A Project Report on Mapping of Focal Mechanism of Earthquakes in Indian Region By TAICHENGMONG RAJKUMAR From Department of Applied Geophysics Indian School of Mines, Dhanbad Carried out under Student Programme for Advancement in Research Knowledge (SPARK) at Council of Scientific & Industrial Research CENTRE FOR MATHEMATICAL MODELLING AND COMPUTATIONAL SIMULATION (CSIR-C-MMACS) NAL BELUR CAMPUS, BANGALORE-560037 Under the guidance of Dr. Imtiyaz Ahmed Parvez Principal Scientist, C-MMACS, Belur Campus (NAL), Bangalore-560037 Department of Applied Geophysics Indian School of Mines, Dhanbad-826004
1. A Project Report on Mapping of Focal Mechanism of
Earthquakes in Indian Region By TAICHENGMONG RAJKUMAR From
Department of Applied Geophysics Indian School of Mines, Dhanbad
Carried out under Student Programme for Advancement in Research
Knowledge (SPARK) at Council of Scientific & Industrial
Research CENTRE FOR MATHEMATICAL MODELLING AND COMPUTATIONAL
SIMULATION (CSIR-C-MMACS) NAL BELUR CAMPUS, BANGALORE-560037 Under
the guidance of Dr. Imtiyaz Ahmed Parvez Principal Scientist,
C-MMACS, Belur Campus (NAL), Bangalore-560037 Department of Applied
Geophysics Indian School of Mines, Dhanbad-826004
2. Date: CERTIFICATE This is to certify that the project
entitled Mapping of Focal Mechanism of Earthquakes in Indian Region
submitted by Taichengmong Rajkumar to Indian School of Mines,
Dhanbad in partial fulfillment of the requirement for the award of
the degree for Int. Master of Science and Technology is a record of
bonafide work carried out by him under my supervision and guidance
at CSIR CENTRE FOR MATHEMATICAL AND COMOUTER SIMULATION (C-MMACS),
NATIONAL AEROSPACE LABORATORIES (NAL), BANGALORE. It is also
certified that the project work has not been submitted for any
purpose elsewhere, in part or full. Dr. Imtiyaz Ahmed Parvez
Signature of the Convenor, SPARK C-MMACS Project Guide CSIR Centre
for Mathematical Modelling and Computer Simulation (Council of
Scientific & Industrial Research) NAL Belur Campus,
Bangalore-560037, India
3. ACKNOWLEDGEMENT I am very grateful to my guide Dr Imtiyaz
Ahmed Parvez who helped me a lot during the whole project. He has
been very kind to me and shared his knowledge with me. I would also
like to thank Dr Anil Kumar, Convenor of SPARK for being so patient
to us. I also convey my heartiest thanks to Stella madam for her
help and support. I would like to thank Prof. Shalivahan sir, HOD,
Department of Applied Geophysics, Indian School of Mines, Dhanbad
who has always been a support to us. Also I would like to thank my
fellow summer interns for helping me throughout these two
months.
4. INTRODUCTION Earthquakes occur on faults. A fault is a thin
zone of crushed rock separating blocks of the earth's crust. When
an earthquake occurs on one of these faults, the rock on one side
of the fault slips with respect to the other. Faults can be
centimeters to thousands of kilometers long. The fault surface can
be vertical, horizontal, or at some angle to the surface of the
earth. Faults can extend deep into the earth and may or may not
extend up to the earth's surface. Based on the nature of relative
movement along the fault it can be classified into three types: (i)
Thrust fault (ii) Normal fault (iii) Strike-slip fault The block
above the fault plane is called the hanging wall and that below the
fault plane is footwall. Dip is the angle between the horizontal
surface and the plane of the fault; hade is compliment of the dip.
A standard nomenclature rake has evolved for describing slip
direction. The actual motion of the two blocks on either side of
the fault plane is defined by a slip vector which can have any
orientation on the fault plane. The direction of slip vector is
given by the angle of slip or rake (). It is measured in the plane
of the fault from the strike direction to the slip vector showing
the motion of the hanging wall relative to the footwall. Thrust
Fault: A thrust fault is a fault along which the hanging wall
(upper side of the fault) has moved up relative to the foot-wall.
The thrust is one that dips less than 45 and an
5. over thrust that dips less than 10. In pure thrust-faulting
the slip vector is parallel to the dip direction and it is upward,
so = 90. Thrust faulting involves crustal shortening and implies
compression. Normal Fault: A normal fault is a fault along which
hanging wall has moved relatively downward. In pure normal faulting
the slip vector is also parallel to the dip direction of the fault
plane but it is downward i.e = -90 (270). Normal faulting involves
lengthening of the crust and implies tension. There are many
possibilities concerning the actual movement; the footwall may
remain stationary and the hanging wall goes down; or the hanging
wall remains stationary and the footwall goes up, or both blocks
move down but the hanging wall moves more than the footwall, or
both blocks move up; but the footwall moves more than the hanging
wall. Some geologists use the term gravity fault in preference to
normal fault. Strike-slip Fault: A strike slip fault is a fault
along which displacement has been essentially parallel to the
strike of the fault, that is the dip-slip component is less or
negligible ( = 0 or 180). For = 0, the hanging wall moves to the
right so that the opposite wall, faced by an observer, moves
relatively to the left. This is called left-lateral slip or
sinistral fault. When = 180, the hanging wall moves to the left and
the opposite wall faced by an observer moves relatively to the
right. This is right-lateral slip or dextral fault. In general will
have a value different than these special cases and the motion is
then called oblique slip.
6. FOCAL MECHANISM Seismologists refer to the direction of slip
in an earthquake and the orientation of the fault on which it
occurs as the focal mechanism. They use information from
seismograms to calculate the focal mechanism and typically display
it on maps as beach ball symbol. This symbol is the projection on a
horizontal plane of the lower
7. half of an imaginary, spherical shell (focal sphere)
surrounding the earthquake source (A). A line is scribed when the
fault plane intersects the shell. The stress-field orientation at
the time of rupture governs the direction of slip on the fault
plane, and the beach ball also depicts this stress orientation. In
this schematic, the gray quadrants contain the tension axis (T),
which reflects the minimum compressive stress direction, and the
white quadrants contain the pressure axis (P), which reflects the
maximum compressive stress direction. The computed focal mechanisms
show only the P and T axes and do not use shading. These focal
mechanisms are computed using a method that attempts to find the
best fit to the direction of P-first motions observed at each
station. For a double-couple source mechanism (or only shear motion
on the fault plane), the compression first- motions should lie only
in the quadrant containing tension axis, and the dilatation
first-motions should lie only in the quadrant containing the
pressure axis. However, first-motion observation will frequently be
in the wrong quadrant. This occurs because a) the algorithm
assigned an incorrect first-motion direction because the signal was
not impulsive, b) the earthquake velocity model, and hence, the
earthquake location is incorrect, so that the computed position of
the first-motion observation on the focal sphere (or ray azimuth
and angle of incidence with respect to vertical) is incorrect, or
c) the seismometer is mis-wired, so that up is down. The latter
explanation is not a common occurrence. For mechanisms computed
using only first motion directions, these incorrect first-motion
observations may greatly affect the computed focal mechanism
parameters. Depending on the distributed and quality of
first-motion data, more than one focal mechanism solution may fit
the data equally well. For mechanism calculated from first-motion
directions as well as some methods that model waveforms, there is
an ambiguity in identifying the fault plane on which slip occurred
form the orthogonal, mathematically equivalent, auxiliary plane. We
illustrate this ambiguity with four examples (B). The block
diagrams adjacent to each
8. focal mechanism illustrate the two possible types of fault
motion that the focal mechanism could represent. Note that the view
angle is 30-degree to the left of and above each diagram. The
ambiguity may sometimes be resolved by computing the two
fault-plane orientation to the alignment of small earthquakes and
aftershocks. The first three examples describe fault motion that is
purely horizontal (strike slip) or vertical (normal or reverse).
The oblique-reverse mechanism illustrate that slip may also have
components of horizontal and vertical motion.
9. Seismic Moment Tensor: The seismic moment tensor M can be
written in the form of 3x3 matrix as where each component
represents one of the nine possible force couples. A force couple
consists of two forces acting together. M12 consists of two forces
of magnitude f, separated by a distance d along 2-axis, that act in
opposite directions along 1. The magnitude of M12 = fd, which has
unit of Nm. In the case of M11, two forces of magnitude f are
separated by a distance d along 1-axis, and act in opposite
directions along 1 axis. This type of couple is sometimes referred
to as vector dipole. There will be no torque in case of M11. The
moment tensor can be described in terms of three orthogonal axes: P
(for pressure; a compressive axis), T (for tension), N (for null).
Fault surface along which the earthquake was generated is 45 degree
from the P and T axes, and contain the N axis. For any moment
tensor there will be two nodal planes. One nodal plane is
perpendicular to other nodal plane and intersects along the N axis.
One of the planes is the fault surface and other is called as
auxiliary plane.
10. Harvard CMT Solution: The Global Centroid-Moment-Tensor
(CMT) Project is overseen by Principal Investigator Gran Ekstrm and
Co-Principal Investigator Meredith Nettles at the Lamont-Doherty
Earth Observatory (LDEO) of Columbia University. The project was
founded by Adam Dziewonski at Harvard University (USA) and operated
there as the Harvard CMT Project from 1982-2006, led first by Prof.
Dziewonski and later by Prof. Ekstrm. During the summer of 2006,
the activities of the CMT Project moved with Prof. Ekstrm to LDEO.
This research effort is moving forward under the name "The Global
CMT Project". The main dissemination point for information and
results from the project is the web site www.globalcmt.org. The CMT
project has been continuously funded by the National Science
Foundation since its inception, and is currently supported by award
EAR-0824694. Focal Mechanism solutions in Indian Region: For focal
mechanism solutions, I have used CMT HARVARD catalog. For this I
have obtained focal mechanism during 01-01-1976 to 31-05-2015. I
have used more than 140 focal mechanism solutions of earthquakes.
The corresponding epicentres are located inside a quadrangle from 8
N to 38 N in latitude and 68 E to 98 E in longitude. The magnitude
range is chosen from 5.5 to 8. Data for plotting Focal Mechanism:
Lon lat depth mrr mtt mpp mrt mrp mtp iexp 88.14 33.00 10 -2.05
-1.74 3.80 -0.62 -3.73 0.98 25 88.4 34.2 89.16 30.69 10 -3.48 -0.82
4.31 -0.12 0.82 0.33 25 89.4 31 89.05 27.42 44 -0.24 -1.83 2.07
1.51 -1.12 -1.26 25 89.4 27 70.99 36.80 228 5.47 -6.20 0.73 1.03
1.30 -0.71 25 69.8 34.5 91.28 34.21 10 -0.04 -0.63 0.66 -0.12 0.20
1.23 25 90.5 33.6 73.48 35.22 10 1.81 -1.57 -0.25 -0.46 0.44 0.71
25 74.2 36.3
16. 70.44 20.98 12 0.02 -4.45 4.42 2.98 0.53 1.98 23 69.8 20.5
73.75 34.88 18 2.94 -0.51 -2.43 -0.62 2.50 -2.17 23 74.4 34 75.60
33.02 20 2.23 -1.08 -1.14 2.17 -1.21 1.23 24 75.3 33 75.95 33.09 22
5.26 -3.62 -1.64 2.32 0.94 3.25 23 76.8 32.6 75.71 33.10 26 3.58
-2.75 -0.82 3.38 0.00 1.80 23 76 32.6 The focal mechanism solutions
are plotted in the map using GMT software and various zones have
been mapped from the clusters of similar focal mechanism. The
radius of the beach balls shown in figure 1 represents the size of
the earthquake. Bigger the size of beach balls corresponds to
higher magnitudes. Different Zones and their characteristic faults:
Zone 1: This zone covers the earthquakes occurred at Hindu Kush
area. These are mostly Thrust faults. Zone 2: It covers the
earthquakes occurred at the boundary of Jammu and Kashmir and
China. These are Strike-Slip faults. Zone 3: It covers the Tibet
region. These are Strike-Slip faults. Zone 4: It covers the
Himalayas. These are Strike Slip faults. Zone 5: It covers Jammu
and Kashmir. These are Thrust faults. Zone 6: It covers Uttarakhand
and Nepal. These are Thrust faults. Zone 7: It covers the boundary
of Nepal and Himalayas. These are Normal faults. Zone 8: It covers
the Himalayas. These are Normal faults. Zone 9: It covers the Tibet
region. These are Strike-Slip faults.
17. Zone 10: It covers the Tibet region. These are Thrust
faults. Zone 11: It covers the earthquakes occurred in Arunachal
Pradesh. These are Thrust faults. Zone 12: It covers the boundary
of NE India and Myanmar. Here the fault type is Thrust fault. Zone
13: It covers the earthquakes occurred in Myanmar. These are
Strike-Slip faults. Zone 14: This zone covers the earthquakes of
Andaman and Nicobar region. These are Thrust faults. Zone 15: It
covers the Andaman and Nicobar region. The fault type is Thrust
fault. Zone 16: It covers the Andaman and Nicobar region. These are
Strike-Slip faults. Zone 17: It covers the Andaman and Nicobar
region. These are Thrust faults. Zone 18: It covers Andaman and
Nicobar region. These are Normal faults. Zone 19: It covers the
earthquakes occurred in Madhya Pradesh and Western Maharashtra.
These are Thrust faults. Zone 20: This zone covers the earthquakes
of Gujrat and Rajasthan. These are Strike- Slip faults. Zone 21: It
covers Sikkim and Bhutan. The fault type is Strike-Slip fault.
18. Fig 1: Map showing Different Zones
19. Another map is plotted showing the Pressure and Tension
axes. Pressure axis Tension axis Fig 2: Focal Mechanisms with
Pressure and Tension axes
20. DISCUSSION AND CONCLUION The upper mantle beneath the
mountainous Hindu Kush region of northeastern Afghanistan is the
site of a tectonically complex area. Although it is not clearly
associated with any island arc system, this region is perhaps the
most active zone of intermediate depth (70-300km) earthquakes in
the world. The region is therefore interesting, since it provides a
setting for examining deep-seated tectonic processes in a collision
zone as well as allowing a study of intermediate depth seismicity
as phenomenon in it. Because of its proximity to the Eurasian-
Indian plate boundary the Hindu- Kush seismic zone is believed to
be grossly related to the convergence of the Indian and Eurasian
sub-continents. We have two different types of fault plane solution
in Hindu Kush region. Fault planes with solutions that in the west
have westward plunging T axes and in the east have eastward or
vertically plunging T axes. We infer that the configuration of the
Hindu Kush seismic zone could possibly be the result of subduction
of oceanic lithosphere from two separate, small basins in opposite
directions. The thrust type faulting is prevalent in the Nepal
region and confirmed that the under thrusting of Indian plate
towards the north along the Himalayan arc has occurred. It is
observed thrust and strike slip components and inferred that there
is under thrusting of Indian plate towards southwest. Some
fault-plane solutions of Tibetan region and observed that there is
a presence of a combination of normal and strike-slip faulting with
T-axes trending approximately east-west. There is a coherent under
thrusting of Indian plate beneath the Lesser Himalaya in the
eastern half of the arc. They inferred that slip vectors are
locally perpendicular to the Himalayan mountain range with very
gentle plunge in the eastern section and more steeply in the
western section. It is apparent that the compressive stress is
acting in N-S to NESW directions which are
21. approximately perpendicular to the major trend of the
Himalaya. It also reveals that the earthquake generation process in
the region is due to the northnortheast compressive stress exerted
by the Indian plate to the Tibetan plate. However, the plunges of
P-axis of a few events show compression from approximately
northwest direction. In the NE region near Nagaland and Manipur we
have found from the focal mechanism and orientation of P and T axes
that the CMF, a geologically older thrust fault, accommodates
motion through dextral strikeslip manner, which is a part of
relative plate motion between the India and Sunda plates.
Therefore, CMF is the present-day active deformation front or plate
boundary fault between the India and Burma plates. In Andaman
region the strike slip events can hardly be designated as
inter-plate events due to (i) the their deep occurrence well within
the underlying Indian plate zone, and (ii) because of their nearly
vertical fault planes oriented NW and NE respectively, contrasting
with the shallow dipping NS trending decollement plane in the
India-Burma subduction zone. Earthquakes with a similar mechanism
do occur in the Andaman arc region, except that they are associated
with the transform faulting in the Andaman sea, the Sumatran fault
zone to the east or the NS trending right- lateral strike-slip
faulting along the Sagaing fault zone farther east. Hence, these
strike-slip events are distinct from the above mechanisms and are
seen to be associated with intra-plate deformation in the
subducting Indian plate. The focal mechanisms of earthquakes in
Gujrat region were the result of movement in a fault at shallow
depth, caused by the stress within the Indian tectonic plate
pushing northward into the Eurasian plate. They are also called
intraplate earthquakes as they occur in the interior of a tectonic
plate different that those Himalayas earthquakes that occur at the
plate boundary.
22. REFERENCES Principles of Seismology: Agustn Udas Lecture
Notes Focussing on MICROEARTHQUAKE INVESTIGATIONS: NGRI, Hyderabad
Discourse on Seismotectonics of Nepal Himalaya and Vicinity:
Appraisal to Earthquake Hazard by D. Shanker1, Harihar Paudyal, H.
N. Singh