A) 25 km
B) 250 km
C) 2500 km
D) 5000 km
Correct Answer: C
Solution :
Suppose d = distance of epicenter of Earth quake from point of observation vS = Speed of S-wave and vP = Speed of P-wave then \[d={{v}_{P}}{{t}_{P}}={{v}_{S}}{{t}_{S}}\] or \[8\,{{t}_{P}}=4.5\,{{t}_{S}}\] Þ \[{{t}_{P}}=\frac{45}{8}{{t}_{S}},\] given that \[{{t}_{S}}-{{t}_{P}}=240\] Þ \[{{t}_{S}}-\frac{4.5}{8}{{t}_{S}}=240\] Þ \[{{t}_{S}}=\frac{240\times 8}{3.5}=548.5\,s\] \ \[d={{v}_{S}}{{t}_{S}}=4.5\times 548.5\]\[=2468.6\approx 2500\,km\]
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