ORI GIN AL PA PER

Probabilities for the occurrences of medium to largeearthquakes in northeast India and adjoining region

R. B. S. Yadav J. N. Tripathi D. Shanker B. K. Rastogi

M. C. Das Vikas Kumar

Received: 2 September 2008 / Accepted: 9 May 2010 / Published online: 25 June 2010 Springer Science+Business Media B.V. 2010

Abstract The return periods and occurrence probabilities related to medium and largeearthquakes (Mw 4.07.0) in four seismic zones in northeast India and adjoining region(2032N and 87100E) have been estimated with the help of well-known extremevalue theory using three methods given by Gumbel (1958), Knopoff and Kagan (1977) and

Bury (1999). In the present analysis, the return periods, the most probable maximum

magnitude in a specified time period and probabilities of occurrences of earthquakes of

magnitude M C 4.0 have been computed using a homogeneous and complete earthquakecatalogue prepared for the period between 1897 and 2007. The analysis indicates that the

most probable largest annual earthquakes are close to 4.6, 5.1, 5.2, 5.5 and 5.8 in the four

seismic zones, namely, the Shillong Plateau Zone, the Eastern Syntaxis Zone, the Hima-

layan Thrusts Zone, the Arakan-Yoma subduction zone and the whole region, respectively.

The most probable largest earthquakes that may occur within different time periods have

been also estimated and reported. The study reveals that the estimated mean return periods

R. B. S. Yadav B. K. Rastogi V. KumarInstitute of Seismological Research (ISR), Raisan, Gandhinagar, Gujarat 382009, India

Present Address:R. B. S. Yadav (&)Indian National Centre for Ocean Information Services (INCOIS), Ministry of Earth Science,Government of India, Near ALEAP, Pragathi Nagar, Near JNTU, Kukatpally, Hyderabad, Indiae-mail: rbsybhu@rediffmail.com; rbsyiitr@indiatimes.com

J. N. TripathiDepartment of Earth and Planetary Sciences, University of Allahabad, Allahabad 211002, India

D. ShankerDepartment of Earthquake Engineering, Indian Institute of Technology Roorkee, Roorkee, Uttaranchal247667, India

M. C. DasShiv-Vani Oil and Gas Exploration Ltd, Agartala, India

123

Nat Hazards (2011) 56:145167DOI 10.1007/s11069-010-9557-y

for the earthquake of magnitude Mw 6.5 are about 67 years, 910 years, 5978 years,72115 years and 88127 years in the whole region, the Arakan-Yoma subduction zone,

the Himalayan Thrusts Zone, the Shillong Plateau Zone and the Eastern Syntaxis Zone,

respectively. The study indicates that Arakan-Yoma subduction zone has the lowest mean

return periods and high occurrence probability for the same earthquake magnitude in

comparison to the other zones. The differences in the hazard parameters from zone to zone

reveal the high crustal heterogeneity and seismotectonics complexity in northeast India and

adjoining regions.

Keywords Extreme value theory Seismic hazard Earthquake probability Return periods

1 Introduction

The earthquake hazard is estimated as the probability of occurrence of an earthquake with

magnitude larger or equal to a particular value within a specified region and a given time

period. The seismic hazard assessment of any region is of great importance to minimize the

seismic risk and to predict earthquakes accurately. The seismic hazard due to an earth-

quake can be minimized to implement the building codes in developing countries like India

and losses of human lives and properties due to the hazardous earthquakes can be pre-

vented. The seismic hazard can be evaluated from the statistical analysis of the seismic

history of region by estimating the probabilities of occurrences of an earthquake in a given

region within a certain time period. Although there is not just one single method for the

evaluation of earthquake hazard, yet several researchers of the world have made efforts for

systematic application of statistical methods. A number of statistical models are being

suggested and their properties are being used to estimate the recurrence intervals and

probabilities of occurrences of earthquakes in different regions in the world (Lomnitz

1974; Utsu 1972, 1984; Rikitake 1976, 1991; Hagiwara 1974; Shanker and Singh 1997;

Shanker et al. 2007; Yadav et al. 2010).

The extreme value theory is a well-known and commonly used method to assess the

seismic hazard parameters like return periods and probability of earthquake occurrences. It

has major advantage that it requires only the extreme value magnitude. The extreme value

theory was proposed by Gumbel (1935) for the flood analysis and applied by various

workers in the world for the estimation of seismic hazard parameters. Nordquist (1945)

attempted this theory first time for the earthquakes in southern California and for the

largest earthquakes in the world. After that a number of researchers of the world applied

this method in different regions (Gayskiy and Katok 1965 in Soviet Union; Epstein and

Lomnitz 1966 in America; Dick 1965 in New Zealand; Milne and Devenport 1969 in

Canada; Karnik and Hubernova 1968 and Schenkova and Karnik 1970 in Europe; Shakal

and Willis 1972 in North Circum-Pacific seismic belt; Rao and Rao 1979; Goswami and

Sarmah 1983; Shanker and Singh 1997 and Shanker et al. 2007 in India and adjoining

region; Bayrak et al. 2005 and Oztrurk et al. 2008 for Turkey region).

In the present study, the earthquake probabilities have been estimated in the whole

region of northeast India and adjoining region divided into four different major seismic

zones. We used the methods given by Gumbel (1958), Knopoff and Kagan (1977) and

Bury (1999) and a comparison of estimated results of return periods, most largest earth-

quakes and occurrence probabilities has been carried out with the help of a homogeneous

and complete earthquake catalogue covering time period of 1897 to 2007.

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2 Regional seismotectonics and zonation of the study region

The northeast India and adjoining region bounded by latitude 2032N and longitude 87100E, which includes Eastern Syntaxis of Himalaya, the Arakan-Yoma subduction belt,the Naga thrust fold belts, the Shillong Plateau, the Main Central Thrust (MCT) and Main

Boundary Thrust (MBT) of the Himalayan Frontal Arc has been considered as study region

(Fig. 1). The Himalayan province of the northeast India has been characterized by a series

of north-vergent thrust faults. The important thrusts among these are the Main Boundary

Thrust (MBT) and the Main Central Thrust (MCT). The Himalayan orogenic belts over-

thrust the down-flexed Indian plate constituting the GangaBrahmaputra foredeep, along

the Himalayan Frontal Fault (HFF). It is divided into four major litho-tectonic units

from its southern distinction above the plains to the Indus-Tsangpo suture, namely, the

Fig. 1 The map shows northeast India and adjoining region, which has been adopted as the study region inthe present investigation. The map is showing several faults, thrusts, lineament and structural features of thestudy region. The area of four active major seismogenic source zones: Zone-I (the Eastern-SyntaxisHimalaya), Zone-II (the Arakan-Yoma subduction zone), Zone-III (the Shillong Plateau) and Zone-IV (theHimalayan Thrusts zone) are also shown with light-yellow region (modified from Gupta et al. 1986; Yadavet al. 2009, 2010)

Nat Hazards (2011) 56:145167 147

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Siwalik-Himalaya, the Lesser Himalaya, the Higher or Central Himalaya, and the Tethys or

Tibetan Himalaya and is marked by four major thrusts or fault systems (Nandy 1986). The

eastern Himalaya is a part of the collision boundary between India and Asia, which is

characterized by the NE vergent thrusts belt such as MBT (Eastern region), MFT, Lohit

Thrust, Mishmi Thrust and Bame Tuting Fault. The Burmese arc is characterized by SE

heading thrusts, chief among which are the Naga and Disang thrusts. The Shillong plateau

(SP), source area of the 1897 great earthquake of Mw 8.1, has a complex tectonic model.The dominating thrust/strike-slip faulting earthquakes in the western plateau could be

explained by the pop-up tectonic model (Bilham and England 2001). The E-W trending

Himalaya and the NS trending Arakan-Yoma subduction belt has been developed as a

consequence of the collision between Indian and Eurasian plates and subduction of Indian

plate within Burmese plate (GSI 2000).

This region is one of the most seismic active regions of the Himalayan seismic belt and

has experienced two great earthquakes of magnitude Mw 8.1 (fatalities 1542) and 8.5(fatalities 1526) in 1897 at Shillong and in 1950 at Assam, respectively (Bilham 2004). The

June 12, 1897 earthquake has occurred in the vicinity of Shillong Plateau on a steep SSE-

dipping reverse fault beneath the northern edge of the central Shillong Plateau (Bilham and

England 2001). The earthquake almost totally destroyed settlements and small towns on

the western part of the Plateau, and caused heavy damage in surrounding districts, mainly

due to the extensive liquefaction of the ground (Ambraseys and Bilham 2003). The Assam

earthquake of August 15, 1950 has occurred in the vicinity of India-China border and it

was the earthquake for which instrumentally determined magnitude 8.6 was measured by

Gutenberg (Abe 1981). In addition to these two great earthquakes, this region has expe-

rienced several dozen large earthquakes, which have caused loss of human lives and

destroyed buildings (Fig. 2).

The northeast India and adjoining region can be divided into four main seismogenic

source zones following Dutta (1964) and Gupta et al. (1986), namely, the Eastern Syntaxis(Zone-I), the Arakan-Yoma subduction belt (Zone-II), the Shillong Plateau (Zone-III) andthe Main Central Thrust (MCT) and the Main Boundary Thrust (MBT) of the HimalayanThrusts Zone (Zone-IV). It is very difficult to demarcate straightforward boundaries for thesemajor seismogenic source zones due to unavailability of detail mapping of neotectonic

structural features and micro-seismicity studies of the region. The causes of seismic activity

of these major source zones may be related to each other, and seismic activity of one zone

may influence the seismic activity of other. Therefore, the Zone-II and Zone-III are over-

lapping to the some parts of Zone-I, and Zone-III is overlapping to the some parts of Zone-II

and Zone-IV. These zones are the major tectonic elements of this region as described by

Yadav et al. (2009) and shown in Fig. 1. These major seismic zones have been delineated on

the basis of three fundamental parameters: the active tectonic structures, focal mechanism of

earthquakes and seismicity level. However, a detail delineation of seismic zones needs

several parameters such as complete and homogeneous instrumental as well as historical

seismicity, detailed mapping of neotectonic faults, geology and history of paleoseismology

of the considered region. It is not always possible to get all these information for certain

seismic regions of the world and seismogenic source zones are divided on the basis of

seismicity level, focal mechanism and tectonics of the region. The Zone-I of this region (the

Eastern Syntaxis) have NWSE trending several thrust faults (e.g. Lohit Thrust, Mishmi

Thrust) which are dipping towards NE direction as evidenced from focal mechanism of

earthquakes occurred in this zone. This zone is devastated by a great earthquake of mag-

nitude Mw 8.5 in 1950 which caused heavy damage in the region. The Zone-II (The Arakan-Yoma subduction zone) is the region where Indian plate subducts within the Burmese plate,

148 Nat Hazards (2011) 56:145167

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and several thrust belts dipping towards east have been developed. The occurrences of

earthquakes in this zone are shallow as well as intermediate in nature, and the seismic

productivity of this zone is high when compared to others. The focal mechanism of earth-

quakes indicates NS striking nodal planes dipping towards east. The Zone-III (the Shillong

Plateau) is situated within the Indian plate and away from the plate collision boundary of

Indian and Eurasian plates. The intraplate shallow seismicity of this region is mainly due to

the differential movement/uplifting of the Shillong plateau along major thrust/strike slip

Dauki fault (Bilham and England 2001). The focal mechanism of earthquakes in this region

shows strike-slip motion with small amount of thrust component. The complex tectonic

regime of the region surrounding this zone reveals that the region has experienced great

compressive stresses and resulting distortions due to northward and eastward slow move-

ment of the Indian plate (Bilham and England 2001). The Zone-IV (the Himalayan Thrusts

Zone) is related to the continentcontinent collision boundary of Indian plate with Eurasian

plate. The seismicity of this region is mainly related to major thrust faults of Main Boundary

Thrust (MBT) and the Main Central Thrust (MCT). The focal mechanism of earthquakes

occurred in this zone shows E-W striking nodal planes dipping northward.

Fig. 2 Epicentral map of earthquakes in the considered region during the period 1897 to 2007 for allmagnitude range (Mw) showing the concentration of seismic activity along the tectonic trends of the region

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3 The applied method

The extreme value theory was originally proposed by Gumbel (1935, 1958) for flood

analysis and widely used in earthquake hazard parameters estimation by various

researchers of the world. It is based on the random variable function. In the present study,

we applied the first type asymptotic distribution of extreme values. This theory assumes

that if earthquake magnitude is unlimited, if activity rate decreases with increase in size of

earthquakes and pattern of occurrence of earthquake is random (Yadav 2009), then the

probability of non-exceedance of earthquake of magnitude m in one year will be

Gm expaexpbm 1This is the cumulative distribution function of the largest annual magnitude. Here a

and b are constants, which may be estimated from the least-square fit of the relationship:

ln ln G ln a bm 2The above equation can be estimated by taking double logarithms of Eq. 1. The term a

exp (-bm) of Eq. 1 is the expected number of earthquakes, Nm, in a given year which havemagnitude exceeding m (Epstein and Lomnitz 1966). It can be written as:

ln(Nm lna bm 3which is as same form as the widely used empirical relationship of Gutenberg and Richter

(1956):

Log10Nm a bmThe parameters a and b are related to a and b as follows:

a ln a= ln 10 and b b= ln 10In addition to Eq. 3, several other useful formulas can be obtained from the model

expressed by Eq. 1. For example, the most probable or most frequently observed annual

maximum magnitude (u) can be estimated in terms of a and b as:

u ln a=b 4The most probable earthquake magnitude, ut, in t year periods can be written as:

ut lnat=b u ln t=b 5The mean return period (i.e. mean interval in years) between earthquakes is defined as

the expected time interval for the occurrence of an earthquake with magnitude greater than

or equal to m and is given by:

Tm 1=Nm expbm=a 6The occurrence probability of earthquakes, Pt (m), the most important result of this

theory, can be estimated for a particular magnitude, m, in t year period by:

Ptm 1 expat: expbm 7For the estimation of constants (a and b) of Eq. 1, the largest annual earthquake

magnitude m1, m2, m3,, mN taken from N-sets are arranged in increasing size. Then theobserved probability of each mi may be written as (Gumbel 1958):

G i=N 1 8

150 Nat Hazards (2011) 56:145167

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where, i varies from 1 to N. According to Knopoff and Kagan (1977), the above estimate isbiased for larger earthquakes. Since, we are interested in obtaining the best fit for high

magnitudes and long return periods, therefore, Knopoff and Kagan (1977) suggested

another method for the estimation of observed probability:

G i 0:5=N 9Recently, a more sophisticated formula was suggested by Bury (1999) which approx-

imates the median of the distribution-free estimate of the sample variate and, even for

small value of N, it produces parameters estimations comparable to the results obtained bymaximum likelihood estimations:

G i 0:3=N 0:4 10The main objective to apply the method of Gumbels extreme value expressed by Eq. 1

in this study is to estimate the earthquake hazard parameters for the northeast India and

adjoining region. The earthquake hazard will be calculated in terms of the expected time

interval for the occurrence of an earthquake, the most probable largest magnitude of

earthquakes in a given time period and the occurrence probability of earthquakes in a

specified time interval. For this purpose, the Eqs. 37 of above method will be used for the

estimation of hazard parameters for four different major seismogenic source zones and

whole region of northeast India and adjoining region.

4 Homogeneous and complete seismicity database

A homogenous and complete earthquake catalogue is one of the most important inputs in

the seismology to estimate seismic hazard of any region of the world. In order to prepare

the seismicity database for present investigation, various existing earthquake catalogues

and lists (historical and recent) pertaining to the region have been considered. In their

present form, however, they are inhomogeneous, inadequate and not arranged in proper

fashion for more detailed seismological investigation and seismic hazard assessment.

Although temporary networks have been operated in recent decades in the wider area under

investigation, and these have revealed many important details for better understanding of

the causes and peculiarities of the regional seismicity, their data usually do not meet the

long-term coverage and stability criteria needed for seismic hazard assessment. Therefore,

for NE India the most accurate, complete, and more or less homogeneous data, especially

with respect to magnitudes, come from permanent global seismic network observations and

related parameter reports in the International Seismological Summary (ISS 19221963) as

well as the follow-up reports or bulletins of global seismological data centres since the

early 1960s. The latter are the International Seismological Centre in the United Kingdom

(ISC, since 1964; http://www.isc.ac.uk/search/bulletin), the NEIC (since 1963;

http://neic.usgs.gov/neis/epic/epic-global.htm), and the HRVD (since 1976; now operated

as the Global Centroid-Moment-Tensor project at Lamont-Doherty Earth Observatory

(LDEO) of Columbia University; http://www.globalcmt.org/CMTsearch.html). Therefore,

supplementation and revision of our catalogue for the time span 19632007 is based

exclusively on magnitude data from these three international data centres. With respect to

moment magnitudes, 186 Mw,HRVD (Mw given by HRVD) values between 4.7 and 7.2,pertaining to earthquakes in the study region, have been available for the period 1976 to

2007. Only for three events with Mw between 5.6 and 5.8, we have taken the respectivevalues from the NEIC.

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For the period 1897 to 1962, Gupta et al. (1986) compiled all available seismicity data

after critical examination of several existing catalogues and assessment of the detection

capabilities of older seismological networks. Descriptions of historic earthquakes prior to

1897 have been added to their catalogue. In total, the Gupta et al. (1986) catalogue includes

502 events with magnitude C4.0, yet magnitude values are given for only 316 events. We

adopted this catalogue for the period 1897 to 1962. Comparing it with Chandra (1992) and

ISC data, we found that the magnitudes M reported by Gupta et al. (1986) are equivalent toISC surface wave magnitudes MS. Therefore, we take M values prior to 1964 as proxies forMS,ISC (Ms reported by ISC).

The whole catalogue has been homogenized for moment magnitude Mw with the help ofdifferent regression relationships developed among various magnitude scales (mb, Ms andMw) given by different agencies. A detail description of compilation, homogenization andcompleteness of earthquake catalogue is prearranged by Yadav et al. (2009) and Bormann

and Yadav (2010). The final earthquake catalogue consists of 3001 earthquakes of mag-

nitude Mw C 3.0 during the period 1897 to 2007.The Gumbel extreme value theory requires the independent sets of extremes. For this

purpose, 1-year interval of extreme has been considered to make extremes independent to

each other. If the interval is shorter or longer than 1 year then it makes the data discon-

tinuous (Shanker and Singh 1997). In the present analysis, the earthquake data is dis-

tributed for the period 19542007 in whole region, the period 1964 to 2007 in Eastern

Syntaxis Zone (Zone-I), 19612007 in the Arakan-Yoma subduction Zone (zone-II), 1982

2007 in the Shillong Plateau Zone (Zone-III) and 19582007 in the Himalayan Thrusts

Zone (Zone-IV).

5 Results and discussion

The extreme value theory has been applied to estimate the earthquake hazard parameters

for the whole area of northeast India and adjoining regions as well as for each of the four

sub-zones. For this purpose, three different methods given by Gumbel (1958), Knopoff and

Kagan (1977) and Bury (1999) have been applied for the estimation of constants a and b ofthis model. The whole study region has been divided into four major seismogenic source

zones on the basis of seismicity level, tectonics and focal mechanism of earthquakes. A

homogeneous and complete seismicity database prepared by Yadav et al. (2009) during the

period 1897 to 2007 has been used to evaluate the most probable largest magnitudes, the

mean return periods and probabilities of occurrences of different magnitudes in different

time. A comparative study of estimated hazard parameters estimated by three different

methods of Gumbel (1958), Knopoff and Kagan (1977) and Bury (1999) has been carried

out to see the susceptibility of the whole region and each zone.

We calculated the constants (a and b) of Gumbels extreme value distribution byestimating observed probabilities (G) using three methods of Gumbel (1958), Knopoff andKagan (1977) and Bury (1999) written in Eqs. 810, respectively. These constants can also

be estimated with the help of famous Gutenberg and Richter relationship (1956) and

related to its constants a and b by equations a = (ln a/ln 10) and b = (b/ln10). However,the constants, estimated using approach of Gumbels extreme value distribution, are more

significance and they are better related to the tectonics of the study region (Makropoulos

and Burton 1984; Tsapanos and Burton 1991; Oztrurk et al. 2008). A plot of largest annual

magnitude (m) versus observed probability (G) estimated by three approaches can be seenin Fig. 3ao for whole region, Zone-I (Eastern syntaxis), Zone-II (Arakan-Yoma

152 Nat Hazards (2011) 56:145167

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subduction zone), Zone-III (Shillong Plateau) and Zone-IV (Himalayan Thrusts Zone). The

least-square method has been applied to estimate the constants a and b of Eq. 1 which arelisted in Table 1 for three methods. The parameters a and b are useful to estimate the mostprobable largest magnitudes, the mean return periods and probabilities of occurrences of

different magnitudes in different time in this region. The minor deviation in the estimated

parameters can be observed in three methods which suggest the biasing nature of Gumbels

(1958) approach (Shanker et al. 2007) and Knopoff and Kagan (1977) method. The median

values of these parameters are estimated by Bury (1999) method among these three

ln(-lnG) = 16.014 -2.7584M -5.00-4.00-3.00-2.00-1.000.001.002.00

-5.00-4.00-3.00-2.00-1.000.001.002.00

5.05.0 5.5 6.06.0 6.5 7.07.0 8.0 7.5

ln(-l

nG)

ln(-l

nG)

MagnitudeMagnitude

G=(i-0.5)/N analysis

ln(-lnG) =15.548 -2.6792M

-5.00-4.00-3.00-2.00-1.000.001.002.00

5.0 5.5 6.0 6.5 7.0 7.5

ln(-l

nG)

Magnitude

G=(i-0.3)/(N+0.4) analysis

ln(-lnG) = 16.292 -3.196M

-5.00-4.00-3.00-2.00-1.000.001.002.003.00

4.0 4.5 5.0 5.5 6.0 6.5

ln(-l

nG)

Magnitude

G=i/(N+1) analysis

G=i/(N+1) analysis

ln(-lnG) = 17.699 -3.468M -5.00-4.00-3.00-2.00-1.000.001.002.003.00

4.0 4.5 5.0 5.5 6.0 6.5

ln(-l

nG)

Magnitude

G=(i-0.5)/N analysis

ln(-lnG)=17.056 -3.3437M

-5.00-4.00-3.00-2.00-1.000.001.002.003.00

4.0 4.5 5.0 5.5 6.0 6.5

ln(-l

nG)

Magnitude

G=(i-0.3)/(N+0.4) analysis

ln(-lnG) = 12.093 -2.1971M -5.00-4.00-3.00-2.00-1.000.001.002.00

4.0 5.0 6.0 7.0 8.0

ln(-l

nG)

Magnitude

G=i/(N+1) analysis

ln(-lnG) = 13.046 -2.3669M -5.00-4.00-3.00-2.00-1.000.001.002.00

4.0 5.0 6.0 7.0 8.0

ln(-l

nG)

Magnitude

G=(i-0.5)/N analysis

ln(-lnG) =12.613 -2.2898M -5.00-4.00-3.00-2.00-1.000.001.002.00

4.0 5.0 6.0 7.0 8.0ln

(-lnG

)Magnitude

G=(i-0.3)/(N+0.4) analysis

ln(-lnG) = 10.759 -2.3152M

-4.00

-3.00

-2.00

-1.00

0.00

1.00

2.00

4.0 4.5 5.0 5.5 6.0

ln(-l

nG)

Magnitude

G= i/(N+1) analysis

ln(-lnG) =11.989 -2.5745M

-5.00-4.00

-3.00-2.00-1.000.00

1.002.00

4.0 4.5 5.0 5.5 6.0

ln(-l

nG)

Magnitude

G=(i-0.5)/N analysis

ln(-lnG) = 11.424 -2.4554M

-4.00

-3.00

-2.00

-1.00

0.00

1.00

2.00

4.0 4.5 5.0 5.5 6.0

ln(-l

nG)

Magnitude

G=(i-0.3)/(N+0.4) analysis

ln(-lnG) = 16.919 -3.2311M

-5.00-4.00-3.00-2.00-1.000.001.002.00

4.5 5.0 5.5 6.0 6.5

ln(-l

nG)

Magnitude

G=i/(N+1) analysis

ln(-lnG) = 18.151 -3.4633M

-5.00-4.00-3.00-2.00-1.000.001.002.00

4.5 5.0 5.5 6.0 6.5

ln(-l

nG)

Magnitude

G=(i-0.5)/N analysis

ln(-lnG) =17.595 -3.3585M

-5.00-4.00-3.00-2.00-1.000.001.002.00

4.5 5.0 5.5 6.0 6.5

ln(-l

nG)

Magnitude

G=(i-0.3)/(N+0.4) analysis

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

(j) (k) (l)

(m) (n) (o)

Fig. 3 Gumbels extreme value distribution of observed probability (G) by three different methods,namely, Gumbel (1958); Knopoff and Kagan (1977) and Bury (1999) for whole region (a, b, c); Zone-I (d, e,f); Zone-II (g, h, i); Zone-III (j, k, l) and Zone-IV (m, n, o)

Nat Hazards (2011) 56:145167 153

123

methods suggests the better method to estimate the hazard parameters of the study region.

The equations of probability distribution for non-exceedance of magnitude m can be

estimated from Eq. 1 for the whole region and four seismic zones using three methods and

can be written as:

Whole region

Gm exp3210701:48 exp2:58m by P i=N 1Gm exp9011390:98 exp2:76m by P i 0:5=N

Gm exp5654713:48 exp2:68m by P i 0:3=N 0:4Zone-I (Eastern Syntaxis Zone)

Gm exp11899417:41 exp3:19m by G i=N 1Gm exp48593483:72 exp3:47m by G i 0:5=N

Gm exp25546222:08 exp3:34m by G i 0:3=N 0:4Zone-II (Arakan-Yoma Subduction Zone)

Gm exp178617:16 exp2:20m by G i=N 1Gm exp463239:74 exp2:37m by G i 0:5=N

Gm exp300438:99 exp2:28m by G i 0:3=N 0:4Zone-III (Shillong Plateau Zone)

Gm exp47051:59 exp2:32m by G i=N 1Gm exp160974:30 exp2:57m by G i 0:5=N

Table 1 Calculated values of lna and b for different zones Zones Methods Hazard parameters

Ln a b

Whole region G = i/(N ? 1) 14.982 2.58

G = (i - 0.5)/N 16.014 2.76

G = (i - 0.3)/(N ? 0.4) 15.548 2.68

Zone-I G = i/(N ? 1) 16.292 3.19

G = (i - 0.5)/N 17.699 3.47

G = (i - 0.3)/(N ? 0.4) 17.056 3.34

Zone-II G = i/(N ? 1) 12.093 2.20

G = (i - 0.5)/N 13.046 2.37

G = (i - 0.3)/(N ? 0.4) 12.613 2.28

Zone-III G = i/(N ? 1) 10.759 2.32

G = (i - 0.5)/N 11.989 2.57

G = (i - 0.3)/(N ? 0.4) 11.424 2.46

Zone-IV G = i/(N ? 1) 16.919 3.23

G = (i - 0.5)/N 18.151 3.46

G = (i - 0.3)/(N ? 0.4) 17.595 3.36

154 Nat Hazards (2011) 56:145167

123

Gm exp91491:38 exp2:46m by G i 0:3=N 0:4Zone-IV (Himalayan Thrusts Zone)

Gm exp22275545:05 exp3:23m by G i=N 1Gm exp76362324:68 exp3:46m by G i 0:5=N

Gm exp43793676:82 exp3:36m by G i 0:3=N 0:4The cumulative probability distribution curves for non-exceedance, G(m), and

exceedance, (1 - G(m)), estimated using three methods are presented in Fig. 4ao for thewhole region and four different seismic zones. These figures show that the cumulative

probability distribution for non-exceedance, G(m), of magnitudes B5.9 is B50%, whereas

0.000.200.400.600.801.001.20

4.0 5.0 6.0 7.0 8.0

Pro

babi

lity

Magnitude

G=i/(N+1) analysis

G(m)1-G(m)

0.000.200.400.600.801.001.20

4.0 5.0 6.0 7.0 8.0

Pro

babi

lity

Magnitude

G=(i-0.5)/N analysis

G(m)1-G(m)

0.000.200.400.600.801.001.20

4.0 5.0 6.0 7.0 8.0

Pro

babi

lity

Magnitude

G=(i-0.3)/(n+0.4) analysis

G(m)1-G(m)

0.000.200.400.600.801.001.20

4.0 5.0 6.0 7.0 8.0

Pro

babi

lity

Magnitude

G=i/(N+1) analysis

G(m)1-G(m)

0.000.200.400.600.801.001.20

4.0 5.0 6.0 7.0 8.0

Pro

babi

lity

Magnitude

G=(i-0.5)/N analysis

G(m)1-G(m)

0.00

0.20

0.40

0.60

0.80

1.00

1.20

4.0 5.0 6.0 7.0 8.0

Pro

babi

lity

Magnitude

G=(i-0.3)/(n+0.4) analysis

G(m)1-G(m)

0.000.200.400.600.801.001.20

4.0 5.0 6.0 7.0 8.0

pro

babi

lity

Magnitude

G=i/(N+1) analysis

G(m)1-G(m)

0.000.200.400.600.801.001.20

4.0 5.0 6.0 7.0 8.0 4.0 5.0 6.0 7.0 8.0

Pro

babi

lity

Magnitude

G=(i-0.5)/N analysis

G(m)1-G(m)

0.00

0.20

0.40

0.60

0.80

1.00

1.20

Pro

babi

lity

Magnitude

G=(i-0.3)/(n+0.4) analysis

G(m)1-G(m)

0.000.200.400.600.801.001.20

4.0 5.0 6.0 7.0 8.0

Pro

babi

lity

Magnitude

G=i/(N+1)analysis

G(m)1-G(m)

0.000.200.400.600.801.001.20

4.0 5.0 6.0 7.0 8.0

Pro

babi

lity

Magnitude

G=(i-0.5)/N analysis

G(m)1-G(m)

0.00

0.20

0.40

0.60

0.80

1.00

1.20

4.0 5.0 6.0 7.0 8.0

Pro

babi

lity

Magnitude

G=(i-0.3)/(n+0.4) analysis

G(m)1-G(m)

0.000.200.400.600.801.001.20

4.0 5.0 6.0 7.0 8.0

Pro

babi

lity

Magnitude

G=i/(N+1) analysis

G(m)1-G(m)

0.00

0.20

0.40

0.60

0.80

1.00

1.20

4.0 5.0 6.0 7.0 8.0

Pro

babi

lity

Magnitude

G=(i-0.5)/N analysis

G(m)1-G(m)

0.00

0.20

0.40

0.60

0.80

1.00

1.20

4.0 5.0 6.0 7.0 8.0

Pro

babi

lity

Magnitude

G=(i-0.3)/(n+0.4) analysis

G(m)1-G(m)

(a) (b) (c)

(d) (e) (f)

(h) (i)

(j) (k) (l)

(m) (n) (o)

(g)

Fig. 4 Cumulative Probability distribution curves of non-exceedance [G(m)] and exceedance [1 - G(m)]for whole region (a, b, c); zone-I (d, e, f); zone-II (g, h, i); zone-III (j, k, l) and zone-IV (m, n, o)

Nat Hazards (2011) 56:145167 155

123

cumulative probability distribution for exceedance, (1 - G(m)), of those magnitudes isC50% in whole region estimated by three methods. The opposite cumulative probability

distributions are observed for magnitude [5.9. The same cumulative probability distri-butions are estimated for magnitudes 5.2, 5.7, 4.8 and 5.3 in Zone-I, Zone-II, Zone-III and

Zone-IV, respectively, for three methods. These magnitude values for whole region and

each zone, where cumulative probability distribution of non-exceedance and exceedance is

50%, are closely related to the most probable largest annual magnitudes (u). Table 2 showsthe most probable annual maxima of largest earthquakes (u) estimated by three differentmethods. The largest annual magnitude of earthquakes have been estimated as 5.8, 5.1, 5.5,

4.7 and 5.2 by Bury (1999) method in whole region and Zone-I, Zone-II, Zone-III and

Zone-IV, respectively. It is observed that there is no significant difference in estimated

values using either method. Rao and Rao (1979) estimated most probable annual maxima

for Assam and NE India region as 5.8 and 6.0, respectively using Gumbels extreme value

method with the help of earthquake catalogue covering period 1926 to 1971. Goswami and

Sarmah (1983) estimated annual earthquake maxima as 5.5 in northeast India using

Gumbels extreme value method with the help of earthquake catalogue during the period

1929 to 1978. Our estimate of largest annual maxima of earthquake is closely related to

these previous studies for the considered region. The most probable largest magnitudes of

earthquakes for different time (ut) have also been estimated and listed in Table 3. Rao andRao (1979) and Goswami and Sarmah (1983) estimated most probable 50 years largest

earthquake magnitude for northeast India as 8.5 and 8.6 which is much larger than our

estimate of 7.3. The last earthquake of magnitude 8.5 has occurred in 1950 in this region

and according to these previous studies this 50 years return period for magnitude 8.5 has

been elapsed. Therefore, our estimate of 50 years largest earthquake magnitude of 7.3 is

realistic. Yadav et al. (2010) have estimated more than 50% probability for the occurrences

of earthquakes of magnitudes C7.0 in 50 years in this region by three probabilistic models

namely, Weibull, Gamma and Lognormal using same earthquake catalogue used for this

analysis.

The yearly expected number of earthquakes (Nm) and mean return periods (Tm) ofmagnitude C4.0 using Eqs. 3 and 6 are estimated and listed in Tables 4 and 5 for the whole

region and four different zones estimated with the help of three different methods. The

mean return period is one of the most important hazard parameters of any region. The

mean return period curves for whole region and four different zones are plotted in Fig. 5

for three different used methods. It is observed that Zone-II (Arakan-Yoma subduction

zone) has higher expected number of earthquakes and smaller mean return periods in

comparison to other three zones for a particular earthquake magnitude. Thus, Zone-II is

seismically more active and vulnerable to future moderate earthquakes in comparison to

the other three zones. The Zone-II is the most seismically active and highly productive

zone in the study region, where seismicity is mainly related to the subduction of Indian

plate beneath Burmese plate. Gupta and Srivastava (1990) estimated return periods of

3.974.44 years for magnitude 6.0 in Arakan-Yoma subduction zone. Shanker and Sharma

Table 2 Most probable annual maxima in whole region and each zone

Methods Whole region Zone-I Zone-II Zone-III Zone-IV

P = i/(N ? 1) 5.8 5.1 5.5 4.6 5.2

P = (i - 0.5)/N 5.8 5.1 5.5 4.6 5.2

P = (i - 0.3)/(N ? 0.4) 5.8 5.1 5.5 4.7 5.2

156 Nat Hazards (2011) 56:145167

123

Tab

le3

Mo

stp

rob

able

larg

est

eart

hq

uak

em

agn

itu

de

for

dif

fere

nt

per

iod

sin

wh

ole

reg

ion

and

each

zon

ees

tim

ated

by

thre

em

eth

ods

Yea

rsW

ho

lere

gio

nZ

on

e-I

Zo

ne-

IIZ

on

e-II

IZ

on

e-IV

Met

ho

ds

Met

ho

ds

Met

ho

ds

Met

ho

ds

Met

ho

ds

G=

i/(N

?1

)G

=(i

-0

.5)/

N

G=

(i-

0.3

)/(N

?0

.4)

G=

i/(N

?1

)G

=(i

-0

.5)/

NG

=(i

-0

.3)/

(N?

0.4

)

G=

i/(N

?1

)G

=(i

-0

.5)/

NG

=(i

-0

.3)/

(N?

0.4

)

G=

i/(N

?1

)G

=(i

-0

.5)/

NG

=(i

-0

.3)/

(N?

0.4

)

G=

i/(N

?1

)G

=(i

-0

.5)/

NG

=(i

-0

.3)/

(N?

0.4

)

15

.85

.85

.85

.15

.15

.15

.55

.55

.54

.64

.74

.75

.25

.25

.2

56

.46

.46

.45

.65

.65

.66

.26

.26

.25

.35

.35

.35

.75

.75

.7

10

6.7

6.6

6.7

5.8

5.8

5.8

6.6

6.5

6.5

5.6

5.6

5.6

5.9

5.9

5.9

15

6.9

6.8

6.8

5.9

5.9

5.9

6.7

6.7

6.7

5.8

5.7

5.8

6.1

6.0

6.0

25

7.1

7.0

7.0

6.1

6.0

6.1

7.0

6.9

6.9

6.0

5.9

6.0

6.2

6.2

6.2

50

7.3

7.2

7.3

6.3

6.2

6.3

7.3

7.2

7.2

6.3

6.2

6.2

6.4

6.4

6.4

75

7.5

7.4

7.4

6.4

6.3

6.4

7.5

7.3

7.4

6.5

6.3

6.4

6.6

6.5

6.5

10

07

.67

.57

.56

.56

.46

.57

.67

.57

.56

.66

.46

.56

.76

.66

.6

12

57

.77

.67

.66

.66

.56

.57

.77

.67

.66

.76

.56

.66

.76

.66

.7

15

07

.77

.67

.76

.76

.56

.67

.87

.67

.76

.86

.66

.76

.86

.76

.7

Nat Hazards (2011) 56:145167 157

123

Ta

ble

4E

stim

ated

ann

ual

nu

mb

ero

fea

rth

quak

es(N

m)

inw

ho

lere

gio

nan

dea

chzo

ne

MW

ho

lere

gio

nZ

on

e-I

Zo

ne-

IIZ

on

e-II

IZ

on

e-IV

Met

ho

ds

Met

ho

ds

Met

ho

ds

Met

ho

ds

Met

ho

ds

G=

i/(N

?1

)G

=(i

-0

.5)/

NG

=(i

-0

.3)/

(N?

0.4

)

G=

i/(N

?1

)G

=(i

-0

.5)/

NG

=(i

-0

.3)/

(N?

0.4

)

G=

i/(N

?1

)G

=(i

-0

.5)/

NG

=(i

-0

.3)/

(N?

0.4

)

G=

i/(N

?1

)G

=(i

-0

.5)/

NG

=(i

-0

.3)/

( N?

0.4

)

G=

i/(N

?1

)G

=(i

-0

.5)/

NG

=(i

-0

.3)/

(N?

0.4

)

4.0

10

5.8

41

44

.60

12

5.3

63

3.3

84

5.9

23

9.1

72

7.2

33

5.8

13

1.6

24

.47

5.4

24

.96

54

.30

73

.54

64

.14

4.5

29

.13

36

.37

32

.84

6.7

58

.109

7.3

59

.08

10

.96

10

.06

1.4

01

.49

1.4

51

0.7

91

3.0

11

1.9

6

5.0

8.0

29

.15

8.6

01

.36

1.4

31

.38

3.0

23

.35

3.2

00

.44

0.4

10

.42

2.1

42

.30

2.2

3

5.5

2.2

02

.30

2.2

50

.27

0.2

50

.259

1.0

01

.02

1.0

10

.13

0.1

10

.12

0.4

20

.40

0.4

1

6.0

0.6

00

.57

0.5

90

0.0

50

.04

0.0

49

0.3

30

.31

0.3

20

.04

0.0

30

.03

0.0

80

.07

0.0

7

6.5

0.1

60

.14

0.1

55

0.0

10

.007

0.0

09

0.1

10

.09

0.1

00

.01

0.0

08

0.0

10

.01

0.0

10

.01

7.0

0.0

40

.03

60

.041

0.0

02

0.0

01

0.0

02

0.0

30

.03

0.0

30

.004

0.0

02

0.0

03

0.0

03

0.0

02

0.0

02

158 Nat Hazards (2011) 56:145167

123

Tab

le5

Est

imat

edre

turn

per

iod

(Tm

)in

wh

ole

reg

ion

and

each

zon

e

MW

ho

lere

gio

nZ

on

e-I

Zo

ne-

IIZ

on

e-II

IZ

on

e-IV

Met

hod

sM

eth

od

sM

eth

od

sM

eth

ods

Met

ho

ds

G=

i/(N

?1

)

G=

(i-

0.5

)/

N

G=

(i-

0.3

)/

(N?

0.4

)

G=

i/(N

?1

)

G=

(i-

0.5

)/N

G=

(i-

0.3

)/

(N?

0.4

)

G=

i/(N

?1

)

G=

(i-

0.5

)/N

G=

(i-

0.3

)/

(N?

0.4

)

G=

i/(N

?1

)

G=

(i-

0.5

)/N

G=

(i-

0.3

)/

(N?

0.4

)

G=

i/(N

?1

)

G=

(i-

0.5

)/N

G=

(i-

0.3

)/

(N?

0.4

)

4.0

3.4

day

s2

.5d

ays

2.9

day

s1

0.9

day

s7

.9d

ays

9.3

day

s1

3.4

day

s1

0.2

day

s1

1.5

day

s8

1.6

day

s6

7.3

day

s7

3.5

day

s6

.7d

ays

4.9

day

s5

.7d

ays

4.5

12

.5d

ays

10

day

s1

1.1

day

s5

4d

ays

45

day

s4

9.7

day

s4

0.2

day

s3

3.3

day

s3

6.3

day

s2

59

.6d

ays

24

3.7

day

s2

50

.9d

ays

33

.8d

ays

28

day

s3

0.5

day

s

5.0

45

.5d

ays

39

.8d

ays

42

.4d

ays

26

7.2

day

s2

54

.9d

ays

26

4.7

day

s1

20.6

day

s1

08

.7d

ays

11

3.9

day

s2

.3y

ears

2.4

yea

rs2

.3y

ears

17

0d

ays

15

8.4

day

s1

63

.5day

s

5.5

16

5.3

day

s1

58

.5d

ays

16

1.9

day

s3

.6y

ears

3.9

yea

rs3

.9y

ears

36

1.7

day

s3

54

.9d

ays

35

8d

ays

7.2

yea

rs8

.8y

ears

8.0

yea

rs2

.3y

ears

2.5

yea

rs2

.4y

ears

6.0

1.6

yea

rs1

.7y

ears

1.7

yea

rs1

7.8

yea

rs2

2.4

yea

rs2

0.6

yea

rs2

.9y

ears

3.2

yea

rs3

.1y

ears

22

.9y

ears

31

.8y

ears

27

.3y

ears

11

.8y

ears

13

.9y

ears

12

.9y

ears

6.5

5.9

yea

rs6

.8y

ears

6.5

yea

rs8

8.4

yea

rs1

26

.8y

ears

10

9.9

yea

rs8

.9y

ears

10

.4y

ears

9.7

yea

rs7

2.9

yea

rs1

15

yea

rs9

3.3

yea

rs5

9.3

yea

rs7

8.3

yea

rs6

9.1

yea

rs

7.0

21

.7y

ears

27

.3y

ears

24

.7y

ears

43

7y

ears

71

8.3

yea

rs5

85

.8y

ears

26

.8y

ears

33

.9y

ears

30

.4y

ears

23

2.2

yea

rs4

16

.8y

ears

31

8.6

yea

rs2

98.5

yea

rs4

42

.3y

ears

37

0.4

yea

rs

Nat Hazards (2011) 56:145167 159

123

(1998) also estimated return periods of 57 years for this region for same magnitude. Our

estimate of return periods of 2.93.1 years for magnitude 6.0 in this zone is closely

comparable to these previous studies. Small difference in values of return periods is

obvious due to the different earthquake dataset, source zone size and observation time

period. The Zone-I (Eastern syntaxis) has smaller expected number of earthquakes and

larger return periods in comparison to the other zones, suggesting less vulnerable to future

moderate earthquakes when compared to others. However, this zone has experienced a

devastating great earthquake of magnitude Mw 8.5 in 1950 which caused a huge loss oflives and properties (Gupta et al. 1986; Bilham 2004; Yadav et al. 2009, 2010). The Zone-

IV (Himalayan Thrusts Zone) is second seismically active zone after Zone-II (Arakan-

Yoma subduction zone) in terms of return periods. The mean return periods for magnitude

6.0 are estimated as 1214 years in this zone. Sharma and Malik (2006) estimated return

period as 16.4 years for this zone which was based on about 99 earthquakes occurred in

this region. Our result is analogous and the difference in return period is due to the

difference in period of observation, number of earthquakes used and source zone size. The

Zone-III (the Shillong Plateau) is the third seismically active zone of the study region with

(a) (b)

(c) (d)

(e)

Fig. 5 Plots of mean return period (Rt) curves for whole region (a); zone-I (b); zone-II (c); zone-III (d);and zone-IV (e) estimated by three methods

160 Nat Hazards (2011) 56:145167

123

Table 6 Probabilities of occur-rences of earthquakes, Pt(m), fordifferent magnitude (m) and timeperiods (t)

Methods

Rt (m) G = i/(N ? 1)

G = (i - 0.5)/N

G = (i - 0.3)/(N ? 0.4)

Whole region

R1 (5.0) 1.00 1.00 1.00

R1 (5.5) 0.89 0.90 0.89

R1 (6.0) 0.46 0.44 0.45

R2 (6.0) 0.70 0.69 0.69

R5 (6.0) 0.95 0.94 0.95

R10 (6.0) 1.00 1.00 1.00

R10 (6.5) 0.81 0.77 0.79

R20 (6.5) 0.96 0.95 0.95

R10 (7.0) 0.37 0.31 0.33

R20 (7.0) 0.60 0.52 0.56

Zone-I (Eastern Syntaxis)

R1 (5.0) 0.74 0.76 0.75

R5 (5.5) 0.75 0.72 0.73

R10 (5.5) 0.94 0.92 0.93

R20 (6.0) 0.67 0.59 0.63

R50 (6.0) 0.94 0.89 0.92

R100 (6.5) 0.68 0.55 0.61

R20 (7.0) 0.04 0.03 0.03

R50 (7.0) 0.11 0.07 0.08

R400 (7.0) 0.60 0.43 0.50

Zone-II (Arakan-Yoma subduction)

R1 (5.0) 0.95 0.97 0.96

R1 (5.5) 0.64 0.64 0.64

R5 (5.5) 0.99 0.99 0.99

R5 (6.0) 0.81 0.79 0.80

R10 (6.0) 0.97 0.96 0.96

R20 (6.5) 0.89 0.85 0.87

R10 (7.0) 0.31 0.26 0.28

R20 (7.0) 0.53 0.45 0.48

R50 (7.0) 0.85 0.77 0.81

Zone-III (Shillong Plateau)

R5 (5.5) 0.50 0.43 0.46

R5 (6.0) 0.20 0.15 0.17

R10 (6.0) 0.35 0.27 0.31

R20 (6.0) 0.58 0.47 0.52

R50 (6.0) 0.89 0.79 0.84

R50 (7.0) 0.19 0.11 0.15

R100 (7.0) 0.35 0.21 0.27

Zone-IV (Himalayan Thrusts Zone)

R1 (5.0) 0.88 0.90 0.89

R1 (5.5) 0.35 0.33 0.34

Nat Hazards (2011) 56:145167 161

123

return periods of 2332 years for moderate size earthquake of magnitude 6.0. This region

has experienced a great earthquake of magnitude Mw 8.1 in 1897 which caused huge loss oflives and properties and it is susceptible for the occurrences of such large earthquakes in

near future (Bilham and England 2001). Sharma and Malik (2006) estimated return period

of 10.323.9 years for this zone, which is analogous to our results. In our study, the mean

return periods of magnitude 6.5 are observed about 67 years, 910 years, 5978 years,

72115 years and 88127 years in whole region, Arakan-Yoma subduction zone (Zone-II),

Himalayan Thrusts Zone (Zone-IV), Shillong Plateau Zone (Zone-III) and Eastern Syntaxis

zone (Zone-I), respectively. Goswami and Sarmah (1983) estimated return periods of

1.9 years for magnitude 6.0 in northeast India region from both methods of Gumbel (1958)

and Knopoff and Kagan (1977) using Gumbels extreme value theory type-I. Our estimate

of return period of 1.61.7 years for magnitude 6.0 in whole region is as good as with the

results of Goswami and Sarmah (1983).

The probabilities, Pt(m), have been estimated for the whole region and four majorseismogenic source zones for certain magnitude (m) and time period (t). Table 6 providesthe information about probabilities of occurrences of earthquakes for different magnitudes

(m) and periods (t). It is observed that occurrence probability is higher in Zone-II (Arakan-Yoma subduction zone) than other three zones for the occurrences of same medium to

large earthquake magnitude. The earthquake hazard curves (expressed in terms of the

probability expected for earthquakes with the maximum observed magnitudes, and plotted

for magnitudes and for the time span of 25-, 50-, and 100-years) for whole region and each

zone is shown in Fig. 6. From these hazard curves, it is clearly observed that probability of

occurrence of an earthquake with magnitude m C 6.0 in 25-, 50- and 100-year returnperiods decreases exponentially with magnitude in all four seismogenic zones. Spatial

variation of earthquake probabilities in next 100 years for magnitude range 6.06.5 and

6.67.0 are shown in Fig. 7a and b, respectively, for four major seismogenic source zones

of considered region. From Fig. 7a, it is observed that Zone-II (Arakan-Yoma subduction

zone) has highest probability (1.00) for the occurrence of an earthquake of magnitude 6.0

6.5 in next 100 years, whereas Zone-I (the Eastern Syntaxis) and Zone-III (the Shillong

Plateau) show lowest probabilities (0.610.97) among four Zones. But, in general, the

probability of occurrences of such magnitude earthquakes in next 100 years is more than

61% in all zones. Similarly, from Fig. 7b, it is observed that the probability of occurrences

of earthquakes of magnitude 6.67.0 in next 100 years is still high (0.971.00) in Zone-II

(Arakan-Yoma subduction zone), whereas other three zones show probability less than

Table 6 continuedMethods

Rt (m) G = i/(N ? 1)

G = (i - 0.5)/N

G = (i - 0.3)/(N ? 0.4)

R5 (5.5) 0.88 0.87 0.88

R10 (5.5) 0.99 0.98 0.98

R5 (6.0) 0.35 0.30 0.32

R10 (6.0) 0.57 0.51 0.54

R20 (6.0) 0.82 0.76 0.79

R50 (6.0) 0.99 0.97 0.98

R50 (7.0) 0.15 0.11 0.13

R100 (7.0) 0.28 0.20 0.24

162 Nat Hazards (2011) 56:145167

123

60% except Zone-IV (the Himalayan thrusts zone) where it exceeded up to 64%. The

lowest probability (0.160.49) is observed for Zone-I (the Eastern Syntaxis). From above

discussion, it is clear that Zone-II (Arakan-Yoma subduction zone) has high probability

Fig. 6 Probability-magnitude curves for whole region (a), Zone-I (b), Zone-II (c), Zone-III (d) and Zone-IV (e) for the 25-, 50- and 100-year return periods

Nat Hazards (2011) 56:145167 163

123

164 Nat Hazards (2011) 56:145167

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([95%) of occurrences of earthquakes of magnitude 6.07.0 in next 100 years among fourmajor seismogenic source zones of northeast India and adjoining region.

Generally, the statistical methods need a large number of observations to estimate

recurrence periods. In this analysis, 26 to 54 years data have been taken to estimate the

seismic hazard parameters. Since, the values of model parameters a and b, which are usedto estimate the probability of occurrences and return periods, do not affect much for short

or long duration of the seismicity data (Shanker and Singh 1997). Therefore, the study may

be considered to be consistent and estimated return periods and probabilities may be used

as quantitative measure of seismicity.

6 Conclusions

The northeast India and adjoining region is one of the most active regions in the Himalayan

seismic belt, which is the triple-junction of three plates, namely, Indian plate, Eurasian

plate and Burmese plate. This region has experienced two great earthquakes of magnitude

Mw 8.1 in 1897 and Mw 8.5 in 1950 and several moderate to large earthquakes, whichcaused widespread destructions in this region. It is a surprising thing that such two great

earthquakes have occurred in such a short-time period in the region. Therefore, this region

is most vulnerable to moderate and large earthquakes in near future. In the present study,

the earthquake hazard parameters (most probable largest earthquake magnitudes in t-years,return periods and probabilities of occurrences of earthquakes of certain magnitude in

certain time period) for moderate to large earthquakes (4.07.0) have been estimated in

northeast India and adjoining region using method of extreme value theory given by

Gumbel (1935, 1958). For this purpose, a homogeneous and complete earthquake cata-

logue covering time period 1897 to 2007 and prepared by Yadav et al. (2009, 2010) is used.

This study introduces a comparative study of four zones by considering different

datasets using three methods of Gumbel (1958); Knopoff and Kagan (1977) and Bury

(1999). From the analysis, it can be concluded that three methods give almost same results

for the lower magnitudes but for larger magnitudes, Gumbel (1958) method overestimates,

Knopoff and Kagan (1977) method underestimates and the Bury (1999) method gives

average-estimate than other two methods, suggesting the better approach than other two.

The results of this study clearly indicate that Arakan-Yoma subduction zone (Zone-II) has

low mean return periods and high occurrence probability for the occurrences of moderate

to large magnitude earthquakes (6.07.0) in comparison to other three zones. The largest

annual earthquake magnitudes are close to 4.6, 5.1, 5.2, 5.5 and 5.8 in the Shillong Plateau

Zone (Zone-III), the Eastern Syntaxis Zone (Zone-I), the Himalayan Thrusts zone (zone-

IV), the Arakan-Yoma subduction zone (Zone-II) and the whole region, respectively. The

Zone-I (the Eastern Syntaxis Zone), Zone-III (the Shillong Plateau Zone) and Zone-IV (the

Himalayan Thrusts Zone) have low probabilities and large mean return period for higher

magnitude earthquakes when compared to Zone-II (Arakan-Yoma subduction zone). It is

observed that Zone-II (Arakan-Yoma subduction zone) has high probability ([95%) ofoccurrences of earthquakes of magnitude 6.07.0 in next 100 years among four major

Fig. 7 a Map view of probabilities of occurrences of earthquakes for magnitude range 6.06.5 in the next100 years in the four major seismogenic source zones of northeast India and adjoining region. Legend atlower-right corner of map shows the probabilities ranges for each zone. b Map view of probabilities ofoccurrences of earthquakes for magnitude range 6.67.0 in the next 100 years in the four major seismogenicsource zones of northeast India and adjoining region. Legend at lower-right corner of map shows theprobabilities ranges for each zone

b

Nat Hazards (2011) 56:145167 165

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seismogenic source zones of northeast India and adjoining region. The return periods and

probability of occurrences of very large earthquake ([7.0) cannot be established using theapproach used in this study due to the biasing of estimates in higher magnitude range. This

is the limitation of this method.

These results have potentially useful implications in the probabilistic seismic hazard

assessment for moderate to large earthquakes (4.07.0) in the considered region. The

reliability of results of seismic hazard parameters estimation in a given region depends on

the methodology and information input used. Here, we applied a proposed approach in

general way which is derived from the commonly accepted assumptions and constraints

related to earthquake occurrences. In the present study a reliable, homogeneous and

complete seismicity database is used to estimate the hazard parameters, thus, results may

be used as quantitative measures of seismicity for the considered region. The seismic

potentiality presented in this study for four major seismogenic zones of northeast India and

adjoining region can be interpreted as long-term seismic hazard assessment only.

Acknowledgment We are thankful to the Department of Science and Technology and Ministry of EarthScience, Government of India for providing financial support. First author is thankful to the Director,INCOIS and HoD, ASG, INCOIS for their support. Sumer Chopra and A. P. Singh helped in preparation ofthe manuscript. We acknowledge thoughtful comments and suggestions by Editor-In-Chief Dr. ThomasGlade and anonymous reviewers which enhanced the quality of manuscript significantly.

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Probabilities for the occurrences of medium to large earthquakes in northeast India and adjoining regionAbstractIntroductionRegional seismotectonics and zonation of the study regionThe applied methodHomogeneous and complete seismicity databaseResults and discussionConclusionsAcknowledgmentReferences

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