Earthquake Engineering GE / CEE - 479/679 Topic 13. Wave Propagation 2

Name: Earthquake Engineering GE / CEE - 479/679 Topic 13. Wave Propagation 2
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Description: Earthquake Engineering GE / CEE - 479/679 Topic 13. Wave Propagation 2

Earthquake Engineering GE / CEE - 479/679 Topic 13. Wave Propagation 2
John G. Anderson Professor of Geophysics March 4, John Anderson: GE/CEE 479/679: Lecture 13

John Anderson: GE/CEE 479/679: Lecture 13
Combining in F=ma In this equation, Xi is a body force acting on the point, if any. March 4, John Anderson: GE/CEE 479/679: Lecture 13

March 4, John Anderson: GE/CEE 479/679: Lecture 13

The Free Surface SH S-waves can have two polarizations: SH - wave motion is parallel to the surface. Causes only horizontal shaking. SV - wave motion is oriented to cause vertical motion on the surface. Amplitudes are approximately doubled Motion in and out of the plane of this figure - hard to draw. SV Motion perpendicular to the direction of propagation causes vertical motion of the free surface. March 4, John Anderson: GE/CEE 479/679: Lecture 13

Two Media in Contact i1 This way of drawing is consistent with horizontal layers in the Earth. Lower velocities near the surface imply wave propagation direction is bent towards the vertical as the waves near the surface. i2 March 4, John Anderson: GE/CEE 479/679: Lecture 13

Two Media in Contact Transmitted SV For an incoming SV wave, the situation gets even more complex. In this case, both P- and SV-waves are transmitted and reflected from the boundary. The P- and SV-waves are coupled by the deformation of the boundary. i1 Transmitted P j1 i2 i2 Reflected P j2 Incoming SV Reflected SV Generalized Snell’s Law March 4, John Anderson: GE/CEE 479/679: Lecture 13

Realistic Earth Model i1 p is the “ray parameter. It is constant along the ray i2 β increases Eventually, as the velocity increases with depth, rays are bent back towards the surface. Waves cannot penetrate into layers where β is too large. March 4, John Anderson: GE/CEE 479/679: Lecture 13

Body Waves: Discussion
The travel time curves of body waves can be inverted to find the velocity structure of the path. March 4, John Anderson: GE/CEE 479/679: Lecture 13

Seismic Refraction * Δ Δ Δ Δ Δ Δ Δ Δ Δ Δ Δ Δ i1 Refracted wave i2 β increases Because velocity increases with depth, rays are bent back towards the surface. Apparent velocity at the array of sensors is the same as the velocity of the refracted ray along the top of the refracting layer. Records from a profile of sensors radial from an explosion can thus be inverted to find velocity with depth. p is constant along the ray March 4, John Anderson: GE/CEE 479/679: Lecture 13

Realistic Earth Model i1 i2 β increases Due to Snell’s law, energy gets trapped near the surface. This trapped energy organizes into surface waves. March 4, John Anderson: GE/CEE 479/679: Lecture 13

Four types of seismic waves Body Waves P Waves Compressional, Primary S Waves Shear, Secondary Surface Waves Love Waves Rayleigh Waves March 4, John Anderson: GE/CEE 479/679: Lecture 13

Surface Waves Love waves: trapped SH energy. Rayleigh waves: combination of trapped P- and SV- energy. March 4, John Anderson: GE/CEE 479/679: Lecture 13

Surface Waves For surface waves, geometrical spreading is changed. For body waves, spreading is ~1/r. For surface waves, spreading is ~1/r0.5. March 4, John Anderson: GE/CEE 479/679: Lecture 13

Surface Waves: Discussion
Body waves are not dispersed. Surface waves are dispersed, meaning that different frequencies travel at different speeds. Typically, low frequencies travel faster. These have a longer wavelength, and penetrate deeper into the Earth, where velocities are faster. Typically, Love waves travel faster than Rayleigh waves. March 4, John Anderson: GE/CEE 479/679: Lecture 13

Surface Waves Surface wave dispersion curves can be inverted to find the velocity structure of the path crossed by the surface waves. March 4, John Anderson: GE/CEE 479/679: Lecture 13

Particle motion in S-waves is normal to the direction of propagation. This is also true of Love waves. However, Love waves would show changes in phase along the direction of propagation that would not appear in vertically propagating S waves. March 4, John Anderson: GE/CEE 479/679: Lecture 13

Motion of Rayleigh waves is “retrograde elliptical”. March 4, John Anderson: GE/CEE 479/679: Lecture 13

Site Response What is site response What causes it What are it’s characteristics. March 4, John Anderson: GE/CEE 479/679: Lecture 13

Classic example of site effect : Mexico City
Mexico City, Mexico March 4, John Anderson: GE/CEE 479/679: Lecture 13

Figure 2 March 4, John Anderson: GE/CEE 479/679: Lecture 13

Physics of Site Response
Layer over half space Multiple layers over half space Basins Topography March 4, John Anderson: GE/CEE 479/679: Lecture 13

Multiple flat layers March 4, John Anderson: GE/CEE 479/679: Lecture 13

Basins: major phenomena
Amplification Energy trapped Conversion to surface waves at basin edge Longer duration March 4, John Anderson: GE/CEE 479/679: Lecture 13

Basin edge Kobe, Japan earthquake disaster. March 4, John Anderson: GE/CEE 479/679: Lecture 13

Liu and Heaton, ~1980 Study of strong motion from the San Fernando earthquake. Published in Bull. Seism. Soc. Am. Demonstration of a basin effect. March 4, John Anderson: GE/CEE 479/679: Lecture 13

Site Characterization
Goal: characterize the average effect of geology on strong motion, and use this to improve predictions. The shallow geology is an almost miniscule part of the total path from the earthquake to the station. However, it has a strong effect on the ground motions, because it is the closest to the station. Geophysical measurements, using wave propagation techniques, are used to measure near-surface site characteristics. Also need to know basin geometry, depth. March 4, John Anderson: GE/CEE 479/679: Lecture 13

Geotechnical Site Classification
Many schemes to classify the site. Encroaching into the territory that Prof. Siddharthan will discuss later. But it’s good to introduce the subject from the viewpoint of the seismologist. March 4, John Anderson: GE/CEE 479/679: Lecture 13

Seed and Idriss (1982) 1. Rock sites 2. Stiff soil sites (< 60 m deep) 3. Deep cohesionless soil sites (> 75 m deep) 4. Sites underlain by soft to medium stiff clays Problem with this approach: Does not recognize that the spectral shape also depends on the earthquake magnitude. March 4, John Anderson: GE/CEE 479/679: Lecture 13

Geotechnical Classification Schemes
Geology Material on a geological map For example, for California one simple approach is the “QTM” approach, using the age of the material. Q = Quaternary T = Tertiary M = Mesozoic Whether the location is “erosion-dominated” or “sedimentation-dominated” (rock, soil) March 4, John Anderson: GE/CEE 479/679: Lecture 13

NEHRP Classification Shear velocity of near-surface materials
NEHRP Category Description Shear velocity (m/s) A Hard rock >1500 B Firm to hard rock C Dense soil, soft rock D Stiff soil E Soft soil <180 F Special studies soils March 4, John Anderson: GE/CEE 479/679: Lecture 13

Pancha, Anderson, Biasi, Anooshepor, Louie
Empirical site response and comparison with measured site conditions at ANSS sites in the Reno area Pancha, Anderson, Biasi, Anooshepor, Louie March 4, John Anderson: GE/CEE 479/679: Lecture 13

Results from Pancha’s ReMi studies in the Central Truckee Meadows March 4, John Anderson: GE/CEE 479/679: Lecture 13

Figure 1a This map is horizontally exaggerated- the distance scale may be accurate for the horizontal or vertical direction, but it cannot be accurate for both. Any map should be plotted so the scale works in any direction. March 4, John Anderson: GE/CEE 479/679: Lecture 13

Figure 1b This map is horizontally exaggerated- the distance scale may be accurate for the horizontal or vertical direction, but it cannot be accurate for both. Any map should be plotted so the scale works in any direction. March 4, John Anderson: GE/CEE 479/679: Lecture 13

Alternate Figure 9 March 4, John Anderson: GE/CEE 479/679: Lecture 13

OLD Figure 7 March 4, John Anderson: GE/CEE 479/679: Lecture 13

Earthquake Engineering GE / CEE - 479/679 Topic 13. Wave Propagation 2

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Earthquake Engineering GE / CEE - 479/679 Topic 13. Wave Propagation 2

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