There are several types of seismic waves. The two that you need to know are P-waves and S-waves . The information below is very important and you must know it.
P-waves - also called primary or compressional waves. Four Important things to remember about P-waves: 1. They travel the fastest. 2. They travel in the same direction as the particles they vibrate (see diagram 1). 3. They travel through solids, liquids and gases. 4. They travel faster through denser materials (density increases with depth on the earth).
S-waves- also called secondary or shear waves. Four important things to remember about S-waves: 1. They travel slower than P-waves. 2. They travel to right angles to the vibration of the particles (see diagram 2). 3. S-waves cannot pass through liquids or gases. 4. They travel faster through denser materials.
Remember that p-waves will always reach the seismograph first because they travel faster and that s-waves only travel through solids.
Locating an Earthquake Epicenter
To find the epicenter you need to use atleast three seismograms (diagram 3). From the seismograms you need to determine the arrival times of the p-waves and the s-waves, calculate the time difference and use the Earthquake P-wave and S-wave Travel Time Graph on page 11 in the reference tables. Turn to this graph in your reference tables now and follow the steps to determine the epicenter distance.
Step 1. Determine the arrival times of the p-wave and s-wave. Based on the sample seismogram the p-wave arrived at 0 minutes and the s-wave arrived at 5 minutes.
Step 2. Calculate the time difference. S-wave arrival time minus P-wave arrival time equals time difference. Based on our sample we would have 5 minutes - 0 minutes = 5 minute time difference
Step 3. Now turn to the Earthquake Time Travel Graph on page 11 in your reference tables. Place a sheet of paper along the Travel Time axis of the graph as shown in diagram 4 (only part of the graph is shown here) and mark the time difference calculated in step 2.
Step 4. Now slide the piece of paper across the graph until the bottom mark lines up with the p-wave line and the top mark lines up with the s-wave line. Make sure the edge of the paper is parallel to the vertical lines on the graph. See diagram 5.
Step 5. Now read straight down the edge of the paper until it crosses the Epicenter Distance axis. This is the answer. Based on the sample seismogram the epicenter distance is 3,400 km. Try this on your own. Use diagram 6 to answer the following questions..
1. What are the arrival time of the p-wave and s-wave? Click Here for the Answer P-wave arrival time = 6:02:00 (HRS:MIN:SEC) S-wave arrival time = 6:07:30
2. What is the time difference? Click Here for the Answer 6:07:30 (s-wave arrival) - 6:02:00 (p-wave arrival) = 5:30 time difference
3. What is the epicenter distance? Click Here for the Answer Epicenter Distance = 3,800 km If you are within 200 km of this value then you understand how to find the epicenter distance.
If you need more help try the Virtual Earthquake.