Thailand Training Program in Seismology and Tsunami Warnings, May 2006

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Theoretical Seismology 1 Sources
Thailand Training Program in Seismology and
Tsunami Warnings, May 2006
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Brief History of Global Seismology in Thailand

• 1960s WWSSN (World-wide Standardized Network
  100 stations)
• CHG
• 1970s SRO (Seismic Research Observatory 1st
  global digital network)
• CHTO
• 1990S GDSN (Global Digital Seismograph Network)
• 2000s Disaster Warning Center

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What is the cause of earth movement?

• Some earth movements are associated with magma
• Or with mine bursts and explosions
• Most shaking is caused by failure of rocks in
  the earth

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Theoretical Seismology 1 Sources
Describe Earth Rupture Elastic Rebound
Fault Geometry Double-couple Force Seismic
Moment Tensor Models of Earthquake
Rupture Rectangular rupture Circular rupture
Distributed slip models Earthquake Size
Magnitudes Seismic Moment Energy
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Concepts and Terminology
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San Francisco Earthquake April 18, 1906 Mw
7.7-7.9 470 km rupture of San Andreas fault

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Elastic Rebound Theory Reid (1910)
(Data in 1851-65, 1874-92, 1906)
Asperity
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Elastic Rebound Loading or deformation cycle

• Four phases
• Interseismic
• Preseismic
• Coseismic
• Postseismic

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Breaking of Brittle Rock

• Build-up of stress (strain energy)
• Rupture at weakest point
• Break along a plane of weakness
• Radiation of seismic waves

(In contrast to ductile rock, which fails by
creep.)
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What does a critical amount of applied stress do
to a rock?
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What does a critical amount of applied stress do
to a rock?
smax smin sint
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Types of faults
Normal fault
Dip Slip
Thrust (Reverse) fault
Oblique-slip fault
Strike, dip, slip
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Strike-Slip Faults
Left-lateral
Right-lateral
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Equivalent Body Forces

Single Force
Dipole
Couple (Single Couple)
Double Couple
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Single-force earthquakes volcanic eruptions and
landslides
Mount St. Helens, USA
Kanamori et al. 1984
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Equivalent Body Forces

Single Force
Dipole
Couple (Single Couple)
Double Couple
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1940 Imperial Valley, California (Ms 7.1)
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P-wave first motions
Auxiliary plane
Fault plane
This type of faulting is more likely to produce
large tsunamis
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Single Couple versus Double Couple
Single Couple
Double Couple

• P polarity pattern same
• S polarity pattern different
• Single Couple resembles fault slip

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Moment tensor dipoles and couples
u(t)i S Gij(t) mj
9 components Symmetric matrix so 6 independent
(LW p.343 AR p.50)
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Moment Tensor for an Explosion


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Moment Tensor for Fault Slip

North
?
Double Couple Fault - Slip
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NEIC fault plane and moment tensor solutions
05 05 18.4 0.587 N 98.459 E 34 G 6.4 6.8 A 1.0
20 695 NIAS REGION, INDONESIA. MW 6.7 (GS), 6.7
(HRV). ME 6.6 (GS). Felt (V) at Padang and
Sibolga (III) at Palembang and Pekanbaru,
Sumatra. Felt (III) in Malaysia. Felt on Nias and
in Singapore. Broadband Source Parameters (GS)
Dep 34 km Fault plane solution NP1 Strike155,
Dip75, Slip90 NP2 Strike335, Dip15,
Slip90 Rupture duration 7.0 sec Radiated
energy 1.61014 Nm. Complex earthquake. A small
event is followed by a larger event about 2
seconds later. Depth based on larger event.
Moment Tensor (GS) Dep 38 km Principal axes
(scale 1019 Nm) (T) Val1.57, Plg65, Azm39
(N) Val-0.02, Plg14, Azm162 (P) Val-1.55,
Plg20, Azm257 Best double couple
Mo1.61019 Nm NP1 Strike10, Dip28,
Slip121 NP2 Strike156, Dip66, Slip74.
Centroid, Moment Tensor (HRV) Centroid origin
time 050524.6 Lat 0.42 N Lon 98.24 E Dep
39.0 km Bdy Half-duration 5.6 sec Principal
axes (scale 1019 Nm) (T) Val1.49, Plg66,
Azm61 (N) Val0.06, Plg1, Azm329 (P)
Val-1.55, Plg24, Azm238 Best double couple
Mo1.51019 Nm NP1 Strike326, Dip22,
Slip88 NP2 Strike149, Dip69, Slip91.
Scalar Moment (PPT) Mo1.31019 Nm.
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Kinematics
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Haskell Line Source

Haskell, 1964 Specifies Fault length L Fault
width W Rupture velocity v Permanent slip D Rise
time T
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Circular Crack Sato and Hirasawa, 1973

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Haskell Line Source

Dislocation Source
Haskell, 1964
sumatra
Sumatra earthquake Ishii et al., 2005
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Complicated Slip Distributions -


1999 Chi-Chi, Taiwan Earthquake
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What is magnitude? Why do we need it?

• Magnitude is a number that represents earthquake
  size no matter where you are located.
• It should be related to released seismic energy.
• It should handle the smallest earthquake to the
  largest earthquake.

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January 26, 2001 Gujarat, India Earthquake (Mw7.7)
Body waves
vertical
Rayleigh Waves
P PP S SS
radial
transverse
Love Waves
Recorded in Japan at a distance of 57o (6300 km)
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Earthquake Size Magnitude
Charles Richter 1900-1985
log of amplitude
Distance correction
M log A log A0
Richter, 1958
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Types of Magnitude Scales
Period Range
ML Local magnitude (California)
regional S and 0.1-1 sec
surface
waves Mj JMA (Japan Meteorol. Agency)
regional S and 5-10 sec
surface
waves mb Body wave magnitude
short-period P waves 1 sec Ms Surface
wave magnitude long-period surface
20 sec
waves Mw Moment
magnitude very long-period
gt 145 sec surface waves Me Energy
magnitude broadband P waves 0.5-20
sec Mwp P-wave moment magnitude
long-period P waves 10-60 sec

Mm Mantle
magnitude very-long period
gt 200 sec

surface waves
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Why are there different magnitudes?

• Distance range
• ML (local, Wood Anderson, 0.8 s)
• Teleseisms (recorded at long distances)
• mB (uses Amax /T, but in practice T is
  short-period)
• MS (uses Amax /T, but in practice T is
  long-period)
• Depth
• MS not useful
• mb still works, as well as Me and Mw
• Physical significance
• More recent magnitudes (Mw and Me) are related
  to different aspects of earthquake size.

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What are the limits of historic magnitudes(ML
,mb, and Ms)?

• Quick and simple measurements
• Usually from band-limited data.
• single frequency may not all frequencies
• Saturation
• single measurement may not represent large
  rupture
• ML and mb 6.5 MS 8.5
• Empirical formulas
• Physical significance not certain
• e.g., from Gutenberg-Richter,
• log ES 11.8 1.5 MS

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More Recent Magnitude Scales
Mw Moment magnitude very
long-period surface waves gt 145 sec Me
Energy magnitude broadband P waves
0.5-20 sec Mwp P-wave moment magnitude
long-period P waves 10-60 sec

Mm Mantle
magnitude very-long period
surface waves gt 200 sec


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MW is derived from - Seismic Moment Mw 2/3 log
M0 - 6.0
Area (A)
Slip (S)
Seismic Moment (Rigidity)(Area)(Slip)
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Seismic moments and fault areas of some famous
earthquakes
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Mw compared to Me
Different magnitudes are required to describe
moment and energy because they describe different
characteristics of the earthquake.
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These two earthquakes in Chile had the same Mw
but different Me
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  Earthquakes with the same Mw can have
different macroseismic effects. For the
Central Chile earthquakes
Earthquake 1 6 July 1997 30.0 S 71. W Me
6.1, Mw 6.9 Felt (III) at Coquimbo, La Serena,
Ovalle and Vicuna. Earthquake 2 15 October 1997
30.9 S 71.2 W Me 7.6 Mw 7.1 Five people
killed at Pueblo Nuevo, one person killed at
Coquimbo, one person killed at La Chimba and
another died of a heart attack at Punitaqui. More
than 300 people injured, 5,000 houses destroyed,
5,700 houses severely damaged, another 10,000
houses slightly damaged, numerous power and
telephone outages, landslides and rockslides in
the epicentral region. Some damage (VII) at La
Serena and (VI) at Ovalle. Felt (VI) at Alto del
Carmen and Illapel (V) at Copiapo, Huasco, San
Antonio, Santiago and Vallenar (IV) at Caldera,
Chanaral, Rancagua and Tierra Amarilla (III) at
Talca (II) at Concepcion and Taltal. Felt as
far south as Valdivia. Felt (V) in Mendoza and
San Juan Provinces, Argentina. Felt in Buenos
Aires, Catamarca, Cordoba, Distrito Federal and
La Rioja Provinces, Argentina. Also felt in parts
of Bolivia and Peru.
 
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Mm Mantle Magnitude
Source Correction
Mm log10(X(w)) Cd Cs 3.9
Distance Correction
Spectral Amplitude
amplitude measured in frequency domain
surface waves with periods gt 200 sec
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Magnitudes for Tsunami Warnings
Want to know the moment (fault area and size)
but takes a long time (hours) to collect
surface wave or free oscillation data
Magnitude from P waves (mb) is fast but
underestimates moment ? If have time
(hours), determine Mm from mantle
waves ? For quick magnitude (seconds to
minutes), determine Mwp from P
waves
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Mwp P-wave moment magnitude
?uz(t)dt ? Mo
Quick magnitude from P wave Uses relatively
long-period body waves (10-60 sec) Some
problems for Mgt8.0
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Magnitudes for the Sumatra Earthquake
mb 7.0 1 sec P wave
131 stations Mwp 8.0 8.5 60 sec P
waves Me 8.5
broadband P waves Ms 8.5 - 8.8 20 sec
surface waves 118 stations Mw 8.9 -
9.0 300 sec surface waves Mw
9.1 - 9.3 3000 sec free oscillations

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Things to Remember
1. Earthquake sources are a double couple force
system which is equivalent to Fault Slip 2.
The moment tensor describes the Force System
for earthquakes and can be used to determine
the geometry of the faulting 3. Earthquake
ruptures begin from a point (hypocenter) and
spread out over the fault plane 4. The size of
an earthquake can be described by different
magnitudes, by moment, and by energy. 5. Quick
determination of magnitude is needed for
tsunami warning systems.
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Relationship between different types of magnitudes
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15 km
M4
M5

M6
10
5
0
M4 M5 M6
Seismicity in NEIC catalog 1990 - 2005
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Log E 1.5Ms 4,8
Log E 1.5 Me 4.4