Finding epicenter from three station values.

1. UNIVERSITY OF ENGINEERING AND
TECHNOLOGY PESHAWAR
Wasim Khan
Engineering Geology & Seismology

2. Locating the Epicenter of an Earthquake
Introduction: The epicenter is the point on Earth's surface directly above an earthquake. Seismic
stations detect earthquakes by the tracings made on seismographs. Tracings made at three separate
seismic stations are needed to locate an earthquake epicenter.
Objective: To identify the location of an earthquake epicenter using a travel time graph and three
seismograph tracings.
Materials:. Ruler map safety drawing compass Earth Science Reference Tables
Procedure: 1. Study the three seismograph tracings below. Notice the time scale below each tracing.
Each mark on the time scale represents one minute.
2. Start with the first tracing, labeled Balboa Heights. Locate the P, which indicates the arrival time
of the
P wave at the seismic station, and the S, which indicates the arrival time of the S wave. Draw a line
straight down from the peak of the P wave to the time scale and another line from the peak of the S
wave to the
time scale. Record the arrival times on the data table.
3. Determine the difference between the arrival times of the P and S waves. For Balboa Heights, the
time difference is about 4 min 50 sec. Do your data match this result?
*4. Turn to the “Earthquake P-wave and S-wave Travel Time” graph on page 11 of your Earth
Science Reference Tables. Lay the edge of a piece of paper along the vertical axis of the graph. Place
a mark on the edge of the paper at 0 on the vertical axis and another mark corresponding to 4 min 50
s. Keeping the edge of the paper straight up and down, slide the paper across the graph to the place
where the P and S curves are the same distance apart as the two points you marked. The point where
the paper meets the distance scale at the bottom of the graph indicates the distance to the epicenter.
Determine the distance to the epicenter from Balboa Heights. The distance for a time difference of 4
min 50 s is about 3200 km. Be sure to enter this information on the data table.
*See the attached example for Balboa Heights.
5. Now locate Balboa Heights on the map. Balboa Heights is marked BH. Place the point of your
drawing compass at 0 on the map scale. Adjust the compass so that the pencil point is at 3200 km on
the map scale. Without altering the compass, place the point at the center of the mark for Balboa
Heights. Draw a circle with a radius of 3200 km around Balboa Heights.
6. Return to the seismograph tracings. Repeat procedure steps 2-6 for Boulder and Mexico City and
complete the data table. Boulder is indicated on the map by BC and Mexico City is indicated by MC.

3. Step wise procedure following:
From the following seismographs
City P-wave arrival
time(tp) in mins
S-wave arrival
time(ts) in mins
Difference (ts-
tp) in mins
Distance to
Epicenter
Balboa 19 23.8 4.8 3200
Boulder 16 18.6 2.6 1700
Mexico city 17.4 20.9 3.5 2400
 Now Distance to epicenterisfindfrom the graph on page 3.
 As in step3 they said 4 min 50 seconds isfor Balboa but my resultis 4 min 48 seconds.

4. Page#3 Graph
BalboaHeights
Boulder
MexicoCity

6. Graph1
Analysis and conclusion:
Q1: According to the reference tables, how long does.it take a P wave to travel 4000 km?

7. Answer: From Graph 1 at 4000km the time taken by P wave (tp) is approximately 7 minutes
10 seconds
Q2: Howlong does it take an S wave to travel 4000 km?
Answer:From the Graph 1 at 4000km the time taken by S wave (ts) is approximately 12 minutes 40
seconds.
Q3: Whatis the difference inarrival timesfor a P wave and S wave that have traveled4000 km?
Answer: Sofrom Q1 and Q2 (ts-tp=5.5minutes) sothe differenceis 5 minutesand30 seconds.
Q4: What is the approximate distance to the epicenterifthe difference inthe arrival timesof the P
wave and S wave is 5.5 min?
Answer: FromGraph 1 the approximate distance tothe epicenter is4100Km.
Q5: As the distance to the epicenterincreases,howdoesthe amount of time betweenthe S wave and
the P wave change?
Answer: Fromthe graph as the distance toepicenterisincreasesthe amountof time betweenPandS
increases.
Q6: Afterdrawing the first circle for Balboa Heights,where is the earthquake epicenterlocated
relative to this one circle?
Answer: Fromonlyone circle the earthquake epicenterisatthe circumference of the circle.Itwill all
aroundthe circle boundary.
Q7: Afterdrawing the circle for Boulder,you now have two circleson your map. Relative to these two
circles,where is the earthquake epicenter?
Answer:From nowthe twocircle itis little biteasier.The earthquake epicenterwill be atthat area
where the twocirclesintersecteachother.
Q8: Afterdrawing the circle for MexicoCity,you now have three circleson your map. Relative to the
three circles,where is the earthquake epicenter?
Answer: Nowfromthe three circlesitisveryeasyto the findthe fixedpointof earthquakeepicenter.It
isat that pointwhere the circlesintersecteachother onpage
Q9: Give the location and year of the major earthquake from the past that occurred in the same area
as the epicenteryouhave just located?
Answer: Fromthe Google map I have takenscreenshots.The epicenterisshownin Screenshot2and
areas nearlytothe epicenterisin screenshot1.
Year Location MomentMagnitude Deaths
1886 SouthCarolina 6.9-7.3 60
1906 San Francisco,California 7.9 3000
1989 Loma Prieta,California 6.9 63
1994 Northridge,California 6.7 61

8. Screenshot 1
Screenshot 2

9. Q10: Wouldit be possible foran earthquake at thissame location to be feltwhere you live?Whyor
why not?
Answer:I live in MalakandDargai. Letwe assume earthquake epicenteratMexicoCitysofrom
Malakandto MexicoCitydistance is 13,934Km soit isverydifficulttofeltearthquake.Butitmayor may
not be feltbecause if level of shakingisveryhighsoitis possible tofeel it.
The End