RELOCATION OF EARTHQUAKES IN MYANMAR BY
MJHD METHOD: AFTERSHOCKS OF LARGE
EARTHQUAKES AND SEISMICITY ALONGTHE SAGAING FAULT
PHYO Maung Maung∗
MEE08159

Supervisor: Bunichiro SHIBAZAKI∗∗
Nobuo HURUKAWA∗∗ABSTRACTTo understand the seismicity in Myanmar, detailed analysis of hypocenters and fault planes
determination are important. First, we relocated seven earthquakes of which magnitudes are Mw > 6.0,
together with their aftershocks using modified joint hypocenter determination (MJHD) method andused P-wave arrival times from International Seismological Centre (ISC). We relocated aftershocks of
one day, one week, or one month based on the aftershocks’ occurrences. Target area is + 1 degreefrom both the latitude and longitude from Global CMT for each large earthquake. After the relocationsof the large earthquakes and their aftershocks, we could identify the fault plane of three earthquakes
but some earthquakes are difficult to identify the fault plane due to aftershocks occurrences.
Furthermore, we relocated earthquakes including the historical earthquakes to investigate theseismicity along the Sagaing fault. We used P-wave arrival times from ISC (1961 to March, 2007) andthose for seven historical earthquakes from International Seismological Summary (ISS). Once thehypocenters relocated precisely, the earthquakes occurrence and seismic gap along the Sagaing fault
would become clear. Some relocated epicenters are closer to the fault than those before relocation. We
could determine the hypocenters of historical earthquakes and recent earthquakes on the fivesub-regions. It means our results are consistent with the geological setting of this region and are usefulfor seismic hazard assessment in this region.
Keywords: Hypocenter relocation, Sagaing fault, Historical earthquakes

1. INTRODUCTION
Myanmar is located at a very active tectonic area, which includes the subduction zone and the active
Sagaing fault. From seismological point of view, Myanmar is one of seismically active countries of theworld. Shallow and intermediate earthquakes occur along the Burma subduction zone and shallow
strong earthquakes occur along the Sagaing fault (Figure 1). The Sagaing fault is an active right lateral
strike slip fault crossing from north to south through Myanmar. Most of historical earthquakes occurredalong the fault. Fifteen large earthquakes (of M > 7.0) hit the country from 1912 to 2003 (Figure 2).
The frequency of the occurrence of large earthquakes is around once in 12 to 16 years. According to
the seismicity map, the northwestern part of Myanmar is one of strong earthquake sources of the world.
The earthquakes are spreading all over the country except the southern coastal regions, which indicatesorientation of active faults. In the present study, we relocate aftershocks of seven large earthquakes
from 1988 to 2003 to determine the fault plane by MJHD method. In order to know the precise
seismicity along the Sagaing fault, we relocated earthquakes along the Sagaing fault including eventsfrom 1961 to March, 2007 and historical earthquakes. We use the data from International
Seismological Centre (ISC) and International Seismological Summary (ISS).
∗Department of Meteorology and Hydrology (DMH), Myanmar.
**International Institute of Seismology and Earthquake Engineering, Building Research Institute, Japan.

1Figure 1. The epicenter distribution map ofMyanmar is used USGS data from 1973 to2009, March 31. The different size andcolor of symbols indicates the size anddepth of earthquakes.

Figure 2. The epicenter distribution map ofhistorical earthquakes in Myanmar occurredfrom 1912 to 2003 and data from Departmentof Meteorology and Hydrology (DMH).

2. METHOD
In this study, we used the Modified Joint Hypocenter Determination (MJHD) method developed by
Hurukawa and Imoto (1990, 1992) for the determination of hypocenters and fault planes in Myanmar.
The equation shown as follows;(O − C )ij = (t ij − To j ) − Tij =

∂t ij

∂λ j

dλ j +

∂t ij

∂ϕ j

dϕ j +

∂t ij

∂z j

dz j + dTo j + dS i…………… (1)where tij is arrival time of jth event at ith station, Tij is the calculated travel time of the jth event at the ithstation, dSi is the station correction at the ith station, Toj is the origin time, O is the observed travel time,
C is the calculated travel time, (O-C)ij is the travel time residual of jth event at ith station, dλj, dϕj, dzjand dToj are correction of trial hypocenter of jth event.
Due to the heterogeneous earth’s structure, when the station coverage is not good, the JHD
solutions become unstable and unreliable because of the trade–off between station corrections andfocal depths of earthquakes. For this reason, Hurukawa and Imoto (1990, 1992) developed using theconstraints below.
n∑ S i Di = 0i =1n∑ S i cos θ i = 0i =1n∑ S i sin θ i = 0i =1n∑ S = 0 ………………………… (2)i =1iwhere Si is the station correction at ith station, Di is the distance between the ith station and the center ofthe region, θi is the azimuth of the ith station from the center of the region and n is the number ofstations.

23. DATA
3.1 Aftershock relocation and its fault plane
Foreshocks, a mainshock and its aftershocks are relocated in this study by using P-wave arrival times
from International Seismological Centre (ISC) in order to determine the fault plane. Then, thecorresponding fault plane is determined based on the obtained aftershock distribution. We relocatedseven large earthquakes from 1988 to 2003 with Mw > 6.0 with data taken from Global CMT catalog.
3.2 Historical earthquakes relocation
We relocate seven historical earthquakes using data from International Seismological Summery (ISS).
These are the January 19th, 1929 (M=7.0), August 8th, 1929 (M=7.0), May 5th, 1930 (M=7.3),
December 3rd, 1930 (M=7.3), January 27th, 1931 (M=7.3), September 12th, 1946 (M=7.3) and July 16th,
1956 (M=7.0) earthquakes. In addition, we relocated the earthquakes to know the precise seismicity
and to determine hypocenter on the five sub-regions along the Sagaing fault using International
Seismological Center (ISC) from 1961 to March, 2007.

4. RESULTS
4.1 Aftershock relocation and its fault plane
It is difficult to determine the fault plane of the mainshock using only ISC hypocenter distribution due
to insufficient accuracy. Therefore, we analyzed the earthquakes to determine the fault plane using
MJHD method. In order to determine the fault plane of the August 6th, 1988 earthquake, we relocatedthe aftershocks from August 6th, 1988 to August 21st, 1988. We cannot determine the fault plane of themainshock using International Seismological Centre (ISC) hypocenter distribution map in Figure 3.

Figure 4. MJHD hypocenter of the August
6th, 1988 earthquake and its aftershocks.
Symbols are same as in Figure 3. The A-Band C-D lines are two nodal planes shown bysolid lines in cross sections. The moment
tensor solution is taken from Global CMT
catalog.

Figure 3. Hypocenter distribution of theAugust 6th, 1988 earthquake and its
aftershocks by ISC. The size of symbols
represents the magnitude of the events. The
color and shape of the symbols denote thedepth range of the events.

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We identified 236 stations worldwide and phase data with travel time residuals (O-C) > 2.0
seconds were excluded. The mainshock and its aftershocks are relocated to identify the fault plane
using MJHD method in Figure 4. The hypocenter distribution of this event suggests the nodal plane
shown in cross section C-D could be the fault plane related to that event as shown in Figure 4, because
the mainshock and all aftershocks were located along this nodal plane. The strike and dip of the fault
plane are 148° and 54° determined by Global CMT.
Similarly, we could determine the fault plane of the November 6th, 1988 earthquake with Mw
7.0 occurred in the China-Myanmar border region as shown in Figure 5. The largest aftershock
followed with Mw 6.9 within 12 minutes after the mainshock and some nine aftershocks followed on
the same day with Mw > 4.5. From the MJHD hypocenters distribution map, we can clearly find thefault plane of the earthquake after relocation from two nodal planes. The nodal plane shown in thecross section A-B is the fault plane because aftershocks concentrated along this plane. The strike anddip angle of this fault plane are 333° and 78° as determined by Global CMT. And also, we could
determine the fault plane of the September 21st, 2003 earthquake as shown in Figure 6. The nodal plane,
which is shown in cross section C-D could be the fault plane of the event. Aftershocks are concentrated
along this nodal plane and all earthquakes are around 20 to 50 km depths. The strike and dip of thefault plane are 100° and 83° determined by Global CMT.

Figure 6. MJHD hypocenters of theSeptember 21st, 2003 Earthquake and its
aftershocks. Symbols are same as in Figure 4.

Figure 5. MJHD hypocenters of theNovember 6th, 1988 Earthquake and its
aftershocks. Symbols are same as in Figure 4.

But, it is difficult of determine the fault plane of the January 5th, 1991 earthquake due to thelack of aftershocks. However, we think that the nodal plane shown in the cross section A-B might be
the fault plane because aftershocks occurred near the nodal plane. We could determine the accurate
hypocenters but we could not determine the fault plane by using MJHD method alone. For the April
23rd, 1992, July 11th, 1995 and June 7th, 2000 earthquakes, we could not determine the fault plane due
to lack of aftershocks even though earthquakes occurred in the shallow depth.
4.2 Historical earthquakes relocation
In order to understand the precise seismicity along the Sagaing fault, we relocate hypocentersof historical large earthquakes from 1929 to 1956 using data from International Seismological
Summary (ISS). Combining historical earthquakes and recent earthquakes from 1961 to March, 2007
using the data from International Seismological Centre (ISC), we relocated them simultaneously inorder to improve accuracy of historical earthquakes location by MJHD method (Hurukawa et al. 2008).

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Figure 7 (a) shows location of the five sub-regions and locations of seven historicalearthquakes by ISS data and Figure 7 (b) shows the relocation results by MJHD method. Only P-wave
first arrivals are used in this hypocenters relocation because the reading accuracy of S-wave arrival
times is worse than P-wave arrivals.

(a)

(b)

Figure 7. Comparison between before and after relocations on the fivesub-regions. (a) ISS locations and (b) MJHD locations. The rectangles with blue
color (A-E) indicate the five sub-regions for relocation of earthquakes along theSagaing fault and red circles indicate the epicenters of historical earthquakes.

S.F

In the region (A), we relocated the 223 earthquakes from 1961 to March, 2007 and two historical
recorded earthquakes: the January 19th, 1929 (M 7.0) and January 27th, 1931 (M 7.3). We selectedearthquakes that had occurred in the rectangle between 25.00°N-27.00°N latitude and 96.00°E-99.00°Elongitude. Phase data with travel time residuals (O-C) > 2.0 seconds were excluded. After therelocation by MJHD method, we can see clearly that the earthquakes concentrate along the active
Sagaing fault and depths of the shallow
earthquakes are around 10-40 km. Two historicalearthquakes are moved closer to the Sagaing
fault with focal depths of around 0-20 km as
shown in Figure 8, while some of earthquakes
are occurred at 40-50 km depth. The differenceof location for the January 19th,1929 (M 7.0)
earthquake is 0.47°N in latitude and 1.10°E in
1931 EQ
1929 EQ
longitude and depth is 20.4 km. We could
determine the hypocenter of historicalearthquakes with accuracy because the errors ofthe hypocenters are not so large. Epicentral
distribution and E-W and N-S cross section
along A-B and C-D lines are shown in Figure 8
Figure 8. Relocated hypocenters by the MJHD and cross bar in the circles represent the standard
method in region A. Symbols are same as in errors of the hypocenters.
Figure 3. S.F is the Sagaing fault.

5One historical earthquake (M=7.3) occurred on September 12th, 1946 in the region (B). The
deeper earthquakes occurred outside of the Sagaing fault. According to the relocation result, thelocation of the 1946 historical earthquake moved to the north and depth is still shallow. We relocated
the July 16th, 1956 historical earthquake in the region (C). We think that the hypocenter would be
mislocated because we relocate this earthquake using few stations data and stations are far from theepicenter. We relocate the earthquakes from 1961 to March, 2007 and the August 8th, 1929 historicalearthquake in the region (D). The earthquake cluster occurred between the Sagaing fault and othersmall fault from epicenter distribution map. About the historical earthquake, we think that thehypocenter would be mislocated because we used few stations data and the stations are far from theepicenter. In the region (E), we relocated 61 earthquakes during from 1961 to March, 2007 including
the two historical earthquakes that occurred on the May 5th, 1930 and December 3rd, 1930. Afterrelocation, we could determine the focal depth of the historical earthquakes to be around 5-40 km.
5. DISCUSSION AND CONCLUSION
We relocated the seven earthquakes from 1988 to 2003 with Mw > 6.0 and their aftershocks todetermine the fault plane using Modified Joint Hypocenter Determination (MJHD) method developedby Hurukawa and Imoto (1992). We obtained high accuracy hypocenters of the earthquakes and ourresults are more reliable than hypocenters of International Seismological Centre (ISC) catalog. We
could determine the fault plane of the August 6th, 1988 earthquake, November 6th, 1988 earthquake andSeptember 21st, 2003 earthquake. However, we could not determine the fault plane of someearthquakes which are the January 1st, 1991 earthquake, April 23rd, 1992 earthquake, July 11th, 1995
and June 7th, 2000 earthquake. These results indicate that we cannot determine the fault plane of small
earthquakes. Moreover, we noticed the deep and shallow earthquakes have aftershocks with different
behavior. Large shallow earthquakes are likely to be followed by aftershocks more than deep
earthquake. This MJHD method is very useful for determining the accurate hypocenter of theearthquake.
In addition, we relocated the earthquakes along the Sagaing fault using ISC data from 1961 toMarch 2007 and ISS data for historical earthquakes from 1929 to 1956. We used the only P-wave
arrival times and determined the hypocenters of earthquakes during 47 years and the historicalearthquakes using the MJHD method. We determined the depths of the January 19th, 1929 (M 7.0) andJanuary 27th, 1931 (M 7.3) historical earthquakes are 5 to 30 km in region (A). The 1946 historicalearthquake epicenter is on the Sagaing fault before relocation. After the relocation, the earthquakemoves to the north although depth is still shallow. We could determine the hypocenters of theDecember 3rd, 1930 (M=7.3) earthquake that moved to the north and May 5th, 1930 (M=7.3)
earthquake that moved to the west. The depths of earthquakes are around 10-50 km. Before relocation,these two earthquakes are located on the same place according to the ISS data. We think that the July
16th, 1956 and August 8th, 1929 earthquakes were mislocated due to the poor azimuth coverage and low
quality of data from ISS. Therefore, we concluded that the azimuth coverage, nearby stations andreading data are important in order to relocate hypocenter using MJHD method. We have to be careful
for the input parameters for relocations and the data to get accurate and reliable results. Finally, weexpect that the results of this study will be useful for the future seismological study in Myanmar.
AKNOWLEDGEMENT
I am sincerely grateful to Mr. Dmitry A. Storchak, Director and Mr. James Harris, System andDatabase Administrator from ISC for their continuous assistance on manipulating computer data.
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Hurukawa, N. and Imoto, M., 1992, Geophys. J. Int., 109, 639-652.
Hurukawa, N., 1995, Geophy. Res. Letters, Vol. 22, No. 23, pp 3159-3162.
Hurukawa, N. M. Popa, and M. Radulian, 2008, Earth Planets Space, 60, 565-572

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