MOMENT TENSOR AND HYPOCENTER
DETERMINATION USING WAVEFORM DATA FROMTHE NEW BANGLADESH SEISMIC NETWORK
Sayeed Ahmed Choudhury*
MEE07160

Supervisor: Tatsuhiko HARA**ABSTRACT
The Bangladesh Meteorological Department (BMD) has established a new digital seismic observation
network in 2007. The data from this digital network have been used to determine a moment tensor using
software developed by Dreger (2003). I analyzed the November 11, 2007 Bangladesh-India borderearthquake. The obtained focal mechanism is consistent with that of the Global CMT catalog. This resultsuggests that it is possible for BMD to monitor and investigate the seismic activities by moment tensor
determination procedure using data from the newly established Bangladesh Meteorological Department
Seismic Network.
I also performed hypocenter determination for the recent three earthquakes. The results for the two
events that occurred in Bangladesh are consistent with the USGS hypocenters and felt reports. As for theevent in Myanmar, the difference between their depths is large.
Keywords: Moment tensor inversion, Hypocenter.

INTRODUCTION
Seismicity
The north and east parts of Bangladesh is seismically active. The seismicity in these regions is attributed
to the collision between the Indian plate and the Eurasian plate, and the subduction along the IndoMyanmar range. Bangladesh has experienced large earthquakes in its history. Figure 1 shows historicaland recent earthquakes in and around Bangladesh. The five historical and recent earthquakes among themoccurred in Bangladesh, and the other five events occurred around Bangladesh. This kind of information
can be used to evaluate possible damages caused by future earthquakes in and around Bangladesh.
New Digital Seismic Network of Bangladesh
In Bangladesh, the first seismic station was established in Chittagong in 1954. An analog seismometer wasdeployed at this station. Recognizing the increasing vulnerability for earthquakes, especially in large cities
due to high density of population, unplanned infrastructure and their close proximity to India andMyanmar’s seismically active areas, the GOB is implementing a project to establish three new seismicstations with upgrading the existing one for earthquake monitoring of in and around Bangladesh in 2007.

* Bangladesh Meteorological Department (BMD), Bangladesh.
**International Institute of Seismology and Earthquake Engineering, Tsukuba, Japan.

1This nation-wide seismic network (shown in Fig. 2) consists of six digital broadband seismometers(two borehole and four vault type), two digital short-period seismometer, six digital triaxial accelerometerswith GPS synchronization and five sets of GPS (Geodetic), etc. Networking among these four stations will
be done by dedicated digital network for data transmission to the central data collection and processing
center in the BMD headquarters in Dhaka.

Figure 1. Some historical and recent earthquakes
in and around Bangladesh.
(the data was obtained from the USGS).

Figure 2. The locations of the four stations of the
Bangladesh seismic network.

TIME-DOMAIN MOMENT TENSOR PROCEDURE
SoftwareIn this study, time-domain moment tensor inversion was carried out following Dreger and Romanowicz(1994) and Pasyanos et al.(1996). The software package of this technique has been used at the Universityof California, Berkeley Seismological Laboratory (BSL) since 1993 to automatically analyze events(ML>3.5) in Northern California. Also, this package has been implemented at the Japan National Institutefor Earth Science and Disaster Prevention (NIED). This time-domain moment tensor inversion software
package (TDMT_INV) is available at International Handbook of Earthquake and Engineering Seismology
Handbook, part B. Green’s functions were computed by using the FKRPROG software developed by
Saikia (1994).
MethodThe time-domain moment tensor inversion is performed as follows. First, waveform data are processedand prepared by the following steps: (i) baseline correction, (ii) deconvolution to obtain ground velocities,(iii) integration to calculate displacement, (iv) rotatation to obtain transverse and radial components, (v)bandpass filtering, and (vi) re-sampling to 1 sps.

2Then, Green’s functions are calculated using the frequency wave number algorithm developed bySaikia (1994). The next step is to calculate time domain Green’s function by the inverse FFT using theresult in previous step. Then the bandpass filter with the same passband as was used for the observed data
is applied.
Finally, linear least squares for deviatory moment tensor is preformed for a given source depth(both a spatial and temporal point source is assumed). The inversion yields the moment tensor which isdecomposed into the scalar seismic moment, a double-couple moment tensor and a compensated linear
vector dipole moment tensor. The decomposition is represented as percent double-couple and percent
CLVD. The double-couple is further represented in terms of the strike, rake and dip of the two nodal
planes. The basic methodology and the decomposition of the seismic moment tensor is described in Jost
and Herrmann (1989).
The source depth is determined by finding the solution that yields the large variance reduction,(Dreger and Helmberger, 1993)(data −synth )⎡VR = ⎢1 − ∑⎢
i⎣

2ii

⎤
⎥ ∗100datai2 ⎥⎦(1)where data and synth are the data and Green’s function time series, respectively, and the summation isperformed for all the stations components.
Another measurement that is useful for determining source depth in regions where explosive
events are unlikely is the RES/Pdc, the variance divided by the percent double-couple where,

RES / Pdc = ∑(datai − synthi )2 / Pdc(2)i

Dividing the variance by the percent double-couple tends to deepen the minimum.
Crust ModelI selected a crust model from CRUST 2.0 (Bassin, Laske, and Masters, 2000) referring to the positon of
the Chittagong station, and used this model for Green’s function computation. CRUST 2.0 is a global
crustal model specified on a 2x2 degree grid. Data were gathered from seismic experiments and averaged
globally for similar geological and tectonic settings. The model and programs are available at
http://mahi.ucsd.edu/Gabi/rem.dir/crust/crust2.html
Data
The event that occurred on the November 7, 2007 (the origin time: 07:10:21.72UTC after USGS) wasselected for moment tensor inversion. The nearest station was Chittagong, and we used waveform data
recorded at this station for the moment tensor inversion.
Table 1 shows the hypocenters and magnitudes issued by Bangladesh Meteorological Department,
USGS, India Meteorological Department, and the Global CMT project (http://www.globalcmt.org/). The
focal mechanism of this event in the Global CMT catalog is the strike skip fault.
ResultFigure 3 shows the result of the inversion for a focal depth of 15 km. We used the epicenter from theUSGS in inversion, and the frequency band 0.03-0.04 Hz. The determining focal mechanism is a strike
slip, which is consistent with the Global CMT solution.
We performed moment tensor inversions for depths of 5, 10, 15, 20, and 25 km, and comparedvariance reductions and variances divided by the corresponding percent double-couples. While the former3

is not sensitive to change of depths, the latter is more sensitive, and the focal depth of 15 km provides thesmallest value.
Table 1. Hypocenters and magnitudes of theNovember 7, 2007 earthquake issued by
Bangladesh Meteorological Department,USGS, India Meteorological Department,
and the Global CMT project (* denotes lackof information).
Organization
USGSBMDIMSGlobal CMTLat
Lon Depth M(deg.) (deg.) (km)22.15 92.39 28.7 5.5
22.14 92.38
*6.0
22.1
92.5
15
5.3
22.15 92.5
25
5.5

Figure 3. The result of the moment tensor inversion(the focal depth is set to 15 km).

HYPOCENTER DETERMINATION
Data
I performed hypocenter determination to investigate whether it is possible to obtain reliable hypocenters
using data from the new Bangladesh seismic network. I selected three events shown in Table 2. The
epicenter of the latest event on the July 26, 2008 earthquake is at Mymenshing, and 120 km from thecapacital city Dhaka. Its magnitude was 4.9. At least 25 people were injured at Dhaka.
I picked P and S arrival times of the Dhaka, Rangpur and Sylhet stations for the January 12, 2008
event (in the Bangladesh-India border region) and the December 7, 2007 event (in the Myanmar-India
border region) using EDSP software. Bangladesh Meterological Department picked P and S arrival timesof the Dhaka, Chittagong and Sylhet stations for the July 26, 2008 event (at Mymenshing, Bangladesh).
Table 2. The events selected for hypocenter determination (after the USGS).
Date
26th Jul. 2008
12th Jan. 2008
7th Dec. 2007

Time (UTC)18:51:48
22:44:47
6:56:34

Lat (deg.)
24.79
22.76
23.50

Lon (deg.)
90.51
92.33
94.57

M
4.9
5.0
4.8

Depth (km)
133.8
113.3

Method and Crust Model
I used the program, HYPOCENTER, (Lienert et al., 1986) for hypocenter determination. This is anearthquake location method for locating local, regional and global earthquakes. Since the structures of thecrust and upper mantle beneath Bangladesh is not precisely studied, I used model iasp91 (Kennett andEngdahl, 1991) as an earth model in this study. The initial depth guess is set to 0 km.

4ResultTable 3 shows the determined hypocenters, and Figure 4 shows comparison between these epicenters andthose from the USGS. The results for the events on the July 26, 2008 and the January 12, 2008 areconsistent with the USGS hypocenters. As for the former event, the obtained epicenter is also consistentwith felt repots. As for the event on the December 7, 2007, which occurred in Myanmar, although theepicenter of this study is close to that of the USGS, the difference between the depths is large. This result
suggests the difficulty to locate events outside the network.
In all the cases, data from the three stations are available, and data from one station is missing. It
is desirable that data from all of the stations are continuously available.

Table 3. The hypocenters determined in thisstudy.
Date
26th Jul. 2008
12th Jan. 2008
7th Dec. 2007

Lat(deg.)
24.98
22.66
23.42

Lon(deg.)
90.47
92.17
94.24

Depth(km)10.810.0
10.0

Figure 4. Comparison between the epicentersdetermin study (red stars) and those from theUSGS (blue stars).

CONCLUSION
In this study, I applied the moment tensor inversion code developed by Dreger to the waveform data fromthe new broadband seismic network in Bangladesh. I also performed hypocenter determination of therecent three earthquakes using data from this network.
The focal mechanism solution obtained in this study is consistent with the Global CMT solution
for the same event. The results of hypocenter determination for the events in Bangladesh are consistentwith hypocenters determined by the USGS.
These results suggest that it will be possible to use both programs for further investigations ofevents in Bangladesh. Based on the achievements in this study, I am going to perform moment tensor
inversion using the procedure of this study. I also plan to relocate previous events using the program
HYPOCENTER.

5RECOMMENDATIONS
Accurate hypocenter is necessary to perform moment tensor inversion used in this study. At present, there
are four stations running in the Bangladesh seismic network. Due to communication problems andunstable power supplies, data from all the stations are not always available. It is desirable to increase thenumber of stations to enhance the capability of hypocenter determination. Also, data exchange amongneighboring countries will be effective to improve accuracy of determination of earthquake source
parameters.
To improve moment tensor determination, accurate crust model is necessary. Analyses using
phase data and waveform data should be carried out to construct an appropriate crust model for earthquake
monitoring. Through these improvements, I believe that our network will successfully work for earthquakemonitoring in my country in the near future.

ACKNOWLEDGEMENTS
I would like to thank Dr. Wang Hongti Prossor, Institute of Earthquake Predication, China Earthquake
Administration who supported my study by giving information about data and Instrument of our network.

REFERENCES
Bassin, C., Laske, G., and Masters, G., 2000, EOS Trans AGU, 81, F897.
Dreger, D.S., 2003, International Handbook of Earthquake and Engineering Seismology, W.H.K.Lee,Kanamori, H., Jennings, P.C., and Kisslinger C. (Eds.), Academic Press.
Dreger, D.S. and Helmberger, D.V., 1993, J. Geophys. Res., 98, 8107-8125.
Dreger, D.S. and Romanowicz, B., 1994, U. S. Geol. Surv. Open-file Rept., 94-176, 301-309.
Jost, M.L. and Herrmann, R., 1989, Seis. Res. Lett., 60, 37-57.
Lienert, B.R., Berg, E., and Frazer, L.N., 1986, Bull. Seism. Soc. Am., 76, 771-783.
Pasyanos, M, Dreger, D.S., and Romanowicz, B., 1996, Bull. Seism. Soc. Am., 86, 1255-1269.
Saikia, C.K., 1994. Geophys. J. Int., 118, 142-158.
Web site: Global Crustal Model is available here http://mahi.ucsd.edu/Gabi/rem.dir/crust/crust2.html

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