SOURCE PARAMETER DETERMINATION USINGA SPARSE BROADBAND NETWORKTun Lin Kyaw∗
MEE06020

Supervisor: SHIBAZAKI, Bunichiro∗∗

ABSTRACTWe determined P-wave moment magnitude Mwp introduced by Tsuboi for M 3-7 events. We comparedMwp obtained by this study with Mw of NIED CMT solutions. In the case where we use correction
factor α = 0.2 for radiation pattern, the calculated Mwp is slightly larger than Mw of NIED CMTsolutions. This may be caused by the effect of the velocity structure which is not considered for
calculating Mwp. We also determined the moment magnitude Moment Tensor Inversion using theprogram by Yagi. We assumed the same velocity structure as that used by NIED. Values of Mw
obtained by this study are very similar to Mw obtained by NIED. We can conclude that the Yagi
program can be used for the moment tensor determination for local events of M3-7. As to the events
which are larger than M 7, the point source approximation can not be held. Therefore, it is difficult todetermine the moment tensor using the program by Yagi for local events larger than M 7. However,for distant events larger than M 7, we could determine Mwp which are close to those of Harvard CMTsolutions.
Keywords: P-wave moment magnitude, Moment tenser inversion, Moment magnitude
INTRODUCTION
Myanmar lies on the earthquake belt of Himalayan range. The northeastern part of the Indian plate
subcontinent is seismically active. The motion of the Indian (IN) plate with respect to that of theEurasia (EU) plate is highly oblique to the margin. The seismic records show that there have been atleast 16 major earthquakes bearing magnitude M ≥ 7.0 within the territory of Myanmar for the past
170 years. Monitoring of earthquakes in Myanmar is an urgent subject for earthquake disasterprevention.
Now Myanmar is planning to expand the number of seismic stations by using 10 digital broadband
seismic stations. Myanmar will have a network of 6 VSAT telemetry broadband seismometers by theend of this year. Since our broadband seismic network is under construction, we can not use ourseismic data. Therefore, we use the broadband seismograms from F-net in Japan. In the present studywe adopted a technique to determine earthquake magnitudes using the broadband P-wave
seismograms (Tsuboi et al., 1995; 1999), which is called broadband P-wave moment magnitude Mwp.
This P-wave moment magnitude Mwp scale has been utilized at the Pacific Tsunami Warning Center(PTWC) and the West Coast/ Alaska Tsunami Warning Center WC/ATWC. We also determined themoment magnitude using the program by Yagi (2004). Then, we compare the P-wave moment
magnitude with moment magnitude. These methods will be applied later after the establishment ofnew sets of broadband digital seismogram in Myanmar.

∗Department of Meteorology and Hydrology, Ministry of Transport, Myanmar.

∗∗International Institute of Seismology and Earthquake Engineering (IISEE, Japan)

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DATA
The data used in this study was obtained from the F-net by the National Research Institute for Earth
Science and Disaster Prevention (NEID). We used 6 broadband seismic stations; Abuyama,Aogashima, Nishitosa, Watarai, Yasaka and Toyota operated by the NIED in this study. We selected
22 earthquakes from the F-net of 2007, March to July which occurred in our study areas (Japan). The
magnitude of the selected events ranges from 3 to 7. We chose the earthquake whose epicenters lie
within 30∘ – 38∘ N 128∘ – 142 ∘E. First of all, we need information about an earthquake, its location(latitude and longitude in degrees) and hypocenter depth. We show the epicentral distribution ofevents and the location of stations in Figure 1.

Figure 1 The epicentral distribution of events

METHODIn this study, we used two methods for data analysis.
1. Determination of Broadband P-wave Moment Magnitude
2. Determination of Moment Tensor
Determination of Broadband P-wave Moment Magnitude
Following Tsuboi et al. (1995, 1999). The P-wave moment magnitude, Mwp is calculated from thevertical component of far-field P-wave displacement and by using the following equation to calculate
the approximate seismic moment M0:(M 0 = max ∫ u z ( xr , t )dt)

4πρα 3p r
Fp(1)

where uz is the vertical component record, xr is the station location, ρ is the density, α p is the Pwave velocity, r is the epicentral distance. Fp is the P- wave radiation pattern. Then, we used thestandard moment magnitude formula (Kanamori, 1977) to calculate the moment magnitude:

Μw =1(log M 0 − 9.1)
1.5(2)For the last step, effect of radiation pattern was taken into account and the following formula was used.
Mwp = Mw + α
α = {log (15/4)}/3 ≈ 0.2
For calculating Mwp, we use the method by Kanjo et al. (2006).

2(3)where α is correction for radiation pattern (Tsuboi 1995). This P-wave moment magnitude Mwp scale
has been utilized at the Pacific Tsunami Warning Center (PTWC) and the West Coast/ Alaska
Tsunami Warning Center WC/ATWC.
Determination of Moment Tensor
We used the code developed by Yagi (2004) for obtaining Mw by moment tensor inversion. We cantreat source and propagation process as linear operators. The moment tensor can be described as
double-couple. Therefore, it is possible to construct observed waveform by summing the momenttensor weighted displacements for each moment tensor (convolution of Green’s function and Source
time function). The number of independent components of moment tensor for double-couple is five.
5u (t ) = ∑ mi × Greeni (t ) ∗ Source(t )

(4)

i =1

where Green(t) and source(t) are to calculate the Green’s function. We can assume the structure anddepth of hypocenter. We can also assume the source time function is delta function. Then, theobservation waveform ( O/ ) can be expressed as

O/ = G/ m(5)⎡u ud (t1 ) ⎤G (t1 ) G (t1 )⎢u (t )⎥G (t 2 ) G (t 2 )⎢ ud 2 ⎥⎥ , G/ =O/ = ⎢MMM⎥⎢m1Gns (t1 )M⎢u ns (t1 ) ⎥⎥⎢MMM⎦⎣m1udm1
ud

m2
ud
m2
ud

⎡m 1 ⎤⎢m ⎥G (t 2 )⎢ 2⎥,m= ⎢m 3 ⎥M⎢⎥
m5Gns (t1 )⎢m 4 ⎥⎢m 5 ⎥M⎣⎦
G (t1 )
m5
ud
m5
ud

LLOL

m1
Gudm1 (t ) = Greeniud(t )If we know the observation waveform ( O/ ) and green’s function ( G/ ), then we can solve the above

matrix equation using least-square approach, which is exactly Moment Tensor Inversion.

RESULT

8.0

8.0

7.5

7.5

7.0

7.0

6.5

6.5

Mwp(this study)

Mwp(this study)

Relationship between Mw (NIED), Mw (this study), Mwp (this study, α = 0.0 and α = 0.2) andMjma
In the Figure 2, we compare Mwp obtained in this study with those by the NIED. Here we take into
account α = 0.2. In this case Mwp (this study) are larger than moment magnitude Mw (NIED).
However, in the Figure 3, we take α = 0.0. We can see that there is a good agreement between Mwp(this study) and those by the NIED. According to the result, we can say that for the local earthquake, it
is not necessary to add α = 0.2.

6.0
5.5
5.0

6.0
5.5
5.0

4.5

4.5

4.0

4.0

3.5

3.5

3.0

3.0
3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

3.0

Mw(NIED)

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

M w(NIED)

Figure 2 Comparison of Mw (NIED)and Mwp (this study, α = 0.2)

Figure 3 Comparison of Mw (NIED)and Mwp (this study, α = 0.0)

38.0

Next, we made comparison between moment magnitude (NIED) and moment magnitude (this study)
as shown in Figure 4. We obtained the good correlation between Mw (this study) and Mw (NIED).
According to the figure, we can conclude moment magnitude by using moment tensor inversion
provided by Yagi (2004) and that of NIED result which are very close.
8.0
7.5
7.0

Mw(this study)

6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

Mw(NIED)

Figure 4 Comparison of Mw (this study) and Mw (NEID)In Figure 5, we compared Mwp (this study, α =0.0) and Mw (this study). We obtained the goodcorrelation between Mwp (this study) and Mw (this study).
8.0
7.5

Mwp(this study)

7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
3

3.5

4

4.5

5

5.5

6

6.5

7

7.5

8Mw(this study)

Figure 5 Comparison between the Mwp (this study, α = 0.0) andMw (this study)

8.0

8.0

7.5

7.5

7.0

7.0

6.5

6.5

Mw(this study)

Mw(NIED)

Finally, we compared the Mw (this study) and Mw (NIED) with Mjma. We found that Mjma gave larger
values than obtained from moment magnitude adopted by Yagi (2004) and Mw by NIED.

6.0
5.5
5.0
4.5

6.0
5.5
5.0
4.5

4.0

4.0

3.5

3.5

3.0
3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

3.0

8.0

3.0

Mjma

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

Mjma

Figure 6 Comparison of Mjma andMwp (this study, α = 0.2)

Figure 7 Comparison of Mjma andMw (this study)

4Determination of Mwp for Larger Events (Mw > 7.1)
We analyzed 4 events that occurred in Japan (2003-2007) with moment magnitude more then 7.1. We
retrieved the digital waveforms from broadband seismometer of F-net. We obtained the P-wave
moment magnitude using the Tsuboi (1995, 1999) method for each event. Then, we compared theirvalues with the magnitude obtained from Mjma, Mw (NIED) and Mw (CMT), respectively, by plotting
the Mwp of this study against the moment magnitude. We obtained that Mwp (this study) is larger thanthe Mw (NIED). On the other hand, the Mwp obtained in this study is very close to the Mjma and Mw(CMT).
We could not determine moment tensor inversion because in the program the source ofearthquake is assumed to be a single point source. For the local event we can determine the moment
tensor for the event less than M7.

Figure 8 Location of stations and epicenter of large events
DISCUSSION AND CONCLUSIONSFirst, we determined Mwp for M 3-7 events. We compared Mwp obtained by this study with Mw
obtained by NIED. In the case where we use correction factor α = 0.2, the calculated Mwp is slightly
larger than Mw obtained by NIED. For the calculation of Mwp, an elastic half-space is assumed.
Therefore, it may be necessary to change the values of correction factor α , considering a velocity
structure. For example, in the case α = 0.0, there is a good agreement between Mwp (this study) andMw obtained by NIED.
We also determined the moment magnitude using the program by Yagi (2004). We assumed
the same velocity structure as that used by NIED. Values of Mw obtained by this study are very similar
to Mw obtained by NIED. This indicates that we can determine the moment tensor using the programby Yagi (2004). By comparing Mwp with Mw obtained by Yagi’s program, we can determine acorrection factor, α for Mwp.
As the events which are larger than M 7.1, the point source approximation can not be held.
Therefore, it is difficult to determine the moment tensor using the program for local events by Yagi
(2004). Actually, we tried to determine the moment tensor of the large events which occurred at
subduction zones. But we failed to determine the moment tensor. However, we could determine Mwp,
although the number of analyzed events is small, we could obtain values of Mwp which are closer tothose of CMT solutions.
We used the seismograms from the stations of which distribution is very similar to our future
seismic network. The noise level of our network will be much greater than that of F-net since thesensor of our network is located at the surface. Although and large. We cannot say that we will be able
to determine the Mwp and Mw for small events such M3 and M4 using our future seismic network. But
we will be able to determine the Mwp and Mw for middle events. We will also be able to determine Mwp
for the distant large subduction zone events.

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ACKNOWLEDGEMENT

I am thankful to the National Research Institute for Earth Science and Disaster Prevention (NEID),
Japan because they provided me with the waveform data for my study.
REFERENCES
Kanjo, K., et al., 2006, Earth Planets Space, 58, 121-126.
Fukuyama, E. and M. Ishida, 1998, IISEE. 51, 149-156.
Tsuboi, S., et al., 1995, Bulletin of the Seismological Society of America. 85, 606-613
Tsuboi, S., et al., 1999, Bulletin of the Seismological Society of America. 89, 1345-1351
Tsuboi, S., 2000, Geophysical Research Letters. 27, 3015-3108.
Web site: F-Net, http://www.fnet.bosai.go.jp/
Yagi, Y., 2006-2007, Lecture notes on Earthquake Focal Mechanism, IISEE/BRI.

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