According to the figure below, what is the approximate S-P travel time?
How Can You Locate The Epicenter of an Convulsion?
Iii Types of Waves
Major earthquakes occur when there is rock motion forth a fault (fissure in the chaff). The sudden slippage of huge stone masses sets upwards shock waves that travel through the world. The betoken within the earth where the actual move takes identify is called the focus. As shown in Effigy ane, the betoken on the surface straight to a higher place the focus is chosen the epicenter .
An earthquake epicenter can be located from records made of convulsion waves on devices chosen seismographs . One blazon of seismograph is a visible recording machine, shown in Figure 2. A pen draws a design of the waves on newspaper that is attached to a revolving drum. The moving ridge record from a seismograph is known as a seismogram - meet Figure 3.
A typical seismogram of an earthquake has three prominent wave patterns. The first waves to arrive are the P-waves (also chosen "main" or "button-pull"). They are followed by the S-waves (also called "secondary," "shear," or "shake"). Finally, the L-waves ("long" or "Honey") go far. This investigation contains the seismograms from iii unlike stations for an earthquake. See how accurately you tin locate the epicenter of this convulse.
Figure ane: Earthquake epicenter and focus
Effigy ii: A seismograph
Figure 3: A seismogram
Calculating LAG TIMES
Remember that seismographs record iii types of earthquake waves which have been described to you in class: one) P-waves (too called push-pull or compressional waves), 2) Due south-waves (also called shear or milkshake waves), and iii) L-waves (as well called long or dear waves). Each of these waves travel at different velocities (speeds), even though they are generated simultaneously by an earthquake at the focus (point of origin within the crust). Since P-waves travel faster than South-waves do, the seismograph volition discover P-waves arriving outset, and S-waves volition follow. The time difference, every bit recorded on a clock, betwixt when the P-waves and Southward-waves arrive is called the lag time. Using the clock time numbers listed in your lab handout, the lag times may be hands calculated.
EXAMPLE
"An convulsion was recorded in San Diego. The seismograph tape shows that P-waves first arrived at 10:02-09 PST (read this is "10:02 and ix seconds, AM, Pacific Standard Time"), and S-waves arrived at 10:03-04 PST. What is the lag fourth dimension for this convulsion?"
Respond
Since S-waves arrived subsequently, you may subtract the fourth dimension of arrival of the P-waves from it. To practice this, you may need to "borrow" extra seconds from the minutes column (much like grade school arithmetics, where fractions may be borrowed from the whole numbers column).
S-moving ridge arrival time = ten:03, 4 seconds => ten:02, 64 seconds
P-moving ridge arrival fourth dimension = 10:02, 9 seconds => - ten:02, nine seconds ( decrease)
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Reply = 55 seconds
CALCULATING THE DISTANCE OF THE EPICENTER FROM THE RECORDING STATION
This lab exercise will compare and contrast two distinctly different methods for calculating the altitude to an epicenter. The first method assumes that earthquake waves travel at constant speed (no speeding upwardly nor slowing down), and uses a mathematical formula to determine velocity, distance, or fourth dimension, for iv earthquake recording stations located in the western United States. The calculated distances for each city are then to be drawn with a drawing compass on the base map (Figure 4). If information technology tin can be shown that earthquake waves do not travel at constant speed, and then this method is invalid.
The 2nd method assumes that earthquake waves speed upward with increasing altitude, and the lag time graph (Figure 6) may be used to find either the lag time or the distance to the epicenter. As you will see, the second method works better considering information technology accounts for the increased density of the earth's drapery, outer cadre, and inner core, which causes earthquake waves to speed up.
To calculated lag time using the arithmetic method, a simplified method using rounded off speed numbers is illustrated beneath. If you lot are given information virtually how fast P-waves and S-waves each travel, a sure lag time will correspond to a sure altitude that may be traveled by earthquake waves. In other words, if P-waves travel at 4.00 miles per 2d, and Due south-waves travel at 2.fifty miles per 2nd, and the lag time is 15 seconds, the distance of the convulsion epicenter volition be 100 miles. The method of this adding is shown below.
| Velocity (speed) = V | VP-waves = iv.00 miles second | VS-waves = ii.50 miles 2nd |
| Velocity = Distance Time | Let distance = 100 miles | |
| Fourth dimension = Distance Velocity | TimeP-waves = 100 miles four.00 miles = 25 sec. 2d | TimeSouthward-waves = 100 miles 2.fifty miles =40 sec. 2nd |
| Distance = Velocity X Time | Lag Time = twoscore - 25 = 15 seconds |
How to Use Proportionality
If a lag time of 15 seconds corresponds to 100 miles of distance to the epicenter, how far is the epicenter from another recording station, if that lag time is 30 seconds?
Since the question is "how far," you should use the distance formula, Distance = Velocity X Time. In this case, the "velocity" is the "lag fourth dimension velocity" or 100 miles/15 seconds.
Distance = 100 miles X xxx seconds = 200 miles
15 sec.
HANDLING MATHEMATICAL CALCULATIONS AND WORD Bug
Practice you retrieve solving word problems in high schoolhouse algebra class? If you lot found these types of problems difficult to solve, it was probably because you didn't know exactly what data you were required to calculate. One of the keys to deciphering word bug is to wait for primal phrases and apply the appropriate formula:
| | | |
| | | time |
| | | speed |
| | | |
Understanding & Calculating Lag Fourth dimension
Compare the relative speeds of 2 vehicles, A and B. Both vehicles leave the same departure point but travel at dissimilar speeds. Vehicle A is traveling at 50 miles/hour. Vehicle B is traveling 25 miles/60 minutes. Assuming that neither vehicles slow down nor stop, how long does information technology take for each to travel 250 miles? Before you blurt out the reply, try using one of the above iii formulas. Which is the correct formula to apply hither? If y'all chose the "time" formula, y'all're right. (Why? The key phrase in the give-and-take trouble is "how long does it accept"). So, if yous take the distance, 250 miles; and divide by the speed of each vehicle, you should go:
| Vehicle A | Vehicle B |
| 500 miles = 10 hours 50 miles hour | 500 miles = 20 hours 25 miles hour |
And then, the lag time difference between the two vehicles (10 hours - v hours) is 5 hours. What would the lag time exist if the distance traveled were 500 miles?
Vehicle A would take x hours to travel 500 miles, but Vehicle B would take 20 hours. The lag time here is x hours. So, the blueprint yous should notation here is "the greater the distance, the longer the lag time."
The aforementioned method of calculation may exist used for earthquake waves (P-waves and S-waves). However, you must use consequent units. If yous are given speed units which are "miles per second," you must non mix them with "miles per hour."
The central assumption for using this methodology for computing the altitude to the earthquake epicenter is that the speed of the convulsion waves does non modify with distance. Nevertheless, in reality, this does not concur true over long distances, specially if the convulsion waves penetrate the denser layers of the globe's interior, which causes earthquake waves to speed up in general.
| At least three earthquake recording stations are required to find the location of the earthquake epicenter. A unmarried recording station can but summate altitude, but not direction; to cover all possibilities, a complete circle is drawn around that station. If only 2 earthquake recording stations are used, the circles will overlap at 2 points. Data from a third recording station will eliminate one of these points. |  |
EXERCISES USING THE CONSTANT-SPEED METHOD
1. Four fractional records of the aforementioned earthquake were recorded at Los Angeles, San Francisco, Table salt Lake City, and Albuquerque, shown below. Determine the lag time for each recording station and enter information technology into the "lag time" column, past subtracting the P-wave arrival time from the Southward-wave arrival time.
| | | | |
| | | | seconds |
| | | | seconds |
| | | | seconds |
| | | | seconds |
2. Assuming an average velocity of 3.fourscore miles/second for the P-waves, and ii.54 miles/2d for the South-waves, how long does it accept for each type to travel 100 miles? Show how yous arrived at your answer.
| P-waves took how many seconds? | S-waves took how many seconds? |
What is the lag time associated with this altitude (100 miles)?
3. Determine the distance from each of the four seismograph stations to the epicenter of the earthquake. Distance may be computed past proportion, using the lag time value to 100 miles that you lot obtained in Problem 2.| Recording Station | Calculated Distance (miles)? |
| Los Angeles: | miles |
| San Francisco: | miles |
| Albuquerque: | miles |
| Salt Lake City: | miles |
4. On the base map of the western U.s. (Figure iv), describe circles or arcs with a compass, locating the needle point at each of the four stations, with each radius corresponding to the calculated distance (employ the graphic scale on the base map for measurement).
Where is the epicenter located?
Between what cities shown on the map?
Which city is the earthquake epicenter closes to, and how far?
5. Considering the crusade of earthquakes discussed during lecture, what major structural feature is probably related to this earthquake?
6. The time at which the P-moving ridge arrived at each of the 4 stations is shown on the seismograph record (Problem one). But when did the earthquake actually occur? Show how y'all obtained your answer.
Figure 4: Base of operations Map of western Usa
SEISMIC WAVES: A "WINDOW" TO THE Globe'S INTERIOR
The study of seismic waves is non simply useful for helping to predict and prepare for earthquakes - it is besides used to assist study the properties of the Earth'due south interior. The deepest drill pigsty accomplished by man is less than about 3 miles into the Globe's chaff. Nosotros thus have no direct observation of the thousands of miles of rock below the surface. Seismic waves may be artificially generated with explosives, and then monitored for changes in travel velocities and intensities. Seismic waves increase their speed when traveling through denser material; S-waves cannot travel through liquids. It has been determined by seismologists that the drape stone is denser than the crust, and the outer core of the Earth is composed of liquid fe, while the still denser inner cadre is solid.
Figure 5: Inferred properties of the Earth's interior
Source: https://www.oakton.edu/user/4/billtong/eas100lab/lab10quake.htm
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