Solid Earth Geophysics-Geop503

Ali Onceloncel@kfupm.edu.sa

Department of Earth Sciences, KFUPM

Seismic Waves and Earth’s Interior

Reading: Fowler Chapter 8- Section 8.1

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

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

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

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):

Wave Animations

Allen Jones

http://bingweb.binghamton.edu/~ajones/#Seismic%20Waves

: A program for the visualization of wave propagation

ontributor: Alan JonesYear: 2006

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

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)

Expected Arrival Times:AKI135 Model

P-waves half way to core Waves from the Peru

3D P-wave Propagation of Peru EQ

P-waves in Inner Core Love & Rayleigh at Brazil

3D P-wave Propagation of Peru EQ

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

The phase PcS takes 724.3 s or 12 minutes and 4 seconds to arrive at the station.

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?

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.

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

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?

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.