When an earthquake occurs…?
Sep 04, 2009
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Earthquake Questions
When an earthquake occurs, two types of sound waves are generated and travel through the earth. The primary, or P, wave has a speed of about 8.0 km/s and the secondary, or S, wave has a speed of about 4.5 km/s. A seismograph, located some distance away, records the arrival of the P wave and then, 78 s later, records the arrival of the S wave. Assuming that the waves travel in a straight line, how far is the seismograph from the earthquake?
The answer for this problem is 8.8 X 10^5 m. I can’t seem to get this answer… can you tell me how you did it if you know how to do it? Thanks!
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3 comments
kveryeffective on September 4, 2009 at 11:29 pm
D= Distance travel
S1= Speed of P waves 8.0 Km/s
S2= Speed of S waves 4.5 Km/s
T1= Time it took travel the distance for the P waves
T2= Time it took to travel the distance for the S waves
T2= T1 + 78 seconds
S1*T1 = S2*T2
S1*T1 = S2*(T1+78 s)
S1*T1 = (S2*T1) + (S2*78s)
(S1*T1)-(S2*T1)= S2*78s
T1*(S1-S2)=S2*78s
T1= (S2*78s)/(S1-S2)
T1 = (4.5km/s*78s)/ (8.0-4.5km/s)
T1 = 351km/75 km/s
T1 = 4.68s
D = S1T1
D = 8.0 km/s* 4.68 sec
D = 37.44 km
Hope this helps.
aaronireland2003 on September 4, 2009 at 11:29 pm
well if it takes 78 secounds to get to the graph then it gotta be slow. just canculate the figer from the number on the radies of the earths diamiter. the s wave will get there sloower than the p wave. if they p wave went for say 2 kilo miles which is 16.0km/s just to say. but the s wave will get 9 kilo miles in the 2 miles. slowly the p wave will leave the s wave behind and that is why it is 78 secounds late. then just canculate how far the p wave got form the s wave and bam you have the answer. plus if you have the answer why canculate? any way good luck.
Mark M on September 4, 2009 at 11:29 pm
The S waves would travel 351 km during that 78 seconds they were behind. (4.5 * 78 = 351) That means the P waves need enough time to get 351 km ahead of the S waves. Every second the P waves travel 3.5 km further than the S waves. (8 – 4.5 = 3.5) That means it would have taken 100.29 seconds for the P waves to get that far ahead of the S waves (351 / 3.5 = 100.2857 seconds). In that amount of time the P waves would have traveled 802.29 km (100.29 seconds * 8 km/s) or 8.02 x 105 km. The answer given is likely wrong.