Exercise 1 Seismology
About the data
In the first problem you are going to analyse recordings of waves from an earthquake beneath
New Zealand, made at an observatory in the Indian Ocean. Initially, the vertical, north-south
and east-west components of ground motion were recorded. For the analysis of seismic
waves, however, it is more convenient to transform the two horizontal components (N-S and
E-W) to radial and transverse components of ground motion in the horizontal plane (cf. sec.
4.5.2 and 5.7 in the textbook). The radial component is oriented along the great circle through
the epicentre and the observatory, being positive away from the epicentre, and the transverse
component is oriented perpendicular to the radial, being positive at 90º clockwise from the
positive radial direction. Figure 1 (next page) shows the different components.
Notice that the displacements on the radial and transverse components are not recorded
directly by the seismometers, but are calculated from the displacements on the north-south
and east-west components (i.e., the vertical component is not concerned).
From the definitions above, it follows that P-waves and Rayleigh-waves do not have any
displacement on the transverse component, Love-waves have displacement on the transverse
component only, whereas S-waves can have displacement on all three components.
Problem 1: Identification of seismic phases
Figure 2 (separate file) shows the vertical (top), radial and transverse (bottom) components
of ground motion caused by the New Zealand event.
i) Identify the P-phase and the S-phase and mark them on the seismogram.
ii) Use the travel time curves in fig. 3 (separate file) to determine the epicentral distance, in
degrees, to the observatory where the seismograms were recorded.
iii) Identify and mark other phases on the seismogram, including surface waves, on the
basis of travel times and displacement on the three components.
In the report: explain how you identified the various phases and how you calculated the
epicentral distance. Enclose the figures.
Locating earthquakes
The next problem, which is about locating an earthquake, has to be solved by means of a
program called SWAP and which is installed on the PCs in the computer lab.
You start the program this way: press Start in the lower, left corner, choose Programmer and
then SWAP. A new window is opened for SWAP. When you move the cursor around in the
window, help texts will appear in the field in the lower right corner, explaining the options on
which you are pointing.
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Figure 1. The upper figure shows the three components of ground motion commonly recorded
by seismometers. The lower figure shows the various horizontal components of ground motion
as defined in the text, seen from above. Red, continuous lines show the north-south and eastwest components of ground motion at a given point of time. The dashed, red lines show their
projections onto the radial and transverse components, and the blue, continuous lines show
the total (or net) radial and transverse components of displacement.
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To be able to locate the earthquake, you first have to determine the arrival times of the Pphase on the recordings made at eight stations. But before you start watching the recordings,
you have to specify a display parameter: Press Display, then Time scale, and choose Absolute.
The recordings will then be shown with a common and absolute time axis, so that the arrival
time of each P-phase reflects the distance from the source to the station (all the P-phases were
generated simultaneously when the earthquake occurred).
Now you can start the identification: Press OPEN FILE, and a small window for file search
appears (see figure below).
Go to the directory Felles on ‘geofag’(K:)/gfgruppe-prosj/geofysikk/GEL2140/swap_data.
There you will find eight files, each containing the vertical component of the recording made
at a Geoscope station of this earthquake (more about the Geoscope network on
http://geoscope.ipgp.jussieu.fr ). The recordings have been bandpass filtered.
Open one file at a time, press DETAIL WINDOW, and then the zoom symbol on the left hand
side (see upper figure next page). Use the cursor to define the portion of the seismogram
which you want to enlarge (click left mouse button and drag), it will then appear in the white
field under the complete seismogram (you can use the zoom option also in this field). When
you have chosen a convenient portion: press the P(ick symbol), move the cursor to where you
think the beginning of the P-phase is (use the enlarged seismogram), and click left mouse
button. The time position will then be marked with a red line. To continue with the next data
file: Press RETURN, OPEN FILE etc.
When all the eight data files have been analyzed: Press LOCATOR WINDOW, and a world
map showing the positions of the stations will appear (see lower figure next page). Below, the
eight vertical components are plotted. Before you start localizing the event, you should
specify the hypocentre depth, which is 42 km. Type the value in the white field on the right
hand side (do not remove the old value before you have typed the correct one).
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Locating the earthquake is performed like this: Based on the differences in travel time to the
various stations, you estimate an approximate epicenter position. Place the cursor in that
position on the map and click the left mouse button. According to that position (and the origin
time of the earthquake which is written in the header of the data files), the program will
calculate the arrival time of the P-phase at each of the stations, which is marked with a green
line. The distances between the red and green lines can then be used to improve the estimate
of the position. The RMS value (given on the right hand side) is a measure of the time
differences between the red and green lines. When you are satisfied with the result, determine
the latitude and longitude of the position. Use the 0° meridian and equator as ‘reference axes’.
The distance between the meridians is 20°, and between the parallels it is 30°.
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Problem 2: Locate an earthquake epicentre
Locate the earthquake as described above. For the location to be acceptable, the RMS value
should not be larger than 15.
In the report: Give the latitude and longitude of the location, together with the RMS value
corresponding to it. Explain how you used the differences in travel time to the various
stations when searching for the correct location.
N
S
Problem 3: The fault-plane solution
The figure above shows the “beach ball” for the earthquake located in problem 2.
In the report, answer the following questions:
i) Which type of fault does the “beach ball” correspond to? What can you say about the
orientation of the nodal planes?
ii) By considering the tectonics of the source region (see f.ex. Fig. 20.25 on p.331), explain
which of the nodal planes is the most likely fault plane for this event.
Seismology links:
http://www.iris.edu/quakes/quakes.htm
Incorporated Research Institutions for
seismology
http://www.orfeus-eu.org
Observatories and Research Facilities for European
Seismology
http://neic.usgs.gov
U.S. Geological Survey - National earthquake information center
http://www.indiana.edu/~usesn/
The U.S. Educational seismology network home page
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