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地球科学前沿讲座 Advancing Forum in Geosciences
Overview of Seismology
and Structure of the Earth
Huajian Yao 姚华建
USTC
Earth’s Differentiation
Distribution of Continents and Oceans
Earth is very heterogeneous!
板块分布及其相对运动
Plate Tectonics and Mantle Convection
Time scale of mantle convection: on the order of 100 Ma
分层地幔对流与全地幔对流的争论
地幔对流的模式将影响地球内部物质的交换混合以及地球内部热量的交换
Seismology provides important constraints
on understanding the dynamics of the Earth
Seismic wave is currently the only effective tool that
can penetrate the entire earth Structural
information of the Earth
Fro
m IR
IS
Historical perspective of seismology
First seismic instrument: the
Chinese Seismoscope Invented
132 AD by Zhang Heng (detected a
four-hundred-mile distant earthquake )
Sir Isaac Newton (1642-1727): 3
laws of motion; Law of Gravity (F ~
m1m2/r2); Invented Calculus
1678: Hooke's Law (“stress is
proportional to strain”)
Isaac Newton
1760 – Mitchell Recognition that ground motion due to earthquakes
is related to wave propagation
1821 – Navier Equation of continuum motion (French)
1828 – Poisson Wave equation → P & S-waves
1868 – Mallet earthquake catalogue (most complete up to date)
1885 – Rayleigh Theory of surface waves predict Rayleigh waves
1889 – Potsdam, Germany first recording of distant earthquake
Earthquake Locations
Mallet, 1868 First recording of a distant
earthquake (April 18, 1889)
the recording of the 1989
earthquake in Japan
1892 – Milne First high-quality seismograph → begin of observational
period
1897 – Wiechert Prediction of existence of dense core (based on
mean density, surface density, and meteorites → Fe-alloy)
1900 – Oldham identified P waves, S waves and Rayleigh waves
from records of the 1897 Assam earthquake (Mw 8.1) in India
1906 – Oldham Confirmed the existence of core by analyzing seismic
arrival times of various recorded earthquakes
1906 – Galitzin First feed-back broadband seismograph
1909 – Mohorovičić Crust-mantle boundary (Moho) from analysis of
data at relatively short distance from earthquake
1911 – Love predict Love waves (surface waves)
1914 – Gutenberg Depth to core-mantle boundary : 2900 km
1922 – Turner location of deep earthquakes down to 600 km (but
located some at 2000 km and some in the air...)
1928 – Wadati Accurate location of deep earthquakes → Wadatai-
Benioff zones
1936 – Lehman Discovery of the solid inner core
1939 – Jeffreys & Bullen First travel-time tables：Jeffreys-Bullen
Seismological Tables → 1D Earth model
1948 – Bullen Density profile
1966 – Aki defined seismic moment: Mo = Area * slip * rigidity
1977 – Kanamori established the moment magnitude scale (Mw)
1977 – Aki First 3D regional lithospheric velocity models
1977 – Dziewonski First 3D global mantle velocity models
1996 – Song & Richards Inner core super-rotation
Jeffreys-Bullen (JB) Earth Model (1939)
Classical Research Objectives in Seismology
Source Studies Structure Studies
After Lay & Wallace, 1995
地震学研究的发展
计算能力和资源的提升
科学或工程问题的提出 理论和技术方法的发展
观测台网和数据的丰富
• 1961 WWSSN (Worldwide Standardized Seismograph
Network) — (analog records, nuclear test monitoring)
• 1964 ISC (International Seismological Centre) — travel times and
earthquake locations (http://www.isc.ac.uk)
• 1969-72 Apollo astronauts place a seismometer on the Moon, and
the first "moonquakes" are registered.
• 1978 GDSN (Global Digital Seismograph Network) — (digital
records) (http://www.iris.edu/hq/programs/gsn)
• 1980 IRIS (Incorporated Research Institutes for
Seismology)(http://www.iris.edu/hq/)
• 2000 Japanese Hi-net (High-Sensitivity Seismograph Network,
~700 bore hole strong motion seismometers)
(http://www.hinet.bosai.go.jp)
• 2004 – 2014 USArray (part of EarthScope project in USA, ~400
stations in the field) (http://www.usarray.org/)
• 2012 – now ChinaArray (China Earthquake Administration)
Seismograph Networks
150 + globally distributed stations : 3-D global structure of the Earth / Earthquake locations, source mechanisms / Nuclear Monitoring
Earthquake location, early warning, regional structure, global seismological studies (array methods)
USArray TA Sites
EarthScope
400 portable 3 component broadband instruments on a regular grid (~70 km)
400 portable 3 component short-period and broadband seismographs and 2000 single channel high-frequency recorders
Permanent array of broadband 3 component stations across the country as part of the USGS Advanced National Seismic System
Seismological Observations in China
National Stations: 148 broadband stations
(http://www.ceic.ac.cn)
Provincial stations (a few hundred)
North China Array
Western Sichuan Array
Tibetan Plateau arrays North China Craton Array
Recent seismic array observations in China
Source Studies Structure Studies
Earthquake rupture (simulation,
observation), earthquake early
warning, slow earthquakes &
non-volcanic tremor, fault zone
physical properties …
After Lay & Wallace, 1995
Research topics of modern seismology
4-D seismology, structural changes
and its relationship to Earth’s dynamic
processes, multi-scale resolution of
Earth’s structure, joint inversion of
multiple dataset for structure …
Seismic waves in the Earth
Lecture Notes from MIT OCW
Spheroidal modes
Seismic waves in the Earth
Shearer, 2009
Body waves (typically higher
frequencies, a few Hz to
several tens of seconds
period) can penetrate into
the deep interior of the Earth
velocity structure (Vp &
Vs, attenuation) of the entire
earth and interface
information
Spheroidal modes
P wave (compressional wave)
S wave (shear wave)
Crust average: Vp ~ 6.3 km/s，Vs ~ 3.6 km/s
Shearer, 2009
Travel time table from ak135 model Travel time picks
Seismic waves in the Earth
Earthquake surface waves (typically
T > 10 s) travel along the Earth’s
surface Vs structure in the crust
and upper mantle
Decreasing wave amplitudes
as depth increases
Shearer, 2009
Spheroidal modes
Love wave
Rayleigh wave
Surface wave dispersion
Seismic waves in the Earth
Earth’s normal modes:
discrete resonant (eigen)
frequencies of the Earth
global /average structure
of the Earth, also sensitive
to density
T = 1277.5 s T = 3223 s
T = 2636 s T = 756.6 s
Spheroidal modes
Spheroidal modes
(550 observations)
From notes of G. Masters
Seismic waves in the Earth
Waveforms (amplitude and phase) give information on Earth’s elastic and
anelastic structure as well as density.
Seismological techniques used to study Earth’s structure:
Seismic tomography usually 2-D or 3-D smooth velocity structure
body and surface wave travel-time tomography
tomography based on waveform inversion
Seismic imaging usually 1-D, 2-D or 3-D sharp interfaces
receiver functions (stacking of converted phases)
migration (more sophisticated waveform stacking)
Waveform modeling usually 1-D or 2-D velocity models
compare observed and synthetic waveforms for given models
Questions?
1-D
Preliminary
reference
Earth model
(PREM)
Dziewonski and Anderson (1981)
Major Layers in the Earth
Transition zone
Lower mantle
Major interfaces in the Earth
Transition zone
Lower mantle
Moho (chem)
LAB (phys)
660 km (phys)
410 km (phys)
CMB (chem & phys)
ICB (phys)
LAB: lithosphere-
asthenosphere boundary
CMB: core-mantle
boundary
ICB: inner core/outer
core boundary
Earth’s Crust probably most complicated part of the Earth
Thickness: several km (oceanic) to ~ 80 km (continental)
Crust 2.0 model: http://igppweb.ucsd.edu/~gabi/crust2.html
top of layer(km) VP(km/s) VS(km/s) RHO(g/cm^3)
0.23 1.50 0.00 1.02
-2.39 3.81 1.94 0.92
-2.45 2.10 0.98 1.92
-2.97 3.73 1.96 2.37
-3.43 5.46 2.92 2.66
-9.45 6.57 3.66 2.89
-15.74 7.10 3.92 3.05
-21.59 8.15 4.66 3.36
water
ice
soft sediments
hard sediments
upper crust
middle crust
lower crust
upper mantle
global average of crust 2.0
1-D PREM global average
top of layer(km) VP(km/s) VS(km/s) RHO(g/cm^3)
0.0 1.45 0.00 1.02
-3.0 5.80 3.20 2.60
-15.0 6.80 3.90 2.90
-24.4 8.11 4.49 3.38
Water
upper crust
lower crust
upper mantle
Seismic Techniques for Determining the Structure of
the Crust (and Uppermost Mantle)
1.Active-Source Data (seismic refraction/wide-angle
reflection profiles, seismic reflection profiles)
1.Passive-Source Data (surface waves, ambient noise,
body waves, receiver functions)
1.Laboratory Studies (velocity-density relationships, Vp-
Vs and poisson’s ratio
Nonseismic Techniques:
gravity anomalies, geoelectrical data, borehole data, etc
Modeling results along the seismic refraction/wide-angle
reflection profile at the northern margin of the Tibetan
Plateau.
After Zhao JM et al.
(2006)
Example of deep seismic reflection imaging of crustal
and upper-mantle fault zones. (after Matthews DH and
BIRPS Core Group, 1990) (The Moho is labeled at a two-way-
time of 10 s (30km depth))
Vp, Vs, and Poisson’s Ratio
Body wave traveltime tomography
Wang et al., 2009, EPSL
Crustal Structure from earthquake surface
waves and ambient noise tomography in SW
China (Yao et al., 2008, GJI)
Crustal structure from Receiver Function (Wang et al. EPSL, 2009)
Crustal
LVZ
High Vp/Vs
Vs as a function of Vp for common lithologies (lab data)
Brocher, 2005 (BSSA)
(Vp: 1.5 – 8 km/s)
Poisson’s ratio as a function of Vp for common lithologies
Brocher, 2005
(excludes mafic and
calcium-rich rocks)
(Mooney et al. 2009)
Basement age of the continental crust,
distribution of mid-ocean ridges, oceanic
crust, and continental shelf.
Fourteen primary continental and oceanic crustal types (Mooney et al. 1998).
Vp (km/s)
(Mooney et al. 2009)
Histograms of crustal thickness for six continental tectonic provinces
calculated from the individual point measurements
Earth’s Mantle upper mantle + transition zone + lower mantle
(lithosphere + asthenosphere)
Transition zone
Lower mantle
Mantle structure
is important for
understanding
plate tectonics
and mantle
convection
Global upper
mantle structure
(Vs) from surface
wave tomography
Debayle et al. (2005).
Slow Vs: mid-ocean ridges
Fast Vs: continental craton
(lateral resolution: a few hundred km)
Global mantle structure from
body wave tomography
There are a lot of global velocity models from body wave
tomography (sometimes also joint with surface waves) (see
comparisons:
http://www.igppweb.ucsd.edu/~gabi/rem2.dir/shear-models.html)
Global body wave tomography usually does not resolve the top
100 km structure due to near vertical incidence of teleseismic
waves to stations at the surface. (vertical smearing …)
Advances in theoretical and computational seismology have
improved our understanding of mantle structure:
Ray theory finite frequency kernel (1-D reference model, fast
but not accurate) finite frequency kernel (3-D model, SEM,
slow, iterative, hot research area)
Vs maps at
depths 140,
925 and 2770
km for the
Harvard,
Scripps,
Berkeley,
Grand1997
model
http://www.igppweb.ucsd.edu/~gabi/rem
2.dir/shear-models.html
slices through 4
models that
show the
Farallon slab
http://www.igppweb.ucsd.edu/~gabi/rem
2.dir/shear-models.html
Slices along
the East
African
Rift/African
Superplume
http://www.igppweb.ucsd.edu/~gabi/rem
2.dir/shear-models.html
Quantitative
Comparisons
RMS Amplitude as
Function of Depth
RMS amplitudes are
highest in the upper
mantle, near the top, and
at the bottom of the
mantle. Differences
between models exist
near the transition zone
(see also above). The
1997 Grand model, that is
shown in the plot, exhibits
significantly lower
amplitudes than the other
models. http://www.igppweb.ucsd.edu/~gabi/rem
2.dir/shear-models.html
Amplitude as
Function of
Wavelength Structural variations
are largest near the
top and the bottom of
the mantle. The
spectrum of structural
variations tends to be
'red' meaning that
most power lies in
longer wavelengths. In
some models,
harmonic degree two
appears to be
dominant throughout
the mid-mantle.
http://www.igppweb.ucsd.edu/~gabi/re
m2.dir/shear-models.html
Global P wave tomography (MIT-Vp 2008)
Adpative cells
crust correction,
Li,van der Hilst et al., 2008
Examples of
regional structure
from global body
wave tomography
Li et al., 2008
Station Distribution
Adapative regularization allows for high
resolution in areas with dense station or
ray path coverage
Vertical profiles
(MIT-Vp 2008)
Slabs subducted to
the lower mantle:
evidence for deep
mantle circulation
Li et al., 2008
Vertical profiles
(MIT-Vp 2008)
Slab flattening at the
transition zone in
eastern Asia:
layered circulation?
Li et al., 2008
Finite frequency traveltime tomography: example (Montelli et al., 2004, Science)
(GJI, 2010)
Banana-doughnut kernel
See a lot more plumes (?)
Questions?
Interfaces in the mantle
Transition zone
Lower mantle
LAB (phys)
660 km (phys)
410 km (phys)
CMB (chem & phys)
D” (~ 200 km above CMB)
LAB: lithosphere-asthenosphere boundary
Lithosphere. Above the asthenosphere, the temperature begins to drop more rapidly. This creates a layer of cool, rigid rock called the lithosphere. The lithosphere includes the uppermost part of the mantle and it also includes all of the crust. That is, the crust is the upper part of the lithosphere, and the upper mantle is the lower part of the lithosphere.
Asthenosphere. Nearer to the surface of the earth the temperature is still relatively high but the pressure is greatly reduced. This creates a situation where the mantle is partially melted. The asthenosphere is a plastic solid in that it flows over time.
LAB: receiver functions (converted phases) P-to-S (Ps) or S-to-P (Sp) converted phases at the interface
P wave
S wave
Surface
Station
Pp Ps PpSs
PsPs
PpPs P
Ps
Station
Surface
Interface
Sp from Moho
Sp from LAB
t = 0
Multiples
from Moho
Courtesy of Ling Chen
LAB: wave receiver functions (converted phases)
P-to-S (Ps) or S-to-P (Sp) converted phases at the interface
Rychert & Shearer, 2009
Ps RF
Global map of LAB depth
Rychert & Shearer, 2009
Rychert & Shearer, 2009
Comparison of
Ps-inferred average
properties of the LAB
and Voigt averaged S-
wave velocity profiles
from the surface-wave
results of for different
tectonic regions.
LAB: Tibetan Plateau
(Zhao et al. 2010, PNAS)
410, 520, and 660
km discontinuities
(transition zone)
phase transition of olivine:
closer packing of atoms into
denser structure: olivine
(alpha) wadsleyite (beta)
ringwoodite (gamma)
perovskite (pv) +
magnesiowustite (mw)
Transition zone effects on
subduction and upwelling C
lap
eyro
n s
lop
e
Global topography of 410 &
660 discontinuities and
transition zone thickness
(from stacks of SS precursors)
(thicker in subduction zone
regions and thinner in regions
with hot upwelling)
Flanagan & Shearer,1998
D” discontinuity in the lower mantle
Variation of seismic
velocities and density
through the lower
mantle formodel
PREM, The D” region
is the lowermost 200–
300 km of the lower
mantle overlying the
core–mantle boundary
(CMB)
After Lay T (1989)
Radial profiles of S-
wave velocity in the
lowermost mantle
D”: phase transition
from perovskite to
post-perovskite at ~
120 GPa
ULVZ
D”
spatial variations of
seismic wave
characteristics of D”.
CMB and ICB
CM
B
ICB
homogeneous
due to vigorous
mixing of the liquid
iron/nickel
Inner core structure
The inner core: higher
velocities and higher
attenuation as they
propagate along the polar
paths (preferential
alignment of anisotropic
Fe crystals).
The eastern hemisphere
exhibits a higher velocity,
higher attenuation, weaker
anisotropy, and a larger
transitional depth from top
isotropy to deep
anisotropy.
Cartoon illustration of hemispheric differences in
seismic velocity, attenuation, anisotropy, and
depth extent of anisotropy in the Earth’s inner core.
Lianxing Wen,2011
Innermost core
Inner core super rotation
X. Song and P. Richards, 1996, Nature;
Zhang et al., 2005, Science Glatzmaier, geodynamo
approximately 0.3-0.5 degrees
faster than the rest of the Earth
predicted by models that explain
Earth’s inherent magnetism
(Glatzmaier and Roberts, 1996)
Questions?
Acknowledgements
I acknowledge the authors who provide some of the
course materials (pictures and sentences), which I
obtained from internet, papers, or books but without
proper references.
