Fault Plane Solutions and Evaluation of Stress

Pattern of Seismotectonically Active Sumatra-

Andaman Subduction Zone

Hamid Hussain v & Balaji S

PRESENTED BY:

HAMID HUSSAIN V

DEPT.OF DISASTER MANAGEMENT

KUFOS

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.

STUDY AREA:

METHEDOLOGY:

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 that plane.

These are found from the first arrivals at a number of receivers encircling the epicenter.

Beach Balls For Various Faults

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 analysis.

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

particular year.

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

analysed.

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.

FOCAL MECHANISM

SOLUTIONS

Angle of dip Fault

0o<45o Thrust fault, compressional,

reverse fault

45o to 90o Normal fault, tensional fault,

gravity fault

≈90o Strikeslip fault, transcurrent fault,

lateral 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 prevalence

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 (ABSC)

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

analysis.

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

BLOCK A

12

1 1 12

13 13

1 12 2

1984 1987 1988 1989 1990 1991 2004 2005 2006 2007 2012 2013

no

.of e

ve

nts

year

2

1 1 1 1

4

6

2

11

1

0

2

4

6

8

10

12

1928 1934 1946 1964 1973 2004 2005 2006 2008 2010

no

.of e

ve

nts

year

BLOCK B

2

1 1

2 2 2

1 1 1

2

1

8

13

2 2

1 1

0

2

4

6

8

10

12

14

19

39

19

60

19

61

19

82

19

83

19

85

19

86

19

89

19

90

19

91

19

93

20

04

20

05

20

06

20

07

20

10

20

12

no

.of e

ve

nts

year

BLOCK C

12

1 1 1 1 1 1 1 1 1 1 1 1

3

1 1 1 1

5

17

12

0

2

4

6

8

10

12

14

16

18

no

. o

f e

ve

nts

year

BLOCK D

1

3

1 1 1 1 1

11

2

1 1

2

0

2

4

6

8

10

12

1928 1941 1962 1968 1978 1981 1983 2004 2006 2007 2008 2010

no

. o

f e

ve

nts

year

BLOCK E

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

year

a

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

year

a

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

year

a

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

A 1.1274

B 0.9836

C 1.0168

D 1.0021

E 1.0946

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 suggestingthat these regions are more vulnerable for future seismicity along tectonically

active faults.

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.

CONCLUSION

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.

THANK YOU

QUESTIONS ??