SeismicAttenuation地震波衰减PPT课件

1、,Haberland et al., GRL 2003,EARTHQUAKE MAGNITUDE,Earliest measure of earthquake size Dimensionless number measured various ways, including ML local magnitude mb body wave magnitude Ms surface wave magnitude Mw moment magnitude Easy to measure No direct tie to physics of faulting,(4) intrinsic attenu

2、ation: due to anelasticity,the real earth materials are always “lossy”, leading to reduced wave amplitudes, or intrinsic attenuation.,Mechanisms to lose energy: (1) Movements along mineral dislocations (2) Shear heating at grain boundaries These are called “internal friction”. ,(3) Scattering: A way

3、 to partition energy of supposedly main arrivals into boundary or corner diffracted, scattered energy.,Attnuation Mechanisms:,(1) Geometrical spreading: wavefront spreading out while energy per unit area becomes less.,(2) Multipathing: waves seek alternative paths to the receiver. Some are dispersed

4、 and some are bundled, thereby affecting amplitudes.,Key: very wavelength dependent.,3,Geometric spreading: light moves outward from lamp in expanding spherical wave fronts. By conservation of energy, the energy in a unit area of the growing wave front decreases as r-2, where r is the radius of the

5、sphere or distance from the lamp.,Analogous to light behavoirs,Expect minimum at =90, and maxima at 0 and 180,GEOMETRIC SPREADING: SURFACE WAVES,From geometric spreading alone, expect minimum at =90, and maxima at 0 and 180 Also have effects of anelasticity,For body waves, consider a spherical wavef

6、ront moving away from a deep earthquake. Energy is conserved on the expanding spherical wavefront whose area is 4 r 2, where r is the radius of the wavefront. Thus the energy per unit wave front decays as 1 / r2, and the amplitude decreases as 1 / r,GEOMETRIC SPREADING: BODY WAVES,MULTIPATHING Seism

7、ic waves are focused and defocused by variations in velocity.,Idea of Ray Tubes in body waves,Ray tube size affects amplitudes, smaller area means larger amplitude,Angle dependent: ray bundle expands or contracts due to velocity structure. Also: amplitude varies with takeoff angle,Effect of Multipat

8、hing,9,Scattering:,Scattering occurs when there are velocity heterogeneities in the medium with wavelengths on the order of of the wave.,Snieder et al., Science, 2002,A simulation of 100 randomly-perturbed scatterers,Moving the scatterers (here, 1/40th of the distance shown, so the perturbation is v

9、isible) mostly changes the shapes and amplitudes of the waveforms,Snieder et al., Science, 2002,Waveforms measured in a granite sample at temperatures of 45C (blue) and 50C (red).,Changing the matrix velocity, by contrast, introduces a shift (delay or advance) that increases with coda time.,Scatteri

10、ng: Interesting aside: Coda energy in the Earth tends to attenuate much more rapidly than on the Moon! This is partly because lunar regolith is highly fractured by impacts, but mostly because it contains no fluids (and consequently much less intrinsic attenuation!),Intrinsic Attenuation: Intrinsic a

11、ttenuation, or anelasticity, describes the process by which elastic energy in the Earth is converted to heat when the seismic wave induces unrecoverable deformation. To examine this, lets consider a spring: For an idealized spring, has solution with oscillation frequency More realistically though, i

12、nternal friction in the spring will damp the system resulting in where is a damping factor and Q 0/ is called the quality factor.,Mass m,u,Spring constant k,Intrinsic Attenuation: This system has a solution with real and imaginary parts; the actual displacement is the real part and takes the form: i

13、.e., a harmonic oscillator with an exponential decay of amplitude. Here, A0 is the initial displacement (at time t = 0) and Important to note: High frequencies attenuate more than low Harmonic frequency is changed by attenuation Higher Q results in less change to frequency and less intrinsic attenua

14、tion for given time,Mass m,u,Spring constant k,Generally, loss of amplitude due to intrinsic attenuation is much greater than that due to partitioning, spreading and the other amplitude effects we have discussed,Definition of the quality factor,Energy dissipated in a cycle,Energy stored,Putting ever

15、ything together,Intrinsic attenuation,(1) Movements along mineral dislocations (2) Shear heating at grain boundaries Affected by temperature, pressure, frequency, and medium properties,Attenuation property is complementary to velocity.,Haberland et al., GRL 2003,Myers et al., 1995,The key here is to

16、 correlate a decrease in Q with fluids in the crust and mantle. The fluid layer again represents melting due to subduction.,22,(Romanowicz & Mitchell, 2007),Nakajima et al., 2003, GRL,How to estimate Q,Code wave Surface wave Body wave Amplitude data .,Coda wave attenuation,Kumar et al., 2005,Estimat

17、ing Q from body wave - using t*,Site Response,Attenuation Model,ABCE Amplitude of horizontal component Period,2005 1 1 O=03 15 40.2 +/- 0.04s LAT=26.92 N +/- 0.30km LONG=100.31 E +/- 0.33km DEPTH= 10 km +/- 0.14km STATIONS USED = 6, STAND DEV= 2.63s ML=2.9/ 6, EYA 0.9 202 Pg 03 15 56.4 0.7 Sg 03 16

18、07.6 -0.1 SMN ML=3.0 0.6 0.20 SME 0.6 0.61 PZH 1.4 108 Pn 03 16 03.1 -2.3 Pg 03 16 05.0 0.9 Sg 03 16 19.4 -3.2 SMN ML=2.9 0.6 0.17 SME 0.7 0.18 XAC 2.1 347 Pn 03 16 15.7 0.5 Pg 03 16 19.6 3.0 Sg 03 16 44.5 -0.4 SMN ML=3.2 1.0 0.11 SME 1.0 0.23 TCG1 2.5 221 Pg 03 16 24.0 0.0 Sg 03 16 55.7 -2.2 SMN ML

19、=3.7 0.6 0.52 SME 0.5 0.42 KMI 2.8 128 Pg 03 16 28.9 -1.1 Sg 03 17 07.4 -1.1 SMN ML=2.8 0.8 0.045 SME 0.7 0.047 ZOT 3.1 82 ePg 03 16 36.8 2.5 Sg 03 17 14.6 -1.5 SMN ML=2.8 1.0 0.031 SME 1.0 0.038,Amplitude Q tomography (Shunping Pei),Theory and Method,ML Tomography in North China,ABCE of 1985 to 199

20、5 (Annual Bulletin of Chinese Earthquakes) 1. Each event was recorded by at least 2 stations, 2. Residuals between -2.0 2.0 , 3. Period T of the Sg wave maximum amplitude (A) is between 0.42.0s, 4. Epicentral distance is between 100km-800km. 5. Event depth is less than 20km. 10899 amplitude data 173

21、2 events 91 stations,10899 Amplitude data 1732 Events (+) 91 Stations (),Histogram of ML distribution,Result,Phillips et al., 2005,Q from the tomographic inversion of 1 Hz Lg amplitude ratios,Liu et al., 2004,Checkerboard Test 2 2,Checkerboard Test 1.5 1.5,Standard deviation 0.67,Standard deviation

22、0.41,Summary,Crustal attenuation can be reconstructed by tomographic imaging method using amplitude data. Attenuation levels are correlated with regional tectonic structure. High attenuation often occurs in active tectonic areas with significant faulting, while attenuation is low in the stable Ordos

23、 Craton. The estimate of attenuation shows a close correlation with topography. Q0 is generally low in basins, whereas high Q0 mostly occurs in mountains and uplift regions where crystalline basement appears in the surface. It is possible that low Q0 in basins is caused by fluid in the upper crust,

24、and deep sediment in basins, while high Q0 in the mountains and uplift regions results from the presence of old, dense rocks there. (Published in BSSA(2006),From geometric spreading alone, expect minimum at =90, and maxima at 0 and 180 Also have effects of anelasticity,Haberland et al., GRL 2003,Theory and Method,10899 Amplitude data 1732