ÖNCEL AKADEMİ: SOLID EARTH GEOPHYSICS
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Seismology Measuring the Interior-3
![Solid Earth Geophysics Ali Oncel [email_address] Department of Earth Sciences KFUPM Today’s class: Seismology Measuring the Interior Reading: Fowler Chapter 4](https://image.slidesharecdn.com/lecture16-091219232815-phpapp01/85/ncel-akademi-solid-earth-geophysics-1-320.jpg)
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ÖNCEL AKADEMİ: SOLID EARTH GEOPHYSICS
- 1. Solid Earth Geophysics Ali Oncel [email_address] Department of Earth Sciences KFUPM Today’s class: Seismology Measuring the Interior Reading: Fowler Chapter 4
- 2. 1992 Landers, California EQ, Mw=7.3
- 5. ..use equal area lower hemisphere projection (stereonet) Station reports location relative to focus: Azimuth : relative to focus Angle of incidence: between ray vector and vertical Azimuth and angle of incidence plot on net and first motion noted Focal Mechanism center From: Kearey and Vine, 1996 From: Rowland and Duebendorfer, 1994
- 6. The angle of incidence, which often called the take-off angle , at the earthquake source is the angle from the vertical at which the ray leaves source, and thus the angle at which the ray intersects the lower hemisphere (Fig.4.2-8 of Stein and Wysession, 2003). How is distance related to take-off angle?
- 7. Source: Stein and Wysession (2003) After Pho and Behe (1972)
- 8. Plotting Station Locations Example 1: Plot seismograph station (azimuth N40°E from the earthquake; angle of first motion ray or take-off angle i= 60°) on an equal-area projection Fig. 4.2.13 of Stein and Wysession, 2003 South North
- 11. The Stations Lon Lat Dist, km 176.35° -40.250° 3.403 176.17° -40.292° 14.414 176.37° -40.061° 19.072 176.28° -40.408° 20.154 176.06° -40.106° 25.886 176.47° -40.453° 27.993 176.63° -40.339° 29.118 176.09° -40.429° 29.880 176.27° -40.618° 43.544 176.81° -39.989° 48.965 176.35° -39.699° 59.008 176.88° -39.665° 78.454 176.82° -39.541° 87.502
- 12. Source: Stein and Wysession, 2003
- 13. Stresses Driven Faulting From: http://earth.leeds.ac.uk/learnstructure/index.htm Anderson’s theory of faulting See pp.402, Bolt and Bullen T P
- 14. P=Maximum Compressive Stress T=Minimum Compressive Stress Stress Orientations Source: Stein and Wysession, 2003
- 15. Let’s come back to discuss earthquake fault mechanism b d
Editor's Notes
- Fowler, pp.132. Satellites are greatly improving our knowledge of the Earth. SAR has provided a spectucalar image of the displacement resulting from the 1992 Landers, California, earthquake. This (Mw=7.3, MS=7.5), right-lateral strike-slip earthquake ruptured some 85 km along a fault system, and resulted in a maximum slip of 6 m. By using two satellite radar images of the region, one taken before the earthquake, the other after, it was possible to calculate the small changes in the distance from the ground surface to the satellite orbiting at an altitute of 785 km. The interferogram (Fig.4.21, Plate 5), with each cycle of shading representing 2.8 cm of change in the ground-to-satellite distance, contours the spatial extent of the ground deformation very clearly. There are at least twenty fringes from the northern edge of the image to the cores of the lobes along the faults, representing a difference in distance of 56 cm – this agrees with the observed surface displacements. The area within 5-10 km of the rupture zone shows no organized fringe pattern, the signal is incoherent. This region was subject to large changes in satellite-to-ground distance, intense secondary faulting and block rotation. Space-geodetic studies will clearly become an increasingly powerful means for studying crustal deformation.
- pp. 130, Fowler, 2005: The length of fault plane, along which the rocks are displayed, varies from meters, for a small earthquake, to about 1000 km, for a large earthquake. The 1960 Chile earthquake had a fault plane 1000 km in length, and the aftershock zone of 1957 Aleutian earthquake (Mw=9.1) was some 1200 km long, the largest aftershock zone known. Most earthquakes occur along plate boundaries as a direct result of plate motions; these are interplate (between plates) earthquakes. Intraplate (within-plate) earthquakes comprise only a small portion of the total number occurring, but they can be large and can produce considerable damage.
- PP.133, Fowler: The length of the arrow in the lobe represents the relative magnitude of the first P-wave at that location.
- To determine focal mechanisms, we plot the points where ray intersect the focal sphere, so that the nodal planes can be found. For example, to plot the point corresponsing a ray whose azimuth is 40 and whose take-off angle is 60. We first rotate the net, placing the equator along azimuth 40. Because take-off angles I are measured from the vertical, they correspond to dips of 90-i. We thus mark the point with dip 30 E, and rotate the net back so that north is at the top (pp224).
- Orientation of s1 relative to Earth’s surface dictates type of fault… …three possibilities yield three types of faults