Crustal strain partitioning and the associated earthquake hazard in the eastern Sunda-Banda Arc

Abstract

We use Global Positioning System (GPS) measurements of surface deformation to show that the convergence between the Australian Plate and Sunda Block in eastern Indonesia is partitioned between the megathrust and a continuous zone of back-arc thrusting extending 2000km from east Java to north of Timor. Although deformation in this back-arc region has been reported previously, its extent and the mechanism of convergence partitioning has hitherto been conjectural. GPS observations establish that partitioning occurs via a combination of anticlockwise rotation of an arc segment called the Sumba Block, and left-lateral movement along a major NE-SW strike-slip fault west of Timor. We also identify a westward extension of the back-arc thrust for 300 km onshore into East Java, accommodating slip of ∼6 mm/yr. These results highlight a major new seismic threat for East Java, and draw attention to the pronounced seismic and tsunami threat to Bali, Lombok, Nusa Tenggara and other coasts along the Flores Sea.

Full text

Geophysical Research Letters
Crustal strain partitioning and the associated
earthquake hazard in the eastern
Sunda-Banda Arc
A. Koulali
1
, S. Susilo
2
, S. McClusky
1
, I. Meilano
3
, P. Cummins
1
, P. Tregoning
1
, G. Lister
1
,
J. Efendi
2
, and M. A. Syafi’i
2
1
Research School of Earth Sciences, Australian National University, Canberra, Australian Capital Territory, Australia,
2
Bandan Informasi Geospatial, Cibinong, Indonesia,
3
Institute of Technology Bandung, Bandung, Indonesia
Abstract We use Global Positioning System (GPS) measurements of surface deformation to show that
the convergence between the Australian Plate and Sunda Block in eastern Indonesia is partitioned
between the megathrust and a continuous zone of back-arc thrusting extending 2000 km from east Java
to north of Timor. Although deformation in this back-arc region has been reported previously, its extent
and the mechanism of convergence partitioning have hitherto been conjectural. GPS observations
establish that partitioning occurs via a combination of anticlockwise rotation of an arc segment called
the Sumba Block, and left-lateral movement along a major NE-SW strike-slip fault west of Timor. We also
identify a westward extension of the back-arc thrust for 300 km onshore into East Java, accommodating
slip of ∼6 mm/yr. These results highlight a major new seismic threat for East Java and draw attention
to the pronounced seismic and tsunami threat to Bali, Lombok, Nusa Tenggara, and other coasts along
the Flores Sea.
1. Introduction
Eastern Indonesia encompasses a complex tectonic environment, involving the convergence of four major
tectonic Plates: the Australian, Pacific, Philippine Sea Plates, and the Sunda Block [Hamilton, 1979] (Figure 1).
In this region the Australian Plate subducts northward beneath eastern Java, Nusa Tenggara (114
∘
E–125
∘
E),
and the Banda Arc (Figure 1). These three arc segments accommodate a transition in the style of plate con-
vergence from ocean-continent subduction in east Java, to arc-continent collision in Nusa Tenggara and then
to the island arc subduction in the Banda Sea. While historical earthquake observations for this region are
poorly known, at least seven large earthquakes have occurred between 1648 and 1891 [Soloviev and Go, 1974;
Musson, 2012]. Six of these events were associated with macroseismic intensities of IX–X and four generated
regional tsunamis in the Flores Sea with estimated runup of 3 m or greater [Soloviev and Go, 1974]. During the
instrumental seismic period, four major events were reported in the area between 112
∘
E and 128
∘
E[Ekström
et al., 2012] (Figure 1). The
M
w
7.9 1992 Flores earthquake was the largest thrust event recorded and gener-
ated a large, destructive tsunami [Beckers and Lay, 1995]. The majority of earthquakes during the last century
are attributed to the back-arc segments of Flores and Wetar and have thrust style focal mechanisms [Ekström
et al., 2012; Beckers and Lay, 1995], suggesting that this fault system is accommodating an important part of
the convergence between the Australian Plate and the Sunda Block.
Marine geophysical surveys [Silver et al., 1983] have revealed evidence for two major back-arc thrusts: the
450 km long Flores thrust north of Sumbawa and western Flores, and the 350 km long Wetar thrust north
of Timor (Figure 1). It has been speculated [Silver et al., 1983] that the thick crust beneath Sumba and Timor,
respectively, facilitates transfer of stress from the fore-arc to the back-arc, while the thinner crust elsewhere
(e.g., Savu basin) enables convergence to be partitioned onto fore-arcand back-arc thrusts and strike-slip faults
that cut the arc at angles oblique to the convergence [McCaffrey, 1988]. However, until now, there has been
no conclusive evidence identifying which if any of these faults are facilitating the transfer of convergence.
Early geodetic investigations [Genrich et al., 1996] concluded that the Timor Trough is inactive and most of the
convergence between Australia, Sundaland, and Eurasia occurs to the north at a rate of 50 mm/yr. In contrast,
later studies [Bock et al., 2003; Nugroho et al., 2009] estimated 15 to 20 mm/yr of motion across the Timor
RESEARCH LETTER
10.1002/2016GL067941
Key Points:
• The Sunda-Banda back-arc thrust
system is a large active plate
boundary that extends over 2000 km
• Strain is transferred from Java
subduction to the back-arc thrusts
via a left-lateral strike slip
• Geodetic strain across the
Sunda-Banda back-arc thrusts
emphasize a high seismic and
tsunami hazard
Supporting Information:
• Text S1, Tables S1 and S2,
and Figures S1–S9
•DataSetS1
Correspondence to:
A. Koulali,
achraf.koulali@anu.edu.au
Citation:
Koulali, A., S. Susilo, S. McClusky,
I. Meilano, P. Cummins, P. Tregoning,
G. Lister, J. Efendi, and M. A. Syafi’i
(2016), Crustal strain partitioning and
the associated earthquake hazard
in the eastern Sunda-Banda
Arc, Geophys. Res. Lett., 43, 1943–1949,
doi:10.1002/2016GL067941.
Received 24 JAN 2016
Accepted 18 FEB 2016
Accepted article online 19 FEB 2016
Published online 11 MAR 2016
©2016. American Geophysical Union.
All Rights Reserved.
KOULALI ET AL. CRUSTAL STRAIN IN THE SUNDA-BANDA ARC 1943

Geophysical Research Letters 10.1002/2016GL067941
Figure 1. Seismotectonic setting of the Sunda-Banda arc-continent collision, East Indonesia. Major faults
(thick black lines) [Hamilton, 1979]. Topography and bathymetry are from Shuttle Radar Topography Mission
(http://topex.ucsd.edu/www_html/srtm30_plus.html). Focal mechanisms are from the Global Centroid Moment Tensor.
Blue mechanisms correspond to earthquakes with
M
w
>
7
(brown transparent ellipses are the corresponding rupture
areas for Flores 1992 and Alor 2004 earthquakes), while the green focal mechanism shows the highest magnitude
recorded in Sumbawa. Red dots indicate the locations of major historical earthquakes [Musson, 2012].
Trough and 60 mm/yr of shortening across the Flores Sea. The discrepancies in these results reflect the degree
of uncertainty in understanding and assigning slip partitioning, mainly due to the lack of observations in the
vicinity of the back-arc fault system. In addition, the lack of long and adequately sampled GPS time series
makes it difficult to recognize and correct for the effects of postseismic relaxation resulting from both nearby
local and regional earthquakes that have occurred since GPS observations began in the 1990s. Thus, there are
many important but hitherto unanswered questions regarding the southern margin of eastern Indonesia: Is
the present-day back-arc deformation localized on the Flores and Wetar segments? How does the partitioning
of convergence between the subduction megathrust and back-arc vary along the Sunda-Banda Arc transition?
By what mechanism does this partitioning of convergence occur? Answering these questions is essential for
understanding the associated seismic and tsunami hazard. In this study, we use GPS velocities plus earthquake
slip vectors to quantify slip partitioning in eastern Indonesia (110
∘
E to 135
∘
E) and we discuss the implications
of strain distribution for earthquakes hazard in the region.
2. Methods
2.1. GPS Data Processing
The GPS velocity field presented in this study (supporting information Data Set S1) is calculated from obser-
vations at 94 GPS stations located in east Indonesia in combination with a global network of 80 International
Global Navigation Satellite Systems Service tracking sites. Campaign GPS sites have been surveyed irregularly
from 1993 to 2014, while most of the continuous sites operated from 2009 to 2014 (supporting information
Figure S3). The raw data were processed using GAMIT-GLOBK software [Herring et al., 2010], and uncertainties
were estimated following standard procedures described by Reilinger and et al. [2006]. The velocities used
in Figure 2 are with respect to a Sunda Block-fixed reference frame, defined using the velocity of only three
continuous sites: BINT, NTUS, and GETI, for the period prior to the Sumatra-Andaman 2004
M
w
9.2 earth-
quake [Vigny et al., 2005]. This approach eliminates the effects of contamination of the Sunda reference
frame by postseismic effects resulting from the 2004–2012 Sumatra earthquake sequence Feng et al. [2015].
The weighted root-mean-square for the north and east horizontal velocity components are 0.66 mm/yr and
0.97 mm/yr, respectively, and 1.2 mm/yr for the vertical rate. For our modeling, we use only horizontal veloc-
ities and we do not include any vertical rate estimates since they have large uncertainties due to different
source of systematic errors, making their usage of less importance for our block model inversions.
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Geophysical Research Letters 10.1002/2016GL067941
Figure 2. GPS velocities determined in this study with respect to Sunda Block. Uncertainty ellipses represent 95%
confidence level. The inset figure corresponds to the area of the dashed rectangle in the map. Light blue arrows show
the velocities for East and West Makassar Blocks.
2.2. Kinematic Block Modeling
We model the observed velocities as a sum of block rotations and elastic strain produced by fault locking
[McCaffrey, 2005]. We chose block boundaries based on our qualitative interpretation of the GPS velocities
themselves as well as independent information from earthquakes [Ekström et al., 2012; Shulgin et al., 2011]
and the available seismic and geologic constraints on active faults in the Sunda-Banda Arc [Hamilton, 1979;
McCaffrey, 1988]. We performed a simultaneous inversion of horizontal GPS velocities and earthquake slip
vectors (supporting information Figure S1) to estimate Euler vectors of six blocks, locking depths at major Plate
boundaries and three components of the strain rate tensor for three blocks (Sumba block, Timor block, and
East Makassar block). The flexibility of this approach allows us to verify the significance and the importance
of each plate boundary in the kinematic model. We have investigated the present-day deformation in this
region using four kinematic models with different geometrical configurations of elastic blocks (Figure S5).
Our preferred block model includes two major boundaries that encompass the Sunda-Banda Arc: the Java
Trench and Timor Trough in the southern part and the back-arc thrusts system extending from the Wetar
thrust to the Kendeng thrust east of Java island in the north. We divide the Arc into three blocks: The Timor
Block [McCaffrey, 1988], Sumba Block, and the Eastern Java Block (Figure 3). In the southern part of Sulawesi,
we divide the Makassar Block [Socquet et al., 2006] into eastern and western blocks following the southern
extension of the Walanae Fault and Selayar Trough [Camplin and Hall, 2014]. The boundary we use to separate
the Banda Sea from the Weber Basin is speculative and may have a different geometry, though we do not
currently have observations constraining its precise location. It is not specified as a fault and treated here as
free-slipping boundary.
For the downdip geometry, the faults along the back-arc thrust are assigned a uniform dip angle of 30
∘
in
accordance with seismic reflection profiles [Silver et al., 1983] and discretized with a 5 km downdip interval
from the surface down to 30 km; however, the geometry of Java Trench and Timor Trough was based on the
U.S. Geological Survey slab 1.0 [Hayes et al., 2012] as well as on seismicity cross sections established across the
main thrust for the eastern part were slab 1.0 model is not available. The nodes downdip are placed at depth
every 2 km in the upper 10 km then every 5 km from 10 to 45 km depth and then every 10 km down to 70 km
depth. In order to reduce the number of free parameters, we have estimated uniform locking depths at Wetar,
Flores, Bali-Lombok, and Kendeng thrusts and we have parameterized the locking along the Java Trench as a
function exponentially varying down depth while inverting for the minimum and maximum locking depths
of the transition zone [Wang et al., 2003; McCaffreyetal., 2007]. The four plausible block model scenarios we
investigate here include different combinations of block boundaries where seismicity or geologic constraints
do not provide a unique solution for the surface expression of an active crustal fault. The assessment of the
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Geophysical Research Letters 10.1002/2016GL067941
Figure 3. Relative slip vectors across block boundaries, derived from our best fit model. Arrows show motion of the
hanging wall (moving block) relative to the footwall (fixed block) with 95% confidence ellipses. The tails of arrows is
located within the “moving” block. Black thick lines show well-defined boundaries we use as active faults in our model
and dashed lines show less well-defined boundaries (green : free-slipping boundaries and black: fixed locked faults) .
Principal axes of the horizontal strain tensor estimated for the SUMB, EMAK, and EJAV are shown in pink. The thick pink
arrow shows the relative motion of Australia with respect to Sunda (AUST/SUND). Abbreviations are Sumba Block
(SUMB), West Makassar Block (WMAK), East Makassar Block (EMAK), East Java Block (EJAV), and Timor Block (TIMO).
The background seismicity is from the International Seismological Centre catalog with magnitudes
≥ 5.5 and
depths
< 40 km.
significance of the geometrical complexity of alternate models was based on F test statistics. The detailed
summary of the fit statistics is provided in the supporting information (Table S1).
3. GPS Velocity Field
The velocity field with respect to the Sunda Block (Figure 2a) reveals an anticlockwise rotation of the whole of
eastern Sunda-Banda Arc with an increase in the motion of the archipelago from 9 mm/yr in Bali to 82 mm/yr
toward the eastern end of the Timor Trough. This general increase in the northward velocity of the fore-arc is
consistent with the convergence between the Australian Plate and the Sunda Block being progressively trans-
ferred north as the Australian Plate collision transitions from ocean arc in the West to continent-arc collision
in the East. Motion decreases north of the back-arc toward south Sulawesi, reflecting the conclusions of
previous studies that the back-arc is accommodating deformation [McCaffrey, 1988]. However, our new GPS
data along East Java, Madura Island, and Bali Island reveal a significant north-south gradient of velocities
across the Kendeng Basin and the volcanic arc (Figure 2b). This confirms that the boundary marking the transi-
tion from the Kendeng Basin to the Sunda shelf, known as the Kendeng thrust [Smyth et al., 2008], is active and
probably defines the westward onshore extension of the Flores back thrust [Simandjuntak and Barber, 1996].
Our kinematic model results show that the along-arc change in partitioning of plate convergence is caused by
the anticlockwise rotation of the Sunda-Banda Arc with respect to Sunda Block. The relative motion, between
the Asutralian Plate and the Sunda Block, along the Java Trench decreases from 70
±1.0 mm/yr west of
the Lombok Basin (115
∘
E) to 33 ± 0.9 mm/yr south of Rote Island (123
∘
E) (Figure 3). Eastward of Timor Island
(124
∘
E), the relative motion along the Timor Trough gradually diminishes from 32 ± 2.0 mm/yr to an insignif-
icant 1.0 ± 1.7 mm/yr at Tual Island (132
∘
E). In contrast, the relative motion along the back-arc increases
from west to east, from 6 ± 1.0 mm/yr in East Java to 26 ± 1.0 mm/yr at Flores, 28 ± 1.7 mm/yr at Wetar
and 30
± 1.8 mm/yr to the North of Timor, where the direction of vectors changes to more NE implying that
a significant component of shearing must be accommodated by the structures at the back-arc in this area.
This correlates well with the left-lateral strike-slip faulting inferred from the earthquake fault plane solutions
(supporting information Table S2 and Figure S2).
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Geophysical Research Letters 10.1002/2016GL067941
4. Discussion and Implications for Earthquake Hazard
The description of the present-day motion within the eastern Sunda-Banda Arc is a debated question, with
proposed models suggesting that the deformation can be described by several independent crustal blocks
[McCaffrey, 1988; Genrich et al., 1996] and others proposing this region as a wide zone of distributed deforma-
tion with diffuse transitional zones [Nugroho et al., 2009]. Our results from the interpretation of the new GPS
velocity field as well as the kinematic model suggest that the eastern Sunda-Banda is segmented into three
blocks (East Java Block, Sumba Block, and Timor Block) separated by NE transitions of left-lateral faults. The
Semau Fault (SF, Figure 1) is one of a series of NNE-SSW trending left-lateral strike-slip faults west of Timor
[Charlton et al., 1991] and may have been associated with a large earthquake in 1814 [Soloviev and Go, 1974].
Our best fit block model requires a boundary at the Semau Fault connecting the Timor Trough with the Wetar
thrust, thus forming the western boundary of the Timor Block. The Semau Fault is a key component of our
kinematic model as it provides the structural link between the fore-arc and the back-arc.
Our model requires the addition of a boundary approximately perpendicular to the Java Trench, allowing
movement of this arc segment, known as the Sumba Block [McCaffrey, 1988], which is independent of the Java
fore-arc. We chose the location of this fault where marine seismic and gravity modeling studies [Shulgin et al.,
2011] indicate fracturing in the upper 2 km of the oceanic crust and a sharp increase in crustal thickness. This
fault accommodates only 3 to 4 mm/yr of strike-slip motion, less than 5% of the total relative motion, but its
inclusion improves significantly the fit of the data (Table S1).
The northwestern corner of the Sumba Block abuts the offshore extension of the Kendeng thrust, where we
estimate 5
± 0.4 mm/yr of convergence. The presence of mud volcanoes [Istadi et al., 2009] in the eastern
part of the Kendeng Basin is consistent with overpressuring caused by active convergence in this area.
Although some historical earthquakes may have occurred on the Kendeng thrust in the nineteenth century
[Simandjuntak and Barber, 1996], the absence of more recent events raises the question of whether this fault
is slipping aseismically or is fully locked. The current spatial resolution of the GPS network is insufficient to
resolve the heterogeneity in the coupling on the back-arc thrusts. Therefore, we chose to estimate a uniform
locking depth at each segment. On the Kendeng thrust, we estimated a locking depth of 9
± 3 km and found
that the segment north of Sumbawa Island is the deepest locked segment (20 ± 1.8 km) along the back-arc,
with a moment deficit of 2.4 ×10
18
Nm, equivalent approximately to an earthquake of magnitude M
w
= 6.3.
Between 2006 and 2009 a sequence of 3 earthquakes with
M
w
>
6.3
occurred in the eastern part of this seg-
ment at depths ranging between 18 and 20 km. A recent study [Fuchs et al., 2014] detected the occurrence of
triggered nonvolcanic tremors beneath Sumbawa Island, which might increase the recurrence time of major
events (
>
M
w
7) by helping to release strain during the interseismic period.
In contrast to the localization of deformation in the Sunda-Banda back-arc at the Flores and Wetar thrusts
inferred by previous studies, we find that active deformation extends along the back-arc for over
∼2000 km.
This accounts for a variation from 5% to 40% of the total convergence between the Australian Plate and the
Sunda Block and explains the distribution of both historical and recorded seismic events. Our results elucidate
the role of the left-lateral Semau Fault west of Timor in transferring the shear stress from the trench to the
back-arc (Figure 4), where NE shearing at 20 mm/yr predominates along the transfer boundary with a normal
convergence component of 4 to 9 mm/yr on this fault. This shear zone also marks a transition where the
convergence obliquity along the back-arc changes from normal at Flores (5
∘
N) to a more oblique direction
along east Wetar (17
∘
N), consistent with the difference in shortening [Silver et al., 1983] observed on the Wetar
(
∼10 km) and Flores (∼30 km) thrusts. This suggests that the increase of stress due to the obliquity reflects a
recent evolutionary stage of underthrusting across the Wetar back-arc segment migrating eastward.
The concept of slip partitioning in obliquely convergent fault systems is used to explain the accommo-
dation of shear strain resulting from the trench-parallel component of the relative motion [Fitch, 1972].
The classic model requires the megathrust plate boundary to accommodate the trench-normal slip, and a
parallel strike-slip fault in the fore-arc to take up the oblique slip, with both structures isolating a continental
wedge known as a sliver [Fitch, 1972; McCaffrey, 1988]. Along the transition from the eastern Java Trench to
Timor Trough, the direction of the plate convergence becomes progressively oblique to the east, where earth-
quake slip vectors show a complex pattern of deformation typical of highly curved margins as documented
by McCaffrey [1996]. Previous studies demonstrated that the degree of the deformation partitioning is a
function of the convergence obliquity [McCaffrey, 1992; Vernant and Chéry, 2006]. However, they showed that
full partitioning is reached only for high-obliquity values larger than ∼70
∘
. In our study we predict that only
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Geophysical Research Letters 10.1002/2016GL067941
Figure 4. Fault slip rate components: (a) fault normal (extension positive) and (b) fault parallel (right-lateral positive).
∼37% of the total lateral shear strain is accommodated on Timor Trough and the back-arc thrusts to the north,
suggesting that full partitioning is less likely to occur, consistent with 3-D mechanical modeling predictions
[Vernant and Chéry, 2006]. However, it is unclear how the remaining unaccounted deformation is accommo-
dated and we speculate it is likely to be partitioned further north on the Seram Trough, where the analysis
of focal mechanisms show signs of active deformation. Quantifying precisely where and how this remaining
motion is accommodated is beyond the scope of this paper requiring more dense GPS observations in the
north Banda Sea region.
Our results show a different structural organization, where the convergence itself is transferred to the back-arc.
As with classical slip partitioning, this results in isolation of an arc segment (the Sumba Block), and partitioning
of convergence is achieved through a combination of anticlockwise rotation of the Sumba fore-arc Block and
left-lateral movement along the Semau Fault (Figure 4). A similar organization is observed elsewhere in the
world where back-arc thrusting is active, such as the great Antilles Arc [Mann et al., 2002], the North Panama
Deformed belt [Kobayashi et al., 2014] and the New Hebrides/Vanuatu [Calmant et al., 2003]. These zones of
back-arc thrusting are approximately 500 km, 600 km, and 200 km, respectively, in length. Our results show a
far more extensive zone of active thrusting along a ∼2000 km section of the eastern Sunda Arc, a potentially
important source of seismic and tsunami hazard.
5. Conclusion
Our results draw a new kinematic framework for active deformation in the eastern Sunda-Banda Arc, high-
lighting the need to reconsider the level of seismic hazard there. Several of the active faults identified here
directly threaten socioeconomic assets vital to Indonesia. The Kendeng thrust passes through the southern
outskirts of Surbaya, Indonesia’s second largest city with a population of over 2.5 million, and traverses a
300 km length of East Java, with a population density of over 800 people per square kilometer. The Semau
Fault skirts the city of Kupang, the main commercial centre of Nusa Tenggara with a population of around
500,000. Finally, earthquakes along the back-arc thrust beneath the sea floor extending 1700 km from eastern
Java to Timor could generate regional tsunamis threatening the coastlines of the Flores Sea. Further studies,
including earthquake, geodetic, and paleoseismic, should be undertaken to better understand these threats.
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Acknowledgments
This research was supported under
the Australian Research Council’s
Linkage Projects funding scheme
(LP110100525). Figures are drawn
with GMT [Wessel and Smith, 1998].
The GPS data were computed on
the Terrawulf II computational facility
at the Research School of Earth
Sciences, a facility supported through
the AuScope initiative. AuScope
Ltd is funded under the National
Collaborative Research Infrastructure
Strategy (NCRIS), an Australian
Commonwealth Government
Program. We are grateful to Nyamadi,
Dodi Sudarmono, Caca Juniarsa,
Budi Parjanto, Heru derajat, Sidik Tri
Wibowo, Munawar Kholil and Putra
Maulida, and all to the personnel
of Badan Informasi Geospatial (BIG),
who participated in GPS surveys
over the past 20 years. We appreciate
constructive comments and
improvements from two
anonymous reviewers.
KOULALI ET AL. CRUSTAL STRAIN IN THE SUNDA-BANDA ARC 1949