Seismotectonics

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Seismotectonics
Dr. Perveiz Khalid
M.Sc. (Q.A.U), M.S. (France), Ph.D. (France)

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Aim
GEOL 541 Dr. Perveiz Khalid
To understand:
 basic concepts of seismotectonics
Seismotectonics zonation
 the role of seismotectonics in earthquake
hazard analysis

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Outline
 Introduction
 Concept of plate tectonics
 Seismicity and plate boundaries
 Seismotectonic maps
 Seismotectonic Zonation
 Seismotectonic zones of Pakistan

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Introduction
 Seismic waves provides the information about active, present
day existing processes in the subsurface.
 Analysis (qualitative & quantitative) of earthquake faulting
characteristics such as
• Fault orientation,
• Sense of slip,
• Net cumulative displacement along the fault plane
played the major role in evolution of the theory of plate tectonic.

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What is Seismotectonics?
 The spatial distribution of earthquake may be
used to determine the location of plate boundaries,
faults, direction of relative motion between the plates,.
The rate of earthquake occurrence in some
particular area, and the cumulative displacement of
earthquake occurrence can be bused to infer the
relative velocity between the plates.

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What is Seismotectonics?
 The study of the relationship between the
earthquakes, active tectonics and individual faults of a
region.
OR
The study of active faulting, and its relationship to
plate motion and lithospheric properties.

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Significance
To understand which faults (seismic source) are responsible for seismic activity
in an area by analyzing a combination of
regional tectonics
 recent instrumentally recorded events
 accounts of historical earthquakes
 geophysical data
 This information can then be used to quantify the seismic hazard of an area.

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Concept of Plate Tectonics (1)
 Earthquakes are closely related to tectonic activities, therefore, it is
necessary to understand basic concept of plate tectonics.
 Mobile lithosphere concept

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Concept of Plate Tectonics (2)
 Lithosphere: Highly viscous region, movement involve shearing
motion at the boundaries and produced shear-faulting
earthquakes.
 Asthenosphere: relatively less viscous
 Mesosphere: relatively viscous

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Seismicity and plate boundaries (3)
 Release of strain energy by seismic events is restricted to inhomogeneous stress
regions. i.e., plate boundaries
Plates
A. Pacific
B. North
America
C. Cocos
D. Nazca
E. South
America
F. Caribbean
G. Antarctic
H. Indian
I. Philippine
J. African
K. Eurasian
L. Arabian
Boundaries
1. Aleutian SZ
2. San Andreas Fault
3. Central American
SZ
4. Antilles Sz
5. South American SZ
6. Scotia SZ
7. East Pacific Rise
8. Tonga-Kermadec
SZ
9. New Hebrides SZ
10. Mariana SZ
11. Japan SZ
12. Indonesian SZ
13. Hindu Kush
14. Mid Atlantic Rise
A
K
J
I
H
G
F
ED
C
B
L
1
9
8
7
6
5
4
1432
13
12
11
10

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 Seismotectonic zones are concentrated near to the plate boundaries.
 Divergent boundaries: Two plates are moving apart and new lithosphere is
produced (or old lithosphere is thinned). E.g., midoceanic ridges, continental rifts
 Convergent boundaries: Litjosphere is thickened or consumed by sinking into
the mantle. E.g., subduction zones, alpine belts
 Transcurrent boundaries: plates move past one another without either
convergence or divergence. E.g., transform faults, strike-slip faults

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Divergent and transcurrent plate boundaries are
characterized by shallow seismicity (focal depth < 30 km).

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Seismotectonic Maps
 Show:
 geology (structure)
 geomorphology
 water features
 faults (orientation, geometry, fault plane etc.)
 lineaments
 shear zone
past earthquake events

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Main elements of map
 Seismicity: A careful study and compilation of data will be devoted to the
historical and instrumental seismicity. Regional and local maps may include
earthquake magnitude as low as M 1 but the final seismotectonic map will have to
consider magnitudes large than M 4.
 Active tectonics and faulting:
Plate movement sketch (with kinematic indicators).
Plate sketch map with principal stress direction.
Explanatory notes (including a more comprehensive description of the
oceanic.

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Seismotectonic Zonation
 A seismotectonic zone is considered to be a geographic region of some
geological, geophysical and seismological similarity with a uniform earthquake
potential.
 In a particular seismotectonic zone, the seismicity (earthquakes etc.) is
assumed uniform & homogeneous throughout the zone.
 Seismotectonics zonation provides the basis for Seismic Hazard Assessment
(SHA) of any area.

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Seismotectonic Zonation
Can a tectonic zone consider as Seismotectonic zone??

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Preparation of seismotectonic zones
 Regional tectonics
 Integrated data sets
 Geomorphology
 Earthquake records
 Scale
 Limitations

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Regional tectonics
An understanding of the regional tectonics of an area is
likely to be derived from
 published geological maps
 research publications on the geological structure and
 seismic reflection data (2D, 3D)

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Geomorphology
Seismically active faults and related fault generated folds have a direct
effect on the geomorphology of a region.
This may allow the direct identification of active structures not
previously known.
In some cases such observations can be used quantitatively to constrain
the repeat period of major earthquakes.

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Integrated data sets
Seismotectonic zonation of an area requires the integration of a large number
of disparate datasets:
1. Satellite images (remote sensing)
2. Digital elevation models
3. Geodic maps (conventional geodesy, GPS data)
4. Earthquake/ seismicity data
 Instrumentally recorded events
 Historical records
 Field investigations
5. Geophysical data (gravity, magnetic, seismic profiles and tomography).

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Earthquake: Instrumental records
 Sufficient information has been available from seismometers to allow the
location,
depth and
magnitude
of earthquakes to be calculated.
 In terms of identifying the fault responsible for an earthquake where
there is no clear surface trace, recording the locations of aftershocks
generally gives a strong indication of the strike of the fault.

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Earthquake: Historical records
 To understand the long-term seismicity of an area, information from
earthquakes before the era of instrumental recording are required.
 Careful assessment of historical data in terms of their reliability is
mandatory.
 In most cases, all that can be derived is an estimate of the location and
magnitude of the event. However, such data is needed to fill the gaps in the
instrumental record, particularly in areas with either relatively low seismicity or
where the repeat periods for major earthquakes is more than a hundred years.

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Earthquake: Field investigations
 Information on the timing and magnitude of seismic events that occurred
before instrumental recording can be obtained from excavations across faults
that are thought to be seismically active and by studying recent sedimentary
sequences for evidence of seismically active faults.
 Seismically active faults and related fault generated folds have a direct effect
on the geomorphology of a region. This may allow the direct identification of
active structures not previously known. In some cases such observations can be
used quantitatively to constrain the repeat period of major earthquakes.

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Seismotectonic zones of Pakistan
1. Kohistan-Kashmir
2. Northern Baluchistan
3. Quetta-Sibi
4. Southern Baluchistan
5. Northern Afghanistan-
Tajikistan
6. Hindu Kush
7. NW Afghanistan-
Tajikistan Border Region
8. Eastern Afghanistan
9. Makran Coast
10. Runn of Kuchch
11. Sindh-Punjab
12. Pamir-Kunlun
13. Indian Kashmir
14. Upper Punjab-NWFP
15. Chitral
16. Koh e Sulaiman
17. South West Iran
18. Western Baluchistan
Based on seismicity, geology, source mechanism and the stress direction of the region,
Pakistan is divided into 19 (18) seismotectonic zones.

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Seismicity map of the NW Himalayan Fold-and-Thrust Belt for pre-
instrumental (pre-1904) and instrumental (post-1904-2006) earthquakes.
Plates
A. Pacific
B. North
America
C. Cocos
D. Nazca
E. South
America
F. Caribbean
G. Antarctic
H. Indian
I. Philippine
J. African
K. Eurasian
L. Arabian
Boundaries
1. Aleutian SZ
3. Central American
SZ
4. Antilles Sz
6. Scotia SZ
8. Tonga-Kermadec
SZ
9. New Hebrides SZ
10. Mariana SZ
11. Japan SZ
12. Indonesian SZ
13. Hindu Kush

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Seismotectonic zonation map of the NW Himalayan Fold-and-Thrust Belt for
the period 1904-2006.
Plates
A. Pacific
B. North
America
C. Cocos
D. Nazca
E. South
America
F. Caribbean
G. Antarctic
H. Indian
I. Philippine
J. African
K. Eurasian
L. Arabian
Boundaries
1. Aleutian SZ
3. Central American
SZ
4. Antilles Sz
6. Scotia SZ
8. Tonga-Kermadec
SZ
9. New Hebrides SZ
10. Mariana SZ
11. Japan SZ
12. Indonesian SZ
13. Hindu Kush

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Seismic Hazard Analysis
In order to understand the seismic hazard of an area it is necessary not only to
know where potentially active faults are, but also the orientation of the stress
field. This is normally derived from a combination of earthquake data, borehole
breakout analysis, direct stress measurement and the analysis of geologically
young fault networks. The World Stress Map Project provides a useful online
compilation of such data

Seismotectonics

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Editor's Notes