RAYLEIGH WAVE PHASE VELOCITIES OBTAINEDUSING DATA FROM THE MALAYSIAN NATIONALSEISMIC NETWORKAtifah Mohd Alwi
MEE10504

Supervisor:Tastuhiko HARAABSTRACTThis study is an attempt to measure phase velocities using broadband waveform data from theMalaysian National Seismic Network (MNSN). We applied two-station method in the measurementsof phase velocities of the fundamental mode Rayleigh waves. We selected 10 distant large (M>=6.5)
earthquakes that occurred in 2009 and 2010. We computed the phase velocities using across-correlation technique for fundamental mode Rayleigh waves which were edited based on a rangeof group velocities between 2.6 and 4.0 km/s with visual checks of the waveforms. We computed twotheoretical phase velocities based on PREM (with and without ocean) in calculation of phase velocities.
The phase velocity measurements are divided into three groups based on the difference of paths (i.e.,the Malay Peninsula, the Borneo Island, and the South China Sea (ocean path)). We computed averagephase velocities for these groups. For the Borneo Island, the standard deviation is relatively larger dueto larger scattering of the measurements. For this reason, we only discuss the results for the ocean pathand the Malay Peninsula. The average phase velocities obtained for the Malay Peninsula and the ocean
path are not largely different from theoretical phase velocities calculated for PREM. In order toinvestigate effects of crust structures for the phase velocities, we constructed seven structure models
referring to CRUST2.0, and computed phase velocities for them and model iasp91. These phasevelocities relatively well explain the observed phase velocities at shorter period range (30-90 s). The
differences between the observed and theoretically computed phase velocities at longer period range(100-140 s) may indicate the differences of mantle structures beneath these regions.
Keywords: Crust structure, Two-station method, Rayleigh wave, CRUST2.0
1. INTRODUCTION
The Malay Peninsula is a part of West Malaysia whilst Sabah and Sarawak are parts of East Malaysia.
Based on the tectonic setting, Malaysia underlies the stable Sundaland of the Eurasian plate and theseismicity in Malaysia is considered to be very low. It is bordered by two of the most seismically
active plate boundaries between the Indo-Australian and the Malay Peninsula and between the Sabahand Sarawak and the Philippines Sea Plate. After the Sumatra earthquake occurred on 26th December
2004, the Malaysia government has established the monitoring center in order to strengthen themonitoring network . This study is an attempt to measure phase velocities using waveform data from
the Malaysian National Seismic Network (MNSN) and to have inferences on crustal structures in andaround Malaysia by comparison between observed phase velocities and those theoretically calculatedfor various models.

Malaysian Meteorological Department (MMD), Sarawak Regional Forecast Office, Malaysia
International Institute of Seismology and Earthquake Engineering, Building Research Institute, Japan.

2. METHODOLOGY
We apply two-station method (e.g., Dziewonski and Hales 1972) in this study, in order to avoid thedifficulties with the source phase shift. By assuming both stations are at the same azimuth from anepicenter, the equations for both stations are given as follows:k ( w ). r1   1 ( r1 ,  ,  )   0 ( ,  )   i1 ( )  2 n  ,(1)k ( w ).r2   2 ( r2 ,  ,  )   0 ( ,  )   i 2 ( )  2 m  ,(2)where, r1 and r2 are distance between epicenter and station 1 and station 2, respectively, i1 and i2
are instrumental phase shift of station 1 and station 2, respectively, 0 source phase shift, m and n
are integer number of the phase cycles. By simplifying the Eq.(1) and (2), we can obtain the following
relations between the phase velocity and the phase differences.

C ( )  (r2  r1 ).
k ( )  2  1   i 2   i1  2l

(3)

3. DATA ANALYSIS AND THEORETICAL PHASE VELOCITY COMPUTATIONThe basic procedure of the analysis for this study was shown below (Figure 1).
Data Preparation

Reference Phase Velocities

Two-Station Method

Integer “ l ”

Phase Velocity Measurements

Figure 1. Procedure of phasevelocity measurement.

3.1. Data Preparation for Phase Velocities Measurements
We need vertical component of long period seismograms. We retrieved data from Incorporated
Research Institution for Seismology (IRIS), Data Management Center (DMC) available at
http://www.iris.edu/wilber. We 10 events in that occurred in 2009 and 2010 (Table 1) from theMalaysian National Seismic Network Database with magnitudes ranging between 6.5 and 8.0. We
considered the fundamental mode of Rayleigh wave and we set the start and end times of data to 2
minutes before P arrival and 100 minutes after P arrival, respectively. All the data are in the Standard
for Exchange of Earthquake Data (SEED) format and we need to convert the data to Seismic Analysis
Code (SAC) using rdseed version 5.0 program in order to start the data analysis.

Table1. List of theevents used in thisstudy.

3.2. Time window of Fundamental-mode of Rayleigh Wave
Before the suitable Rayleigh wave train was obtained for further analysis, we listed all the possibilitiesof station pairs that situated near or on the same great circle path. In this case, we set the thresholdvalue for the azimuthal difference between the two stations to 1.0 degree. (e.g., Yoshida and Suetsugu
2004). Therefore, the epicenter and the two stations are near or on approximately the same great circle
path. In this study, a time-window for fundamental mode Rayleigh wave was computed based on arange of group velocities between 2.6 km/s and 4.0 km/s. After visual checks, we changed this range ifnecessary.
3.3. The Computation of Theoretical Phase Velocity
We computed theoretical phase velocities for reference models in order to select an appropriate value
for an integer l in Eq.(3). An integer l is chosen so that the phase velocities are consistent with thosecomputed for the reference model. As for the reference models, we used Preliminary Reference EarthModel (PREM) (Dziewonski and Anderson 1981) with and without ocean for the ocean and the landpaths, respectively. As for the calculations, we used MINEOS package from CIG (ComputationalInfrastructure for Geodynamics, http://www.geodynamics.org/cig/software/mineos).
4. RESULTS AND DISCUSSION
For the calculation of phase velocities, we used the formula in Eq.4 (e.g., Yoshida and Suetsugu 2004).
We computed the phase velocities using a cross-correlation technique for fundamental mode Rayleighwaves which were edited based on a range of group velocities between 2.6 km/s and 4.0 km/s andperformed the visual checks of the waveforms.

C( f ) 2fx,
2ft 0   ( f )  2n

(4)

where, C ( f ) is phase velocity at a frequency f, x is distance between two stations,  ( f ) phasedifference at a frequency, t 0 is reference time difference between the two stations and n is integer
number of the phase cycles.
4.1. Analysis of the Ocean Path(a)

(b)

(c)

Figure 2. (a) The locations of the epicenter (star) and stations KSM (red triangle) andIPM (blue triangle) and the ray paths. The event is occurred at Tonga Islands, on 19March 2009, Origin time:18:17:53, Magnitude:7.6, Depth: 49.1km. (b) The observed raw
vertical component recorded at KSM and IPM stations and the edited waveforms areshown respectively.(c) The obtained phase velocity.

Figure 2(a) shows the locations of the epicenter and the stations and the ray paths. Figure 2(b) shows
the observed raw vertical component and the edited waveforms recorded at KSM and IPM stations,respectively. The edited waveforms (shown in red) are obtained from the group velocity in the range
2.6 and 4.0km/s. Figure 2(c) shows that the dispersion curve is slower than the reference model PREM
in the range period 30–90s. The dispersion curve in the period range longer than 100 s (pink shaded)
was discarded considering a large fluctuation in this case.
4.2. Analysis of the Malay Peninsula (land path)
(a)

(b)

(c)

Figure 3. (a) The locations of the epicenter (star) and stations KOM (red triangle) andIPM (blue triangle) and the ray paths. The event is occurred at Off the W.Coast ofS.Island, NZ , on 15 July 2009, Origin time: 09:22:50, Magnitude:7.8, Depth: 23.5km. (b)The observed raw vertical component recorded at KOM and IPM stations and the edited
waveforms are shown respectively.(c) The obtained phase velocity.
Figure 3(a) shows the locations of the epicenter and the stations and the ray paths. Figure 3(b) shows
the observed raw vertical component and the edited waveforms recorded at KOM and IPM stations,respectively. Figure 3(c) shows the dispersion curve is slower than the reference model PREM in therange period 30s – 110s and faster in the range period 115s – 140s.
4.3. Average of Phase Velocities Measurement(a)(b)(c)Figure 4. The average phase velocities obtained in this study compared with the model
PREM (green broken curve). (a) The average of the phase velocities for ocean pathmainly the station pair crossing the South China Sea. (b) The average of the phasevelocities for the Malay Peninsula (land path). (c) The average of the phase velocities for
Borneo Island (land path).
Figure 4 shows the average phase velocity and their standard deviations computed at respectiveperiods. Based on Figure 4(c), the standard deviation is much larger for the Borneo Island pathcompared with the ocean path (Figure 4(a)) and the Malay Peninsula (Figure 4(b)). This is due tolarger scattering of the measurements. For this reason, we only discussed the results for the ocean pathand the Malay Peninsula. The average phase velocities obtained for the Malay Peninsula and the ocean
path in the period range 30-90 s are not largely different from those computed for PREM considering
their standard deviations. In the longer period range (100s-140 s) they show some differences and it
may indicate the differences of mantle structures beneath these regions.

4.4. The Computation of the Reference Model from CRUST2.0
In order to investigate the effects ofcrust structures for the phase velocities, wecomputed phase velocities for seven models
which we constructed referring to theCRUST2.0(Bassinetal.
2000;
http://igppweb.ucsd.edu/~gabi/crust2.html).
Inaddition, we showed the theoretical phasevelocity for model iasp91 in Figure 6(b).
In Figure 5(a), model C7 provided thebest fit for the ocean path in the period range
between 30-140 s. Considering the standard
deviations, the observed phase velocity is
consistent with the phase velocity for this model,
although in the shorter period (30-80 s) theobserved phase velocity is faster than that ofmodel C7 whilst in the longer period (90-130 s)
the observed phase velocity is slower than that ofmodel C7. Figure 5(b) showed that model J3, J4
and iasp91 provided the best fit for the Malay
Peninsula path. For the land path, in the shorter
period (30-90 s) the observed phase velocity isslower than those for these models whilst in thelonger period (100-130 s) the observed phasevelocities is faster than those for these models.
The effects of the crust structures for phasevelocities are significant for the shorter periods
and they become smaller in the longer period
range. We can consider the phase velocities
computed for models constructed from
CRUST2.0 relatively well explained the observed
phase velocities at shorter period range.

The differences between the observed and thetheoretically computed phase velocities at longer
period range may indicate the differences ofmantle structures beneath these regions.
(a)

(b)

Figure 5. A various theoretical models
constructed from CRUST2.0. (a) The oceanpath and (b) The Malay Peninsula, respectively.

4.5. The Comparison with other studies
Lebedev and van der Hilst (2008) applied the Automated Multimode Inversion (AMI) of
surface and S-wave forms to obtain a Sv-velocity model of the upper mantle (crust-660km). The
Sv-velocities beneath the ocean paths of this study is slower in their model at depths range 80-150 km.
As we discussed above, the phase velocity for the ocean path is a little slower in the period range90-130s, therefore, the result that we obtained in this study for the ocean paths is qualitatively
consistent with their model.
Din (2010) performed the receiver function analysis and suggested that the crust thickness
beneath the IPM station is around 35 km which is similar to the model iasp91. Mat Said (2011)
showed the travel time data for the Malaysian National Seismic Network can be well explained bymodel iasp91. The results of this study are consistent with their result of studies.

5. CONCLUSIONSIn this study we measured phase velocities using waveform data from the MalaysianNational Seismic Network. We used 10 selected events that occurred in 2009 and 2010 withmagnitudes in the range between 6.5 -8.1. We retrieved the vertical component long period waveform
data from IRIS, DMC. We computed the phase velocities using a cross-correlation technique forfundamental mode Rayleigh waves which were edited based on a range of group velocities between
2.6 and 4.0 km/s with the visual checks of the waveforms. We computed two theoretical phasevelocities models based on PREM (with and without ocean) to apply the two station method.
We divided the phase velocity measurements into three groups based on the difference ofpaths (i.e. the Malay Peninsula, the Borneo Island and the South China Sea (ocean path)). Theaverage phase velocities obtained for the Malay Peninsula and the ocean path in the period range
30-90 s are not largely different from theoretical phase velocities calculated for PREM. For theBorneo Island, the standard deviation is much larger due to larger scattering of the measurements.
We compared the average phase velocities obtained in this study with the phase velocities
computed for various theoretical models constructed from CRUST2.0 and for iasp91. The phasevelocities computed for these models relatively well explain the observed phase velocities at shorter
period range (30-90 s). For the ocean path, model C7 provided the best fit whilst for the Malay
Peninsula, models J3, J4, and iasp91 provided the best fit.
The effects of the crust structures for phase velocities are significant for the shorter periods
and they become smaller in the longer period range. The observed phase velocities for the Malay
Peninsula are faster in the longer period range (100-130 s) whilst for the ocean path the phasevelocities are slightly slower in the longer period range (100-130 s).This differences may indicate thedifferences of mantle structures beneath these regions.
AKNOWLEDGEMENT
I would like to express my appreciation to Dr. Toshiaki Yokoi and Dr. Nobuo Hurukawa for theirvaluable comments in my study. In this study we used Seismic Analysis Code (SAC, Goldstein et al.,2007) for seismic waveform data analyses and the Generic Mapping Tools (GMT, Wessel and Smith
1998) to make some figures in the manuscript.
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