EARTHQUAKE ACTIVITY IN THE SOUTHEAST ASIAN REGION BEFORE
AND AFTER THE 2004 GREAT SUMATRA EARTHQUAKE.
CHE NOORLIZA LAT
Geology Department
University of Malaya
50603 Kuala Lumpur
Abstract. Southeast Asia is composed of zones of varying degrees of seismicity,
ranging from high in the Sunda Sea, to low in the South China Sea. The seismic
activity in the Southeast Asia region is largely related to the tectonic boundaries. The
great earthquake of Sumatra in 2004 occurred along the seismically active Sunda
trench where the Indian plate subducts under the Sunda plate. In contrast, Malaysia,
which is located on a stable craton, experiences little or no seismic activity. Judging
from the media, there seems to be more and more earthquakes nowadays, especially in
the Southeast Asia region. Seismicity data from 1973 to present, however, shows that
there is only a slight increase in the earthquake activity of Southeast Asia over the last
30 years.
INTRODUCTION
In the classic textbook, “Elementary Seismology”, Richter (1958) defined an
earthquake as any earth disturbances of natural origin recorded by a seismogram.
Seismology, or the study of earthquakes and related phenomena, began with the
reporting of the Lisbon earthquake in 1755. The causes of earthquakes can be of
volcanic or tectonic origin, or minor activities, like a landslide. Most of the
earthquakes recorded are of tectonic origin, associated with the movement of the
tectonic plates in the earth’s crust (Richter, 1958).
Southeast Asia consists of a stable craton in its interior surrounded by zones of
active boundaries. Countries like Malaysia and Thailand, which are located within the
stable craton, experience little or no earthquake activity. On the other hand, countries
like Indonesia and the Philippines, which are situated within the seismically active
area known as the Pacific Ring of Fire, experience a lot of tremors either from
tectonic movement or associated with volcanic eruptions.
Most of the earthquakes in this region are of tectonic origins. Earthquake
activities are concentrated at subduction zones, expanding ridges and fault areas.
Earthquakes can also be caused by volcanic eruptions, landslides and water reservoirs.
Volcanic tremors are usually smaller in magnitude compared to tectonic earthquakes.
Their depths are shallower and they are more clustered in nature. Some earthquakes,
like the Kenyir earthquake in 1984, were caused by the construction of the Kenyir
dam (Che Noorliza Lat, 1997). Earthquakes triggered by human activity are also
known as induced earthquakes. Reservoir-induced earthquakes are earthquakes that
occur because of the damming of tremendous amount of water.
For this paper, the SEA region is defined as being bounded by latitudes 29o
North to 11o South, and longitudes 92o – 141o East, encompassing an area more than
24 million square kilometres (Figure 1). According to the USGS database from 1973
to present, there has been more than sixty-eight thousand earthquakes occurring in this
area with Richter magnitude larger than 3.5. That amounts to an average of almost
2000 earthquakes per year.
REGIONAL TECTONICS
Southeast Asia is a part of the Eurasian tectonic plate extending into
Sundaland, bounded to the west and south by the Indian-Australian plate and to the
northeast by the Philippine Sea plate. The Eurasian plate can be further broken down
to several smaller plates (also known as microplates) such as the Sunda plate and the
Burma plate. The majority of the earthquakes occur along the plate fringes where the
Eurasian plate subducts under its neighbouring plates. Figure 2 shows the tectonic
setting of the region. The tectonic movements include subduction, where the IndianAustralian plate goes under the Sunda plate, the spreading ridge areas such as where
Burma and Sunda plates move away from each others creating ridges, and also some
strike-slip fault zones such as in central Sumatra and southern Thailand.
Earthquakes at subduction zones increase in depth away from the plate
boundary. The subducting plate could be mapped using the earthquake depth profile.
The subduction zones are also indicated by the presence of trenches. Ryukyu trench,
Manila trench and the Philippine trench in eastern Southeast Asia are created as the
Philippine plate subducts under the Eurasian plate as shown in Figure 2 (Hutchison,
1996). On the western front, the Australian plate is moving against the Eurasian plate
at about 8 cm a year. This northward movement gave rise to the active Sunda trench
which is where most of Sumatra earthquakes are located.
EARTHQUAKE DISTRIBUTION IN SOUTHEAST ASIA
From Figure 1 and Figure 2, we can see how seismicity is influenced by the
tectonic framework of the region. Shallow earthquakes (depth < 30km) occurred at or
near plate boundaries and within the interior. Depth of epicentres increases away from
the plate boundaries. Cross section plots of the earthquakes in the subduction zone
areas would reveal the image of the subducting plate. Deep earthquakes (depth up to
600km) are shown as black dots.
There are also earthquakes associated with faults and volcanic activity. In
Figure 1, they are buried amongst the tectonic earthquakes. The two earthquakes on
Peninsula Malaysia are examples of dam-induced earthquakes.
EARTHQUAKES STATISTICS
To ease interpretation, we would look at the earthquakes with magnitude more
than 5. These earthquakes are more likely to be a threat to humans, in terms of their
destructive potential. Table 1 shows the yearly seismicity of Southeast Asia for the
last 30 years, sorted according to magnitudes. The data is extracted from the USGS
National Earthquake Information website. The data chosen is limited to magnitudes
larger than 4 and post 1977 because international seismic catalogues tend to have a
less complete record at lower magnitude ranges and earlier times.
Table 1 gives an average of 361 magnitude 5 earthquakes, 24 magnitude 6 and
3 magnitude 7 earthquakes per year in this region. The average for magnitude 8
earthquake is only 0.2 per year. The values are expected as smaller earthquakes do
happen more frequently than larger earthquakes.
Figure 3 shows the seismicity of Southeast Asia in a graph form. In Figure 3a,
the top curve shows the total number of earthquakes occurring each year. The lower
curve is for earthquakes magnitude 5. Here we can see that the magnitude 5 curve
basically mirror the total seismicity curve. This indicates that the seismic activity is
dominated by earthquakes in the lower magnitude range. Figure 3b shows the curves
for magnitude 6 and 7. The magnitude 6 curve clearly shows an upward or increasing
trend while the magnitude 7 curve lack any obvious inclination one way or the other.
Both curves show alternating high and low peaks.
In this 30-year period, a magnitude 8 earthquake was first recorded in 1997,
then 2 years later in 1999, followed by another one in 2001. There was a three-year
gap before the magnitude 9 earthquake, another magnitude 8 in 2005, followed by 2
in 2007. It seems like we were having one magnitude 8 earthquake every other year
but the activity has picked up recently. Only one magnitude 9 recorded all this time.
The values conform to the world average occurrence of magnitude 9 earthquake of 1
every 40 years or so.
Another way to analyse the seismicity is to calculate the recurrent rate. This is
done by plotting the frequency of earthquakes against the magnitudes. The slope is
represented by the equation: log N = a + bM, where N is the frequency, a is the
intercept on the y-axis, b is the slope and M is the magnitude. This b-value simply
relates the frequency of earthquakes based on their sizes, obtained by dividing log N
(number of earthquakes) by M (magnitude). Each region has its own, unique b-value.
Figure 4 shows the recurrent curve for this region for earthquakes with
magnitudes larger than 4. From more than fifty-four thousand events, there are more
than forty-two thousand earthquakes with a magnitude between 4 and 4.99 compared
to just 1 earthquake with a magnitude 9 or above (Table 3). The curve illustrates the
fact that the number of earthquakes decrease with magnitude, giving the negative
relation between magnitude and frequency. For this dataset, the b-value for Southeast
Asia for earthquakes from 1978 to 2007 is 0.93. The intercept or the a-value cannot be
determined due to lack of data for the lower magnitude range.
Chana and Chandler (2001) reported a similar b-value for Hong Kong of 0.85.
The difference in the b-value could be attributed to several factors such as
completeness of the earthquake catalogue, the actual formula used, and the limits of
location, time duration and depth of the earthquakes. The higher the b-value means
there are more earthquakes with smaller magnitude occurring for each bigger one. A
b-value of 1 loosely means that the frequency of earthquakes increase a ten-fold for
each increase in magnitude, i.e. there are 1000 magnitude 6 earthquakes compare to
100 magnitude 7. Hong Kong is expected to have a lower b-value compared to
Southeast Asia because it is only a small part of the seismically active zone.
THE 2004 SUMATRA EARTHQUAKE
On December 26, 2004, northern Sumatra was hit by several large
earthquakes, the largest being a magnitude 9 located offshore near Banda Aceh. The
shock was felt in several nearby countries like Malaysia and Thailand. The earthquake
and the resulting tsunami claimed more than a quarter million lives. It was one of the
worse natural disasters in recent history. By comparison, a magnitude 9 earthquake is
10,000 times more powerful than a magnitude 5, releasing more than 1 million times
energy. Even the atomic bombs dropped in Hiroshima and Nagasaki seem minute
compare to the destructive force of this quake.
The tectonic setting of the 2004 earthquake is shown in Figure 5. The quake
epicentre is located at a shallow depth within the Sunda trench as a result of a rupture
that happened as the Indian-Australian plate subducts under the Sunda plate. This
earthquake and its subsequent aftershocks created a rupture zone of more than
1000km long. The destructive effect was compounded by the presence of combination
fault and spreading ridge system in the Andaman Sea where the Burma plate moves
along the Sunda plate. This fault system extended to the south into Sumatra. The
Sunda trench has been riddled with earthquakes; it is just a matter of where and how
big.
DISCUSSION
Even after three years of its occurrence, the Southeast Asia region is still
recovering from its effects, not only in terms of economics and infrastructure but also
in terms of tectonics stability. Recent events in Bengkulu could be indicative of the
restabilizing effort.
Is there truth to the notion that seismicity increases in the Southeast Asia
region after the 2004 quake? The evidences seem to point positively in that direction.
If we calculate the average number of quakes per year before the 2004 great
earthquake, we would get 356 earthquakes. Data from 2004 till now gives an average
of 592 earthquakes per year. This equates to an increase of about 24%.
The change in seismicity before and after the quake is summarized in Table 2.
Here we can see that the average increase in all magnitude levels. For example, before
2004, we only have an average of 21 earthquakes of magnitude 6 per year. The
average for the last 4 years has increased to 38 magnitude 6 earthquakes per year.
This year (up to 31 October 2007), we’ve already experienced 35 earthquakes in the
region. The average of a magnitude 8 quake has increased 8 times! In fact this year we
experience not one but two magnitude 8 quakes. As mentioned earlier, increase in the
number of large earthquakes will equate in multi-fold increase in the number of
smaller earthquakes.
A few words of caution. The results presented in previous paragraphs have
their flaws. Averages before and after the great quake are calculated using data from
different time duration. Before 2004 data spanned 26 years whilst the ‘after’ only
covers time short of 4 years. This inequality would create bias in the data. It would
have been better if we have a longer span of data after the quake for comparison. The
4 year span may not have been enough to establish a reliable average.
On the other hand, Figure 3a also shows an increasing trend. The slope
through the magnitude 6 curve gives the rate of increase of 1.63 events per year. This
escalating trend, however, has been in place since 1978 and may not be totally the
effect from the magnitude 9 quake.
Therefore, as a conclusion, there is definitely an increase in the seismicity of
Southeast Asia as shown by the data from 1978 – present. As humans, it is not in our
power to stop the force of nature. The seismic activity should be diligently monitored
and scrutinised to better prepare for the future. Some quakes do provide information
about things to come. Volcanic tremors have long been used to predict volcanic
eruptions. Some of the quakes have unique foreshock and aftershock sequences. Even
tsunamis can be somewhat predicted by studying the quakes that cause them.
The data used in this study is severely insufficient for this kind of statistical
inferences. To make these all possible, we need a better local network of seismic
stations. Important phase data and information about earthquakes of lower magnitudes
are needed for reliable interpretation. There should be collaboration between the
nations involved in sharing of information and expertise. With the synergy of working
together, we could at least lessen the destructive effect of another magnitude 9 quake,
whenever and wherever it might strike.
REFERENCES
BURMA PLATE, 2007. Available at http://en.wikipedia.org/wiki/Burma_Plate
CHANA, L.S. AND A. M. CHANDLER, 2001. Spatial bias in b-value of the
frequency–magnitude relation for the Hong Kong region, Journal of Asia Earth
Sciences, Vol 20, Issue 1, pp 73-81. (available online at www.sciencedirect.com)
CHE NOORLIZA LAT, 1997. Dam-induced seismicity of Kenyir Terengganu.
Abstract. Warta Geologi. 23(2), 68. Presented at the Geology Department,
University of Malaya, 10 March 1997.
HUTCHISON, Charles S., 1996. Geological Evolution of South-East Asia,
Geological Society of Malaysia, 368p.
NATIONAL EARTHQUAKE INFORMATION CENTER, 2007. Earthquake Data
Base. Available at http://earthquakes.usgs.gov
RAJ, John K., 2007. Tsunami threat to coastal areas of Sabah, East Malaysia.
Geological Society of Malaysia, Bulletin 53, June 2007, pp 51 – 57.
RICHTER, CHARLES, 1958. Elementary Seismology, W.H. Freeman & Co., San
Francisco and London, 768p.
Table 1. Seismicity of South East Asia (1978 – present) (data from USGS/NEIC
Website)
YEAR
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
5
336
316
307
247
265
413
391
475
422
290
331
379
470
405
347
342
408
304
358
213
202
272
294
279
284
276
534
784
464
415
Sum
Average
10823
360.8
MAGNITUDE
6
7
12
3
18
3
12
1
7
0
13
1
27
1
12
1
21
1
16
2
18
1
9
3
14
1
21
4
20
4
33
4
19
1
40
4
39
9
37
4
18
2
21
6
24
3
32
3
21
5
26
4
27
1
36
6
47
3
35
4
34
3
709
23.6
88
2.9
8
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
0
1
0
1
0
0
0
0
1
0
2
SUM
351
337
320
254
279
441
404
497
440
309
343
394
495
429
384
362
452
352
400
233
230
299
330
305
314
304
576
835
503
454
6
0.2
11626
387.5
Note: Data for 2007 is until 31/10/07. The Magnitude 9 earthquake in 2004 is not
included in the table.
Table 2. Comparison between the averages.
Before 2004
Magnitude
Average
5
331.8
6
21.4
7
2004 - present
Magnitude
Average
5
549.3
6
38.0
7
8
2.8
0.1
Total
356.1
0.8
Total
592.0
8
4.0
Figure 1. Earthquakes in Southeast Asia from 1978 – present (data from USGS/NEIC
database)
Figure 2. Active plate margins of Southeast Asia (Hutchinson, 1996).
Yearly Seismicity of Southeast Asia
(1978 - present)
900
800
700
Frequency
600
Total seismicity
500
400
300
Magnitude 5 earthquakes
200
100
0
1978
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
2004
2006
2008
Year
(a)
Yearly Seismicity of Southeast Asia
Magnitude 6 and larger (1978 - present)
50
45
40
35
Magnitude 6
Frequency
30
25
20
15
Magnitude 7
Magnitude 8
Magnitude 9
10
5
0
1978
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
2004
2006
2008
Year
(b)
Figure 3. Yearly seismicity plot for Southeast Asia (1978-present). (a) For total
number of earthquakes and magnitude 5 only. (b) For magnitudes 6 to 9. (data from
USGS/NEIC database.
Southeast Asia Earthquakes
(1978 - present)
100000
42429
10823
Frequency, N
10000
1000
706
Slope = log N/ M = 0.9255
100
88
10
6
1
1
3
4
5
6
7
8
9
Magnitude, M
Figure 4. Recurrence curve for the Southeast Asia Region (data from USGS/NEIC
website)
Figure 5. Tectonic setting of the 2004 Sumatra Earthquake (from Wikipedia)
10