Focus— point beneath the Earth’s surface where fault movement releases seismic waves.
Epicenter— the point on Earth’s surface directly above the focus.
P-Waves (Compressional) --primary (comes first) --travels through solid, liquid and gas --fast (arrives first at station) Liquid—outer core
S-Waves (shear) --secondary (comes 2 nd ) --travels through solid only --slow (arrives at station second) Cannot travel through liquid!!
LOCATING THE EPICENTER 1.Need at least 3 stations 2.Determine when the P wave arrives 3.Determine when the S wave arrives 4.Difference between S and P wave arrival 5. Locate on travel-time curve the difference in S and P wave arrival times in correspondence to distance (x-axis) 6. Draw circles around each station with distance determined in step 5 7. Intersection of the three circles is the epicenter
1. Approximately how long does an earthquake P- wave take to travel the first 6500 kilometers after the earthquake occur? 10 minutes
2. Compared to the thickness and density of the continental crust of South America, the oceanic crust of the Pacific floor is… a. thinner and less dense b. thinner and more dense c. thicker and less dense d. thicker and more dense
3. A seismograph station records a difference in arrival time between the S- and P-wave of 4 minutes. About how far away is the earthquake epicenter? 2,600 km
4. A P-wave arrives at a station located in Seattle, Washington at 2:35:20. An S-wave arrives at 2:37:10. What is the distance to the epicenter of the earthquake? 2:37:10 -2:35:20 1:50 1000 km
5. A P- wave arrived at a station 4 minutes and 40 seconds before the S- wave. What is the distance to the epicenter of the earthquake? 3,200 km
6. The arrival of P- waves and S- waves at a seismic station indicated that an earthquake occurred 4,000 kilometers from the station. The P- wave arrived at 3:32:30 p.m. When did the earthquake occur? 1.How long does it take a P-Wave to travel 4,000 km? 7 minutes 2.3:32:30 7:00 3:25:30 pm
7. A seismic station receives a P-wave at 12:07 a.m. and an S- wave at 12:12 a.m. What is the station’s approximate distance from the epicenter? 1.What is the difference between arrival times? 12:12 am 12:07 5 minutes 2.Measure and Match 3,500 km
8. At which station is the difference in time between the arrival of P- and S-waves the greatest? D. P-waves travel faster so they will arrive first. S-waves would never make it to stations E and F.
9. What explanation do scientists give for the reason that station F did not receive S-waves? 2.The Earth's outer core is liquid, which does not allow S-waves to pass.
10. Seismograph station D is 7,700 kilometers from the epicenter. If the P- wave arrived at this station at 2:15 p.m., at approximately what time did the earthquake occur? 1.How long does it take an earthquake P-wave to travel 7,700 km? 11 minutes 2.2:15:00 pm 11:00 min 2:04:00
11. Seismograph station B recorded the arrival of P-waves at 2:10 p.m. and the arrival of S-waves at 2:15 p.m. Approximately how far is station B from the earthquake epicenter? 1.What is the difference between arrival times? 2:15 - 2:10 5 minutes 2.Measure and Match 3,400 km