Interpreting Seismograms - A Tutorial
for the AS-1 Seismograph 1
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Larry BraileProfessor,
http://web.ics.purdue.edu/~braile/ October, 2006, updated January, 2007 |
Objective: This tutorial is intended as a resource for the interpretation of seismograms recorded by educational seismographs. The tutorial provides a description of the main features of the Earth that affect seismic wave propagation and therefore controls the character of seismic signals recorded on seismographs. A catalog of selected seismograms is also presented to illustrate the variation in signal properties with distance, magnitude, and depth of focus. After initial visual analysis of an earthquake seismogram, one can often determine additional information about the event by identifying phases (individual arrivals on the seismogram that travel a distinct path through the Earth) and measuring amplitudes to estimate the magnitude of the earthquake.
This tutorial is available for viewing with a browser (html file) and for downloading as an MS Word document or PDF file at the following locations:
http://web.ics.purdue.edu/~braile/edumod/as1lessons/InterpSeis/InterpSeis.htm
http://web.ics.purdue.edu/~braile/edumod/as1lessons/InterpSeis/InterpSeis.doc
http://web.ics.purdue.edu/~braile/edumod/as1lessons/InterpSeis/InterpSeis.pdf
A PowerPoint presentation for the Interpreting
Seismograms document is available for download at: http://web.ics.purdue.edu/~braile/edumod/as1lessons/InterpSeis/InterpSeis.ppt
Contents (click on topic to go directly
to that section; use the red up arrows to return to the list of contents):
1. Introduction
2. Seismic Wave Propagation in the Earth
3. Catalog of Seismograms
at Various Distances – Screen Images
4. Catalog of
Seismograms at Various Distances – 60-minute Seismograms
5. Catalog of
Seismograms for Different Magnitudes – Distance ~ 30o
6. Catalog of
Seismograms for Different Magnitudes – Distance ~ 60o
7. Catalog of
Seismograms for the Same Magnitude (~6.7) at Different Distances
8. Catalog of
Seismograms for Large Earthquakes at About the Same Distance
9. Catalog of
Seismograms for the Same Distance (~65o) for Different Depths of
Focus
10. Analysis of Noise on
Seismograph Records
11. Mystery Events
12. References
1. Introduction:
Interpreting
earthquake seismograms generally requires considerable experience and study of
seismology. However, there are some
fundamental principles that provide a basic understanding of seismic wave
propagation and seismogram characteristics.
Furthermore, some experience can be quickly obtained by systematic study
of selected seismograms illustrating variations in amplitude and signal
character related to source-to-station distance, the magnitude of the
earthquake, and the earthquake’s depth of focus.
This tutorial utilizes seismograms recorded over the last six years at
the WLIN station in
An earthquake
catalog (Excel file) for the WLIN station can be downloaded at: http://web.ics.purdue.edu/~braile/edumod/as1lessons/InterpSeis/EqList.xls. Sample AS-1 seismic data for the WLIN station
for the years 2004 and 2005 (data for days that have no significant events have
been deleted from the files to reduce the total file size; files are compressed
and are zip files; the 2004 file is 17.2MB; the 2005 file is 46.4MB; files must
be unzipped [extracted] and placed in your AmaSeis folder to view with the
AmaSeis software as folders named “2004” and “2005”) are available at: http://web.ics.purdue.edu/~braile/new/2004.zip
and http://web.ics.purdue.edu/~braile/new/2005.zip. You can use these data with the AmaSeis
software to view and analyze seismograms, determine the epicenter-to-station
distance using the S minus P method and calculate magnitudes.
Return to list of contents
2. Seismic Wave Propagation in the Earth: Four main types of seismic waves propagate in elastic materials including the Earth. A simplified model of Earth’s interior structure is illustrated in Figure 1. A hands-on Earth structure activity is available at: http://web.ics.purdue.edu/~braile/edumod/earthint/earthint.htm. Two types of body waves, compressional or P-waves and shear or S-waves, travel through the Earth’s interior. S-waves do not travel through fluids so they are not present in the Earth’s liquid outer core. The two types of surface waves are Rayleigh waves and Love waves. Surface waves travel approximately parallel to the Earth’s surface and their particle motions decrease in amplitude with depth below the surface. Additional information of seismic waves can be found in standard seismology and Earth science reference books including Bolt (1993, 2004) and Shearer (1999). Hands-on activities for exploring seismic waves and seismic wave propagation using the slinky are available at: http://web.ics.purdue.edu/~braile/edumod/slinky/slinky.htm and
http://web.ics.purdue.edu/~braile/edumod/slinky/slinky4.htm.
Seismic wave animations and related hands-on activities are be found at:
http://web.ics.purdue.edu/~braile/edumod/waves/WaveDemo.htm.
Seismic waves in the Earth can be represented by specific raypaths and
wave types that result in distinct arrivals, called phases, on seismograms
(Figure 2). Several raypaths for seismic
phases and the concept of geocentric angle (angular distance) and distance
along the Earth’s surface are illustrated in Figures 2, 3 and 4.
Travel times for seismic waves are well known from many years of recording seismograms all over the world from earthquake and explosive sources. Examples of standard travel time curves are shown in Figures 5, 6 and 7. These curves can be used to estimate the epicenter-to-station distance from the S minus P time (Figures 7 and 8) and for identifying phases (arrivals) on recorded seismograms. Examples of using the AmaSeis software and AS-1 seismograms for the S minus P distance estimation and epicenter location method are given at:
http://web.ics.purdue.edu/~braile/edumod/as1lessons/UsingAmaSeis/UsingAmaSeis.htm
http://web.ics.purdue.edu/~braile/edumod/eqdata/eqdata.htm
http://web.ics.purdue.edu/~braile/edumod/as1lessons/EQlocation/EQlocation.htm.
The magnitudes (mb, MS and mbLg) of earthquakes recorded on the AS-1 seismograph can also be estimated using methods described at:
http://web.ics.purdue.edu/~braile/edumod/as1lessons/EQlocation/EQlocation.htm
http://web.ics.purdue.edu/~braile/edumod/as1lessons/magnitude/CalcMagnElect.htm
http://web.ics.purdue.edu/~braile/edumod/as1mag/as1mag.htm
and the AS-1 online magnitude calculator:
http://web.ics.purdue.edu/~braile/edumod/MagCalc/MagCalc.htm. Magnitudes can also be calculated directly with the AmaSeis software.
Results of many magnitude calculations for WLIN seismograms are illustrated at:
http://web.ics.purdue.edu/~braile/edumod/MagCalc/AS1Results.htm.
Additional raypath diagrams for seismic wave propagation through the Earth are shown in Figures 9-12.
Figure 1.
Schematic diagram illustrating the major spherical shells of the Earth's
interior structure. The circles
(representing spherical shells in the 3-D model) are drawn at true scale except
for the circle representing the base of the crust. The thickness of the crustal layer is
exaggerated so that a distinct layer is visible at this scale (the scale of this
diagram is approximately 1:120 million).
In the real Earth, the crust is also of variable thickness with
significant differences between the crustal thickness of oceanic and
continental regions and increased crustal thickness beneath mountainous areas.
Figure 2. Segment of Earth model
showing main boundaries and layers, and approximate compressional- or P-wave
velocity with depth. Raypath shows
approximate travel path for the first arriving P-wave (and the S-wave) for the
seismogram shown above. The seismogram
was recorded by the
Figure 3. Cross
section through the Earth showing important layers and representative raypaths
of seismic body waves. Direct P and S raypaths (phases), including a
reflection (PP and pP), converted phase (PS), and a phase that travels through
both the mantle and the core (PKP). P raypaths are shown by heavy
lines. S raypaths are indicated by light lines. Additional
information about raypaths for seismic waves in the whole Earth and
illustrations of representative raypaths are available in Bolt (1993, p.
128-142) and Shearer (1999, p. 49-60). Surface wave propagation (Rayleigh
waves and Love waves) is schematically represented by the heavy wiggly
line. Surface waves propagate away from the epicenter, primarily near the
surface and the amplitudes of surface wave particle motion decrease with depth.
Figure 4. Earth structure and
raypaths (Figure 3) with the addition of a raypath for the seismic phase PKIKP
that travels through the Earth’s inner core.
Figure 5. Standard Earth travel
time curves for a source depth of 0 km (can be used for shallow focus earthquakes
at distances of ~20 to 120 degrees).
Travel times for many different phases (types of seismic waves and paths
through the Earth) are shown. Note that
the difference between the S and the
P times increases smoothly with distance.
Therefore, a seismogram with a given S minus P time will only match the
travel time data at one specific distance.
This diagram is available at: http://neic.usgs.gov/neis/travel_times/ttgraph.html.
Figure 6. Standard travel time
curves for the Earth for several seismic phases. Travel times for some primary phases are
highlighted.
Figure 7. Overlaying a seismogram
(station KIP, M7.5, 1999
Figure 8. KIP seismogram for the
Figure 9. Raypaths and wavefronts for selected primary
(compressional) wave phases which travel through the Earth. The travel times (in minutes) along the
raypaths and the corresponding wavefronts (short dashed lines; lines or surfaces
of equal travel time) are given by the small numbers adjacent to the
wavefronts. The raypaths are
perpendicular to the wavefronts and represent the direction that a specific
point on the wavefront is propagating.
The raypaths in this real Earth model are curved because the seismic wave
velocity varies with depth. Note the
strong refraction (bending) of the raypaths and wavefronts caused by the
velocity change across the core-mantle boundary. The primary wave types (phases) illustrated
in this diagram are:
P Raypaths
for waves which travel through the mantle with a relatively direct path; 0°-103° distance
range.
Pdiffracted Raypaths for
waves which travel through the mantle and are diffracted at the core-mantle
boundary by the reduced outer core velocity; 103°-150° distance range.
PKP Raypaths for
waves which travel through the mantle, are strongly refracted at the
core-mantle boundary and travel through the outer core; 110°-187° distance
range.
PKIKP Raypaths for
waves which travel through the mantle, the outer core and the inner core; 110°-180° distance
range.
PKiKP Raypaths for
waves that are reflected from the inner core.
In more recent models of the Earth's interior, the PKiKP arrivals are
observed for distances less than about 120°.
Figure
10. IRIS “Exploring the Earth Using
Seismology” poster illustrating seismic wave propagation through the Earth (http://www.iris.edu/about/publications.htm#p).
Figure 11. Close-up diagram of a
portion of the IRIS poster (Figure 10) showing raypaths through the Earth’s
interior for several seismic phases.
Distances in geocentric angle are noted using the degrees scale.
Figure 12. Close-up diagram of a
portion of the IRIS poster (Figure 10) showing a seismogram record section with
several phases identified.
3. Catalog of Seismograms at Various Distances – Screen Images: The partial screen images (from the 24-hour display in AmaSeis for station WLIN) included below and labeled A through R show seismograms from shallow focus earthquakes at epicenter-to-station distances from 1.81 degrees to 143.49 degrees. All screen displays have the same gain factor of 30. The selected seismograms illustrate the change in character of seismic signals with increasing source-to-station distance. It is immediately clear that as the distance increases, the seismograms have longer time duration. This feature is caused be the fact that different wave types travel at different velocities which causes the difference in time between phases to increase with distance. An example is the S minus P times as illustrated in Figures 6 and 7. Also, surface waves travel slower than S waves and are dispersive (velocity is a function of frequency) further increasing the duration of the seismogram with increasing distance of travel. Furthermore, greater source-to-station distance tends to result in many phases representing different wave types and travel paths to have similar amplitudes so that the seismograms are often long and complex. Seismograms also often show a relatively abrupt first arrival (P wave energy), a small number of distinct arrivals, and then a slow “tapering off” of amplitudes as time increases. This last part of the seismogram is called the “coda.” Some of the records shown below also include signals from other events and several noise sources.
Additional information about the events represented by the seismograms can be found in the Excel spreadsheet station catalog (http://web.ics.purdue.edu/~braile/edumod/as1lessons/InterpSeis/EqList.xls).
A.
D = 1.81o, 2004 6/28,
B.
D = 2.59o, 2002 6/18, Near
C.
D = 5.31o, 5/1/05,
D.
D = 9.30o, 2002 11/3,
E.
D = 14.68o, 2004 8/1, N.
F.
D = 19.39o, 2005 7/26,
G.
D = 24.10o, 2006 1/4,
H.
D = 29.97o, 2005 6/17, Off
I. D = 42.04o, 2002 10/23,
J.
D = 51.92o, 2003 2/19, Unimak
Island Region,
K.
D = 61.17o, 2005 6/14, Rat
Islands, Aleutians,
L.
D = 67.70o, 2003 5/21,
M.
D = 81.08o, 2000 8/4,
Sakhalin,
N.
D = 96.20o, 2004 9/5, Near
O.
D = 103.21o, 2005 10/8,
P.
D = 112.99o, 2001 1/26,
Q. D = 127.24o, 2003 8/21, S.
Island,
R.
D = 143.49o, 2000 6/4,
4. Catalog of
Seismograms at Various Distances – 60-minute Seismograms: In the following 3-trace plots (extracted using AmaSeis), the A through R seismograms shown above are displayed as 60-minute records (with
different amplitude scales – note the vertical scales on the left) to see a
direct comparison of the signals at various distances and the same time
scale. For some of the seismograms, one
could “zoom in” further using the AmaSeis extract seismogram tool to see more
detail. The digital, SAC-format
seismograms are listed (with Internet links) in Table 1 so that one can change
the view by “zooming in” and perform additional analysis and display the
results. Seismograms P, Q
and R are also shown in 2-hour
records because of the duration of these records due to the large
source-to-station distances. The
increase in duration with distance, characteristic phases and seismogram
complexity are apparent from the comparison of these seismograms.
Seismograms A (D = 1.81o), B (D = 2.59o) and C (D = 5.31o).