FOCAL MECHANISM DETERMINATION OF LOCAL
EARTHQUAKES IN MALAY PENINSULA
Siti Norbaizura MAT SAID ∗MEE10505

Supervisor:

Tatsuhiko HARA∗∗

ABSTRACT
Since November 30, 2007, small local earthquakes have been observed in the Malay Peninsula near
the Bukit Tinggi area. The total number of these events in the Malay Peninsula is 30 until the end of
2010, including the newly recorded small earthquakes in Jerantut, Manjung and Kuala Pilah, which
occurred in March, April and November in 2009, respectively. Although hypocenters and magnitudes
are determined for these events using data from the Malaysian National Seismic network, focal
mechanisms have not been determined. In this study, we determined their focal mechanisms. We
selected three crust structure models, and compared the observed travel time differences between P
and S waves to those computed for these models. Although they agree relatively well for all of thesemodels, model iasp91 explained the observations better. We analyzed four events that occurred in the
Bukit Tinggi area for focal mechanism determination. We used three-component broadband waveform
data recorded at stations IPM and KOM of the Malaysian National Seismic network. We determined
their focal mechanisms using polarity data of the first motions of P and S waves and their amplitude
ratios. We used iasp91 for take off angle calculations. We obtained relatively well-constrained
solutions for all four events. The focal mechanisms of the largest 3.5mb event, which occurred on
November 30, 2007 is a mostly strike slip with some dip slip mechanism, while those of three events
are strike slip mechanisms. The maximum compressional (P) axes of the November 30, 2007 event isin the NNW-SSE direction, while those of three events are in the NW-SE direction. The minimum
compressional (T) axes of the strike slip events are in the NE-SW direction. Since there is no surface
trace of ruptures observed, this result is important to improve our understanding of these seismic
activities.
Keywords: Focal Mechanism, Polarity, Bukit Tinggi fault.

1. INTRODUCTION
Malay Peninsula is considered as inactive earthquake zone since only induced earthquake activities
around Kenyir Dam, Terengganu were recorded between 1984 and 1985, with magnitudes ranging
from 2.4 to 4.6 on Richter scale. However, in November 2007 until January 2008, there were 13
tremors recorded in the Bukit Tinggi area, with magnitude up to 3.5. The Malay Peninsula seems tohave its fair share of local earthquakes which generated weak motions when these Bukit Tinggi
earthquakes have been continued occurring. As a result of these, the total number of events recorded in
the Malay Peninsula increased to 30 until the end of 2010, including the newly recorded small
earthquakes in Jerantut, Manjung and Kuala Pilah, which have occurred in March, April andNovember in 2009, respectively. Although hypocenters and magnitudes for these local earthquakes are
determined by using data from the Malaysian National Seismic Network, solutions of the focal
mechanisms have not been determined. The purpose of this paper is to determine their focal
mechanism of these local earthquakes using polarities of the first motion of body waves and theiramplitude ratios.
∗

Malaysian Meteorological Department (MMD), Malaysia.
International Institute of Seismology and Earthquake Engineering, Building Research Institute, Japan.

∗∗

2. THEORY AND METHODOLOGY
2.1. Focal Mechanism Solution using Polarities and Amplitude Ratios
It is important to determine accurate focal mechanism solutions for earthquakes with small magnitudes
or those recorded by a small number of seismometers. A method using the initial motion polarity of P
and S waves, and their amplitude ratios is described below. We used the FOCMEC software package(Snoke 1984) from the Incorporated Research Institutions for Seismology (IRIS) which can be
downloaded from http://www.iris.edu/software/downloads/processing/. This software package uses
polarities of P and S waves (P, SV, SH) and amplitude ratios (SV/P, SH/P, SV/SH) as the input set for
determination and display of earthquake focal mechanisms through its two main program, the Focmec
and the Focplt. Directions of polarities in the FOCMEC package are considered in the position ofobserver facing the station with his/her back to the epicenter. Accordingly, the first motion of P arrival
is either up or down, while the SH is either left or right, and upward or backward for SV. Using the
Seismic Analysis Code (SAC), horizontal components of three-component seismographs are rotatedinto radial and transverse component to separate SV and SH. For crustal events recorded at local orregional distances, the first arrivals may be the refracted phases Pn and Sn and if the Poisson’s ratio isconstant in the crust and uppermost mantle, Pn and Sn will leave from the same point on the focal
sphere and have identical travel paths. When choosing polarities and amplitudes for local mechanism
determinations we must consider the fact that an SP arrival that comes in just before direct S affects
the observed waveforms, particularly for SV incidence and also a low-velocity zone at the surface cancomplicate the waveforms (Booth and Crampin 1985).
2.2. Seismic Velocity Model for Malay Peninsula
We checked the effect of the three velocity models available for Malay Peninsula (J1 and C2 from
CRUST2.0, and iasp91) on the travel-time differences between P and S waves and on the P-phasearrivals picked up by the MMD (Figure 1). The calculation was performed using the TauP Toolkit(Crotwell 1999). The following root mean square(RMS) is calculated for the comparison between
the observed and the theoretical S - P travel time,and P-phase arrival,∑n (𝑜𝑏𝑠i − 𝑐𝑎𝑙i )2
RMS = � i=1nwhere 𝑜𝑏𝑠i is the i-th observed value, and 𝑐𝑎𝑙i is
the i-th calculated value and n is the total numberof values. Although the differences of the RMS
among three velocity models, J1, C2 and iasp91,
are not so large, the lowest values are consistently
obtained by iasp91. These results suggest thatmodel iasp91 predicts the behavior of the seismic
velocity in the Malay Peninsula better than themodels from CRUST2.0. This is also consistent
with results obtained by Din (2011) who suggeststhat the crustal thickness beneath IPM station issimilar to the thickness of iasp91 (35km). Thus we
used a velocity model from iasp91 for the
calculation of take off angles; for epicentral
distances of Moho headwave the calculation oftake off angle will be robust since the crustal
waves travelling along the Moho discontinuity
have the same take off angles.

(a)

(b)

Figure 1. (a) Comparison of the observed
and the theoretical (calculated) S-P using
data from IPM station (b) Comparison of
the observed and the theoretical(calculated) P-phase arrival using datafrom IPM station.

3. DATA
Table 1.List of the Bukit Tinggi events analyzed for focal mechanism determination.
YEAR MONTH

2007
2007
2007
2008

Nov
Nov
Dec
Jan

DAY

30
30
12
10

TIME
(UTC)
02:13
12:42
10:01
15:38

EPICENTER
LAT

LONG

3.36°N
3.31°N
3.47°N
3.39°N

101.80°E
101.84°E
101.79°E
101.73°E

DEPTH MAGNITUDE
(KM)
2.3
6.7
10.0
3.0

3.5 mb
3.2 mb
3.2 mb
3.0 mb

We used polarity and amplitude ratio fromthree-component broadband data from two
stations, IPM and KOM; the locations of thestations and the events are shown in Figure
2. Table 1 shows information of the events.
Data were retrieved through BREQ_FAST
Request by electronically mailing a speciallyformatted
filetobreq_fast@iris.washington.edu.
Thehorizontal components were rotated into
radial and transverse components to separate
SV and SH waves. The polarities in the
FOCMEC package are designated by the
directions with respect to an observer facing
the station with his/her back to the epicenter.
We used the Seismic Analysis Code (SAC)(Goldstein et al. 2007) for seismic waveform
Figure 2. Locations of the broadband stations data analysis. Figure 3 shows the component(IPM and KOM) and the events used for for the P and the S arrivals at IPM station for
determination of focal mechanisms. Inverted the 02:13UTC, November 30, 2007 Bukit
triangles and circles denote stations and events Tinggi event. The basis for the polarity
choices for IPM station for Focmec inputof Bukit Tinggi, respectively.
file is D (down on vertical) for P arrival andas for S arrival, F (up on radial and vertical)
for SV, and L (down on transverse). We
made corrections for amplitudes assuming
𝑄𝛼 as 600 and 𝑄𝛽 as 300. The Focmec
program output all acceptable solutions(“possible” mechanisms) based on input
parameters (station identifiers, azimuths,
take off angles at the source, polarities andamplitude ratios from among P, SV, and SH
arrivals). As for selection criteria, no error isallowed. The maximum number of solutions
is set to 100,000. We allowed 0.1 for the
maximum log10 of ratio, and selected 0.01
and 0.02 for lower bound (cutoff value) of P
Figure 3. Observing polarities of the largest Nov and S radiation factor, respectively. The
30, 2007 Bukit Tinggi event. Five traces from up ratio was indeterminate when both
4. RESULTS
AND DISCUSSION
to bottom are the E-W, N-S,
Radial, Transverse
calculated values were less than the chosen
and Vertical component of data recorded at IPM
cutoff values.
station.

4. RESULTS AND DISCUSSION
4.1. Bukit Tinggi, November 30, 2007 (02:13 UTC) 3.5 mb
We obtained a relatively well-constrained solution for thelargest 3.5mb event which occurred at 02:13 UTC, November
30, 2007. The focal mechanism of this event is a mostly strike
slip with some dip slip and the maximum compressional (P)axes are in the NNW-SSE direction (Figure 4). We obtained
58 acceptable solutions, for which the dip, strike and rake of
one nodal plane fall in the range of 45° to 75°, 10° to 60°, and
20° to 50° respectively, while for the second nodal plane theyfall in the range of 50° to 70°, 270° to 310°, and 130° to 170°,
respectively.

Figure 4. Lower-hemisphere
projections of the focal sphere for
the November 30, 2007, 02:13
UTC Bukit Tinggi event .

4.2. Bukit Tinggi, November 30, 2007 (12:42 UTC) 3.2 mb
The solutions of the November 30, 2007, 12:42 UTC Bukit
Tinggi event are well-constrained; we obtained a strike slip
faulting with the P axes distributions in the NW-SE
direction and the T axes distributions in the NE-SW
direction (Figure 5). We obtained 10 acceptable solutions
for which the dip and rake angles of the first nodal plane
fall within the range of 72° to 85° and of -40° to 0°,
respectively while for the second nodal plane, the dip andrake angles fall within the range of 58° to 82° and -180° to-160°. The strike angles, falling in two ranges, between
170° and 190°, and between 340° and 360° for the first
nodal plane, and between 80° and 100°, and between 270°
and 280° for the second nodal plane.

Figure 5. Lower-hemisphere
projections of the focal sphere for
the November 30, 2007, 12:42
UTC Bukit Tinggi event .

4.3. Bukit Tinggi, December 12, 2007 (10:01 UTC) 3.2 mb
As for the Bukit Tinggi event that occurred at 10:01 UTC,
December 12, 2007, the results are close with those of
Bukit Tinggi, 12:42 UTC, November 30, 2007 event in thatthe focal mechanism of this event is strike slip. P axes
distributions are in the NW-SE direction and the T axes
distributions are in the NE-SW direction (Figure 6). We
obtained 6 acceptable solutions with a range of 70° to 82°
for dip angles, and -40° to -10° for rake angles while strikeangles fall in two ranges, of 160° to 180° and of 330° to
360° for the first nodal plane and 55° to 80° for dip angles,-180° to -150° for rake angles, while strike angles fall in
two ranges, of 70° to 90° and 260° to 280° for the second
nodal
plane.
4.4
Bukit
Tinggi, January 10, 2008 (15:38 UTC) 3.0mb

Figure 6. Lower-hemisphere
projections of the focal sphere forthe December 12, 2007, 10:01
UTC Bukit Tinggi event .

4.4. Bukit Tinggi, January 10, 2008 (15:38 UTC) 3.0mb
The focal mechanism of the January 10, 2008, 15:38 UTC
Bukit Tinggi event shows similar results with the previous
two Bukit Tinggi events (the 12:42 UTC, November 30,
2007, and the 10:01 UTC, December 12, 2007 Bukit
Tinggi event) in which the obtained focal mechanism is of
strike slip faulting with P and T axes distributed in the
NW-SE and NE-SW, respectively (Figure 7). We obtained
2 acceptable solutions with a range of 80° to 85°, 0° to 10°
and 170° to 180°, and -20° to -10° for the distributions ofdip, strike, and rake angles, respectively, for the first fault
plane. As for the second fault plane, the dip and the rake
fall in the range of 75° to 80° and -180° to -170°,
respectively, while the strike fall in two ranges, 90° to 100°
and 260° to 270°.

Figure 7. Lower-hemisphere
projections of the focal sphere
for the January 10, 2008, 15:38
UTC Bukit Tinggi event.

4.5 Comparison of the Orientation of P and T axes, and the Observed Polarities
The P axes of the largest 3.5mb Bukit Tinggi event are in the NNW-SSE direction while those of thethree strike slip events are consistently in the NW-SE direction. The T axes for the three strike slip
events are also consistent; in the NE-SW direction. We also compared the polarity of the first motionsof P-phases for six Bukit Tinggi events, of which four are events described earlier while two others are
events that occurred on December 9, 2007 and March 15, 2008. The consistent dilatational first
motions of P waves observed for all events recorded at IPM station imply that the mechanisms of theevents are similar. Hardebeck (2002) suggests that the observed polarity at a given station should be
the same for each event in a cluster.
4.6 Relationship between Focal
Obtained and the Existing Faults

Mechanism

Figure 8 shows the epicenter of the four events that
we analysed, their focal mechanisms and the faults inthe study area, namely the Bukit Tinggi fault and theKuala Lumpur fault. Comparing the fault orientations
and the strike of the nodal planes, the relationshipbetween these two faults and the focal mechanisms
obtained is not clear. The difference in the movement
rate between the west and the east based on the GPSdata observed in the Southeast Asia within a ten-year
span (between 27 November 1994 and 25 December
2004 ), may cause some pushing that could releaseenergy to trigger movement on any weak plane, suchas in the Bukit Tinggi fault zone (Lat 2009). The
stress regime for this area inferred from GPS data
within a ten-year span (between 27 November 1994
and 25 December 2004) indicates that the force inplay is compressional in the NE-SW direction(Simons et al. 2007). This is contradict with theorientation of compressional axes obtained from thedetermination of focal mechanisms , in which in the
NW-SE and NNW-SSE direction. One possible
explanation to these Bukit Tinggi earthquakes is that

Figure 8. Map showing locations of four
Bukit Tinggi events (yellow circles),focal mechanisms obtained and theexisting ancient and inactive faults,
namely the Bukit Tinggi fault and theKuala Lumpur fault.

they are due to the weak-zone-normal extension mechanisms (Hurukawa and Imoto 1992). Fatt et al.
(2011) conducted a relocation study of local earthquakes in the Malay Peninsula and showed that
hypocenters of these events are better improved by using a method of modified joint hypocenter
determination (MJHD) developed by Hurukawa and Imoto (1990), and HYPOCENTER program(Lienert and Havskov 1995). The MJHD program shows that epicenters are scattered along the Bukit
Tinggi fault (striking NW-SE directions and dipping toward NE) and Kuala Lumpur fault(striking NW-SE directions and dipping toward SW) with depth between 6 and 30 km. The
HYPOCENTER program shows that the epicenters are scattered at the Bukit Tinggi fault with depth
between 0 to 10 km with similar orientation of nodal plane to the MJHD program. Their study
however, did not relocate the four Bukit Tinggi events that we analysed. Thus, relocation of theseearthquakes will be effective to improve the accuracy of the results of this study in future works.

5. CONCLUSION
We determined focal mechanisms for four Bukit Tinggi events that occurred in 2007 and 2008 using
the polarity data of the first motions of P and S waves, and their amplitude ratios fromthree-component broadband waveform data recorded at stations of IPM and KOM. Althoughconstraint for dip angles is weaker for two Bukit Tinggi events on November 30, 2007 (12:42UTC)
and on December 12, 2007 (10:01UTC), we obtained relatively well-constrained solutions for thesefour events; three events have strike slip faulting while one (and the largest) has a mostly strike slip
with some dip slip. The T axes of the strike slip events are in the NE-SW direction while theirmaximum P axes are in the NW-SE direction. The P axes of the largest event are in the NNW-SSE
direction. The consistency of the first motion polarity of P-waves observed for all Bukit Tinggi events
together with relatively well-constrained P and T axes direction, suggest that these earthquakes might
have similar mechanisms. This fits the characteristics of cluster earthquakes or earthquake swarms(Lat 2009). The validity of the relative motion of the strike slip mechanism, unfortunately, cannot be
confirmed geologically since these Bukit Tinggi earthquakes share significant similarities with otherintraplate earthquakes in that there are lack of surface ruptures (Shuib 2009). Due to an absence of thissurface movement, the result of this study serves for an important basis in ascertaining the focal
mechanisms of the earthquakes near Bukit Tinggi area, and will improve an understanding of seismic
activities in the Malay Peninsula.
AKNOWLEDGEMENT
We would like to thank Dr. Hurukawa and Dr. Yokoi for their constructive comments that improved
the clarity of the manuscript. We used the Generic Mapping Tools (GMT) (Wessel and Smith 1998) tomake some figures in the manuscript.
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