Synopses of Master Papers

Bulletin of IISEE, 47, 25-30, 2013

RELOCATION OF LARGE EARTHQUAKES ALONGTHE SUMATRAN FAULT AND THEIR FAULTPLANES
Biana Rahayu Wulandari ∗MEE11605

Supervisor: Nobuo HURUKAWA∗∗ABSTRACT
In order to identify fault planes of large eartquakes along the Sumatran Fault in Sumatra, western
Indonesia, fifteen M ≥ 6.0 earthquakes and their aftershocks, from 1964 to 2011, were relocated using
the Modified Joint Hypocenter Determination (MJHD) method. The results of earthquake relocation innorthern, central and southern Sumatra for this period, show that the fault planes of M ≥ 6.0
mainshocks, with a few exceptions, were along the Sumatran Fault. We found that in many cases
mainshocks originated near the segment boundaries and propagated to the end of the fault segments.
The 1967 (M 6.1) and 2003 (M 6.0) earthquakes that occurred in northern Sumatra are off-Sumatranfault earthquakes.
We also relocated eleven M ≥ 6.8 earthquakes from 1900 to 1963, and their aftershocks,
together with the above-mentioned 15 M ≥ 6.0 earthquakes from 1964 to 2011 in order to obtain
precise hypocenter locations of all earthquakes and identify the fault planes of M ≥ 7.0 events. It waspossible to relocate hypocenters precisely except for the 1900, 1907 and 1909 events. We found thatfault segments related to M ≥ 7.0 events are Kumering for 1933 (M 7.5), Tripa for 1935 (M 7.0) and1936 (M 7.2), Ketaun for the first event of 1943 (M 7.4), Sumani and Suliti for the second event of1943 (M 7.6). We considered fault segments related to the 1900 ad 1909 event are Dikit and Siulak,
respectively.
From the results of earthquake location between 1900 and 2011 and the expected fault
lengths of M ≥ 7.0 earthquakes, we obtain earthquake history along the Sumatran fault since the 1892
(M 7.3) event that occurred along the Angkola segment. Possible seismic gaps along the Sumatran
Fault are Sunda, Semangko, Manna, Musi, Sianok, Sumpur, Barumun, Toru, Renun, central part of the
Tripa, Aceh, and Seulimeum segments.

1. INTRODUCTION
The Sumatran Fault is formed as a result of the oblique subduction of the Indo-Australian plate
beneath the Sunda plate. The 1900 km long structure of the right lateral strike slip Sumatran Fault isparallel to the oblique convergence of the Sumatran Subduction on the western coast of Sumatra (Siehand Natawidjaja, 2000). Many large earthquakes occurred along the Sumatran Fault (Figure 1).
By comparing fault segments and historical large earthquakes, Sieh and Natawidjaja (2000)
and Natawidjaja and Triyoso (2007) identified unbroken fault segments, where future large
earthquakes are expected. In a study by Aydan (2007) on crustal deformation and straining of Sumatra
island by using GPS deformation rates, it is found three high stress rate concentration regions along
the Sumatran Fault, which are Sunda, Semangko, Kumering, Ketaun, Dikit, Sianok, Sumpur, Barumun,
Angkola, and Toru.

∗

Agency for Meteorology, Climatology and Geophysics, Indonesia.
*∗ Building Research Institute, Japan.

In northernmost of the Sumatran Fault, Aceh and Seulimeum segments, has notexperienced a major eartquake at least last 170 years. Therefore, a seismic gap is considered there(Bellier et al., 1997; Petersen et al., 2004; Sorensen, 2008). In a very recent study by Ito et al. (2012)
on the Aceh segment, which contains 200-km-long segment, found shallow creeping and locked zonethat appears to be capable of producing a significant earthquake. Their estimation of locking depth andslip deficit rate in this segment suggested that the accumulated seismic moment in 170 years
correspond to an earthquake of magnitude 7.
2. DATA

Figure 1. Epicenters of M ≥ 6.0 earthquakes near
the Sumatran Fault from 1900 to 2011.

We relocated large earthquakes and theiraftershocks along the Sumatran Fault from1900 to 2011. We selected area 8ºS – 7ºN and94ºE – 106ºE for all strike-slip inland
earthquakes with shallow depths (Figure 1).
According to the purposes, twodata set were used in this study. Firstly,
earthquakes with M ≥ 6.0 and theiraftershocks from 1964 – 2011 were relocatedin order to identify their fault planes.
Secondly, earthquakes with M ≥ 6.8 from1900 – 2011 and aftershocks available in ISS
bulletins were relocated in order to identify
the fault planes of old earthquakes and obtain
earthquake history.
The intial P-wave arrival times atworldwide stations are taken from various
references as British Association for theadvancement of Science, Seismology
Committee; ISS (International Seismological
Summary); ISC (International SeimologicalCentre); USGS (United States Geological
Survey). Note that Global CMT solutions
after 1976 were used as Mw magnitude.
Since the CMT solution of the 2003 event is
not available, this event used Ms magnitude
from ISC.

3. THEORY AND METHODOLOGY
The Modified Joint Hypocenter Determination (MJHD) method was used for a precise relocation ofearthquakes along the Sumatran Fault. The MJHD method was developed by Hurukawa and Imoto(1990, 1992) and Hurukawa (1995) from the Join Hypocenter Determination (JHD) method. The
MJHD method calculates hypocenters of a group of earthquakes and station corrections
simultaneously. Station corrections remove the effects of lateral heterogeneity and reflect a travel-timedifference between the assumed velocity structure and the actual one.
They used constraints that station correction is independent of distance and azimuth fromthe center of the region to each station, then minimizes their absolute values. Although sometimes thiswill sacrifice absolute hypocenters, it makes the method stable.
Since we used only P-arrival times and no nearby station, sometimes the focal depth become
deeper than it should be and this is inaccurate. We already know that the focal depths of earthquakes atthe Sumatran Fault are about 0 – 20 km. We can improve the focal depth by changing the additionalcoefficient a 1 in the MJHD method proposed by Hurukawa el al. (2008). Therefore, the group ofearthquakes become shallower.

4. RESULTS AND DISCUSSION
4.1 Relocation of M ≥ 6.0 Earthquakes between 1964 – 2011 and their Fault Planes
In this study, mainshocks of M ≥ 6.0 and their immediate aftershocks after 1964 along the Sumatran
Fault were relocated. In order to know relative locations of large earthquakes and their fault planes, werelocated all earthquakes simultaneously. Sumatra, however, is a large region and about fifteen
earthquakes of M ≥ 6.0 occurred in this region. Therefore, to facilitate the process of relocation, it wasdevided into three regions: Northern, Central and Southern Sumatra.
4.1.1 Northern Sumatra
There were six earthquakes of M ≥ 6.0 occurred in the Norhern Sumatra (Figure 2). The 1964
earthquake occurred close to the Simeuleum segment. The 1990 and 1996 mainshock and its
aftershocks event occured on the Tripa segment. The 1997 occured near the Renun segment. The
fault plane of 1964, 1990, 1996 and 1997 are in the Sumatran Fault. The 1967 and 2003 events are off
Sumatran Fault. The fault plane of 1967 is in Nortwest (NW) – Southeast (SE) direction, which isparallel to the Sumatran Fault.
4.1.2 Central Sumatra
Four of the M ≥ 6.0 earthquakes occurred in the Central Sumatra region. They are the 1987 (M 6.4),
2008 (M 6.0) and the 2007 doublet earthquakes (M 6.4 and M 6.3) (Figure 3).
The fault planes of 1987 and 2008 events are along the SE end of the Renun segment and
NW end of the Toru segment in the Sumatran Fault, respectively. However, it is still difficult to judge
the fault plane of the 2007 doublet from the immediate aftershocks distribution because standar errors
of aftershock location are large. Although we could confirm that the doublet occured at the boundary
between the Sianok and Sumani segments, we could not identify the fault where the doublet occured.
However, our relocation result does not consistent to previous studies. Therefore, further study
necessary for the 2007 doublet.
4.1.3 Southern Sumatra
There are four earthquakes of M ≥ 6.0 occurred in Southern Sumatra region as shown in Figure 4.
The 1995 earthquake of M 6.7 occurred along the SE end of the Siulak segment. The 2009 earthquakeof M 6.6 occurred on the NW end of the Dikit segment. The 1979 earthquake of M 6.5 occurred near
the Ketaun segment. Then, the 1994 earthquake of M 6.8 occurred near the Kumering segment.
The fault plane of 1979, 1994, 1995 and 2009 are on the Sumatran Fault. The 1994
earthquake have no immediate aftershocks. Though the fault plane of 1994 earthquake difficult todetermine. According to Widiwijayanti et al. (1996) who caried out local temporal aftershockobservation of the 1994 earthquake, most of events were located in the broad zone of 55 x 20 km2
along to the Kumering segment of the Sumatran Fault. By comparing their locations and the relocated1994 earthquake in this study, we can conclude that the 1994 earthquake was mislocated about 30 km
NE and apart of the Kumering segment. In the same manner, there is high possibility that other events,
1979, 1995 and 2009 were also mislocated about 30 km NE. If we correct about 30 km for them, theysituated very close to the Sumatran Fault.

Figure 2. Distribution of relocated M ≥ 6.0
earthquakes in Northern Sumatra region between
1964 and 2011 using MJHD. The magnitude anddepth of the earthquakes are represented bydifferent sizes of circle and colored shapes,
respectively. The momment tensor solutions aretaken from Global CMT catalog. Cross indicatesstandard errors of hypocenters.

Figure 3. Distribution of relocated M ≥ 6.0
earthquakes distribution in Central Sumatra
region between 1964 and 2011 using MJHD.
Symbols are same as in Figure 2.

Figure 4. Distribution of relocated M ≥ 6.0 Figure 5. Distribution of relocated earthquakes
earthquakes distribution in Southern Sumatra in Sumatra between 1900 and 2011.
region between 1964 and 2011 using MJHD.
Symbols are same as in Figure 2.

4.2

Relocation of large Earthquakes between 1900-2011

We tried to relocate all earthquakes. However, since travel time residuals of P-arrival times for the1900 (M 7.0), 1907 (M 6.8), 1909 (M 7.3) earthquakes were too large, these earthquakes were
excluded from MJHD relocation.
As there are no nearby stations and only P-arrival time readings are available, we fixed
focal depths of all earthquakes to be 15 km in this relocation. Figure 5 shows MJHD hypocenters.
Almost all earthquakes were located on the Sumatran Fault.
4.3 Earthquake History and Seismic Gaps Along The Sumatran Fault
There are eight M ≥ 7.0 occurredalong the Sumatran Fault since1892 . They are 1892 (M 7.5),
1900 (M 7.0), 1909 (M 7.3), 1933
(M 7.5), 1935 (M 7.0), 1936 (M7.2), and the doublet of 1943 (M7.4, M 7.6). Since we need to
know
the
fault
lengthcorresponding
earthquake
magnitude in order to identify fault
segment and to obtain the seismic
gaps, fault lengths of the magitude≥ 7.0 earthquakes are estimated by
the equation for large earthquakes
in the Sagaing Fault in Myanmar(Hurukawa and Phyo, 2011). Fromthe equation, we obtain the faulthlength of M 7.0, M 7.2, M 7.3, M
7.4, and M 7.6 earthquakes are 62
km, 84 km, 97 km, 113 km, 132
km and 153 km, repectively. These
lengths were used in Figure 6.
Sevenearthquakes
with M ≥ 7.0 have occured along
the Sumatran Fault and rupture thefault segments between 1900 2011. On 1933, an earthquake with
M 7.5 has ruptured all part of the
Figure 6 Earthquake history and the Sumatran Fault. Yellow
Kumering segment in the Southern
Sumatra. On 1935, an earthquake
ellipses indicated M≥7.0 earthquakes after 1930. Blue ellipses
with M 7.0 ruptured northmost
represent M ≥ 7.0 earthquakes before 1930. Red ellipses withpart of the Tripa segment indashed lines indicate possible seimic gaps. Segment names ofNorthern Sumatra. On 1936,
the Sumatran Fault was taken from Sieh and Natawidjaja
another earthquake with M 7.2(2000).
ruptured southmost part of the
Tripa segment. On 1943, doublet earthquakes occured in the Cental Sumatra. The first earthquake with
M 7.4 has ruptured the Ketaun segment, while the second earthquake with M 7.6 has ruptured the
Sumani and Suliti segments. Concerning two M ≥ 7.0 events on 1900 and 1909, we considered from
catalog locations that these two events ruptured the Dikit and Siulak segments, respectively. In
addition, the 1892 earthquake occurred along the Angkola Segment (Prawirodirjo, 2000).
The above results and expected fault length of M ≥ 7.0 earthquakes, we obtain earthquake
history since 1892 and possible seismic gaps along the Sumatran fault in Sunda, Semangko, Manna,Mussi, Sianok, Sumpur, Barumun, Toru, Renun, center part of the Tripa, Aceh and Seulimeum
segment.
These results are different from the recent study of Aydan (2007) on crustal deformation andstarining of Sumatra island using GPS deformation rates. It is pointed out that there are three highstress rate concentration along the Sumatran Fault. They are Sianok, Sumpur, Barumun, Angkola,
Toru, Dikit, Ketaun, Sunda, Semangko, and Kumering segment.

5. CONCLUSIONSWe relocated M ≥ 6.0 earthqukes and their immediate aftershocks near the Sumatran Fault from 1964
– 2011 by using Modified Joint Hypocenter Determination (MJHD) metthod. The fault plane of 1964,
1979, 1987, 1990, 1994, 1995, 1996, 1997, 2007, 2008, and 2009 are in the Sumatran Fault. The 1967
and 2003 are off Sumatran Fault.
We also relocated M ≥ 6.8 earthquakes from 1900 to 2011. We found that sevenearthquakes with magnitude M ≥ 7.0 have occured along the Sumatran Fault and rupture the following
fault segments. On 1933, 1935, 1936, first event of 1943 doublet earthquakes occured along theKumering, northmost of Tripa, southmost of Tripa, Ketaun segment, respectively and the second eventof 1943 doublet occured at the Sumani and Suliti segment. The 1900 and the 1909 earthquake
occurred along the Dikit and Siulak segments, respectively. In addition, the 1892 earthquake occurredalong the Angkola segment.
The result of M ≥ 7.0 earthquake locations for period 1892 to 2011 and expected fault
length of M ≥ 7.0 earthquakes, we obtain earthquake history and possible seismic gaps along theSumatran fault in Sunda, Semangko, Manna, Mussi, Sianok, Sumpur, Barumun, Toru, Renun, center
part of the Tripa, Aceh and Seulimeum segments.

ACKNOWLEDGEMENT
I would like to express my sincere gratitude to Dr. Tatsuhiko Hara and Dr. Bunichiro Shibazaki for
their valuable suggestion for my individual study.

REFERENCES
Aydan, O., 2007, Areconnaisance report on the 2007, Earthquake Disaster Investigation Sub-Comittee,
Japan Society of Civil Engineers.
Bellier et al., 1999, J. Geodyn., 24, 169-183.
Hurukawa, N. and Imoto, M., 1990, J. Seism. Soc. Japan, Ser. 2, 43, 413-429.
Hurukawa, N. and Imoto, M., 1992, Geophys. J. Int., 109, 639-652.
Hurukawa, N., 1995, Geophy. Res. Letters, Vol. 22, No. 23, pp 3159-3162.
Hurukawa, N., M. Popa, and M. Radulian, 2008, Earth Planets Space, 60, 565-572.
Hurukawam,N and Phyo, 2011, Geophys Res Lett, 38, L01310.
Ito, T., et al., 2012, J. Geophys. Res., 117, B06409, doi:10.1029/2011JB008940.
Natawidjaja and Triyoso, 2007, J. Earthquake and Tsunami, Vol. 1, No. 1, 21-47.
Petersen et al., 2004, Tectonophysics, 390, 141-158.
Prawirodirjo, 2000, J. Geophysics Res, 105, 28,343-28,361.
Sieh, K. and Natawidjaja., D.,2000, J.Geophysics. Res, 105, 28295-28326
Sorensen et al., 2008, American Geophysical Union, 89, 14, 133-134,
Widiwijayati, C., et al., 1996, Geophy. Res. Letters, Vol. 23, No. 21, pp 3051-3054.