Seismic Waves & Earth’s Interior Seismograph Seismometers and Seismograms.
Seismic Waves and Earth’s Interior-4
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Transcript of Seismic Waves and Earth’s Interior-4
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Solid Earth Geophysics-Geop503
Department of Earth Sciences, KFUPM
Seismic Waves and Earth’s Interior
Reading: Fowler Chapter 8- Section 8.1
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Summary: Lecture 5
Weekly Seminar: Mid-Oceanic Ridge: The lack of Low Velocity Zone is surprised?
Seismic Waves
Arrival time for PcS
P-wave velocity for Inner Core and Outer Core
Probing the Earth’s Interior with Seismic waves
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Guttenberg (1959) inferred its existence from changes in the amplitude of arrivals, at distances of around 15 degree, which he attributed to the defocusing effect of a low-velocity region.
There are two possible scenarios that produce hidden layers: Low velocity layers and thin layers underlain by a large velocity contrast.
Layers that can not be distinguished from first arrival time information are known as hidden layers.
Recall: Low-Velocity Layer
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Antipodal Focusing
Top: Ra path for a surface reflection. The reflection is a maximum-time phase, because the travel time for reflection at the midpoint (delta/2) is longer than on nearby alternative paths.
Bottom: Ray paths for a surface reflection in a homogeneous medium, in which all reflections off the eliptical surface have the same travel time. The reflection off the midpoint is a minimum-time phase if the surface is flat, and a maximum-time phase if the surface is circular.
Fig. 3.5- 6 of Stein 2003
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Example: May 23, 1968
Antipodal Focusing Amplification
New Zealand
Porto
Malaga
The waves are focused and strongly amplified up to 1 order of magnitude with respect to the normal phase recorded 2° or more away.
Source: Rial, J. A.; Cormier, V. F. (1980, JGR):
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Wave Animations
Allen Jones
http://bingweb.binghamton.edu/~ajones/#Seismic%20Waves
: A program for the visualization of wave propagation
ontributor: Alan JonesYear: 2006
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Ray paths from the Peru event for major P and S phases through the AK135 model
Mw=8.1Depth= 33 km Date: Saturday, June 23
Visualizing Body Waves: Peru EventVisualizing Body Waves: Peru Event
www.rses.anu.edu.au/seismology/SHon2002/sq1sw.pdf
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Peru Earthquake
Despite its large size, the earthquake left a death toll of only 75 persons, including 26 who died as a result of the subsequent tsunami, which also caused the disappearance of 64 people. (Source: Wikipedia)
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Expected Arrival Times:AKI135 Model
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P-waves half way to core Waves from the Peru
3D P-wave Propagation of Peru EQ
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P-waves in Inner Core Love & Rayleigh at Brazil
3D P-wave Propagation of Peru EQ
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On a seismometer located at an earthquake epicenter PcP and ScS arrives 8 minutes, 34 seconds and 15 minutes, 36 seconds respectively after the earthquake. If the earth’s radius at this point is 6371 km, and the core’s radius is 3471 km, find the average P and S wave velocities in the earth’s mantle. (Remember it takes time to go both up and down!). When would the phase PcS arrive? (Note: assume that both paths are vertically incident).
Ray Tracing Exercise
T PcP=514s
T ScS=936s
5 minutesR
ad
i us
=34
86
km
Rad
ius
=12
16k
Outer Core
InnerCore
Mantle
Crust (thickness exaggerated)
6371
3471
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The phase PcS takes 724.3 s or 12 minutes and 4 seconds to arrive at the station.
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16.6 min
8.5 min
tt PcP (Δ=0) = 511.3 s tt PkikP (Δ=0) = 994.6 s
αoc =2 (3480-1222)
994.6 – 511.3
αoc = 9.34 km/s
Reference: Problem 13 a of Stein and Wysession in Chapter 3
5 minutes
Ra
diu
s=
348
6
km
Rad
ius
=12
16km
Outer Core
InnerCore
Mantle
Earthquake
Crust (thickness exaggerated)
Seismograph
Pc
P
PK
iKP
Ra
diu
s=
348
6
km
Rad
ius
=12
16km
Outer Core
InnerCore
Mantle
Earthquake
Crust (thickness exaggerated)
Seismograph
Pc
P
PK
iKP
Use the travel times for PcP and PKiKP at a vertical incidence to estimate the average P-wave velocity in the outer core?
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Reference: Problem 13 b of Stein and Wysession in Chapter 3
se the travel times in the left and previous slide for PKiKP and PKIKP at vertical incidence to estimate the average P-wave velocity in the inner core? Upload your solution to class page due to next class, March 11.
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Fig.3.5-7 of Stein and Wysession
Use the travel times in the left and previous slide for PKiKP and PKIKP at vertical incidence to estimate the average P-wave velocity in the inner core.
Reference: Problem 13 b of Stein and Wysession in Chapter 3
5 minutes
tt PKIKP (Δ=0) = 1212.1 s tt PkikP (Δ=0) = 994.6 s
αic = 2 (1222)
1212.1-994.6
αic = 11.24 km/s
20.20 min
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Body Wave Velocity Structure
P-wave Data
S-wave Data
.What is the P wave velocity in this area (km/s)?
. What is the origin time of the earthquake (using only graph 1) ?
Plot your P wave arrival time data on the first graph and plot your S wave arrival time data on the second graph.
. What is the S wave velocity in this area (km/s)?
. What is the origin time of the earthquake (using only graph 2) ?
. Are the two origin times equal ? Why or why not?
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P-wave Data
S-wave Data
What is the P wave velocity in this area (km/s)? 5.51km/s
What is the origin time of the earthquake (using only graph 1)? ?27.41/ 5.51 = 4.97, so answer is 10:33:0.4.97
What is the S wave velocity in this area (km/s)? 3.26 km/s
What is the origin time of the earthquake (using only graph 2) ?
16.23/3.26 = 4.97, so answer is 10:33:0.4.97
Are the two origin times equal ? Why or why not? The origin times are equal, but only because I did the problem mathematically. If you did the problem graphically you can expect to get slightly different answers.
Y= Distance in kmX= Time in sec.