Citation: Da Costa Oliveira, V., T. Itoi (2022), Probabilistic seismic hazard assessment of Timor-Leste,
Synopsis of IISEE-GRIPS Master’s Thesis.

PROBABILISTIC SEISMIC HAZARD ASSESSMENT
OF TIMOR-LESTE
Vitoria Maria de Lillia da Costa Oliveira1
MEE21710

Supervisor: Tatsuya ITOI2
ABSTRACTThis dissertation examines the purpose of defining the seismic hazard level in Timor-Leste for seismicbuilding codes based on past earthquakes and tectonic settings. The historical earthquake occurrencesfrom 1960 to 2021 were collected from the United States Geological Survey (USGS) and the Instituteof Geology and Petroleum (IPG) with a magnitude between 2 and 8.1 Richter Scala. Earthquake source
zoning was based on the occurrences of historical earthquakes surrounding Timor-Leste, and thefrequency magnitude distribution by using the Gutenberg-Richter recurrence law was estimated. The
kernel density function has been used to modulate the probability distribution of hypocenter distancefrom different source zones. The simulation procedure was to determine peak ground acceleration using
the Ground Motion Prediction equation adapted from Si et al, 1999. The result gave the seismic hazard
curves, and a seismic hazard map plotted for the return period of 475 and 2500 years (10% and 2%
exceedance in 50 years, respectively). The highest total hazard observed in the Atauro site was a very
high hazard level with a PGA value of 440 gals and 209 gals, which was included in class V and class
III, corresponding to a 2% and 10% probability of exceedance in 50 years. However, the smallest total
hazard observed in the southern part of the Viqueque site was a low hazard level with a PGA value of
171 gals for a 2% probability of exceedance for 50 years also for 10% probability of exceedance for 50
years discovered in Viqueque, Lautem and RAEOA sites with the PGA value less than 100 gals.
Keywords: Timor-Leste, Earthquake Source Zones, Earthquake Parameters, PGA, Seismic Hazards
1. INTRODUCTION
Timor-Leste is a young independent country situated in the eastern half of the island of Timor, between
Indonesia and Australia. It consists of 13 municipalities, one special administrative region, Oe-Cusse,
and the islands of Atauro and Jaco. Timor-Leste is approximately 14, 874 sq km, with about 70% of its
land mass covered by mountains or hills.
Timor-Leste is vulnerable to several natural hazards and disasters, including tropical
cyclones, flooding, drought, landslides, earthquakes, and tsunamis. Fortunately, these events are
relatively localized with no historical widespread devasting impacts. This notwithstanding, the country
does not have a seismic building code or formulated policies.
Tectonically, the island of Timor lies in the outer arc region between the Timor Trough and theBanda Arc. It is the most mature part of the transition from subduction to the collision of the Australianplate with the Banda Arc and is the orogenic product of that collision [Carter et al., 1976; Hamilton,
1979]. The subsidence of Banda Trench has rollbacked southeastwards about 30 km from 4 to 0 Ma.
This subducted slab is traceable by earthquake seismology to depths beyond 600 km [Engdahl et al.,1998]. Therefore, the collision creates the area surrounding Timor Island, which is prone to earthquakes.
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Earthquake and Seismology/Geohazard and Geological Engineering Division of Institute of Petroleum andGeology2
Associate Professor of Graduate School of Engineering, The University of Tokyo1

Regarding the seismicity on Timor Island, the shallow earthquakes are in the northern and eastern partsof Timor Island, and the intermediate and deep earthquakes are located in the western part of Timor
Island. The purpose of this study is to define seismic hazard levels in Timor-Leste for building code bydetermining peak ground acceleration with the Probabilistic Seismic Hazard Assessment method (PSHA)

2. DATAThe data was obtained from The United States Geological Survey (USGS) and the Institute of Geology
and Petroleum (IPG) seismic catalog. The composite catalog of this study area spanned from 1960 to
2021 and was used for the seismicity of the Timor-Leste region and the surrounding area between
coordinates 0° - 18° South Latitude and 114° - 135° East with a total of 11237 earthquakes events. The
earthquake catalog considered completeness with ranges of magnitude from 2 SR to 8.1 Ritcher Scale.
The background earthquake model is employed in this study since information about the individual fault
is not identifiable in Timor-Leste.
3. METHODOLOGY
3.1. Identification of Earthquake Source Zones
Identifying the earthquake source zone is based on observing the distribution of past earthquakes andtectonic settings in the region and considering all earthquakes surrounding Timor-Leste. Because theindividual faults are not identifiable, we assumed that the frequency of earthquake events that occurredin the past would happen in the future. The target area focuses on 14 points that cover 14 municipalities
in Timor-Leste, such as Dili, Atauro, Manatuto, Baucau, Lautem, Covalima, Ainaro, Aileu, Ermera,
Liquisa, Bobonaro, Manufahi, Viqueque and RAEOA.
3.2 Processing Earthquake Data
3.2.1 Conversion of Magnitude
Earthquake data collected from the USGS and IPG catalogs have different magnitude types, such as
body-wave magnitude (mb), surface-wave magnitude (Ms), and local magnitude (ML). Meanwhile, in
the a-value and b-value analysis, one type of magnitude is required, namely moment magnitude (M w),
which for this reason is needed to convert all magnitude types from the catalog to moment magnitude(Mw), which were derived by Badan Meteorologi, Klimatologi dan Geofisika (BMKG), 2017:
Mw = 1,0107mb + 0,0801(1)

Mw = 0,6016Ms + 2,476 for Ms ≤ 6.1(2)

Mw = 0,9239Ms + 0,5671 for Ms ≥ 6.2(3)

3.2.2 De-Clustering
De-clustering is to separate mainshocks, foreshocks, and aftershocks in the catalog and remove
dependent events to get the accuracy of the poison model. The process of separating the main andbackground earthquakes from foreshocks and aftershocks uses the empirical method proposed by
Gardner and Knopoff (1974) to find out the sensitivity of the results to changes in the use of the empiricalapproach. The process of the separation uses ZMAP7 software (Wiemar, 2001).

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3.3 Estimation of Earthquake Parameters3.3.1 a and b Value Parameters
Determination of a and b values is chosen from Guttenberg & Richter’s (1944) first study of observationof earthquake magnitudes:
log10λm = a-bm

(4)

Where λm is, the rate of earthquakes with magnitudes greater than m; a is the constant of the overall rateof the earthquake in a region, and b indicates the relative likelihood of large and small earthquakes(typical b values are approximately equal to 1).
The cumulative distribution function (CDF) for the magnitudes of earthquakes that arelarger than some minimum magnitude mmin Eq. (4) can be used for (Baker, 2008) equation if amaximum magnitude can be determined as:=(5)
Where FM (m) is the cumulative distribution function for M, the probability density function (PDF) for
M can also be computed by taking the derivative of the CDF.

(6)
Where fM(m) is a probability density function for M, mmax is a maximum magnitude earthquake. For
continuous distribution of magnitudes into a discrete set of magnitude for a and b values can be used:(7)
Where mj is a discrete set of magnitude, order that mj < mj+1. We can calculate the probabilities
associated with all magnitudes between mj and mj+1 to discrete value mj. If the discrete magnitude isclosely spaced, the approximation will not affect the numerical result.
3.4 Estimation of Ground Motion Parameters
The selection of the GMPE equation in this study is based on the similarity of the region’s tectonics andthe individual fault, not identifying where the new attenuation relationship for peak ground accelerationis adapted from (Si et al.,1999).
= 0.5*Mw+0.0043*D+di+0.61-Log10(X+0.0055*100.5Mw)-0.003X

3(8)

Where Mw is moment magnitude, D is a depth, and di is a constant depending on earthquake
types, 0.00 is for shallow crustal, 0.22 is for intra-plate, the standard deviation σ for log PGA is 0.27.
The natural logarithm of PGA is seen to be normally distributed. So, we can determine theprobability of exceeding any PGA level using mean and standard deviation (G&R)
(9)
Where
() is the standard normal cumulative distribution function, P(PGA) > a|Mw, X, D
is the probabilistic distribution function.
We also assumed the fault distance X according to (Tjanima & Hayashida, 2018):
X=HD-100.5Mw-1.85/2(10)

3.5 Hazard Curve
With all information above, we can now combine it, using the probabilistic seismic hazard assessment
equation, to get the annual probabilistic of exceedance.
(11)
Where ν(PGA >a) is the Annual frequency, that PGA is larger than a.
The result of this hazard analysis includes a hazard map for 10% probability of exceedanceof peak ground acceleration of 10% in 50 years and 2% probability of exceedance in 50 years.
4. RESULTS AND DISCUSSION
We have obtained evidence of the seismicity in the observed area between 1960 and 2021, ranging from
2 to 8.1 Richter scale with depths from shallow to deep. Since the distances between the earthquake
source zones and the sites are very close with a range of less than 400 km and the complexity of thetectonic setting, it is therefore suggested that the cumulative earthquake that occurred in Timor-Leste
has increased over the years, although until now there has been no destructive earthquake. We identify
the earthquake source zones into six zones.
The highest a and b values are represented in Zone 2 (intra-plate), Zone 3 (shallow crustal
and intra-plate), Zone 4 (shallow crustal) and Zone 6 (intra-plate), while the lowest a and b values are
represented in Zone 1 (intra-plate and shallow crustal), Zone 2 (shallow crustal), Zone 4 (intra-plate),
Zone 5 (shallow crustal), and Zone 6 (shallow crustal), respectively. The fix of depths used for the
ground motion prediction equation is generated using MATLAB software.
In this study, the probability target for the seismic hazard and structural performance design
limit state for engineering approaches divides
into two parts, such as the ultimate limit state
and the collapse limit state. According to ACI(American Concrete Institute) 318-02
Standard, the corresponding probability of theincipient collapse is less than 2%/50 years, or
4 x 10-4/year and the probability of theultimate is less than 10%/50 years, or 2x103/year.
The results for the PSHA
analysis for Timor-Leste can be appreciated infigure 1, for a 10% probability of exceedancein 50 years, the PGA ranges from 80-209 gall.

4Based on the peak ground acceleration classification that the PGA value of 0 to 100 gals belongs to class
I by category of low hazard level for Lautem, Viqueque, and RAEOA sites. PGA goes from 101 to 200
gals is observed around the southern part of the Atauro, Dili, Aileu, Manufahi, Manatuto, Ermera,
Liquisa, Covalima, Ainaro, and Bobonaro sites with the class II and categorized as moderate low hazard
level.
However, for a 2% probabilistic
exceedance in 50 years, the PGA varies from171 to 440 gals. The highest PGA is observedin the northern part of the Atauro site, and thePGA value is 440 gals which correspond toclass V with a very high hazard level. PGA
value of 303 to 304 gals is observed around thesouthern part of Atauro, the northern part ofLiquisa and Covalima sites which belong toclass IV by categorized as high hazard levels.
PGA value of 207 to 287 gals is observed in thesouthern part of Liquisa, Dili, Lautem, andAileu, Manufahi, Manatuto, Ermera, Ainaro,
Bobonaro, RAEOA, which belongs to class III
with moderate-high hazard level, and sites Figure 1.Peak ground acceleration for 2% and 10%
Viqueque and Baucau, the ranges of PGA are probability of exceedance in 50 years.
from 171 to 192 gals which include class II with
Moderate low hazard levels.
On the other hand, the hazard level is higher in site Atauro, due to the hypocenter distanceof less than 400 km from the respective site and produced a PGA value the estimated the maximum
value and also Atauro is formed from uplifting pressure of tectonic plates.

5. CONCLUSIONSThe seismic hazard level in Timor-Leste was successfully obtained for future building code purposes.
For comparison between 14 sites of the target area, the very high risk categorized as class V
is observed in the northern part of Timor-Leste, the Atauro Municipality for 2% probability of exceedancein 50 years (which corresponds to a return period of 2500 years), and the lowest hazard level is observedin the southern part of Viqueque Municipality.
Furthermore, for the 10% probability of exceedance in 50 years (which corresponds to areturn period of 457 years), the ranges of hazard levels are from low up to Moderately high.
The highest hazard level is observed in Atauro and the lowest hazard level is observed in
Lautem, Viqueque, and RAEOA.
The hazard level is high in Atauro, due to the hypocenter distance of less than 400 km fromthe respective site and produced an increase in PGA. However, Zone 2 and 3 affected the most to theobserved site because based on the tectonic setting of Timor-Leste is located in the subduction zone, andAtauro is formed from the uplifting of the tectonic setting.
ACKNOWLEDGEMENTS
I would like to express my deep gratitude to Dr. Tatsuya Itoi for his patient guidance, enthusiastic
encouragement, and valuable critiques of this individual study. I would also like to thank my advisor,
Dr. Toshihide Kashima, for his advice and assistance in facilitating my progress and all the other staff
members at IISEE/BRI for their support and encouragement during this training program. My grateful5thanks are also extended to Mr. Eugenio Soares for permitting me to use seismic data from the Instituteof Petroleum and Geology. I would like to express my gratitude to JICA for giving me the chance tofollow this course with financial support.

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