DETERMINATION OF EARTHQUAKEPARAMETERS USING SINGLE STATIONBROADBAND DATA IN SRI LANKA
S.W.M. SENEVIRATNE*
MEE07166

Supervisors: Yasuhiro YOSHIDA**Tatsuhiko HARA***ABSTRACT
We determined epicenters and magnitudes using data from the single broadband seismic stationin Sri Lanka, PALK. First, we analyzed earthquakes to which epicentral distances from PALK
were within 40 degree. Most of the events are in and around Andaman and Nicobar Islands in theIndian Ocean. The obtained results show that it is possible to determine epicenters and surfacewave magnitudes with acceptable accuracy using a single station method.
We also analyzed an earth tremor in Sri Lanka using data from the PALK station. Despitehigh frequency noises contained in the data, the epicenter determined by the single station method
is consistent with felt reports. The calculated local magnitude is 3.2.
Keywords: Single station method, Epicenter, Surface wave magnitude.

INTRODUCTION
In Sri Lanka, we have not faced with many natural disasters except for landslides based on theknown history. In a several decades, there have been implemented preparedness and preventive
measures for landslides considering that they are possible natural disasters in the country. When it
comes to earthquakes, it is said that Sri Lanka is considered fairly safe and not likely to be prone
to major earthquakes. This idea has been changing recently in the society after the Great Tsunami
in 2004. Although we are considerably far away from an active plate margin of Sumatra, wesuffered from the historical tragedy a several hours after the 2004 M=9 earthquake occurred. Thisincident proved that seismic activity in subduction zones can transmit a disastrous tsunami formore than two thousand kilometers away from its origin in the Indian Ocean. The seismicity ofthis area is likely to affect the internal peacefulness of earth in the country. Many off shore
earthquakes have made significant shakings inside the country according to the reports after 2004
great earthquake.
A number of inland tremors have been reported by general public within the country inpast few years. Most of these tremors were very local, confined to particular region within thecountry (Fernando et al. 1986). Monitoring of such micro seismic activities is very important and,since there have been recorded a number of tremors within the country, it is essential for us tolearn and study local earth dynamics and seismicity.
The great Indian Ocean Tsunami in 2004, reminds us that, although Sri Lanka is locatedin a relatively stable area, the secondary events like tsunamis can make significant damages.
Therefore it is expected to study the seismicity of that particular region using available dataresources.
* Geological Survey and Mines Bureau, Sri Lanka.
** Meteorological Research Institute, JMA, Tsukuba, Japan.
*** International Institute of Seismology and Earthquake Engineering, BRI, Tsukuba, Japan.

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PALK is the single reliable seismic station in Sri Lanka, data from which was used in thestudy. We used the software DIMAS to analyze waveform data from 39 regional and teleseismic
events and one local event.
DATA
We selected 39 earthquakes recorded at PALK whose moment magnitudes are larger than 6.
Although we are mainly interested in events in and around Andaman and Nicobar Islands, someevents in other regions are also included in our dataset.
The basic information about events were initially collected using JWEED developed by
IRIS data center. The selected events can be further reviewed for the purpose of the study. Once
we decide the events of the study, the electronic request is generated and sent to the data center
through JWEED. A few minutes later, the requested waveform data are obtained on the clientcomputer and we can save them as mini seed or SAC data formats according to our propose.

METHODWe used S-P time differences and an assumed velocity structure of the region to find epicentral
distances. The P waves and S waves were picked from the vertical component and two horizontal
components, respectively. Appling these results with appropriate regional P and S wave velocities
in Wadatii diagrams, we can find the epicentral distances quickly. The particle motion of a P
wave recorded in all three components can be used to determine the direction of the P wave.
Surface wave magnitudes (Ms) were measured using Raleigh waves that were recorded onvertical components. Furthermore, owing to the availability of waveform data, Local Magnitude(ML) was calculated for a recent local earth tremor.
The software DIMAS (Display, Interactive, Manipulate and Analysis of Seismogram) isused in determination of earthquake parameters in this study (Droznin, 1997). The program has
been developed based on the following algorithms.
Origin time
The equation (1) shows the relationship of origin time with arrival time of P wave, S-P timedifference and ratio of the velocity.
To = Tp – Ts-p/V,(1)

whereTo = Origin Time,Tp = Arrival Time of P wave,
Ts-p = Difference of arrival times of S-wave and P-wave,
V = (Vp /Vs) -1,
Vp = Velocity of P wave,
Vs = Velocity of S wave.
Epicentral Distance
After finding the P wave travel time, assuming the source depth, the epicentral distance isdetermined by the travel time table. The program uses the default value for depth, which is set tozero. Depending on waveforms, a user can assume the depth based on depth phases as well. The
program uses the travel time table for model iasp91 (Kennett and Engdahl, 1991), and theepicentral distance is calculated based on this table.

2Source Azimuth
The azimuth from the station to the seismic source is determined by the investigation ofpolarization of the initial motion of the P waves. Analyzing the particle motion of the P wave inthe first few seconds of the three component records and assuming the lateral homogeneity of theregion, the direction of the P wave can be obtained.
Magnitude Calculation
Surface and local magnitudes are calculated by the following formula.
Surface wave Magnitude M S = log 10 (A/T) + 1.66 {log 10 (delta)} + 3.33
A – The maximum amplitude of the surface wave (µm)
T – The dominant period of the measured wave (seconds)
delta – The distance from the station to epicenter (degree)
Local Magnitude M L = log 10 (A/T) + 2.56 {log 10 (dist)} + 0.67
A – The maximum amplitude (µm)
T – The dominant period of the measured wave (seconds)
dist – The distance from the station to source (km)

(2)

(3)

RESULTS
Epicenters
Figure 1 shows the epicenters determined in this study and those from the PDE catalog. We foundthat the quality of PALK data were good enough for epicenter determination. We calculatedistances between the corresponding epicenters of this study and the PDE catalog, and theirfrequency distributions are shown in Fig. 2. The distances for most events are in the range
between -1 and 1 degree, from which we may expect relatively good estimates for magnitudes.

Distribution of Epicenter distance difference
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Frequency

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Epicenter distance difference ( DIMAS -PDE indegree)

Figure 1. The epicenters determined in the study Figure 2. Frequency distribution of distances(red stars) and those from the PDE (blue stars) between epicenters determined in this study
catalogues
areshown,respectively. and those by the USGS.
Corresponding epicenters areconnected bysolid lines.

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Magnitude calculation
Figure 3 shows the comparison between Ms determined in this study and those from the PDE
catalog, and Fig. 4 shows the distribution of their differences. We find a good agreement between
them for the magnitude range between 6 and 8. Above M=8, their agreement becomes poorer.
Their magnitude differences are within ±0.2 for most of the events.
Figure 5 shows the comparison between Ms determined in this study and Mw from theGlobal CMT catalog, and Fig. 6 shows the distribution of their differences. Again, we find a good
agreement between them for the magnitude range between 6 and 8. Their magnitude differences
are within ±0.3 for most of the events.
Distribution of Ms DIMAS & Ms PDE difference

Com parison of Ms-DIMAS w ith Ms-PDE
1014129Frequency

Ms (DIMS)

1087864265678

910

0.5

0.4

0.3

0.200.1

-0.1

-0.2

-0.3

5-0.4

-0.5

0

Ms DIMAS -Ms PDE

Ms (PDE)

Figure 3. The comparison of Ms obtained fromthe Study and those from the PDE catalogue.

Figure 4.
Frequency distribution ofdifferences Ms in the study and those fromthe PDE catalogue.
Distribution of Magnitude Difference between
Mw & Ms(DIMAS)

Com parison of Ms-DIMAS w ith Mw -GCMT
1098

97

frequency

Ms (DIMAS)

68754
32

61

5678

9100.4
0.5

0.2
0.3

-0.1
00.1

-0.4
-0.3
-0.2-0.6
-0.5

-0.9
-0.8
-0.7

05

Mw(GCMT) - Ms (DIMAS)

Mw (GCMT)

Figure 5. The comparison of Ms obtained in thisstudy and Mw from the Global CMT catalogue.

4Figure 6.
Frequency distribution ofdifferences between Ms of this study andMw from the Global CMT catalogue.

Analysis of a local tremor
On the April 7, 2008, there was an earth tremor in the central part of Sri Lanka. According toreports from local authorities and public, shakings were felt around Dambulla and Sigiriya inCentral Province.
We applied high band-pass filter to the waveform data from PALK, and found a signal
around the time where the shakings were felt. This is likely to a record of the earth tremor.
Around the time when this local event occurred, there was another regional event in the IndianOcean. Therefore, the data of the local tremor data has noises of the considerable level. However,using high band-pass filtering, we could analyze this local event.
The latitude and longitude of the estimated epicenter was 7.84 degree and 80.75 degree,respectively. This result is consistent with the felt reports issued by local authorities. The
calculated local magnitude of this event was 3.2.
Back azimuths
Figure 7 shows the distribution of differences between the back azimuths in this study and those
calculated using the epicenters from the PDE catalogue. They distribute around 10 degree. One ofthe possibilities for this discrepancy is laterally heterogeneous earth structure. However, this is
not likely in this case, because the differences of the back azimuths are observed not only for theevents around Andaman and Nicobar Islands but also for the events in other regions.
Another possible reason is inaccurate information on configuration of the seismometer. Itwill be useful and interesting to use an artificial event, the precise epicenter of which we know, todetermine the actual directions of the sensors.
Distribution of Azim uth difference

Frequency

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135791113

15

17Azim uth DIMAS - Azim uth PDE (degree)

Figure 7. The differences of azimuth values of catalogue (PDE)and calculated in DIMAS

CONCLUTIONSIn this study, we investigated uncertainty of epicenters and surface wave magnitudes determinedby a single station method using broadband data from the PALK station. The epicenters obtainedin this study agreed well with those of the PDE catalogue in general. The distances between theircorresponding events are between –1 and 1 degree for most of the events. We observed thesystematic difference of the back azimuths. The plausible reason is likely to be inaccurate
information on configuration of the seismometer.

5We compared Ms determined in this study to those from the PDE catalogue and Mw
from the Global CMT catalogue. In both cases, they agreed well in general in the magnitude
range between 6 and 8. Above the magnitude of 8, their differences become larger. One of thereasons may be saturation of Ms for huge events. However, this initial information will be usefuland beneficial for tsunami disaster mitigation, because the estimated Ms for the events whose
moment magnitudes are larger than 8 are larger than 8, based on which we can consider thepossibility of occurrence of devastating events.
We analyzed one local event on the April 7, 2008 using the single station method.
Although the data contained considerable noises, the determined epicenter was consistent withfelt reports. The calculated local magnitude was 3.2.

RECOMMENDATIONSThe study showed that it was possible to determine epicenters and magnitudes by the single
station method using data from PALK with relatively good accuracy. However, uncertainty ofthese estimates is still large, and it is necessary to enhance earthquake monitoring capability
through upgrade and deployment of seismic networks with relevant infrastructures, andestablishment of data centers. In addition, it is important to study local and regional seismicity,
since many earth tremors has been felt recently in the country. I would like to propose thefollowing suggestions future development.
(1) The current University seismic network, which at present is not working properly, should beupgraded so that it is possible to monitor earthquakes continuously. The upgrade plan wasstudied and proposed by Thaldena (2008). It is desirable to establish a real-time data transfer
between these stations and GSMB National Seismic Data Center.
(2) In order to better understand seismicity in and around the country, another seismic networkwith sufficient number of short period seismometers is desirable.
(3) The PALK station is governed by international agencies and there are certain limitations fordata usage. It is highly desirable to establish our own broadband seismic station. This would
be definitely beneficial for earthquake and tsunami disaster mitigation of the nation.

ACKNOLEGEMENTS
I would like to express sincere gratitude to Ms. Nilmini Thaldena at Geological Survey and Mines
Bureau of Sri Lanka for providing me data and information during the study.

REFERENCES
Fernando M.J. and Kulasinghe A.N.S., 1986, Phys. Earth planet. Inter., 44, 99-106.
Droznin, D., 1997, Operations manual for USGS, Kirtland, Albuquerque.
Kennett, B.L.N. and Engdahl, E.R., 1991, Geophys. J. Int., 105, 429-465.
Thaldena, S.N.B., 2008, Bulletin of IISEE, 42, 37-42.

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