focal plane solutions and stress pattern of Sumatra Andaman subduction zone
Fault Plane Solutions and Evaluation of Stress
Pattern of Seismotectonically Active Sumatra-
Andaman Subduction Zone
Hamid Hussain v & Balaji S
HAMID HUSSAIN V
DEPT.OF DISASTER MANAGEMENT
AIM AND OBJECTIEVES:
To enumerate the focal plane solutions of Andaman and
Nicobar islands based on earth quake parameters.
(Epicenter, strike and dip angle)
To estimate the b value for Andaman and Nicobar islands
using earth quake magnitude data and epicenter data.
To analyse the fault slip rate/stress accumulation on
Regional seismogenic faults.
1. Data archiving focal plane solutions
2. Data archiving b Values.
3. Ground Penetrating Radar Survey
1. Data archiving focal plane solutions
In order to determine where and what depth an earth quake
has occurred, it is possible to make a fault plane
solution/focal mechanism solutions which is deducing the
orientation of fault plane and direction of displacement of
These are found from the first arrivals at a number of
receivers encircling the epicenter.
For the present study faults are identified from the focal mechanism of earth quake
data from International center for seismology (ICS) by analyzing the strike, dip,
magnitude, and epicenter of the earthquakes.
Only the earthquakes which has a magnitude above 6.0 were considered for
The azimuth or structural trend of all the earthquakes were plotted using the
Geo-Orient/stereonet7 in the form of Rose diagram.
The orientation and direction of strike were inferred and the direction of principle
stresses were identified from the strike bearings.
The epicenter of the earth quake that occurred from 1977-2005 and subsequent
focal plane mechanisms were plotted in the map using Arc Gis version 9.3
Each faults were identified in relation with the regional fault systems.
The results were analysed.
b value analysis
b value tells about the relative occurrence of large and small earthquakes during a
particular year in a particular area.
Its given by the relation Log N = a – bM
N = Number of Events
a = a constant, which is the highest Magnitude of the occurred earthquake during a
M = magnitude of the Earthquake.
This Relation is popularly known as Guttenberg Ritcher Relation
If the b value is less for a region it indicates that there is a possible chance of large
magnitude earthquakes for that region.
If the b value is high there is a possibility of occurrence of small magnitude
earthquake more than high magnitude earthquakes or less probability of large
earthquake in compared to small ones.
b value tells about the accumulation of stress also.
The earth quake data were archived from USGS and magnitudes above 5.0 are
In the present work the whole Andaman and Nicobar Islands were divided in to
5 blocks according to latitude and longitudes , b values and corresponding a
values were calculated for the years 1918-2014.
The b values for each block were plotted in the map by using Arc Gis version 9.3.
The b values showing exceptional values are then analysed in association with
the regional fault systems.
The results are analysed.
RESULTS AND DISCUSSION
The earthquake focal mechanism data of Andaman and Nicobar Islands between 1977
and 2004 were acquired from International Centre for Seismology (ICS), UK. Magnitudes
of greater than 6.0 were used for the study purpose. A number of 35 earthquakes were
collected, analysed and plotted. Characteristic seismogenic faults were interpreted from
the earthquake data using strike, dip, magnitude, and epicenter of each earthquake.
Angle of dip Fault
0o<45o Thrust fault, compressional,
45o to 90o Normal fault, tensional fault,
≈90o Strikeslip fault, transcurrent fault,
Block modal for fault identification on the basis of
angle of dip (Bath, 1979)
The azimuth or structural trend of all the earthquakes were plotted by
using the Geo-Orient/stereo net 7, in the form of Rose diagram.
Rose diagram showing the strike of the faults and their
Dip direction of the faults in stereo net
The strike of the faults trends in NNE-SSW, NNW-SSE, NE-SW, E-W and NW-SE direction.
The direction of Principle stresses were identified
NNW-SSE, indicating the direction of maximum principal stress (σ1)
NE-SW, indicates the direction of least principal stress (σ3)
All the dips are trending towards Easterly direction
Major tectonic features and fault plane solutions for the earthquakes
(Magnitude 6.0 and above) occurred between 1977 and 2005. Thick red line
With triangles indicates the Trench (AST), black line indicates West Andaman
Fault (WAF) and thick red line indicates Andaman Backarc Spreading Center
In comparison to the trenchward earthquakes (12 numbers), the backarc
extension earthquakes are more numerous (19 numbers), suggesting active
down-dip extension of the pre-seismic plate boundary.
Six earthquakes observed were ruptured on West Andaman Fault (WAF)
which is deep seated active strikeslip fault and is acting as lithospheric scale
boundary. Subsequently, six earthquakes were observed along Andaman
Backarc Spreading Centre (ABSC).
Twelve number of trenchward earthquakes were observed in the vicinity of
the subduction zone due to tectonic activity along the trench.
Three earthquakes ruptured along Seulimeum strand of Sumatra fault
system East of ABSC.
showing a sharp thrust contact between overriding sandstone
and the shale near Port Mout (N 110 38’678”; E 92o 39’ 40”)
b value analysis
The earthquake data of Andaman and Nicobar Islands and its surroundings
were acquired from USGS (United States Geological Survey) and ICS
(International Centre for Seismology) website. The data from 1918 to 2014
were collected and only the magnitudes 5.0 and above were taken for the
Block Latitude Longitude
A 4o -6o N 90 o -96 o E
B 6.1o -8 o N 90 o -96 o E
C 8.1o -10o N 90 o -96 o E
D 10.1o -12o N 90 o -96 o E
E 12.1o -14o N 90 o -96 o E
BLOCK WISE CALSSIFICATION OF STUDY AREA
year a value b value
1939 5.6 0.987
1960 5.2 1.121
1961 5.2 1.121
1982 5.5 1.06
1983 5.2 1.063
1985 5.2 1.121
1986 5.8 1.005
1989 5.4 1.079
1990 6 0.971
1991 5.9 0.987
1993 5.7 1.022
2004 7 0.937
2005 6.5 0.949
2006 5.8 1.032
2007 5.9 1.005
2010 7.8 0.747
2012 5.5 1.06
Year a value b value
1984 5.2 1.1
1987 5.6 1.025
1988 5.8 0.989
1989 5.3 1.83
1990 5 1.148
1991 5.2 1.103
2004 8.6 1.185
2005 6.2 0.941
2006 5.1 1.125
2007 5.4 1.062
2012 5.6 1.034
2013 5.9 0.9895
BLOCK A BLOCK B
value b value
1918 6.2 0.9596
1932 6 1.0267
1938 6.2 0.9596
1941 5.8 1.0258
1965 5.7 1.043
1967 5.7 1.043
1968 5.5 1.081
1970 5.5 1.081
1986 5.6 1.062
1992 5.9 1.008
2004 6.1 1.03
2005 7.2 0.943
2009 6.1 0.975
2012 5.8 1.025
value b value
1928 8 0.934
1934 6.5 0.986
1946 6.7 0.956
1964 6.3 1.017
1973 5.6 1.44
2004 5.8 1.113
2005 6 1.076
2006 5.9 1.035
2008 6.7 1.044
2010 6.6 0.971
value b value
1928 6.2 0.983
1941 7.7 1.75
1962 6.1 1
1968 5.5 1.1
1978 5.6 1.08
1981 5.7 1.07
1983 6.1 1
2004 6.2 0.996
2006 6.9 1.051
2007 5.6 1.089
2008 5.8 1.051
2010 6.8 0.956
BLOCK C BLOCK D BLOCK E
BLOCK Cumulative b value
In latitudes between 4o-6o N (Block A) encircling Northern Sumatra, the
calculated b values which are less than 1 that are parallel to the strike
of West Andaman Fault (WAF) suggest accumulation of potential stress
and severity and probability of future earthquakes by rupturing the
segment of the fault accompanied by stress release.
In latitudes between 6.1o to 8 o (Block B) which includes Great Nicobar
and Nancowry group of Islands, the estimated b values are less than 1
and 6 numbers of this kind falls along ABSC and 6 numbers fall parallel
to WAF and 7 numbers fall in the vicinity of subduction zone (AST).
These tectonically active features (ABSC, WAF, and AST) provide an
ideal tectonic settings for the occurrence of future seismicity.
In latitudes between 8.1 o to 10 o N (Block C) which includes Car Nicobar and
adjacent Islands, the estimated b value which are less than 1.0 along WAF-5,
ABSC-3 and AST-5 and are at risk for the occurrence of mega earthquakes.
In latitudes between 10.1 o to 12 o N (Block D) yielded b values which are less
than 1.0 around Little Andaman and South Andaman will probably experience
moderate to large magnitude earthquakes in the future.
In latitudes between 12.1 o to 14 o N (Block E) which encircles Middle and North
Andaman, the plotted b value are below the threshold magnitudes and the
region is highly susceptible for future tectonic activities especially along
tectonically active Jarwa thrust, Button thrust and in the vicinity of active faults.
The estimated cumulative b value of Block B (Great Nicobar and Nancowry
group of Islands) show exceptionally low b values (0.9836) which is suggesting
that these regions are more vulnerable for future seismicity along tectonically
The fault plane solutions for Andaman and Nicobar has identified mainly
three types of faults viz, Normal, Thrust and Strike slip.
Based on the earthquake focal mechanism analysis, it was found that
most of the earthquakes are ruptured in the forearc and backarc region
than trench ward region in the Western side of the Islands, this is suggesting
that the West Andaman Fault (WAF) is acting as a lithospheric scale
boundary and the back arc region is in the state of down dip extension.
Most of the earthquakes falling in the back arc region suggests that the
region is active, as evidenced by the opening of the Andaman Sea.
The b value analysis of Andaman and Nicobar Island suggests the
occurrence of future seismicity along the active regional faults i.e. the
low b values corresponding to the vicinity of regional active fault
systems such as WAF, ABSC and AST will experience future seismicity by
rupturing the segments of the faults accompanied by stress release.