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```Shaking Up the Classroom –
Activities with Earthquake
Seismograms 1
L. Braile, Purdue University
S. Braile, Happy Hollow
School, West Lafayette, IN
October, 2005
braile@purdue.edu,
http://web.ics.purdue.edu/~braile
The web page for this document is:
http://web.ics.purdue.edu/~braile/edumod/as1lessons/EQlocation/EQlocation.htm.
This PowerPoint file:
http://web.ics.purdue.edu/~braile/edumod/as1lessons/EQlocation/SminusP0304.ppt.
Partial funding for this development provided by the National Science Foundation.
 Copyright 2005. L. Braile. Permission granted for reproduction for non-commercial uses.
1
S minus P Times: Epicenter
Location on a Globe
Using P (compressional phase;
“first arrival”) and S (shear wave)
arrival times from 3 or more
seismograph stations, one can
estimate the epicenter to station
distance (using standard travel
time curves or table) from the S
minus P times. The earthquake
location can then be determined by
the intersection of arcs (with radius
equal to the epicenter to station
distances) drawn on a globe.
The EQlocation document is also available as an MS Word file so that the
seismograms can be printed at the correct scale (1 cm = 1 minute) and so that
http://www.eas.purdue.edu/~braile/edumod/as1lessons/EQlocation/EQlocation.doc.
Three step procedure to determine
epicenter from S minus P (S – P) times:



Measure the S – P times on the seismograms and
record the times*
Determine the inferred distance for each S – P time from
the travel time curves (similar to estimating the distance
to lightning from the difference in time between the
lightning flash and the thunder)*
Triangulate using circles (of radius = distances from step
2) drawn on a globe to determine the epicenter*
* (The S – P times can be measured on paper records of seismograms or
using digital seismograms and the AmaSeis software travel time tool to
determine the epicenter to station distance. Triangulation can also be
performed using an online mapping tool. Also see the Virtual Earthquake
website: http://www.sciencecourseware.com/VirtualEarthquake/ )
Seismogram from station CCM
(Cathedral Cave, Missouri) for
Oaxaca, Mexico earthquake of
September 30, 1999. Prominent P, S
and surface waves are visible.
the IRIS WILBER II web tool and is
displayed using AmaSeis software.
Seismogram from station
TUC (Tucson, Arizona) for
Oaxaca, Mexico earthquake
of September 30, 1999.
Seismogram from station
NNA (Nana, Peru) for
Oaxaca, Mexico
earthquake of September
30, 1999.
Seismogram from station KIP
(Kipapa, Hawaii) for Oaxaca,
Mexico earthquake of
September 30, 1999.
P arrival
S arrival
The S wave is often of lower frequency and about one half
way between the P wave and the surface waves.
These are vertical component seismograms (seismograph is
sensitive only to vertical motions). Why might the S wave not be
prominent on a vertical component record (but be well recorded
on a horizontal component record)?
Note that for
distances greater
degrees, the S
waves travel along
a raypath from the
epicenter to the
station that has a
steep angle near
the surface (“near
vertical”), and the S
wave motion is
perpendicular to the
direction of
propagation (the
raypath direction).
Seismic travel times in
the Earth (determined
from a very large number
of observations of
earthquakes and
explosions). Note that
the difference between
the S and P arrival times
(the “S minus P time”)
increases with distance.
So, the S minus P time
on a single seismogram
can be used to estimate
the distance of the station
from the epicenter (1
degree = 111.19 km).
Simplified graph of the S minus P times in the Earth (reproduced in the
EQlocation.doc file at the same scale as the seismograms (1 cm = 1 minute).
Example of determining the epicenter to station distance from the S – P time.
In this case, the S – P time is 8 minutes and a cm scale is placed on the S –
P graph; the inferred distance is about 58 degrees.
Table 1. Data table for the S minus P earthquake location information.
Station (Latitude and
Longitude, in
degrees):
TUC (32.310, -110.785)
Tucson, AZ
CCM (30.056, -91.245)
Cathedral Cave, MO
NNA (-11.987, -76.842)
Nana, Peru
KIP (21.423, -158.015)
Kipapa, HI
Measured S minus P
times (minutes; measure
to nearest tenth of a
minute = 1 mm on the
seismogram):
Inferred distance (degrees and
kilometers; convert degrees
to km by multiplying by
111.19 km/degree):
Degrees:
Kilometers:
Distance Scale
Distance P
In degrees
S
KIP seismogram with P and S arrival times
(vertical lines) picked with AmaSeis picking
tool and positioned on standard travel time
curves using the AmaSeis travel time curve
tool. The S minus P time is a function of
distance so that positioning the seismogram
on the curves so that the P and S arrival times
line up with the P and S travel time curves
infers the epicenter to station distance, in this
case 58.25 degrees (6477 km).
Time Scale
Close-up of seismogram positioned (P and S arrival
times) on the travel time curves (in AmaSeis software).
Table 2. The Oaxaca earthquake data set:
M7.5 September 30, 1999 Oaxaca, Mexico earthquake recorded at GSN stations CCM
(Cathedral Caves, MO), TUC (Tucson, AZ), NNA (Nana, Peru), and KIP (Kipapa, HI) – click
Seismograms:
CCM.00.BHZ.D.SAC, TUC.00.BHZ.D.SAC, NNA.00.BHZ.D.SAC, KIP.00.BHZ.D.SAC
the EQlocation files and then viewed and analyzed using the AmaSeis
software. The AmaSeis software (Windows) can be obtained from Alan
Jones’ website: http://www.geol.binghamton.edu/faculty/jones/. A tutorial
on using the AmaSeis software is available at:
http://www.eas.purdue.edu/~braile/edumod/as1lessons/UsingAmaSeis/U
singAmaSeis.htm (seismograms for two additional data sets that can be
used for S – P location [using digital seismograms and AmaSeis] and for
magnitude calculation [for the AS-1 seismograms using the MagCalc
online tool] can be found in Section 6 of the Using AmaSeis document).
Earthquake
epicenter location
on a globe –
triangulation
Determining
distance on a
globe.
Drawing an arc on the globe (center of circle/arc is the
station location; radius = distance in degrees inferred from
the S – P time at the station).
Drawing an arc on the globe for
Oaxaca EQ for TUC station.
Epicenter (red dot)
located by
intersection of
circular arcs
(triangulation)
Earthquake
epicenter location
using an online
mapping tool –
triangulation
The data shown in the table are
entered into the online tool to
create a map showing the
station and epicenter locations
and the S – P circles. The
the S – P times.
Instructions and an example for using the online S – P mapping tool and
for using the IRIS Event Search tool are available at:
http://www.eas.purdue.edu/~braile/edumod/eqdata/eqdata.htm.
Online instructions on the IRIS website are also available at:
http://www.iris.edu/quakes/eventSearchInstructions.htm.
Map produced using the online
S – P mapping tool in the IRIS
Event Search for the Oaxaca
event. Epicenters of historical
using the mapping tool in Event
Search.
A magnitude calculation exercise (using paper copies of
seismograms or digital seismograms analyzed with the AmaSeis
software) is also included in the EQlocation document.
L. Braile web page (links to USGS earthquake site, SpiNet, Alan Jones’ site,
USGS travel time calculator, AS-1 seismograph information, AS-1
seismograph magnitude calculator, etc.):
http://www.eas.purdue.edu/~braile/
http://web.ics.purdue.edu/~braile/ (new)
AS-1 seismograph information:
Seismograph and seismogram information (including EQlocation exercise):
http://www.eas.purdue.edu/~braile/edumod/as1lessons/as1lessons.htm
use, and very similar to the methods that are actually used to locate
earthquakes) is described at:
http://www.eas.purdue.edu/~braile/edumod/eqlocate/tutorial.htm. The
digital data that can be analyzed using AmaSeis and used for S – P
earthquake location are also included (see Section 4).
Shaking Up the Classroom –
Activities with Earthquake
Seismograms 1
L. Braile, Purdue University
S. Braile, Happy Hollow
School, West Lafayette, IN
October, 2005
braile@purdue.edu,
http://web.ics.purdue.edu/~braile