Presentation on theme: "How to Use the Earthquake Travel Time Graph (Page 11 of the Earth Science Reference Tables). The graph on page 11 can be used to find several different."— Presentation transcript:
How to Use the Earthquake Travel Time Graph (Page 11 of the Earth Science Reference Tables). The graph on page 11 can be used to find several different variables. Lets start with: TRAVEL TIME or DISTANCE from EPICENTER (Given one, find the other) Sample Problem: A seismic station is 3000 kilometers from the epicenter of an earthquake. How long will it take P waves from this earthquake to reach the seismic station? You must always ask yourself two questions: 1) What information do I have? 2) What information am I looking for? If you dont know what youre looking for its very unlikely that youll find it! 3000 kilometers from the epicenter How long (time)
We know the distance (3000km) Were trying to find the time it will take P waves to travel this distance. Find 3 (really 3000km) on the bottom axis and, using a straight edge (a ruler) draw a line up to meet the P wave line. Now, using the ruler, draw a line over to the left axis and read the travel time: 5 minutes and 40 seconds. Notice that each small box is worth 20 seconds. Now try the same problem in reverse: It takes P waves 5 minutes and 40 seconds to travel from an epicenter to a seismic station. How far is the seismic station from the epicenter?
We know the P wave travel time: 5 minutes and 40 seconds. We are trying to find the distance between the epicenter and the seismic station. Using a ruler, draw a line from the time (5:40) to the P wave line. Now draw a line straight down to the bottom axis and read the distance kilometers! All of these problems are the same. Given the time, find the distance. Given the distance, find the time. And it doesnt matter whether you are given P or S wave travel time as long as you are careful to use the correct line and, most important: TAKE THE TIME TO BE CAREFUL AND ACCURATE!
Lets take this one step further. A seismic station is 3000 km from the epicenter of an earthquake. If P waves from that quake arrive at the station at 4:25:40 PM (4 hours, 25 minutes and 40 seconds), at what time did the earthquake occur? We do exactly the same thing we did before, we find the travel time which is 5:40 (5 minutes, 40 sec.) Now some math. If the waves arrived at 4:25:40 and theyve been traveling for 5:40, when did they start out? Subtract: 4:25:40 5: :20:00 The quake occurred at 4:20:00 PM
OK, lets try another type of problem involving both P and S waves. Heres an example It takes P waves 7:20 (7 minutes and 20 seconds) to travel from an earthquake epicenter to a seismic station. How long will it take S waves from the same earthquake to reach the seismic station? What do we know?We know P wave travel time is 7:20 What are we trying to find?We want to find S wave travel time. Do we have enough information to do the problem? NO! We have to divide the problem into 2 parts. First, we do just what we did in the previous problem. We use the P wave travel time to find the distance to the seismic station and then We use the distance to the seismic station to find how long it took S waves to travel that same distance. Lets see how its done
We know P wave travel time is 7:20 so, using a ruler, draw a line from 7:20 over to the P wave line. Now draw a line straight down to find the distance from the epicenter The distance is 4200 km. Notice that each small box on the bottom axis is 200 km. Now we can get on with the second half of the problem. If the seismic station is 4200 km from the quake epicenter, how long (time) did it take the S waves to travel that same distance? Draw a line from 4200 km straight up to the S wave line. Now draw a line over to the vertical axis and read the time minutes exactly (13:00) And now for something completely different
Another type of question involves the difference in travel time between P waves and S waves. Whenever you are given the difference in arrival times OR you are given both times so that you can subtract and find the difference, you should immediately realize that you will be dealing with the shaded area (yellow) between the two lines. Heres a sample problem: P waves arrive at a seismic station 4:30 (4 minutes, 30 seconds) before the arrival of S waves from the same earthquake. How far from the epicenter is the seismic station?
Heres what you know: P & S waves arrive at a seismic station 4 minutes and 30 seconds (4:30) apart. You are asked to find the distance to the epicenter. Heres how: 1) Take a sheet of paper and line up the left edge with the vertical axis (time). Be sure that most of the paper is hanging down below the graph. This is important. 2) Make a small, thin, and accurate mark on the paper at 0 time. Make another small, thin, and accurate, mark at 4:30 (4 minutes, 30 seconds). Your 2 marks are now 4:30 apart!
Now slide your paper to the right until one of your marks is exactly on the S wave line and the other is exactly on the P wave line. It is very important to be sure your paper is straight (vertical). Now look to see where the bottom of your paper crosses the lower (epicenter distance) axis. In this case it crosses at exactly 3000 km which is the answer. When P & S waves arrive 4 minutes and 30 seconds apart it means that the seismic station is exactly 3000 km from the epicenter of the quake. Of course we can do the same problem in reverse!
A seismic station is 3000 km from the epicenter of an earthquake. How long after P waves arrive will S waves from the same earthquake arrive at the station? This time place your paper on the 3000 km mark taking care to keep the paper straight and to cover both the P and S wave lines. Make small, thin, and accurate marks where your paper crosses the P line and the S line. Then
Slide your paper to the left so that your lower mark is on time 0. Now just read the time indicated by your top most mark: 4:30 (4 minutes and 30 seconds) which is the answer. This kind of question may also be phrased this way: P waves arrive at a seismic station at 12:32:00. S waves from the same earthquake arrive at 12:36:30. How far from the epicenter is the seismic station? All you need to do is subtract in order to get the difference in arrival times. 12:36: :32: :04:30 0 hours, 4 minutes, and 30 seconds. Just remember, if youre given 2 times, just subtract to get the difference.
One last kind of problem to become familiar with. Sometimes, instead of giving you the difference in arrival times, you will be given a seismogram (a record made by a seismograph) instead. Using this seismogram find when the P waves and the S waves arrive. Do this by carefully making a mark at the P wave and S wave arrival times. Now count the number of minutes between the arrival of P and S waves at the station. From this point on its just like the previous problem: If the difference in arrival times between P and S waves is 6 minutes, how far is the seismic station from the epicenter of the earthquake?
Using the seismogram (on the previous page) you determined that P & S waves arrived at a seismic station 6 minutes apart. Place your paper along the vertical axis (time) and make small and accurate marks at time 0 and time 6. Remember to keep most of the paper hanging down below the graph.
Now slide your paper until one mark is on the S wave line and the other is on the P wave line. Be absolutely sure that your paper is straight (vertical). Read the distance at the point where the edge of the paper crosses the bottom (distance) axis. Remember that each small box is 200 km. The answer is km! That all there is to it. But doing this once or twice wont help. The ONLY way to get good at anything is to practice, practice and then practice some more!
One last type of problem: Find the time of origin of the earthquake. In other words, use the information given to find out when the quake occurred. Heres a sample problem: A seismic station is 4000km from the epicenter of an earthquake. P waves arrive at the station at 2:48:00 PM. At what time did the earthquake occur? First, use the distance to find the P wave travel time. OK, the P waves took 7 minutes to travel the 4000km distance. If they arrived at 2:48:00 and the trip took 7 minutes, they must have started out 7 minutes before 2:48 PM So subtract. 2:48:00 - 7: Origin time: 2:41:00 PM
Lets try another one: P waves arrive at a seismic station at 4:22:10 AM. S waves from the same earthquake arrive at 4:28:50 AM. What is the time of origin of the earthquake? First subtract to get the difference in arrival times: 4:28:50 - 4:22: difference = 6:40 (6 min, 40 sec) Now, as we did before, get a piece of paper and carefully and accurately make marks at time 0 and time 6:40
Slide the paper until one mark is on the P wave line and the other is on the S wave line. Now read the distance from the epicenter on the bottom: 4000 km. Now that we know the distance to the epicenter we can easily tell how long it took P waves to travel that distance. P waves took 7 minutes to travel the 4000 km and since they arrived at 4:22:10 we can subtract to find out when they started. 4:22:10 - 7: Origin time: 4:15:10
Best advice: Many students read a question and immediately realize that they do not know what to do. They feel they are completely lost. In that case theres one important thing to remember: DO SOMETHING! Just dont sit there! Do something with the information you have. For instance, if the question mentions P waves and gives a distance, use that to find a travel time. If the question gives a travel time for S waves use that to find a distance to the epicenter. Now you have a little more information than you had before. Re-read the question. Be sure you know what youre looking for and see if you now have enough information to get the answer. Never give up. Never let yourself sit there doing nothing!