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Spatial variation of seismic b-value is estimated in the Indo-Myanmar subduction zone of northeast (NE) India
using the homogeneous part of earthquake catalogue (1996–2015), recorded by International Seismological
Center (ISC), consisting of 895 events of magnitude MW?3.9. The study region is divided into 1°×1° square
grids and b-values are estimated at each grid by maximum likelihood method. In this study, the b-value varies
from 0.75 to 1.54 in the region. Significant variation of low b-value in the respective location may indicate high
stress accumulation in that region. Spatial variation reveals intermediate b-value anomalies around the epicenter
of the Mw=6.7 Manipur earthquake which occurred on 3rd January at 23:05 UTC (4 January 2016 at 04:35
IST). The variations of b-values are also estimated with respect to depth. The low b-value associated with the
depth range?15–55 km, which may imply crustal homogeneity and high stress accumulation in the crust. Since,
NE India lies in the seismic zone V of the country; this study can be helpful to understand seismotectonics in the
region. The study of seismicity patterns is crucial for evaluation of seismic
hazards and better understanding of different seismotectonic aspects
within a seismogenic zone (Bernard et al., 2006; Schorlemmer et al.,
2005; White et al., 2009; Sorbi et al., 2012). In the Gutenberg and
Richter (1944) frequency-magnitude distribution relation, the b-value is
an important seismic parameter that describes the characteristics of an
ensemble of earthquakes. The parameter ‘b’ is believed to be dependent
on the stress regime and tectonic character of a region (Allen et al.,
1965; Mogi, 1967; Scholz, 1968). Many studies reveal that the b-value
indicates stress distribution and level of heterogeneity in the Earth’s
crust (e.g. Mogi, 1962; Scholz, 1968; Wyss, 1973; Wiemer and
Katsumata, 1999; Schorlemmer et al., 2005; Mousavi, 2017). As such, it
can provide significant information on the seismotectonic and hazard
potential of a region. Laboratory studies show, b-value are governed by
several factors, such as, the degree of material heterogeneity (Mogi,
1962), the level of applied stress (Scholz, 1968), the degree of stress
concentration (Meredith and Atkinson, 1983), the chemical reactivity
of pore fluid (Meredith and Atkinson, 1983), and the pore fluid pressure
(Sammonds et al., 1992; Lockner and Byrlee, 1991; Power et al., 1995;
Wiemer and McNutt, 1997; Wyss et al., 1997; Wiemer et al., 1998; Jollyand McNutt, 1999; Murru et al., 1999; Sanchez et al., 2004; Bridges and
Gao, 2006; Farrell et al., 2009; Van Stiphout et al., 2009; Bachmann
et al., 2012; Mousavi, 2017). Increase in the material heterogeneity or
an increase in the thermal gradient results in high b-values (Mogi, 1962;
Warren and Latham, 1970), while b-value diminishes with rise in applied
shear stress or effective shear stress (Wyss, 1973; Scholz, 1968;
Kayal, 2008; Singh et al., 2008). The b-values systematically vary with
earthquake focal mechanism. For example, normal faulting events exhibit
the highest, thrusts events have the lowest and strike-slip events
show intermediate b-values, respectively (Schorlemmer et al., 2005).
The areas of crustal homogeneity and high stress correspond to smaller
b-values (b 1) imply crustal heterogeneity
and low stress (Bridges and Gao, 2006; Mousavi, 2017). In
volcanic region with earthquake swarms, b-values are rather high, and
can have values as high as 3.0 (McNutt, 2005). For tectonic events bvalues
are in the range of 0.8–1.2 (e.g., Schorlemmer et al., 2005;
Kamer and Hiemer, 2015). Spatial variations of b-value have been
studied by several researchers in a number of seismically active areas
(e.g., Wiemer and Katsumata, 1999; Singh et al., 2009). For NE India,
few researchers have performed the spatial variation of b-values accompanied
by depth (e.g., Bhattacharya et al., 2002; Bhattacharya and
Kayal, 2003; Khan et al., 2011). The Indo-Myanmar subduction zone of NE India lying between latitude
22–27°N and longitude 93–97°E, is tectonically one of the most
earthquake-prone zones in the world (Fig. 1). The region has experienced
several large earthquakes of magnitude M ; 6.0 in the past few
decades (Kundu and Gahalaut, 2012). Subduction of the Indian lithosphere
plate beneath the Burmese plate makes this region tectonically
very complex. In this paper, an attempt has been made to evaluate the
spatial variation, and depth distribution of b-value in the Indo-
Myanmar subduction zone before the occurrence of 3rd January 2016
Manipur earthquake of magnitude MW=6 (Gahalaut et al., 2016). The NE India (Fig. 1) is characterized by high seismic activity. The
region is flanked by the ongoing India-Asia collision in the north and
the Indo-Myanmar subduction in the east, the Brahmaputra river alluvium
in the Assam valley, the Shillong-Mikir plateau in between these
two arcs, and thick sediments of the Bengal basin in the south (Bilham
and England, 2001; Kayal et al., 2012). The Indo-Myanmar subduction zone of NE India is one of the most
seismically active fold and thrust belts in the world, which extends from
Mishmi hills to the Bay of Bengal in NE-SW, NNE-SSW and N-S directions,
where the Indian plate subducts under the Burmese micro-plate
and shows very complex deformation (Barman et al., 2014). Since
1897, the NE India has experienced 20 large (M?7.0) and two great
earthquakes (M?8.5); one on June 12, 1897 (Oldham, 1899) and the
other on August 15, 1950 (Tandon, 1954). The western Indo-Burma
fold and thrust belt is segmented into N-S blocks along the E-W transverse
zones and exhibits dextral slip between the Nagaland salient and
the Manipur recess and sinistral slip between Manipur recess and the
Tripura-Mizoram salient (Jade et al., 2007). Towards the east of the
Indo-Burma fold and thrust belt (IBFTB), the India Sunda relative plate
motion of about 36 mm/year is partitioned between the Churachandpur-
Mao fault through dextral strike-slip of 16 mm/year and the
remaining motion (?20 mm/year) is accommodated at Sagaing fault
through dextral strike-slip motion (Kundu and Gahalaut, 2013). The
Sagaing fault is one of the most prominent active strike-slip faults in the
Indo-Burmese arc. It extends over 1000 km from north to south. In this way, it touches the Andaman spreading center (ASC) towards its
southern termination (Hurukawa and Maung, 2011). The largest
earthquake that had occurred along, or near, the Myanmar portion of
this fault is the Burma earthquake of Mw ?8 (Kundu and Gahalaut,
2013). The earthquakes in the Indo-Myanmar arc are referred to as
subduction tectonics and normal, thrust and strike-slip faulting have
been reported for the region (Kayal, 1996; Kumar and Rao, 1995).
Kayal (2008) and Le Dain et al. (1984) reported that intermediate depth
earthquakes dip eastward beneath the Indo-Myanmar ranges and they
reach a depth of ?200 km. The tectonic stress is much complicated in
the region due to collision tectonics in the north and subduction tectonics
in the east. The detail of the regional stress was studied by several
researchers (Jade et al., 2007; Kundu and Gahalaut, 2012, 2013;
Gahalaut et al., 2016). In this study, an earthquake data set containing 2648 events has
been selected of different magnitude scales within Indo-Myanmar
subduction zone. The magnitude of these events ranges from 2.5 to 7.0.
The data-set was prepared from the International Seismological Center
(ISC) catalogue during the period 1964–2015. While selecting the
events, duplication of events was kept minimum. Among the magnitude
scales as prepared by ISC catalogue, MW, mb, ML, MD and MS are the
significant ones, and this entails different conversion relations. So far,
several conversion relations of magnitude have been suggested by different
authors for NE India. In this study, Das et al. (2012) and Bora
et al. (2013) have been followed to convert the seismicity catalogue
into a homogeneous one (moment magnitude, Mw). Still, there is an
apparent influence of Magnitude of completeness (MC). This is considered
to be crucial for all seismicity-based studies as it leads to reliable
result by optimizing number of events available (Wiemer and
Wyss, 2000). As per definition, Mc refers to minimum magnitude of
complete recording at which 100% of the earthquakes in a space-time
volume are detected (Wiemer and Wyss, 2000; Woessner and Weimer,
2005; Rydelek and Sacks, 1989). In general, there happens to be spatial
and temporal change in Mc which corresponds to the ability of detection
of networks augments with passage of time (Mousavi, 2017). Methods
like Entire-magnitude-range (EMR) or the best fit suggested by ‘Zmap’
(Wiemer, 2001) are very instrumental. Although, they result in high Mc,
but their accuracy is limited upto some extent. However, while conducting
a standard analysis, best combination (MC ?95, MC ?90
Maximum Curvature) method (Wiemer and Wyss, 2000) emerges to be
a good choice and it is widely accepted. For entire data, temporal
variation of MC has been estimated (Fig. 2a). A sample window of size
200 samples has been chosen based on the number of earthquakes and
their distribution with time in the catalog. From Fig. 2a, it can be seen
that MC decreases sharply from 1996. This can be seen from the cumulative
curve as well (Fig. 2b). This may be either due to network
change or the use of more stations in the recent years that resulted in
detection of more events. We have selected the part of the catalog from 1996, where cumulative curve shows constant and steep slope, and MC
is computed accordingly using maximum curvature method (Wiemer
and Wyss, 2000) and with bootstrapping (Schorlemmer et al., 2003).
The value is estimated to be 3.9 (Fig. 3). After considering MW?MC,
total 895 events have been considered as final database and are used to
calculate the b-values in the area. Epicenters of 895 earthquakes (Mw ?
3.9) are shown in Fig. 4. Epicenters of some strong earthquakes
(M?6.0) are also shown in Fig. 1. These events mostly show thrust/
reverse faulting; a few solutions with strike slip components (Kundu
and Gahalaut, 2013). The Gutenberg-Richter relation (Gutenberg and Richter, 1944) is
one of the well known empirical relations in seismology. It represents
the frequency of occurrence of earthquakes as a function of magnitude
as: log10N (M)=a?bM, where N (M) is the cumulative number of
earthquakes having magnitude equal to or larger than magnitude M. ‘a’
is a measure of seismic activity that depends on the size of the area and
constant ‘b’ describes the relative size distribution of earthquakes. Numerous
methods have been proposed to calculate out b-value and its’confidence limit (e.g. Aki, 1965; Utsu, 1965, 1992; Bender, 1983; Shi
and Bolt, 1982; Frohlich and Davis, 1993; Okal and Kirby, 1995; Scholz,
1997; Mousavi, 2017). In this study, the modified maximum likelihood
method of Aki (1965) and Utsu (1965) is utilized which is as follows:

Here, M denotes the average magnitude of a group of earthquakes
with M > MC, MC is the magnitude of completeness and Mbin is the
binning width. For spatial distribution, the study area is divided into
square grids having dimension of 1°×1° (geographical window) each
with an overlapping window of dimension 0.5°×0.5° (moving
window). To maintain the inherent grid to grid continuity of data
points, the moving window has been opted. Due to insufficient data, bvalue
calculations are not performed in the grids having Nmin (minimum
number of events) 120 km), the number of events are decreasing. This
may be due to low tectonic stress level at deeper depths, resulting the
increase in b-values. The b-values obtained in the present study are
compatible with the earlier reported b-value for the NE India (Khan
et al., 2011). In some segments of the study region reflect higher stress
accumulation compared to others, which may indicate the possibility of
future large rupture at these locations. Since, the Indo-Myanmar subduction
zone is an active region with the ongoing subduction of Indian
plate beneath the Burmese plate, the present study will provide useful
information to estimate seismic hazard assessment in the study region.
However, comprehensive catalogue will be essentially important in
future for more detailed analysis in the region. In this study, spatial and depth distribution of b-values are estimated
in the Indo-Myanmar subduction zone of NE India using earthquake
catalog from 1996 to 2015. Spatial variation map showed that the bvalue
varies from 0.75 to 1.54 within the region. Intermediate variation
of b-values were observed around the epicenter of the 3rd January,
MW=6.7, Manipur earthquake. These b-value anomalies can be interpreted
as a high stress accumulated zone before the Manipur earthquake.
Low b-value anomalies are observed along Sagaing fault and
Kabaw fault region. Low b-values (0.94–1) are found in the upper crust,
which indicate homogeneity and high stress accumulation in the upper
crust. Also, this low b-value may be due to high seismic moment release
in the uppermost crust. As b-value is directly related to the local and
regional factors including tectonic stresses, seismic moment release,
and earthquake focal mechanisms, our study on it could provide valuable
information of precursory signal of seismic hazard of the Indo-
Myanmar subduction zone. The authors JB and DB acknowledges financial support from
University Grant Commission, New Delhi, India, vide sanctioned No. F.
No.-43-522/2014 (SR). The figures were made using Generic Mapping
Tools (GMT) (Wessel and Smith, 1998). The Editor-in-Chief Prof. Mei-
Fu Zhou, Associate Editor Prof. Dapeng Zhao and three anonymous
reviewers are gratefully acknowledged for their constructive comments
and suggestions which helped us in upgrading manuscript significantly
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