Surface Wave Propagation…
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Surface Wave Propagation
Around the Earth A Problem Solving Exercise
L. Braile, Professor
Purdue University
braile@purdue.edu,
web.ics.purdue.edu/~braile
January, 2001
Background:
Examine the seismograms shown in Figure 1. The display is called a seismic record
section. Each trace is a seismogram recorded at a seismograph station. Most of the station
locations (letter code adjacent to each trace) can be found on the seismograph network maps
shown in Figures 2 and 3. On the record section (Figure 1), the seismograms are plotted
according to the distance (in degrees, geocentric angle) from the earthquake location and time
from the earthquake origin. The traces are of the vertical component of ground motion, and have
been filtered to include only periods longer than 125 seconds. The prominent arrivals (often
called phases) that “angle across” the record section and are labeled R1, R2 and R3 are longperiod Rayleigh (surface) waves that travel along the surface of the Earth. Surface waves
penetrate (have particle motion) to depths of tens to hundreds of km (depending on the type of
wave and the period of oscillation of the phase) but travel approximately parallel to the Earth’s
surface.
Questions:
1. What explains the unusual pattern of the R1, R2 and R3 phases across the record section?
(That is, what are the surface wave propagation paths for R1, R2 and R3 that explain this
pattern?). Hint: For our purposes we can assume that all of the stations are located in a line
(actually a great circle) around the Earth. The cross section diagram in Figure 4 illustrates
this situation. A few of the seismograph stations are shown at the appropriate distance from
the earthquake. (The actual locations of the stations are shown in Figures 2 and 3 and are not
arranged in a line or great circle path through the epicenter of the Loma Prieta Earthquake.
However, because the surface waves propagate in all directions from the source, the arrival

Copyright 2001. L. Braile. Permission granted for reproduction for non-commercial uses.
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times are approximately the same as if the stations were all located along a great circle path
from the epicenter.)
2. What is the approximate velocity of propagation of the long period Rayleigh waves
illustrated in Figure 1? Hint: the velocity along the surface (often called the apparent
velocity, which for the surface waves is the actual propagation velocity because they are
propagating parallel to the surface) can be determined from the slope (velocity = 1/slope) of
the arrivals for the R1, R2 and R3 phases on the record section. What is the velocity in
degrees/minute? For the spherical Earth of approximate radius 6371 km, one degree of
geocentric angle corresponds to a distance along the surface of approximately 111.19 km.
Using this information, what is the approximate velocity of the long-period Rayleigh waves
in units of kilometers/second? (The conversion to km/s is useful because these units are
commonly used to describe the velocity of propagation of seismic waves through Earth
materials.)
3. Using the velocity found in question 2 (in degrees/minute or km/s), how long does it take for
the long period Rayleigh wave to propagate completely around the Earth (360 degrees)?
Reference (and source of Figures 1, 2 and 3, pp. 14, 189, 190):
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Lay, Thorne, and Terry C. Wallace, Modern Global Seismology, Academic Press, San Diego,
California, 521 pp., 1995.
Figure 1. Seismograms of the 1989 Loma Prieta (central California) earthquake in record section form
showing the long period Rayleigh wave (largest amplitudes) labeled R1, R2, R3. The
seismograms are vertical component records and have been filtered to include only periods
longer than 125 seconds. Seismograph stations that recorded the seismograms, in order of
distance, are: AMNO, COL, KIP, HRV, SJG, PPT, RPN, AFI, CAY, MDJ, HIA, TOL, BDF,
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WMQ, TAM, KMY, CAN, TWO, HYB, BCAO, NWAO, SLR, RER. Locations of most of these
stations are shown on the maps in Figures 2 and 3. (Modified from Lay and Wallace, 1995).
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60o
30o
0o
-90o
0o
90o
180o
-30o
-60o
Figure 2. Locations of modern, digital, broadband seismograph stations of the IRIS/USGS-GSN and
IRIS-IDA networks. (Modified from Lay and Wallace, 1995).
60o
o MDJ
TOL o
-90
o
0
o
30o
o WMQ
BCAO
90
0o
o
SLR o
-30o
-60o
Figure 3. Locations of modern, digital, broadband seismograph stations of the Project GEOSCOPE
network. Dots and triangles are GEOSCOPE stations. Open circles are other digital stations
recording seismograms shown on the record section in Figure 1. (Modified from Lay and
Wallace, 1995).
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Loma Prieta
Earthquake
ANMO
r
Su
fac
av e
eW
Surf a
c
eW
Seismograph
Stations
HRV
av
e
PPT

90°
Center of
the Earth
TAM
sS
t h’
Ear
Cross section
through the
Earth
fa
ur
NWAO
c
e
180°
RER
Figure 4. Schematic diagram illustrating a cross section through the Earth and the locations of
seismograph stations. Distance from the earthquake epicenter to a station can be measured in
kilometers along the surface or by the geocentric angle () in degrees.