Overview of Seismology and Structure of the...

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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

China Array

http://www.chinaarray.org

(not available to most people)

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.