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

L1

9

87

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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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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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


Science

Seismotectonics