ÖNCEL AKADEMİ: SOLID EARTH GEOPHYSICS
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Seismic Waves and Earth’s Interior-4
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ÖNCEL AKADEMİ: SOLID EARTH GEOPHYSICS
- 1. Solid Earth Geophysics-Geop503 Ali Oncel [email_address] Department of Earth Sciences, KFUPM Seismic Waves and Earth’s Interior Reading: Fowler Chapter 8- Section 8.1
- 3. Recall: Low-Velocity Layer 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.
- 6. Wave Animations Allen Jones http:// bingweb.binghamton.edu /~ajones/#Seismic%20Waves Seismic Waves : A program for the visualization of wave propagation Contributor: Alan Jones Year: 2006
- 7. Ray paths from the Peru event for major P and S phases through the AK135 model Mw= 8.1 Depth= 33 km Date: Saturday, June 23 Visualizing Body Waves: Peru Event www.rses.anu.edu.au/seismology/SHon2002/sq1sw.pdf
- 8. 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 )
- 9. Expected Arrival Times:AKI135 Model
- 12. 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 5 minutes 6371 3471 T PcP =514s T ScS =936s
- 13. The phase PcS takes 724.3 s or 12 minutes and 4 seconds to arrive at the station.
- 14. tt PcP ( Δ =0) = 511.3 s tt PkikP ( Δ =0) = 994.6 s α oc = 9.34 km/s Reference: Problem 13 a of Stein and Wysession in Chapter 3 5 minutes Use the travel times for PcP and PKiKP at a vertical incidence to estimate the average P-wave velocity in the outer core? 16.6 min 8.5 min α oc = 2 (3480-1222) 994.6 – 511.3
- 16. 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 α ic = 11.24 km/s 20.20 min Fig.3.5-7 of Stein and Wysession α ic = 2 (1222) 1212.1-994.6
- 17. Body Wave Velocity Structure 1.What is the P wave velocity in this area (km/s)? 2. 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. 3. What is the S wave velocity in this area (km/s)? 4. What is the origin time of the earthquake (using only graph 2) ? 5. Are the two origin times equal ? Why or why not? P-wave Data S-wave Data
- 18. 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 km X= Time in sec. P-wave Data S-wave Data
Editor's Notes
- This low An important seismic discontinuity occurs as a low-velocity zone in the Upper Mantle velocity zone is called the Asthenosphere It is not sharp but stretches from a depth of about 100km to 250 km below the surface Both P- and S-wave velocities are sharply reduced within the Asthenosphere Thought to be partly molten (1-10% liquid) which accounts for its low velocities and make it very weak Most basalt magmas have their origin in the asthenosphere
- Fig. 3.5- 6 of Stein 2003 Rial, 1978: Volume 55 Issue 3 Page 737-743 , December 1978 Summary. This note reports on the remarkable focusing of seismic body waves at or near the antipode (Δ = 180°) of an earthquake's epicentre. The particular seismic velocity structure and sphericity of the Earth cause body-wave phases such as P (diff), PKP, PP, PPP, PcPPKP, SKSSKS, SS , etc. to converge individually at antipodal distances after being diffracted, reflected or refracted at discontinuities. This focusing strongly amplifies each signal up to almost one order of magnitude with respect to the normal phase recorded two or more degrees away. Since the signal/noise ratio is enhanced in the same proportion, seismograms at antipodal distances provide clear and strong arrivals of otherwise weak phases. Antipodal monitoring of seismic waves is suggested as a powerful means of exploring the Earth's interior. The study of these ‘seismic images’ generated at focal points of seismic rays will yield information on the departures from lateral homogeneity and sphericity of the core, as well as stronger constraints on earth models. To interpret the observations correctly, the data must be compared with theoretically generated seismograms . Since the appropriate ray theory equations (see, e.g. Scholte; Gilbert & Helmberger; Richards) are singular at Δ =180°, a corrective measure is taken which provides a formal expression for the wave amplitude that remains finite at the antipode, and reproduces the usual expressions at other distances.
- Figure 3.5.11 of Stein, pp. 169: It shows that seismograms recorded at PTO (Porto, Portugal) and (Malaga, Spain) from earthquake in New Zealand . Phases like PP and PKP are focused at the antipode, because paths in any direction from the source arrive at the same time. Note the larger arrivals at PTO, only 0.7 degree from the antipode. Rial, J. A. ; Cormier, V. F. (1980, JGR): The antipodal region (178°?Δ?180°) of a seismic wave source is investigated in detail and shown to provide a new set of remarkable data to use in the exploration of the earth's interior . Body and surface waves converge individually at antipodal distances after having sampled laterally the totality of the planet. The waves are focused and strongly amplified up to 1 order of magnitude with respect to the normal phase recorded 2° or more away. The delicate interference patterns thus formed yield information on departures from lateral homogeneity and sphericity of the core and mantle, the structure of the inner core, global dissipation characteristics of the upper mantle, and provide strong constraints on earth models. Seismograms have been synthesized that closely reproduce the phases P diff , PKIKP, PKIIKP, PKP(BC), PKP, and PP observed at World-Wide Standard Seismographic Network long-period instruments located within 5° from the antipode of the New Zealand Inangahua earthquake of May 23, 1968. Preliminary results indicate that the lower mantle and upper core are laterally homogeneous as seen by 15-s waves, but the core-mantle boundary region is probably laterally inhomogeneous. The inner core-outer core boundary appears to be a sharp transition with a P wave velocity jump of the order of 0.8 km/s. The resolution of the long-period data is poor, but the potential richness of the method when better data sets are available strongly motivated the investigation. Suggested future lines of research using antipodal observations include monitoring of inner core phases
- Allen Jones http://bingweb.binghamton.edu/~ajones/
- Ring of Fire strikes Peru A magnitude-8.1 earthquake at 33 kilometers deep shook Peru on Saturday, June 23, says the USGSs National Earthquake Information Center, upping previous estimates that put the quake's magnitude at 7.9. The focus was just off the Peruvian coast, about 120 miles west of Arequipa, Peru's second largest city. Three major aftershocks followed ranging from magnitude 5.5 to 6.3. Debris and collapsed homes killed at least 70 people, most in Arequipa, and over 30 people are missing from Camana, a nearby coastal town deluged by a quake-induced tsunami. A landslide blocked the main road into Moquegua, 60 miles from Arequipa, keeping emergency food and medicine from a town where 17 people were killed and 80 percent of the houses seriously damaged or flattened. Stretched out against an active continental margin, Peru is no stranger to big quakes. The most memorable was the devastating magnitude-7.7 event in 1970 that killed approximately 70,000 people during an avalanche of rock and snow on Mount Huascarán, a little over 200 miles north of Lima.
- Expected Arrival Times: AKI135 Model
- P-waves half way to core
- Incidence: Ray path in a medium with velocity increasing smoothly with depth. The ray parameter is constant along the ray path, so the angle of incidence changes as the velocity changes. The angle of incidence is smallest at the surface, where velocity is lowest, and is 90 at the bottoming depth.
- Y= Distance in km X= Time in sec.