Field Trip/Discussions 96 年 4 月 28 日 -- 古生物地層野外地質（苗栗磚場剖面） -- 魏國彥老師

1. Field Trip/Discussions 96年4月28日--古生物地層野外地質（苗栗磚場剖面）--魏國彥老師 96年5月26日--火成岩野外地質（觀音山）--楊燦堯老師

3. More on mantle plumes • Plume distorted by mantle convection? Rising speed of plume ? • Diameter of plume conduit and plume head ? • Plume hypothesis – a vague theory ?

4. Chapter 10: Earthquakes and Earth’s Interior

5. Seismology • Seismology is the study of the generation, propagation, and recording of elastic waves in the Earth (and other celestial bodies) and of the sources that produce them.

6. How Earthquakes Are Studied (1) • Seismometers are used to record the shocks and vibrations caused by earthquakes. • All seismometers make use of inertia (慣性), which is the resistance of a stationary mass to sudden movement. • This is the principal used in inertial seismometers.

7. Figure 10.1 Figure 10.2

8. How Earthquakes Are Studied (2) • Three inertial seismometers are commonly used in one instrument housing to measure up-down, east-west, north-south motions simultaneously.

9. Earthquake Focus And Epicenter • The earthquake focus (or hypocenter) (震源) is the point where earthquake starts to release the elastic strain of surrounding rock. • The epicenter (震央) is the point on Earth’s surface that lies vertically above the focus of an earthquake.

10. Figure 10.3 Rupture FrontThe rupture front is the instantaneous boundary between the slipping and locked parts of a fault during an earthquake. Rupture velocityThe speed at which a rupture front moves across the surface of the fault during an earthquake. (~ 0.8 Vs) Vel ~ 3 km/s

11. Finite Source Modeling of 1999 Taiwan Chi-Chi Earthquake and its Tectonic Implications Wu-Cheng Chi, Douglas Dreger, and Anastasia Kaverina

12. Source rupture process of the 2003 Tokachi-oki earthquake (Yagi, 2004, EPS)

14. The First Seismogram from a Distant Earthquake Seismology (地震學) is a fairly young science; recordings of earth motion (seismograms) have only been made for about 100 years. Shown is what is widely considered to be the first remote (teleseismic) seismogram, made on April 17, 1889, in Postdam, Germany by E. von Rebeur-Pacshwitz (Nature, 1889). The earthquake was in Japan and had a magnitude of about 5.8. Source: http://www.iris.edu

15. Earth structure instrument Some facts about seismology • Seismic tomography/Inverse theory • The recorded motions can be viewed as the output response of a sequence of linear filters with properties we wish to determine. Source mechanism

16. Some facts about seismology • The history of seismological advances is one of the alternating progress in describing source properties or in improving models of Earth structure. Seismic Source mechanism Earth Structure

17. Some facts about seismology • Earthquake faulting and its role in global plate tectonics. • Kinematic and dynamic characteristics of shear faulting sources, their scales of variation, and measures of energy release such as seismic magnitudes and earthquake moment.

18. Seismic Waves (1) • Vibrational waves spread outward initially from the focus of an earthquake, and continue to radiate from elsewhere on the fault as rupture proceeds.

19. Seismic Waves (2) • There are two basic families of seismic waves. • Body waves(體波) can transmit either: • Compressional motion (P waves), or • Shear motion (S waves). • Surface waves(表面波) are vibrations that are trapped near Earth’s surface. There are two types of surface waves: • Love waves, or • Rayleigh waves.

20. Body Waves (1) • Body waves travel outward in all directions from their point of origin. • The first kind of body waves, a compressional wave, deforms rocks largely by change of volume and consists of alternating pulses of contraction and expansion acting in the direction of wave travel.

21. P-wave: Longitudinal wave (縱波) Compressional waves are the first waves to be recorded by a seismometer, so they are called P (for “primary”) waves.  Dan Russel

22. Body Waves (2) • The second kind of body waves is a shear wave (剪力波). • Shear waves deform materials by change of shape, • Because shear waves are slower than P waves and reach a seismometer some time after P waves arrives, they are called S (for “secondary”) waves.

23. Figure 10.4 S-wave: Transverse wave (橫波) SV motions on the vertical plane parallel to the propagation direction  Dan Russel

24. Body Waves (3) • Compressional (P) waves can pass through solids, liquid, or gases. • P waves move more rapidly than other seismic waves: • 6 km/s is typical for the crust. • 8 km/s is typical for the uppermost mantle.

25. Body Waves (4) • Shear waves can travel only within solid matter. • The speed of a S wave is times that of a P wave. A typical speed for a S wave in the crust is 3.5 km/s, 5 km/s in the uppermost mantle.

26. Seismic body waves, like light waves and sound waves, can be reflected and refracted by change in material properties. Figure 10.5

27. Body Waves (5) • For seismic waves within Earth, the changes in wave speed and wave direction can be either gradual or abrupt, depending on changes in chemical composition, pressure, and mineralogy.

28. Body Waves (6) • If Earth had a homogeneous composition and mineralogy, rock density and wave speed would increase steadily with depth as a result of increasing pressure (gradual refraction). • Measurements reveal that the seismic waves are refracted and reflected by several abrupt changes in wave speed.

29. c : reflection at the core-mantle boundary K: P wave transimission in the outer core i: P wave reflection at the inner-outer core boundary I: P wave transmission in the inner core J:S wave transmission in the inner core Figure 10.6

32. PKIKP P K P K I PKJKP J P K P K

33. Surface Waves (1) • Surface waves travel along the surface of the ground or just below it, and are slower than body waves, but are often the largest vibrational signals in a seismogram. The two most important are Rayleigh (雷利波) and Love (洛夫/勞夫/樂夫 … 波) waves, named after British scientists, Lord Rayleigh and A.E.H. Love who discovered them.

34. Surface Waves (2) • Rayleigh waves combine shear and compressional vibration types, and involve motion in both the vertical and horizontal directions. The velocity of Rayleigh waves is about 0.92 times that of S waves..

35. Surface Waves (3) Love waves consist entirely of shear wave vibrations in the horizontal plane, analogous to an S wave that travels horizontally, so they only appear in the horizontal component of a seismogram. The velocity of Love waves is approximately equal to that of S waves, so they arrive earlier than Rayleigh wave.

36. Rayleigh wave retrograde (counter-clockwise) motions P > S > Rayleigh wave velocity

37. Surface Waves (2) • The longer the wave length of a surface wave, the deeper the wave motion penetrates Earth. • Surface waves of different wave lengths develop different velocities. This behavior is called Dispersion (色散).

38. Figure 10.7

39. Determining The Epicenter (1) • An earthquake’s epicenter can be calculated from the arrival times of the P and S waves at a seismometer. (P-S differential travel time) • The further a seismometer is away from an epicenter, the greatest the time difference between the arrival of the P and S waves.

40. Determining The Epicenter (2) • The epicenter can be determined when data from three or more seismometers are available. • It lies where the circles intersect (radius = calculated distance to the epicenter).

41. Figure 10.9 Figure 10.8

42. Earthquake Magnitude (1) • In 1931 Kiyoo Wadati constructed a chart of maximum ground motion v.s. distance for a number of earthquakes • In 1935 C. Richter constructed a similar diagram of peak ground motion versus distance and used it to create the first earthquake magnitude scale. - Richter magnitude (芮氏地震規模)

43. Earthquake Magnitude (2) What do you observe from the figure ?

44. Earthquake Magnitude (3) In 1935 C. Richter constructed a similar diagram of peak ground motion versus distance and used it to create the first earthquake magnitude scale.

45. Earthquake Magnitude (4) Richard Magnitude Assumptions Given the same source-receiver geometry and two earthquake of different size, the “larger” event will “on average” generate larger amplitude arrivals. The amplitudes of arrivals behave in a “predictable” fashion. i.e. the effects of propagation are known in a statistical fashion.

46. Earthquake Magnitude (5) ML=log(A) + f(Δ,h) + Cs + Cr A – Ground displacement of the measured reference phase f– a correction for epicentral distance Cs – correction for the sitting of a station. (site effects) Cr– correction for the source region. Multiple stations are used to reduce the amplitude biases caused by radiation pattern, directivity, and anomalous path properties.

47. Earthquake Magnitude (6) • Later seismologists devised more general magnitude estimate based on either on P wave (T~1 s), called mb, surface wave trapped in the crust (T~20 s), or surface trapped in the upper mantle (T~200 s), called Ms.

48. Seismic waves in frequency domain Estimates of Stress Drop of the Chi-Chi, Taiwan, Earthquake of 20 September 1999 from Near-Field Seismograms Ruey-Der Hwang et al. BSSA, 2000.

49. Magnitude saturation

50. A large-sized earthquake occurs over a larger fault, requires more time to rupture. • Measures of earthquake size based on the maximum ground shaking do not account for an important characteristic of large earthquakes - they shake the ground longer.