Citation: Rahman, Md., M., T. Hara (2019), Determination of Hypocenters of Earthquakes in and aroundBangladesh, Synopsis of IISEE-GRIPS master report.

DETERMINATION OF HYPOCENTERS OF
EARTHQUAKES IN AND AROUND BANGLADESH
Md. Mominur RAHMAN1,2

Supervisor:

Tatsuhiko HARA3

ABSTRACTWe analyzed 56 earthquakes that occurred in and around Bangladesh between June 2016 and February
2019. We picked arrival times from waveform data of the local network of Bangladesh MeteorologicalDepartment. Then we determined hypocenters of those events. We investigated the detectability of thelocal network. Some events that occurred in Bangladesh especially in the western part that are notregistered in the ISC bulletin were detected by the local network. On the other hand, the local network
did not detect some events which occurred in Bangladesh that were registered in the ISC bulletin. Then
we compared the hypocenters determined by the local network data to those retrieved from the ISC
bulletin and the USGS catalog. The differences between hypocenters are less than about 50 km in theepicentral distance from the centroid of the local network up to 300 km. The differences are larger at thegreater epicentral distance for some events. We found the tendency that intermediate depths events werelocated as shallow earthquakes. We combined the arrival times from the local network and the arrival
times recorded in the epicentral distance up to 10 degrees from the ISC bulletin and the USGS catalog.
Then we determined the hypocenters using the combined data and compared the hypocenters with thoseretrieved from the ISC bulletin and the USGS catalog. The large differences between hypocenters foundfor the analysis using only local data are reduced.
Keywords: Hypocenter determination, Detectability, Comparison of hypocenters, Combination ofarrival time data.

1. INTRODUCTION
Bangladesh is an earthquake-prone country, and it is very vulnerable to earthquake (e.g., Zaman andMonira, 2017). Bangladesh lies at the junction of three tectonic plates, the Eurasian plate, Indian plate,and Burma plate (e.g., Munima, et al., 2018). Bangladesh Meteorological Department (BMD) is the onlyorganization in Bangladesh which is responsible for issuing the earthquake information and warning. In
this study, we determined the hypocenters of earthquakes in and around Bangladesh using arrival times
from the local seismic network of BMD. We investigated the detectability of the local network throughcomparison of the hypocenters determined by the local network and those from the ISC bulletin. Then
we compared the hypocenters to those from the ISC bulletin and USGS catalog to examine the accuracyof hypocenters. We also determined hypocenters using the local and regional data to investigate whether
it is possible to improve hypocenter determination. The systematic investigation on the detectability ofearthquakes and accuracy of hypocenter determination in this study is the first attempt for analyses using
the local network data of BMD.
1

Bangladesh Meteorological Department, Dhaka, Bangladesh.
IISEE-GRIPS master course student
3
International Institute of Seismology and Earthquake Engineering, Building Research Institute.
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2. DATA
2.1. Data from the local network of BMD
We analyzed 56 earthquakes that occurred in and around Bangladesh from June 2016 to February 2019.
Waveform data from the 10 broadband seismic stations of BMD were used to determine hypocenters ofthese events. The magnitude range of these 56 earthquakes is from 2.5 M L to 6.9 ML determined byBMD.
2.2. Data from ISC bulletin
Weusedearthquake
informationfromtheISCbulletin(http://www.isc.ac.uk/iscbulletin/search/bulletin/) in the period from June 2016 to February 2019 toinvestigate the detectability of the local network. We also used data from the ISC bulletin in the period
from June 2016 to December 2016 for hypocenter comparison and determination. We retrieved origin
times, hypocenters, P and S wave arrival times and magnitudes from the ISC bulletin.
2.3. Data from USGS catalog
We retrieved origin times, hypocenters, P and S wave arrival times and magnitudes from the USGScatalog (https://earthquake.usgs.gov/earthquakes/search) in the period from January 2017 and February
2019 for hypocenter comparison and determination.

3. METHODOLOGY
3.1. Program HYPOCENTER
We used the program HYPOCENTER, which was developed by Lienert and Havskov (1995). This
program can determine both local and distant events. It was written in FORTRAN77. The program
HYPOCENTER first collects a set of parameters which control hypocenter determination procedure andstations information. Next, it conducts a travel time consistency test and starts to locate events by
iteration processes. It calculates travel time and their derivatives, conducts residual weighting andcalculates hypocenter corrections to be applied. The iteration is repeated until the solution is converged.
3.2. Hypocenter determination using waveform data from the local network
First, we converted local waveform data to SAC format data by using CSDP-IAS software. Then we used
Seismic Analysis Code (SAC, Goldstein and Snoke 2005) to pick P and S arrivals. The picked arrival
times are used for preparing the input file for the program HYPOCENTER. After preparing all thenecessary data files, we run the program
HYPOCENTER to determine hypocenters. We used iasp91 (Kennett and Engdahl, 1991) as a velocity
model for hypocenter determination. We obtained all the location information with hypocentral errors.
4. RESULTS AND DISCUSSION
4.1. Hypocenter determination using local waveform data
We determined hypocenters of 56 earthquakes that occurred in and around Bangladesh in the period
from June 2016 to February 2019 by HYPOCENTER using arrival time data from the local network.

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Figure 1 shows the hypocenters of these events.
Based on the distribution shown in Figure 1, we set
the region defined by the latitude range from 19o N
to 28o N and the longitude range 87o E to 95o E toexamine the detectability of the local network,
which is shown by the rectangle in Figure 1. There
are 51 events in this region among the 56 events that
we analyzed.

Figure1. Hypocenters of 56 events determined by
HYPOCENTER (Lienert and Havskov, 1995) using
data from the local network of BMD.

4.2. Detectability of local network of BMD
In order to investigate the detectability of the local network, we compared the hypocenters determinedby the local network to those of the events registered in the ISC bulletin. 510 events are registered in thearea shown by the rectangle of Figure 1 in the period from June 2016 to February 2019 in the ISC
bulletin. Among the 510 events, 28 events were detected by the local network of BMD (Figure 2) and482 events were not detected by the local network of BMD (Figure 3).
The agencies that reported the locally undetected events in Bangladesh are International
Data Center (IDC, Comprehensive Nuclear-Test-Ban Treaty Organization, CTBTO, Viena), National
Center for Seismology (NDI, India), National Seismological Center (DMN, Nepal). The minimum of
the body wave magnitudes of the common 28 events is around 4, which suggests the lower bound of thedetectability of the network.

Figure 2. Hypocenters of 28 events detected by thelocal network of BMD among 510 events in the area
shown by the rectangle.

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Figure 3. Hypocenters of the 482 locallyundetected events that are registered in the ISC
bulletin.

Among 51 events, 23 events aredetected by the local network of Bangladesh
Meteorological Department (BMD) but notregistered in the ISC bulletin, the magnitude
range of these 23 events is from 2.3 to 4.1 ML
obtained from the local network of BMD. Figure
4 shows the hypocenters of these 23 earthquakes.
The focal depths of most of them are in between
0 to 20 km. Several earthquakes are located in thewest and south parts of Bangladesh while a few
events in these regions are registered in the ISC
bulletin.

Figure 4. Hypocenters of 23 events that are notregistered in ISC bulletin but were detected bythe local network in the area shown by therectangle.

4.3. Comparison of hypocenters of local events with that of the ISC/USGS catalog

Distance between hypocentersof local and ISC/USGS.

We investigated the accuracy of the hypocenters determined by the local data. Among the 56 events, weanalyzed 33 events that are registered in the ISC bulletin and USGS catalog. Figure 5 shows thecomparison of these hypocenters. There is a tendency that intermediate-depth events that occurred
beneath of Myanmar and Myanmar-India border region are located as shallow events. Choudhury (2008)
determined hypocenters for 3 events that occurred in and around Bangladesh using data from theBangladesh seismic network deployed at that time. Among them for the 1 event, which occurred on
December 7, 2007 beneath Myanmar, the difference between the focal depth determined using the local
data and that of the USGS catalog was large. This result is consistent with the result of the present study.

Figure 5. Comparison of hypocenters of 33
events determined using the local data andthose from the ISC/USGS catalog which aredenoted by the circles and the solid stars,
respectively.

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1401201008060402000 100 200 300 400 500 600 700 800 9001000

Distance between epicenters of centroid
and ISC/USGS

Figure 6. The distances between the hypocentersobtained from the local network data and those fromthe ISC/USGS catalog as a function of the distance ofepicenters from the centroid of the local network.

Figure 6 shows the distances between the hypocenters obtained from the local network data
and those from the ISC/USGS catalog with respect to the distance of an epicenter from the centroid of
the local network. The differences between hypocenters are less than 50 km in the epicentral distance
range up to 300 km. The differences are larger up to about 140 km at the greater epicentral distance forsome events. The average distance between hypocenters is 46.9 km.
4.4. Comparison of hypocenters determined using the combined data to those of the ISC/USGSDistances between hypocentersof combined data and ISC/USGSAs a preliminary attempt to improve hypocenter determination, we combined arrival time data from thelocal network and those from ISC bulletin and USGS catalog recorded in the epicentral distance up to10 degrees. This analysis corresponds to hypocenter determination using local and regional data in thestudy area.
For the combined data, we found the location errors of 3 events among 33 events are greaterthan 100 km (i.e., the error of at least one of the three coordinates exceeds 100 km). We exclude these 3
events for comparison because the hypocenters are not well determined.
Figure 7 shows the comparison of hypocenters of 30 events determined using the combined
data with those of the ISC/USGS catalog. Figure 8 shows the hypocentral distances for the local data
analysis and those for the analyses of the combined data.
The large distances between the hypocenters determined using the local data and those fromthe ISC/USGS catalogs are reduced, as is shown in Figure 8, while for some events the distances between
the hypocenters increased for the combined data. The average of the distances between the hypocentersof 30 events is 34.0 km in case of the combined data, and that is 48.6 km in case of the local data,respectively. The average of the differences is reduced in the case of the combined data for these 30events.
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1401201008060402000 20 40 60 80 100120140160

Distances between hypocenters oflocal and ISC/USGS
Figure 7. Comparison of hypocenters of 30
events determined using the combined data andthose from the ISC/USGS catalog which aredenoted by the circles and the solid stars,respectively.

Figure 8. Comparison of the hypocentral
distances of 30 events between the local data
and the ISC/USGS data with those of thecombined data and ISC/USGS data.

5. CONCLUSIONSWe analyzed 56 earthquakes that occurred in and around Bangladesh between June 2016 and February
2019. We used CSDP-IAS software to convert waveform data from the local network to waveform datain SAC format. We picked P and S arrival times from these local waveform data. Then we executed
HYPOCENTER to determine hypocenters of earthquakes using local data.

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We investigated the detectability of the local network. 510 events are registered in the ISC
bulletin in the area defined by the latitude range 19oN-28oN and the longitude range 87oE-95oE. Amongthem, 28 events were detected by the local network, and 482 events were not detected by the local
network. The minimum of the body wave magnitudes of the common events is around 4, and this is
likely to correspond to the lower bound of the detectability of the local network. Among 56 events thatwere analyzed, 23 events are detected by the local seismic network but are not registered in the ISC
bulletin. The magnitude ranges of these 23 events are 2.9 to 3.9 ML.
Then we compared the hypocenters determined by the local network data to those ofretrieved from the ISC bulletin and USGS catalog. We found the tendency that intermediate-depth events
that occurred beneath Myanmar and Myanmar-India border region are located as shallow earthquakes.
The differences between hypocenters are less than 50 km within the epicentral distance from the centroidof the local network up to 300 km, while the differences become large up to 140 km at the greater
epicentral distance for some events.
We combined the local data and the ISC/USGS data whose epicentral distances were ≤ 10
degrees. We executed HYPOCENTER to determine hypocenters using the combined data. In the caseof the combined data, the location errors for 3 events are greater than 100 km. We excluded these eventsfrom 33 common events and compared the hypocenters of the 30 events determined by the combined
data with those retrieved from the ISC/USGS catalog.
This comparison shows that the large differences between the hypocenters at the distance
range between 300 and 600 km obtained for the analysis using the local data are reduced in the analysis
using the combined data, while the differences between hypocenters increased for some events. The
average of the differences between the hypocenters is smaller for the analyses using the combined data.

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 would like to express my sincere gratitude to the supervisors Dr. TatsuhikoHara, Chief research scientist, IISEE, BRI for his continuous support, valuable suggestion, andinstruction during my study. I would like to thank Dr. T. Yokoi (Director of IISEE, BRI) and allothers sensei of IISEE, BRI for their valuable lectures and support. I also would like to thank all
the other staff members at IISEE/BRI and my colleagues for their support and encouragement
during this training program. We used data from ISC (2019) bulletin for this study. We used
Generic Mapping Tools (Wessel and Smith, 1998) for preparing some of the figures in this study.

REFERENCES
Choudhury, S. A., 2008, Bulletin of IISEE, 43, 7-12.
Goldstein, P., and Snoke, A., 2005, Seismology Data Management Center Electronic Newsletter.
International Seismological Center, 2019, On-line bulletin, http:// doi.org/10.31905/D808B830.
Kennett, B.L.N., and Engdahl, E.R., 1991, Geophys. J. Int. 105, 429-465.
Lienert B. R., and Havskov, J., 1995, Seismological Research Letters, 66, 5.
Munima, T., Salman, M.A., and Akhter, S.H., 2018, International Journal of Science, 3, 939 – 949.
Wessel, P., and Smith, W.H.F., 1998, EOS Trans. AGU, 79, 579.
Zaman, M.A., and Monira, N., 2017, J Geol Geophys 6, 300.
Website: https://earthquake.usgs.gov/earthquakes/search.
Website: http://www.isc.ac.uk/iscbulletin/search/bulletin.

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