Citation:
Ngun Za Iang, S. Kita (2019), Completeness Magnitude of Earthquakes and b-Value in
Myanmar, Synopsis of IISEE-GRIPS master report.

COMPLETENESS MAGNITUDE OF EARTHQUAKES
AND b-VALUE IN MYANMAR
Ngun Za Iang1,2

Supervisor:

Saeko KITA3ABSTRACT
In order to estimate completeness magnitude of earthquakes and b-values, the NEDC local dataset was usedduring 2014-2018. This study aims to estimate the completeness magnitude of earthquakes in Myanmar and to
examine the b-value with its relations: depth, faulting style, and stress accumulating in a sophisticated tectonic
setting of Myanmar. The completeness magnitude Mc was estimated as ML 2.8 with a b-value 0.68±0.02 in
the whole study region. Regionally, the b-values vary from 0.52 to 1.0. The tendency relation between b-value
and faulting styles was found along the Sagaing Fault. The southern part with a smaller b-value indicates athrust mechanism, whereas the northern part with a larger b-value shows a strike-slip mechanism. The b-value
generally increases with the increase of depth in the whole study region. In the active fault region, the b-value
decreases with depth within the upper crust at depths of ~15 km, which might be related to high-stressaccumulation. Below at a depth of 15 km, the b-value then increases with a function of depth. In contrast, asignificant decrease of b-value with depth was observed at depths of 75-85 km in the subduction zone of thestudy region, which might be discussed with the process of dehydration in the ocean crust. The methodologyof this study might be fruitful to understand the seismically complex tectonic system beneath Myanmar.
Keywords: b-value, completeness magnitude, maximum likelihood method, stress, Myanmar.

1. INTRODUCTION
b-value is one of the most functional seismic parameters to study the seismicity within a specified region andtime window. The b-value is calculated from Gutenberg and Richter (1944) frequency- magnitude distribution
as log10 N=a - bMmin, where N is the cumulative number of earthquakes with the magnitude M (≥Mmin), ‘a’ and‘b’ are constants. The parameter ‘a’ is related to seismic activities, depending on many parameters; the size ofthe region, time intervals, a total number of earthquakes and its magnitude and the value of ‘b’. ‘b’ is the bvalue, indicating the slope of the cumulative number against magnitude. To understand seismic characteristics,
previous studies have revealed b-value and its correlations: heterogeneous materials (Mori and Abercrombie,
1997), focal mechanism dependence (Schorlemmer et al., 2005), dependence of depths and strength profile ofthe crust (Spada et al., 2013), the age of a subducting plate (Nishikawa and Ide, 2014) and differential stress(Scholz, 2015). Nanjo et al. (2019) and Nanjo et al. (2012) indicated that the b-values of earthquakes in
the Kumamoto and Tohoku-Oki decrease just before the mainshocks, respectively. Therefore, theexamination of b-values of events can offer valuable information about the future activity of earthquakes
in the specific region.

1

Department of Meteorology and Hydrology (DMH), Myanmar.
IISEE-GRIPS master course student
3
International Institute of Seismology and Earthquake Engineering, Building Research Institute.
2

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Myanmar is one of the most earthquake-prone regions, located in the Alpide Himalayan Seismic
Belt. Mainly, four tectonic plates exist in and around Myanmar: The Eurasian, the Indian, the Sunda, and theBurma Plates (Hurukawa et al., 2012) as shown in Figure 1. Also, the Indian Plate subducts beneath the Burma
Plate. Moreover, the Sagaing Fault is a major active fault, located as a boundary between the Burma andSunda Plates, and runs over 1000 km from north to south. It is a right-lateral strike-slip fault with a displacement
rate of 18 mm/yr (Socquet et al., 2006). Due to the seismically complex tectonic system, the shallow tointermediate-focus earthquakes occurred in the subduction zone (the western part), whereas most of the shallow
earthquakes occurred in the active fault region (the eastern part) as shown in Figure 1. In order to comprehendcharacteristic of seismic activities beneath in Myanmar, the main goal is to estimate the magnitude ofcompleteness of earthquakes in Myanmar using the dataset of National Earthquake Data Center (NEDC) andto evaluate b-value with its correlations: depth, focal mechanisms and stress accumulation in the study region.

Figure 1. The tectonic setting of Myanmar
and neighbor regions (Hurukawa et al. (2012)
and Wang et al. (2014) ) and epicenters of events
during 201-2018 (NEDC).

Figure 2. Locations of broadband seismic
stations (inverted-red triangles) in Myanmar
and neighbor countries’ stations (invertedblue triangles). Red lines: Active faults.

2. METHODOLOGY
To evaluate the b-value, the maximum likelihood method (Utsu, 1965) was adopted as follows;b=

log 10 e,
M − Mc(1)where M is the average magnitude of cumulative earthquakes with M > Mc. Mc is the magnitude ofcompleteness or the smallest earthquake magnitude and log10 e = 0.4343. A larger b-value corresponds tosmaller events while a smaller b-value is related to larger events.
Moreover, to calculate the error δb of b-value, the method developed by Aki (1995) wasused as follows;b = b N ,(2)where N is the total number of earthquakes of the given sample.
The goodness-of-fit method (Wiermer and Wyss, 2000) was also used to estimate Mc bycomputing the absolute difference R of the number of events in each magnitude bin as follows;

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M

  Bi − Si MiR(a, b, M i ) = 100 − 
100  ,
 i Bi





max

(3)

where Bi and Si are the observed and predicted cumulative number of events in each magnitude bin.
3. DATAA total of 21 broadband seismic stations has been installed throughout Myanmar between 2008 and 2018
(Figure 2), which provide a valid dataset with local magnitude (ML) within a distance of 1,000 km(Shiddiqi et al., 2019). Since 2014, the Department of Meteorology and Hydrology (DMH) has beenusing SEISAN program developed by Havskov and Ottemoller (2010) from the University of Bergen to
determine hypocenter locations and magnitude of events. The study area was selected between thelatitudes of 16-28º N and the longitudes of 92-102º E depending on including the number of events
within the study period (2014-2018). This study could not cover the Andaman Islands region due to thelimitation of event number during this study. A total of 1350 events was collected from the NEDC local
dataset with local magnitude listed from 1.0-6.3 (ML) to estimate Mc and b-values. Moreover, 197
events were also collected from the Global Centroid Moment Tensor Catalog between 2009 and 2018
to investigate focal mechanism solutions which show faulting styles.
4. RESULTS AND DISCUSSIONS
4.1. Completeness Magnitude and b-Value Estimation in the Whole Study Region
By using the NEDC local data (2014-2018), including 1350 events of homogeneous magnitude (ML), themagnitude of completeness was estimated as ML 2.8 with 690 events and a b-value 0.68±0.02 in the wholestudy region (Figure 3). When the change of Mc with time was calculated by using a three-year window, theMc decreases with time during this study period (Figure 4). The decrease of Mc with time might be related
to the improvement of local seismic networks in Myanmar during this study period. A total of 690 events
satisfied Mc 2.8 was taken as a final database to estimate the b-values in this study (Figure 5).

Figure 3. Estimation of b-value and Mc in thewhole study region. Solid red line: the line of thebest fit. Circles: the cumulative number of events.
Crosses: the number of events of eachmagnitude.

Figure 4. The change of Mc with time during this study
period (2014-2018). ML: Local Magnitude.

34.2. Regional Variations of b-values in the Study Region
To examine differences of b-values between the subduction and active fault regions, the study region (Figure5) was divided into three sub-regions: the western part (the subduction region) between the latitudes of 16-28º
N and the longitudes of 92-95.4º E, the region along the Sagaing Fault between the latitudes of 16-28º N andthe longitudes of 95.4-97.5º E and the eastern part
(active fault region), the latitudes of 97.5-102º E. Thenthe b-values were obtained as 0.74±0.03 in the
subduction region, 0.62±0.05 in the region along theSagaing Fault and 0.65±0.06 in the eastern partrespectively. The b-value of the subduction region ishigher than that of other active fault regions. Oncel et al.
(2001) also showed that lower b-values are associatedwith the increase of possible occurrence of moresignificant future earthquakes because of theconcentration of stress in the extremely faulted area.
Moreover, when dividing the study region(Figure 5) into four, the b-values were obtained as:
0.74±0.03 in the northwestern part between 22-28º N
and 92-97º E, 0.66±0.05 in the southwestern part
between 16-22º N and 92-97º E, 0.52±0.06 in thenortheastern part between 22-28º N and 97-102º E and
1.0±0.13 in the southeastern part between 22-28º N and Figure 5. The epicenter locations of selected 690
92-97º E, respectively. The variations of b-values from events (ML ≥ 2.8) within the study region, from
0.52 to 1.0 in this study might be related to regional the NEDC dataset (2014-2018). Red lines:
stress variations resulting from the characteristics of the Active faults.
tectonic plate and active fault.
Moreover, the tendency correlation of b-values andfaulting styles was also found along the Sagaing Fault (Figure 6).
The b-value in the southern part containing thrust mechanisms is
0.61±0.07, whereas the b-value in the northern part showing
strike-slip mechanism is 0.67±0.07 (Table 1). Schorlemmer et al.
(2005) also indicated that the b-values of thrust mechanisms arelower than those of strike-slip mechanisms.
Table 1. Difference of b-values along the Sagaing Fault.
Region
Northern part
Southern partb-value
0.67
0.61

Error
0.07
0.07

4.3. Regionally Variations of b-Values with depths
In the whole study region, the variation of b-values with depths
was calculated by using moving 5 km depth with 150 events ineach bin starting from the surface to the depth of 150 km (Figure7). Generally, the trend of b-values increases with the increase of
depth. The depth histogram (Figure 8) showing the decrease of event number with depth might affectthe increase of b-values with depths as well. The b-value slightly decreases within the depths of ~15 km,
and below at 15 km depth, the b-value increases with depth. The lower b-value within the upper crust
may be associated with the higher accumulation of stress because the b-value is negatively correlated tostress. Spada et al. (2013) also indicated that the b-value decrease within the brittle upper crust before
Figure 6. Focal mechanism solutions(global CMT catalog) of events along
the Sagaing Fault.

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stopping the decrease in depth near the brittle-ductile transition zone, and increases in the deeper depths
again.

Figure 7. Variation of b-values (yellow circles) with depth andits error (red vertical bar) in the whole study region.

Figure 8. Depth histogram of theselected 690 events (ML ≥ 2.8).

Moreover, the variation of b-values with depths was examined depending on the differenceof tectonic features. In the active fault regions, the variation of b-values with depths was examined byusing moving 5 km depth with a 50-event bin due to the limitation of event number during this study.
As a result, the b-value decreases within depths of ~15 km, and below at a depth of 15 km, the b-value
increases with the increase of depth (Figure 9). Moreover, the depth histogram of events (Figure 10) of
the active fault regions shows the decrease of event number with depth below at a depth of 20 km.

Figure 9. Variation of b-values (yellow circles) with depth
and its error (red vertical bar) in the Sagaing Fault region.

Figure 10. Depth histogram of events(ML ≥ 2.8) in the Sagaing Fault region.

When the variation of b-values was examined in the subduction region, the computation
was performed by using moving 5 km depth with a 100-event bin. In contrast, a significant decrease ofb-value was observed at depths of 75-85 km, which is similar with the result of the whole study region
showing the decrease at depths of 80-85 km (Figure 6). Shiddiqi et al. (2018) reported that the source
process of the Chauk earthquake (Mw 6.8) with an intraslab intermediate-depth of 84.1 km, which shows
sub horizontal mechanism faulting, corresponds to the dehydration fragility. The significant lower bvalue at depths of 75-85 km might be associated with the hypocenter depth of the Chauk earthquake (ata depth of 84.1 km) as well as the process of dehydration fragility at intraslab intermediate-depths.

5. CONCLUSIONSIn this study, the b-value and the completeness magnitude Mc of earthquakes in Myanmar were
estimated by analyzing the NEDC local dataset (2014-2018) and obtained Mc 2.8 (ML) and b=0.68±0.02
in the whole study region. The Mc also decreases with time. Moreover, by investigating the regionalvariation of b-values, the b-values vary from 0.52 to 1.0. The b-value of the active fault region is lower

5than that of the subduction region, which could show higher stress accumulation in the active fault region
than the subduction region. Besides, the tendency correlation of b-values and different faulting styles
was investigated along the Sagaing Fault. The southern part of the Sagaing Fault related a smaller bvalue shows thrust mechanisms, whereas the northern part of the Sagaing Fault correspondent to a larger
b-value indicates strike-slip mechanisms.
Furthermore, when the temporal variation of b-values as a function of depths in the study
region was explored, generally, the b-value trend increases with respect to depth in the whole region. In
the active fault region, the b-value decreases with depth within the brittle upper crust at depths of ~15km, and the decrease of b-value stops near the brittle-ductile transition, which shows the higher stressaccumulation in the upper crustal rock. Then below 15 km, the b-value increases with the increase of
depth. In contrast, a significant decrease of b-value was found at intraslab intermediate- depths of 75-85
km in the subduction zone.
This study shows that the examination of b-values and following procedure are useful for evaluating
seismic observation network and understanding seismic activities in Myanmar. The installation of thelocal seismic observation networks might play an essential role in the study of seismic hazard assessment
and the mitigation of devastating natural disasters caused by earthquakes in earthquake-prone regions.
ACKNOWLEDGEMENTS
This research was conducted during the individual study period of the training course “Seismology,
Earthquake Engineering and Tsunami Disaster Mitigation” by the Building Research Institute, JICA,
and GRIPS.I am deeply thankful to my advisor and supervisor, Dr. Saeko KITA, for always giving
valuable advice and guiding me to complete my individual study successfully. I would like to express
my gratitude to Dr. Tatsuhiko HARA and Dr. Bunichino SHIBAZAKI for their invaluable suggestion
throughout my research work. I would like to express thanks to all JICA and IISEE/ BRI staff members
for their support and encouragement through this training course. I am grateful to GRIPS for giving mea chance to attend this master course.
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