HYPOCENTER RELOCATION AND MOMENTTENSOR ANALYSIS OF EARTHQUAKES IN
MYANMAR: TOWARD THE INVESTIGATION OF
THE BURMA SUBDUCTION-SAGAING FAULTSYSTEM
Pa Pa Tun∗
MEE07164

Supervisor:

Bunichiro SHIBAZAKI∗∗
Nobuo HURUKAWA∗∗ABSTRACTFor understanding tectonics and precise earthquake hazard assessment, the detail analysis of
hypocenters and moment tensor are necessary. First, present study determined the moment tensor
solutions around Myanmar using teleseismic body wave form by the inversion program by Yagi.
Moment tensor solutions obtained in this study are almost consistent with the global CMT solutions.
The general orientations of P-axis for shallow earthquakes are consistent with the NNE direction of the
Indian plate motion with respect to the Eurasian plate. Next, we relocated hypocenters in and around
Myanmar, using the modified joint hypocenter determination (MJHD) method developed by
Hurukawa and Imoto. This technique allows us to relocate many events simultaneously in order toimprove hypocenter location compared with ISC hypocenters; the MJHD method gave the surprisingly
clear features of seismicity. Along the Burma arc, the hypocenters are concentrated on narrow inclined
zone. The results show the configuration of the subduction zone. Also we clarified the features ofseismicity along the inland fault zone. The Sagaing fault system is located at the area where the cut-off
depth of seismicity becomes shallower and the strength of the crust is weaker. Furthermore, we foundthat the earthquakes are concentrated on the active spreading axis and transform fault in the southern
opening region. The results give us new constraints on understanding tectonics in and around
Myanmar.

Keywords: Moment tensor inversion, Hypocenter relocation, Burma subduction, Sagaing fault
INTRODUCTION
Myanmar is located at the very active tectonic area which includes the Burma oblique subduction, theSagaing strike slip fault system and the southern opening region. For understanding the tectonic
features of Myanmar, we determined the moment tensors using the body wave form inversion. The
focal mechanism bears information on tectonics around the earthquake source region: plate motion andthe tectonic stress which causes the earthquake. Seismic moment M o represents the size of anearthquake. Hypocenters will be useful for determining the subduction interface. Along the Burma
subduction zone, there is a high potential for occurrence of large tsunamigenic earthquakes. Therefore,
we need to urgently determine the location of the subduction interfaces. The purpose of this study is toconfirm the plate boundary between the Indian plate and Eurasian plate along the subduction zone andto understand the tectonic features in Myanmar.
∗

Department of Meteorology and Hydrology (DMH) of Myanmar.
International Institute of Seismology and Earthquake Engineering, Building Research Institute, Japan.

∗∗

DATA
Moment tensor inversion
We retrieved data from the Data Management Center of the Incorporated Research Institution for
Seismology (IRIS-DMC) via internet. In this study, we analyzed only teleseismic body wave (Pwaves) from 54 events in the range of Latitude between 8.00° N and 30.00°N and Longitude between
90.00°E and 102.00°E with the body wave magnitude (Mb) above 5.5.
Hypocenter determination
In this study we used the earthquake phase data from International Seismological Center (ISC) catalog
from January of 1980 to December of 2004. The data include the P-arrival times from worldwide
stations.

THEORY AND METHODOLOGY
Moment tensor inversion
We represented the seismic source process as point source model. We can write the verticalcomponent of the seismic waveform at the station j as
6

u j (t ) = ∑ M q ∫ G jq (t − τ , xc , y c , z c )T (t )dτ + eo + em ………………. (1)q =1where G jq is the complete Green’s function, M q

is the moment tensor at centroid ofsource ( x c , y c , z c ) , T (t ) is source time function, e0 is observation error, and em is modeling error.
Also we can write this equation in simple vector form as.

d = G (T (t ), xc , y c , z c )m + e ................................................ (2)where d is the observation waveform, G is the convolution Green’s function with source timefunction and m is the moment tensor solution.
Moment tensor inversion needs an assumption of location of hypocenter and information ofsource time function (duration, shape). We determined the depth of hypocenter and duration and shapeof source time function by grid search method. In this study, we used the prem-modify-model velocity
structure to find the moment tensor solution. We calculated the Green’s function and performed body
waveform inversion using programs developed by Yagi (2004).
Hypocenter determination
The equation which is used in the determination of hypocenters is as follows.

(O − C ) ij = (t ij − Tooj ) − Tij =

∂t ij

∂x j

dx j +

∂t ij

∂y j

dy j +

∂t ij

∂z j

dz j + dTo j + dS i …………… (3)where t ij and Tij are the arrival times and the calculated travel time of the j -th event at the i -th
station, respectively. dS i is a correction of a station correction at the i -th station. To is the origin
time. O is the observed travel time. C is the calculated travel time. dx , dy , dz ant dTo are thecorrections to the trial hypocenter

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 a modified
joint hypocenter determination (MJHD) method using the constraints below.
n∑ S i Di = 0
i =1n∑ S i cos θ i = 0
i =1n∑ S i sin θ i = 0
i =1n∑ S = 0 ………………………… (4)i =1iwhere S i is the station correction at the i -th station, Di is the distance between the i -th stationand the center of the region, θ i is the azimuth to the i -th station from the center of the region, and n
is the number of stations.

RESULTS
Moment tensor inversion
In this study, we determined moment tensors to understand various behavior of tectonic and related
effect in the region near the subduction and active fault area. We analyzed the 54 events for moment
tensor inversion and compared our results (Figure 1) with those from global CMT. Also we used thegrid search method developed by Yagi (2004) to find out the best depth and source duration with theminimum variance. Generally almost all the results of moment tensor inversion are consistent with theglobal CMT results. But some of the results are different in depth and seismic moment from those of
global CMT. One of the reasons is that we used the body wave although global CMT is determinedusing long-period wave. We found that the focal mechanism solution from moment tensor inversion isin agreement not only with global CMT solution but also with the previous studies for regional
tectonic setting. According to the generation orientation of P-axis from focal mechanism solution, thedirection of P-axis is the same as the Indian plate motion (Figure 2). So this result is consistent withthe other results of the Indian plate motion.

.

Figure 1. The results of moment tensor
inversion.

Figure 2. General orientation of P-axis
and T-axis.

Hypocenter determination
We relocated the hypocenters for all the events that occurred in and around Myanmar and wereobtained from ISC data catalog from 1980 to 2004 including all of the depth range. In this method, tomake accurate location, it is better to take a small region with no significant variations in structures. If
there is any strong variation in the velocity structures, the station correction strongly depends on thelocation of events. Therefore, we selected the several small regions for the analysis (Figure 3). Mainly,
there are three regions: the Burma subduction zone, the region from the Burma subduction zone to
Sagaing fault zone, and the Southern opening region.
Based on the two criteria, we selected the stations and events for each region. These two
criteria are minimum number of stations (MSTN) and minimum number of events (MEVN). We
remove phase data of which travel time residuals are larger than 2 seconds. To obtain the precise
hypocenters, the epicentral distances of nearby stations were checked. There are valuable nearby
stations for all events. Therefore absolute locations are reliable for all regions. The station
corrections for all of events are almost same.
Seven regions from (A) to (G) are selected for
the Burma subduction zone. The region between Latitude
from 15.00°N to 24.00°N and Longitude 90.00°E to96.00°E was included. Figure 4 shows the result in region(A). Before relocation all of events are scattering. (Figure4a). But after relocation using the MJHD method some ofthe events are concentrated along the subduction zone andAsome are related with in inland earthquakes. In Figure 4b,
BH
the dotted line are represented the possible plate boundaryCwhich are in between Longitude around 91.0°E for thisDIregion. However, MJHD method gave the significant of
the tectonic features for all regions.
J
E
Region (H), (I) and (J) are included in the areafrom the Burma subduction zone to the Sagaing fault. The
F
K
tectonic features for those regions are similar each other.
G
L
Figure 5 illustrates the result in region (I). In this region,
we found that the earthquake clusters which seem to beforeshocks and aftershocks for a large event. Before
relocation these events are generally at a depth ofapproximately 10 km and concentrated in horizontal
distribution. But after relocation this earthquake cluster
appeared at a depth between 0-50 km and down dip
direction to the west. The epicenters are muchconcentrated in around the center of the cross section and
Figure 3. Map showing the 12 subseem to be distributed vertically. We can assume that thisregions. The rectangulars (A-G)
earthquake cluster seem to be the Taungdwingyi
with red color include the Burmaearthquake ( M w 6.6) which occurred at Latitude 19.90°N
subduction zone, the rectangulars
and Longitude 95.73°E on September 21st 2003. Moreover,(H-J) with blue color include thefour shallow earthquakes occurred just on the Sagaing
Burma subduction zone to Sagaing
fault.
fault and the rectangulars (K-L)
The regions (K) and (L) are included in thewith black color include theSouthern opening zone. In these regions the earthquakes
Southern opening zone.
clusters occurred systematically along the spreading axis
and transform fault respectively. Figure 6 show that the
MJHD hypocenter distribution for region (L).

Figure 4a. ISC hypocenter in region (A). The
epicenter distribution and two vertical crosssections along A-B and C-D lines areillustrated. A dotted rectangular in epicentral
map indicates the epicentral area of selectedearthquakes. The size of the symbols
represents the magnitude of the events. The
color and shape of the symbols denotes thedepth range of the events

Figure 5. The relocated hypocenters by
the MJHD method for region (I). The
symbols are the same as in Figure 4b.

Figure 4b. Relocated hypocenters by the
MJHD method in region (A). Cross
represent the standard errors of
hypocenters. The symbols are the sameas in Figure 4a.

Figure 6. MJHD hypocenters for region (L).
The symbols are the same as in Figure 4b.

5. DISCUSSION AND CONCLUSION
To investigate the tectonic setting around Myanmar, we obtained moment tensor solutions in andaround Myanmar using the teleseismic waveform inversion code developed by Yagi (2004). We
analyzed 54 events and compared the results with the solutions from global CMT. Some differences
are seen in centroid depth and seismic moment between our result and global CMT solution. Thosedifferences may be due to the difference in method for determining moment tensors. The P-axis
determined in the present study is consistent with the NNE motion of Indian plate relative to Eurasianplate.
Since the subduction zone at the shallower part is not clear at the northern segment of the
Burma trench. Depth ranges of seimicity become narrower after the relocations by MJHD. It’s difficult
to judge exactly where the plate boundary is. Around the deformation front suggested by Cummins(2007), seismic activity is quite low. Deformation front seems to exist on the east side of the suggested
deformation front. The previous study by Khan (2005) analyzed the hypocenter distribution along the
subduction zone using the ISC data between 1964 and 1999. Since the hypocenter distribution wasvery scattered, they could not get exact images of seismicity along the subduction zone. Our study got
much improved hypocenter distribution which is useful for understanding the tectonics in Myanmar.
In the Burma subduction zone to the Sagaing fault, the hypocenters are relocated very well.
There are two groups of earthquake hypocenter distributions. One is representing the subduction zone
earthquakes, and the other one is related to the Kabaw and Sagaing faults. According to depth
distribution of earthquakes, subduction zone earthquakes are located on intermediate depth. On theother hand, earthquakes near the Kabaw and Sagaing faults are located on the shallow depth. Before
relocation, we can not clarify the accurate depth of earthquakes clusters. But after relocation, all ofthese earthquakes clusters are concentrated very well. These events seem to be aftershocks of theTaungdwingyi earthquake which occurred at Latitude 19.90°N and Longitude 95.73°E on September
21st 2003.
As to the Southern opening zone, before relocation, hypocenters of ISC are rather scattering.
After relocations, hypocenters are clustering. To clarify the cause of seismic activity around this zone,
we compared seismicity with the fault lines which are related with the Sagaing fault (Nielsen, 2004;
Curry, 2005). We found that the two seismic activities occurs around Latitude 16.00°N and Longitude
95.70°E, and Latitude 15.00°N and Longitude 95.70°E and these are close to the fault lines drawn by
Nielsen (2004). Hypocenters distribution from Latitude 13.00°N to 15.00°N follows systematically theactive spreading axis and the transform fault which is the southern extension of the Sagaing fault,
which is supported by Curray (2005).

AKNOWLEDGEMENT
I am grateful thank to Professor Yagi (Tsukuba University) for his valuable guidance, suggestion andsupporting the programs.

REFERENCES
Bhattacharya, P.M., Pujol, J., Majumdar, R.K. and Kayal, J. R., 2005, Current Science, 89, 1404-1413.
Cummins, P. R., 2007, Nature, 449, doi: 10.1038/nature06088.
Curray, J. R., 2005, Journal of Asian Earth Sciences 25, 187-232.
Hurukawa, N. and Imoto, M., 1990, J. Seism. Soc. Japan, Ser. 2, 43, 413-429.
Hurukawa, N. and Imoto, M., 1992, Geophys. J. Int., 109, 639-652.
Khan, P. K, 2005, Geo. Scie. Journal, 9, 227-234.
Nielsen, C., Chamot-Rooke, N., Rangin, C and the Andaman Cruise Team, 2004, Geology, 209,303-327.
Rao, N.P., and Kalpna, 2005, Geophys. Res. Lett. , 32, L05301, doi: 10.1029/2004GL022034.
Sahu, V. K., Gahalaut, V. K, Rajput, S., Chadha, R. K., Laishram, S. S. and Kumar, A., 2006, Current
Science, 90, 1688-1692.
Socquet, A., Vigny, C., Chamot-Rooke, N., Simons, W., Rangin, C. and Ambrosius, B., 2006, J.
Geophys. Res., 111, B05406, doi: 10.1029/2005JB003877.
Yagi, Y., 2004, Earth Planets Space, 56, 311-316.
Yagi, Y., 2006, IISEE Lecture Note 2007-2008, IISEE, BRI.