Name: ________________________
Sumantra Earthquake & Tsunami Lab
Partners: _______________________
Adapted from D. Robinson & S. Kluge
Introduction: In this lab you’ll study seismograms from six different seismic stations
recording the magnitude 9.0 Sumatra earthquake of December 26th, 2004. By comparing
the arrival times of the P and S waves on each seismogram, you’ll be able to determine the
distance from the epicenter to each station. Once you know the distance to the epicenter
from three stations you can pinpoint the epicenter using a process known as triangulation.
Procedure Part 1: Finding the Epicenter
1. Read the time of the P and S waves at each station and place that information in your
data table below. Read each arrival time to the nearest second.
2. Determine the amount lag or lapse time between the arrival of the P and S waves at
each station. Subtract the P-wave arrival time from the S-wave arrival time. Record your
results in DATA TABLE 1.
3. Use the P and S wave travel-time curves (ESRT p.11) to find the distance from each
station to the earthquake epicenter and record that distance in your data table.
4. On MAP #1, use the map scale and your safety compass to draw circles around each
station of a
radius equal to the epicenter distances that you just found using your reference tables.
5. Label the intersection of the 3 circles as the epicenter of the earthquake.
Data: Seismographs
Station #1: IC. LSA (Tibet, China) 29.7 N, 91.15 E
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Station #2: KMBO Latitude: 1.13 S, Longitude: 37.25 E
Station #3: GUMO Guam, Mariana Islands Latitude: 13.59 N, Longitude: 144.87 E
Data Table 1:
Seismic
Station
P-Wave
Arrival
S-Wave
Arrival
IC.LSA
1:04:29
1:09:04
Lag Time
(S – P)
Distance to
the Epicenter
P-Wave
Travel Time
KMBO
GUMO
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Partners: _______________________
Adapted from D. Robinson & S. Kluge
MAP #1
Name: ________________________
Sumantra Earthquake & Tsunami Lab
3
Questions:
1. Which station was farthest from the epicenter? What if the time markers were not
printed on the seismograms, how could you figure out which station was farthest
away then? _________________________________________________
_______________________________________________________________________
2. Using p.5 of the ESRT’s what two plates were involved in this earthquake?
_______________________________________________________________________
3. What kind of plate boundary is between these two plates?
_______________________________________________________________________
4. How do the plates move relative to one another at this type of plate boundary?
________________________________________________________________________
5. The earthquake occurred approximately 100 km east of the boundary, and at a
depth of 30 km. With this information can you figure out which plate is being
subducted? Explain and draw a sketch of this kind of boundary.
_________________________________________________________________________
6. Subtract the P-wave travel time from the P-wave arrival time for each station to
determine when the earthquake occurred. If your data from the seismograms is
accurate then you should get the same time for each station. When the earthquake
occurred is called the origin time.
IC.LSA
KMBO
GUMO
P-Wave arrival time
-- P-Wave travel time
Origin Time
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Name: ________________________
Sumantra Earthquake & Tsunami Lab
Partners: _______________________
Adapted from D. Robinson & S. Kluge
Procedure Part 2: Finding the Epicenter again using closer stations.
Repeat the procedure from part 1 using PALK, DGAR, & COCO. Complete Data Table #2.
Use the compass to plot the epicenter on Map #2.
Data:
Seismograph #1: PALK, Sri Lanka, 7.3° N, 80.7° E
Seismograph #2: DGAR, British Indian Ocean Territory, 7.4° S, 72.5° E
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Seismograph #3 COCO, Cocos (Keeling) Islands, Australia: Coordinates: (12.2° S, 96.8° E)
Data Table 2:
Seismic
Station
P-Wave
Arrival
S-Wave
Arrival
Lag Time
(S-P)
Distance to
Epicenter
P-Wave
Travel Time
Origin Time
PALK
DGAR
COCO
Questions:
7. Are the origin times the same as the first set of seismographs?
___________________________________________________________________________
8. What could cause the small differences in each stations calculated origin time?
___________________________________________________________________________
9. The earthquake actually occurred at 00:58:53 GMT. Which set of seismographs
were gave a more accurate origin time? Why?
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Partners: _______________________
Adapted from D. Robinson & S. Kluge
MAP #2
Name: ________________________
Sumantra Earthquake & Tsunami Lab
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