1. Introduction to Seismology
Geology 5640/6640
Introduction to Seismology
13 Feb 2015
Last time: Seismology as Investigative Tool
Deep-Earth investigations use earthquakes as sources,
three-component broadband seismometers as receivers,
and analysis tools include:
• Normal modes • Precursors
• Waveforms • Receiver Functions
• Tomography • Anisotropy
• Ambient Noise
Normal modes give radial velocity/density structure (and
perturbations can image laterally)
Precursors are early arrivals with different reflection points
Waveforms use “all” of the information content in waves
Receiver functions difference the arrival times of
transmitted and converted phase arrivals
© A.R. Lowry 2015

2. Negative Velocity Gradients (faster over slower): called
“lithosphere/asthenosphere
boundary (LAB)”; “mid-
lithosphere discontinuity
(MLD)”…
Interpreted as partial melt,
water, metasomatized layer?
(Levander & Miller, G3, 2012)

3. (Levander & Miller, G3 2012)

4. An alternative estimate of crustal thickness… Perhaps just
as interesting is variation in VP/VS ratio of the crust.

5. Types of Seismic Analysis:
Tomography

6. This looks more complicated than it is; and that’s my point.
Frederick Simons

7. This looks simpler than it is; and that’s my point.
Frederick Simons

8. X-Ray attenuation tomography
Projections from all angles:
X-ray intensity
Reconstructed image:
X-ray attenuation constants
Frederick Simons

9. Frederick Simons

10. Seismic wavespeed tomography
Projections from all angles:
Waveforms and arrival times
Reconstructed image:
Wavespeed variations
Frederick Simons

11. Seismic tomography Near surface Old oceans fast Young oceans slow
Cratons (old parts of continents) fast
Core-mantle boundary
Past subduction fast
African and Pacific anomalies slow
Cause?
Ritsema
Frederick Simons 11

12. Recent imaging with EarthScope data reveals a relict
strand of Farallon slab still attached to lithosphere
that docked during accretion of Siletzia…

13. Types of Seismic Analysis:
Anisotropy

14. Anisotropy Central Pacific West East Seismic wave velocity depends on
direction of:
• Propagation (P- & S-waves)
• Polarization (S-waves)
Explanation (for the mantle):
Elastic anisotropy of olivine
Alignment of olivine crystals
VP anomaly (km s-1)
South
PROPAGATION DIRECTION
Christensen & Salisbury (1979) JGR
Conrad & Lithgow-Bertelloni (2002) Science 14

15. Polarization anisotropy aka shear-wave splitting
Upper mantle xenolith
Calcite: CaCO3 15

16. Angle between backazimuth and seismic fast axis
Shear-wave splitting
If backazimuth coincides with a seismic symmetry axis, the shear wave is not split (Null case)
Angle between backazimuth and seismic fast axis
Delay time
Anisotropic layer
Incoming SKS phase
Invert the splitting by grid-searching for combination of fast axis and delay time which best removes the splitting 16

17. Shear-wave splitting Remove splitting:
Radial
Transverse
1. Minimum Energy on Transverse: Remove transverse Energy
2. Rotation-Correlation: Searching for maximum correlation
3. Eigenvalue criteria: Searching for most linear particle motion 17

18. Polarization anisotropy
Most of shallow Pacific mantle:
Horizontally polarized shear wave faster than vertically polarized shear wave
VSH>VSV
If origin of anisotropy is related to plate motion, might expect VSH-VSV to increase systematically westward. It doesn’t!
Ekstrom and Dziewonski (1999) Nature 18

19. These images actually used ambient noise tomo:
Vertical
Horizontal
Moschetti, Ritzwoller et al. in Nature
Lin, Ritzwoller et al., Nature Geoscience

20. Types of Seismic Analysis:
Ambient Noise

21. Ambient Noise (= freedom from the “tyranny of earthquakes”…)
Seismic Interferometry
uses the cross-correlation of
signals at two sites to create
a “virtual source-receiver
pair”… Averaging over time
periods of weeks to months.
Shapiro et al., Science 2005

22. Lin
et al.,
Geophys.
J. Int.,
2008

23. Noisin’ around Yellowstone!
Derek Schutt