Download Frederick Simons

Geology 5640/6640 15 Feb 2017
Introduction to Seismology
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
Read for Wed 17 Feb: S&W 62-75 (§2.5)
© A.R. Lowry 2017
Types of Seismic Analysis:
Waveform Modeling
Waveform modeling of a D” triplication from the
Ethiopia/Kenya array from a deep Sandwich Island event
Sun D et al. PNAS 2007;104:9151-9155
©2007 by National Academy of Sciences
Types of Seismic Analysis:
Receiver Functions
Jessie Lawrence
• All signals can be
represented as sums
of sines & cosines
• Waveform is a source
wavelet convolved with
Earth structure!
• “Divide” S-waveform
direction sinusoids by
P-waveform direction
sinusoids–this removes
source wavelet and
leaves structure!
Consider a seismogram to be a
signal from the earthquake source,
s(t), that goes through a series of
filters:
Earth e(t)
Instrument i(t)
Each has an impulse response.
The earth’s impulse response is
the Green’s function…
U(t) = s(t)  i(t)  e(t)
(where  denotes convolution)
f (t) Ä g( t) º
¥
ò f (t )g(t - t )dt
-¥
A receiver function tries to
take i(t) and s(t) out of U(t),
leaving us with e(t)
Alan Levander
Ps
PpPs
PsPs
PpSs
(From an
EarthScope
Transportable
Array site in
northern
Nevada)
Negative Velocity Gradients
(faster over slower): called
“lithosphere/asthenosphere
boundary (LAB)”; “midlithosphere discontinuity
(MLD)”…
Interpreted as partial melt,
water, metasomatized layer?
(Levander & Miller, G3, 2012)
(Levander & Miller, G3 2012)
An alternative estimate of crustal thickness… Perhaps just
as interesting is variation in VP/VS ratio of the crust.
Types of Seismic Analysis:
Tomography
Frederick Simons
This looks more complicated than it is…
Frederick Simons
This looks simpler than it is…
X-Ray attenuation tomography
Projections from all angles:
X-ray intensity
Reconstructed image:
X-ray attenuation constants
Frederick Simons
Frederick Simons
Seismic wavespeed tomography
Projections from all angles:
Waveforms and arrival times
Reconstructed image:
Wavespeed variations
Frederick Simons
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?
Frederick Simons
Ritsema
Recent imaging with EarthScope data reveals a relict
strand of Farallon slab still attached to lithosphere
that docked during accretion of Siletzia…
Types of Seismic Analysis:
Anisotropy
West
South
Explanation (for the mantle):
• Elastic anisotropy of olivine
• Alignment of olivine crystals
Central
Pacific
VP anomaly (km s-1)
Seismic wave velocity depends on
direction of:
• Propagation (P- & S-waves)
• Polarization (S-waves)
East
Anisotropy
PROPAGATION DIRECTION
Christensen & Salisbury (1979) JGR
Conrad & Lithgow-Bertelloni (2002) Science