1. Introduction to Seismology
Geology 5640/6640
Introduction to Seismology
17 Feb 2017
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
Waveforms use “all” of the information content in waves
but are computationally intensive!
Receiver functions difference the arrival times of
transmitted and converted phase arrivals
Tomography uses travel-times for large numbers of
cris-crossing rays to image velocity structure
© A.R. Lowry 2017

2. Assignment I is now posted on the course
website…
Due Monday, Feb 27 at the beginning of class
(submit to
Note: Not all of the relationships you’ll need have been
covered in course notes, but if you’ve been reading
your text you’ll know where to look!

3. Types of Seismic Analysis:
Anisotropy

4. 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 4

5. Polarization anisotropy aka shear-wave splitting
Upper mantle xenolith
Calcite: CaCO3 5

6. Angle between back-azimuth and seismic fast axis
Shear-wave splitting
If back-azimuth coincides with a seismic symmetry axis, the shear wave is not split (Null case)
Angle between back-azimuth 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 6

7. 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 7

8. Polarization anisotropy Most of shallow Pacific mantle:
Horizontally polarized shear wave is
faster than the vertically polarized
shear wave
VSH > VSV
If the origin of anisotropy is related to
plate motion, might expect VSH – VSV
to increase systematically westward.
It doesn’t!
Ekstrom and Dziewonski (1999) Nature 8

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

10. Example of deep Earth seismology: Earth has an “inner inner core”…
Wang et al. (2015) Nature Geoscience
Expressed as
“east-west”
anisotropy in the
innermost core,
distinct from
north-south directed
outermost inner
core.

11. Types of Seismic Analysis:
Ambient Noise

12. 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

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

14. Noisin’ around Yellowstone!
Derek Schutt