m/s = 240s；

(2) The time when the second wave of seismic waves arrived in our city:

T2 = sv2 = 1.44× 106m 4000m/s = 360s，

Time when the main wave arrives earlier than the secondary wave:

△t = T2-t 1 = 360s-240s = 120s。

Answer: (1) It takes 240 s for the main wave of this seismic wave to reach our city;

(2) The main wave of seismic wave reaches our city 120s earlier than the secondary wave.