Exploring Planet Earth Larry Braile, Purdue University
[email protected], www.eas.purdue.edu/~braile CSTA Convention, Oct.
2011, Pasadena, CA
http://web.ics.purdue.edu/~braile/new/SminusP2009.ppt S minus P
Times: Epicenter Location on a Globe Slide 2 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 changes
or additions can easily be made:
http://web.ics.purdue.edu/~braile/edumod/as1lessons/EQlocation/EQlocation.doc
http://web.ics.purdue.edu/~braile/edumod/as1lessons/EQlocation/EQlocation.doc
This PowerPoint file:
http://web.ics.purdue.edu/~braile/new/SminusP2009.ppthttp://web.ics.purdue.edu/~braile/new/SminusP2009.ppt
Slide 3 S minus P time earthquake location method is similar to
using the lightning-to- thunder delay time to estimate the distance
to a lightning strike; and, if three observations are available,
triangulating to find the location of the lightning. Slide 4 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/eec/Earthquake/)
http://www.sciencecourseware.com/eec/Earthquake/ Slide 5 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)? Slide 6 Seismogram from station KIP (Kipapa,
Hawaii) for Oaxaca, Mexico earthquake of September 30, 1999. P
arrivalS arrival The S wave is often of lower frequency and about
one half way between the P wave and the surface waves. Slide 7
Seismogram from station CCM (Cathedral Cave, Missouri) for Oaxaca,
Mexico earthquake of September 30, 1999. Prominent P, S and surface
waves are visible. Seismogram was downloaded using the IRIS WILBER
II web tool and is displayed using AmaSeis software. Slide 8
Seismogram from station TUC (Tucson, Arizona) for Oaxaca, Mexico
earthquake of September 30, 1999. Slide 9 Seismogram from station
NNA (Nana, Peru) for Oaxaca, Mexico earthquake of September 30,
1999. Slide 10 Seismogram from station KIP (Kipapa, Hawaii) for
Oaxaca, Mexico earthquake of September 30, 1999. P arrivalS arrival
The S wave is often of lower frequency and about one half way
between the P wave and the surface waves. 8 minutes Slide 11 Note
that for distances greater than about 40 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). Slide 12 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). KIP
seismogram Slide 13 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. KIP seismogram S P time = 8 minutes; distance is
58 degrees Slide 14 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). KIP seismogram S P time = 8
minutes; distance is 58 degrees Slide 15 Table 1. Data table for
the S minus P earthquake location information. Station (Latitude
and Longitude, in degrees): 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: 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
8.0586449 Slide 16 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). P S Distance In degrees Distance Scale Time Scale Slide 17
Close-up of seismogram positioned (P and S arrival times) on the
travel time curves (in AmaSeis software). Slide 18 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 on the
SAC files below to download. Seismograms:
CCM.00.BHZ.D.SACCCM.00.BHZ.D.SAC, TUC.00.BHZ.D.SAC,
NNA.00.BHZ.D.SAC,
KIP.00.BHZ.D.SACTUC.00.BHZ.D.SACNNA.00.BHZ.D.SACKIP.00.BHZ.D.SAC
The seismograms used in this activity can be downloaded from links
in the EQlocation files and then viewed and analyzed using the
AmaSeis software. The AmaSeis software (Windows) can be obtained
from Alan Jones website: http://bingweb.binghamton.edu/~ajones/. 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).http://bingweb.binghamton.edu/~ajones/
http://www.eas.purdue.edu/~braile/edumod/as1lessons/UsingAmaSeis/U
singAmaSeis.htm Slide 19 Determining distance on a globe.
Earthquake epicenter location on a globe triangulation Slide 20
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). Slide 21 Drawing an arc on the globe for Oaxaca EQ
for TUC station. Slide 22 Epicenter (red dot) located by
intersection of circular arcs (triangulation) Slide 23 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 calculated radius comes from 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.
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.
http://www.iris.edu/quakes/eventSearchInstructions.htm Slide 24 Map
produced using the online S P mapping tool in the IRIS Event Search
for the Oaxaca event. Epicenters of historical events and title
were added using the mapping tool in Event Search. KIP TUC CCM NNA
Epicenter
