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

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  • Badan Riset dan Inovasi Nasional

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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.
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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
E125
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 IXX 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 S1S9
•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 20042012 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.
KOULALI ET AL. CRUSTAL STRAIN IN THE SUNDA-BANDA ARC 1944
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; McCareyetal., 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
KOULALI ET AL. CRUSTAL STRAIN IN THE SUNDA-BANDA ARC 1945
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).
KOULALI ET AL. CRUSTAL STRAIN IN THE SUNDA-BANDA ARC 1946
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
KOULALI ET AL. CRUSTAL STRAIN IN THE SUNDA-BANDA ARC 1947
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.
KOULALI ET AL. CRUSTAL STRAIN IN THE SUNDA-BANDA ARC 1948
Geophysical Research Letters 10.1002/2016GL067941
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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
... This fault, which has two segments: the Flores thurst (450 kilometers) and the Wetar thrust (350 kilometers), extends from Wetar in the east to the Bali Basin (Silver et al., 1983). Koulali et al. (2016) offered a different interpretation, claiming that the fault can be extended to part of East Java. Because of this type of fault, Lombok Island is a hanging wall block. ...
... The convergence rate of the subduction zone is around 70 mm/year based on the GPS measurement (Altamimi et al., 2017;Bock, 2003). The Australian Plate is moving at varying velocities to the east (Koulali et al., 2016;Nugroho et al., 2009;Robiana et al., 2018). While the north's back-arc thrust fault moves at a rate of roughly 5.6 to 6 mm/year (Koulali et al., 2016;Susilo et al., 2018). ...
... The Australian Plate is moving at varying velocities to the east (Koulali et al., 2016;Nugroho et al., 2009;Robiana et al., 2018). While the north's back-arc thrust fault moves at a rate of roughly 5.6 to 6 mm/year (Koulali et al., 2016;Susilo et al., 2018). While the two strike-slip faults, which are located in the west and east of island, move at roughly 0.5 mm/year (Irsyam et al., 2017). ...
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This study qualitatively elaborates fault characteristics causing earthquakes in Lombok Island. Historically, Lombok Island has been 12 destructive and significant earthquakes in period of 1979 – 2018. Therefore, the island is in an earthquake-prone region. The hazard factor is one of the key factors in assessing risk. This study become important as their potential to have a big impact. A source of activity that has not been fully investigated in detail was the location of the recent major and devastating earthquake (East Lombok earthquake 2019). Consequently, there is a chance that the level of risk on the Island of Lombok will rise. According to the study’s results, a thorough investigation is required to identify and pinpoint the cause of the 2019 Lombok earthquake in order to improve the earthquake risk index and help the local government to reduce losses due to earthquakes.
... In general, we find that the GPS velocity vectors rotate counterclockwise, increasing in value toward the east. This is in line with the findings of the previous studies by Koulali et al. (2016) and Susilo et al. (2016). ...
... Previous studies have used a uniform grid approach to define their seismogenic zones (Palano et al. 2018;Pan et al. 2020). Here, we divide Banda Arc into five zones, considering the limitation of the GPS distribution, as well as summary from previous studies (Argus et al. 2011;Koulali et al. 2016;Susilo 2017), which have defined these zones based on the main tectonic features, seismic data, and velocity patterns with respect to the Australian plate. The segmentation is coded BAND, BRHD, SSUL, SUMB, and TIMO, which represent the Banda Sea, Papuan Bird's Head, South Sulawesi, Sumba, and Timor areas, respectively ( Figure 5). ...
... meters above mean sea level, causing 87 fatalities and 1400 deaths (Tsuji et al. 1995). Another possible reason why TIMO has the lowest moment rate ratio is that this region's relative motion gradually decreases along the Timor and Tanimbar Troughs, from 32 ± 2.0 mm/year to 1.0 ± 1.7 mm/year (Koulali et al. 2016). This decrease in rates is also related to the continental-arc collision beneath the Timor and Tanimbar Troughs, which is portrayed by the low strain accumulation in these zones in Figure 4. ...
Article
Full-text available
The Banda Arc region has produced several large destructive earthquakes, some of which have been followed by tsunamis. To better understand the earthquake potential in this area, we performed a comparison between geodetic and seismic moment rates. Data were collected from 110 continuous and campaign GPS stations observed for approximately ten years. The results show that the derived velocity field indicates that the Banda Arc deformation is characterized mainly by crustal shortening caused by the interaction of the Australian, Pacific, and Philippine Sea plates. Meanwhile, the contraction strain pattern dominates the Banda Arc area except around Papuan Bird’s Head. Areas with high strain rates have a history of significant seismicity, such as the Flores-Wetar Back Arc, the area around Ambon, and the Papuan Bird’s Head. The ratio of the geodetic moment rate to the seismic moment rate in the Banda, Bird’s Head, South Sulawesi, and Sumba zones are ∼1.5–7.0, indicating a moment deficit rate. The moment deficit rate provides an equivalent earthquake potential of Mw 7.7–8.1. This potential may be related to an aseismic deformation or stress accumulation, the under-sampling of long-term earthquake rates within the seismic catalogs, or a composite of these factors.
... The transcurrent structure bounding the west side of Sumba Ridge can be envisioned as a conjugate of the NE-SW left-lateral transpressional zone of western Timor Island (Charlton et al., 1991;Koulali et al., 2016;Nugroho et al., 2009;Fig. 1B and 9A). ...
... 1B and 9A). Right-lateral shear also occurs farther north within the Banda arc according to GPS data (Koulali et al., 2016), focal mechanisms (Fig. 14 in Lüschen et al., 2011), and enéchelon structures along the Lesser Sunda Islands (Muraoka et al., 2002). Convergence and incipient collision with the Australian continent therefore appears to be accommodated by pervasive shortening of the Australian continental margin in eastern Timor and by more localized wrenching on discrete transpressional zones farther west (Harris, 1991;Duffy et al., 2013). ...
... Convergence and incipient collision with the Australian continent therefore appears to be accommodated by pervasive shortening of the Australian continental margin in eastern Timor and by more localized wrenching on discrete transpressional zones farther west (Harris, 1991;Duffy et al., 2013). These later allow incipient southwestward lateral escape of the Savu-Sumba block (Fig. 9A), which had already been identified as a semi-rigid block by Nugroho et al. (2009), Koulali et al. (2016 and Hengesh and Whitney (2016). The extrusion of this block could explain the southward migration of the deformation front in the Savu Rote accretionary prism, and the apparent dextral offset of this deformation front west of the Sumba ridge. ...
Article
The transition along the strike of the Sunda subduction zone, from oceanic subduction in the west to subduction of continental Australian lithosphere in the east is envisioned as one of the canonical examples of the structural changes that take place within an overriding plate when a continental lithosphere wedge enters a subduction zone. Yet, the along-strike offset of the trench toward the Australian margin represents a structural response opposite to the predictions of numerical models. To understand this paradox, we analyse the morphotectonic evolution of Sumba island located at the transition from oceanic to continental Indo-Australian lithosphere subduction. Drainage evolution allows us to constrain the topographic evolution of the island since the Pliocene. Flights of uplifted coral reef terraces document Quaternary deformation. Focal mechanisms of shallow crustal earthquakes constrain the current stress field. Together, these data reveal that the island is affected by dextral en-échelon folding. Offshore, west of the island, reverse and strike-slip focal mechanisms evidence an active dextral transpressional zone. The emergence of the island and dextral shearing of the accretionary prism were triggered by subduction of the western lateral boundary of the Australian continental margin. We contend that the Plio-Quaternary tectonic evolution of the region, with transpression and migration of the trench toward the Australian margin is primarily dictated by shear stress transfer from the lower plate to the overriding plate, favored by strong interplate coupling, and by southwestward escape of the Savu-Sumba block following the impingement of the Australian continental margin against Timor island.
... Global Positioning System (GPS) survey is successfully mapped an active extension of Flores backarc thrust westward along 300 km throughout East Java (Included Surabaya) common as the Kendeng Fault [3]. Additionally, Surabaya and its surroundings included the western region of East Java, is passed through by the Kedung Waru faults which are associated with the Lidah Anticline, the Gayungan Anticline and the Kedung Waru Anticline [4]. ...
... Regional tectonic map of the main fault of Java Island[3] ...
Article
Full-text available
The Magnetotelluric data acquisition is performed with a certain coordinate system orientation, thus the possibility of the field measurement data would be not proper with the geological strike direction. Therefore, the information of geoelectrical strike is needed to enhance the quality of Magnetotelluric data interpretation where if the data had the similar direction with the geological strike thus it will be properly sufficient with the actual subsurface condition. A magnetotelluric study by utilized polar diagram was conducted to analyze the orientation of geoelectrical strike that are correlated with the geological structures in the Surabaya and Gresik areas. However, polar diagram analysis using phase tensor data has an ambiguity of +90° therefore additional analysis is carried out using tipper data. The results showed the geoelectrical strike direction is N315°E and it is slightly similar with the strike trend of geological structures anomaly. Meanwhile, the geological structures is generally characterized by low to medium value of resistivity contrasts around 30-500 ohm.m consisting of the Surabaya Kendeng Fault, Kendeng Kedung Waru Fault, anticline, and other expected faults.
... As part of Lesser Sunda Islands, seismicity in the provinces is influenced by two possible sources of earthquake: the Flores Back-arc to the north [16] and the subduction zone [17] close to the Sunda Trench to the south of the islands. These sources are seismically active, making the regions potentially vulnerable to seismic threats [18]. Thus, the main aim of this study is to find the most appropriate algorithm and c value suitable for characterising seismicity in the regions of interest. ...
... Using these zones, we are able to capture seismicity rate and vulnerability in each province and in the northern side or the southern side of the provinces. This has considered two possible sources of past and future events, the one associated with the Flores Back-arc Thrusting Fault to the north and the other, the subduction zone to the south of the provinces [16,17,18,24,25]. ...
Article
Full-text available
Data declustering separates mainshocks from both foreshocks and aftershocks while a reliable estimate of completeness magnitude is a key point in seismic parameter determination. These play a role in seismicity-related work. In this preliminary study, we reported seismicity in two Indonesian provinces, namely NTB and NTT, as part of eastern Sunda Arc using the USGS catalogue during 1970-2021 based on performance of three declustering methods (Gardner and Knopoff, Reasenberg, Uhrhammer). These methods were tested along with three techniques of M c determination (MAXC, EMR, BC) provided by ZMAP to estimate minimum magnitude cut-offs, leading to an accurate completeness magnitude. After careful examination, the Reasenberg and BC techniques were proved to be suitable for characterising seismicity in the regions of interest, where M c was calculated under a linear assumption of the cumulative frequency-magnitude distribution (FMD), widely known as the Gutenberg-Richter law. The results revealed that b and a parameters are influenced by the choice of a specific declustering algorithm and calculation of M c . NTT was found to have a higher level of seismicity than NTB and seismicity rates in the southern part of both provinces were higher than those in the northern part. However, the number of strong ground motion with M w ≥ 6.5 in the northern area was larger than that in the southern, indicating the potency of Flores Back-arc Thrust for generating large earthquakes hence possible tsunamis.
... The Lombok island (Indonesia), which is a part of the Sunda Arc ( Fig. 1), is regularly facing many generated natural disasters such as earthquakes, tsunamis, and volcanic eruptions due to the subduction from the Indo-Australian plate toward beneath the Eurasian plate (north) with slip rate 60-70 mm/year in N-NE dipping (Hamilton 1979;Hanifa et al. 2014;Hall and Spakman 2015;Koulali et al. 2017;Ferrario 2019, Yang et al. 2020). On the other hand, the Flores thrust, which lies from Flores to the north of Bali as the back-arc system, dips to the south with ~ 26 mm/year near Flores, decreasing to ~ 10 mm/year toward the west near Lombok (Koulali et al. 2016). The accumulation of slip rate has generated many destructive earthquakes in the last decade such as the 1992 M w 7.9 Flores earthquake (Yeh et al. 1993;Beckers and Lay 1995;Hidayat et al. 1995;Pranantyo and Cummins 2019) followed by the tsunami phenomena and the 2018 Lombok sequence (Yeh et al. 1993;Beckers and Lay 1995;Hidayat et al. 1995;Pranantyo andCummins 2019, Salman et al. 2020). ...
... The E-W-trench length of Flores backthrust is situated north of Lombok Island and extends 450 km as the consequence of convergence between the Indo-Australia plate and the Eurasian plate (Hamilton 1979, Yang et al. 2020. The geodetic observation indicates that the slip rate along the Flores backthrust is approximately 20-30 mm/year to the east, while the slip rate is approximately 10-15 mm/year to the west which indicates that the Flores backthrust is seismically active (DeMets et al. 1990, Koulali et al. 2016. Statistically, the Flores backthrust has been categorized as a seismogenic fault with historically 25 M 6 + earthquakes since 1690 and one of the famous devastating earthquakes that followed by tsunami phenomena, namely, the 1992 Flores M w 7.9 earthquake (Ekstrom et al. 2012, Zhao et al. 2021. ...
Article
The Lombok island is a part of the Sunda Arc, tectonically controlled by the subduction system of the Indo-Australian Plate in the southern part while the Flores thrust in the northern part. A sequence of major earthquakes consisting of Mw 5.9-6.9 struck in July-August 2018 and generated massive loss and damage in the Lombok and neighboring regions. In the current study, we analyzed the seismicity clustering of this series of shallow earthquakes including the seismic data from 2009 to 2021. We applied the logarithmic transformation of the nearest-neighborhood distance (log10η) for the seismic data within the domain of space (magnitude), time (magnitude), and depth (magnitude). The combined relationship between these factors has been estimated, which showed that the seismicity in Lombok island was prominently unimodal and indicates the existence of a single type of statistically distribute earthquake. These relationships also point out a contribution of space, time, and depth around 32.410%, 54.447%, and 13.143% respectively. Later we utilized the Welch power spectrum analysis for log10η and analyzed the performance and complexity of the seismicity. The analysis clearly showed the highest peak that was corresponding to the frequency of series earthquake distribution from 25 July to 8 August 2018.
Article
This study utilizes a range of complementary geological, geophysical, and geochemical data to analyze the structural, deformational, and compositional variations along the Sunda-Banda volcanic arc transitional zone from oceanic to continental subduction. The core component is a synthesis of three station-based anisotropy inferences and two 3-D shear wave velocity models in the crust and upper mantle, all of which are derived based on ∼5 years of broadband seismic data collected across eastern Indonesia. These seismic results are based on receiver functions, shear wave splitting, ambient noise tomography, and a new teleseismic surface wave tomography model. We observe distinct along-arc spatial variations in the shear wave velocity and anisotropic structure within the crust and mantle wedge above the subducting slab. An intriguing arc-perpendicular orientation is observed in crustal fabrics and mantle anisotropy consistently in central Flores, contrasting with the arc-parallel orientations on the western and eastern sides. The orientations correlate with the change in strike of the highest topography as well as the presence of ∼N-S aligned cinder cones in central Flores. The 3-D shear wave velocity model in the crust and uppermost mantle also presents a sharp structural change in central Flores that indicates a relatively thinner crust. At deeper depths, the newly imaged upper mantle structure exhibits along-arc variations, but the change across central Flores appears to be distributed more broadly and outlines a relatively slow mantle wedge beneath eastern Flores. These new observations roughly correlate with previously identified changes in volcanic geochemistry characteristics across central Flores. Our study links the surface topography, geology, and volcanism with subsurface crust and mantle structures, unraveling an enigmatic structural and/or compositional boundary along the Banda volcanic arc.
Chapter
The National Earthquake Center (PusGeN) in 2017 stated that East Java was traversed by several active faults that extended towards Bali and Flores. The gravity research was carried out to map the fault structure in the western part of East Java. Gravity method is applied to identify subsurface geological structures by utilizing variations in Earth's gravity caused by differences in density of subsurface rocks. The processed data is satellite gravity data (TOPEX) as many as 110 data spread in Lamongan, Gresik, and Surabaya City. Gravity Topex is a Geodesy satellite altimeter launched by NASA to measure the altitude of the satellite above the closest sea surface point to a very high precision. Gravity data that collected from Topex/Poseidon satellite are already processed into free air anomaly, further, it is performed Bouguer Correction and Terrain Correction, resulting in Bouguer Anomaly data which is then filtered using Second Vertical Derivative (SVD). The results of SVD 2-D modeling have a high level of accuracy because the faults are well identified and highly correlated with the PuSGeN faults map (2017). The curve of the slice profiling results on the Surabaya Fault shows that the maximum value of SVD is 0.1 mGal/m2 and the minimum value is 0.2 mGal/m2 which is identified as a reverse fault. The curve of the slice profiling results on the Waru Fault shows that the maximum value of SVD is 0.2 mGal/m2 and the minimum value is 0.3 mGal/m2 which is identified as a reverse fault.KeywordsBouguer anomalyTOPEX gravityFaultSecond vertical derivative (SVD)
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We investigate seismic anisotropy across southeastern Indonesia where the Indo-Australian plate subducts beneath and collides with the Sunda-Banda arc. Geochemical, geodetic, and tomographic studies reveal an along-strike transition from oceanic subduction to continental subduction and collision near central Flores that is due to a change of lithospheric composition in the subducting plate. To investigate the anisotropic fabric and dynamics of the upper mantle surrounding this young (∼5 Ma) arc–continent collision, we perform shear wave splitting analysis on local and teleseismic S waves recorded by an array of broadband seismometers that crosses the subduction–collision boundary. Seismic anisotropy inferred from our local S dataset shows that anisotropic sources above the slab extend to depths exceeding 100 km. Analysis of teleseismic SKS and SKKS waves reveal a shift in subslab fast axes from trench-parallel to trench-perpendicular near the ocean–continent boundary in the lower plate, which we relate to regional subslab mantle flow being deflected around the subducted continental lithosphere. Along-strike variations in anisotropic fast axes from teleseismic phases overlap with distinct structural and tectonic boundaries that divide distinct regions of the collision, implying the effects of the collision transcend any one dataset and highlighting the complexity of collisional boundaries. These results shed light on the interaction between tectonic structure and mantle dynamics in an emergent collision, and help constrain the nature of upper mantle deformation in the early stages of collision.
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The region offshore Eastern Java represents one of the few places where the early stage of oceanic plateau subduction is occurring. We study the little investigated Roo Rise oceanic plateau on the Indian plate, subducting beneath Eurasia. The presence of the abnormal bathymetric features entering the trench has a strong effect on the evolution of the subduction system, and causes additional challenges on the assessment of geohazard risks. We present integrated results of a refraction/wide-angle reflection tomography, gravity modelling, and multichannel reflection seismic imaging using data acquired in 2006 south of Java near 113°E. The composite structural model reveals the previously unresolved deep geometry of the oceanic plateau and the subduction zone. The oceanic plateau crust is on average 15 km thick and covers an area of about 100 000 km2. Within our profile the Roo Rise crustal thickness ranges between 18 and 12 km. The upper oceanic crust shows high degree of fracturing, suggesting heavy faulting. The forearc crust has an average thickness of 14 km, with a sharp increase to 33 km towards Java, as revealed by gravity modelling. The complex geometry of the backstop suggests two possible models for the structural formation within this segment of the margin: either accumulation of the Roo Rise crustal fragments above the backstop or alternatively uplift of the backstop caused by basal accumulation of crustal fragments. The subducting plateau is affecting the stress field within the accretionary complex and the backstop edge, which favours the initiation of large, potentially tsunamogenic earthquakes such as the 1994 Mw= 7.8 tsunamogenic event.
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Bone Gulf is one of the inter-arm basins of the unusual K-shaped island of Sulawesi. Its age, character and origin are disputed. This study is based on recently acquired 2D seismic lines, seabed multibeam mapping and limited well data, and is linked to stratigraphy on land. The gulf is probably underlain by pre-Neogene volcanogenic, sedimentary, metamorphic and ultramafic rocks, and includes crust of Australian origin. We favour basin initiation in the Miocene rather than Eocene, by extension associated with strike-slip deformation. The main basin trends N-S and is divided into several sub-basins and highs. The highs segment the gulf and their WNW-ESE orientations reflect pre-Neogene basement structures. They are interpreted as strike-slip fault zones active at different times in the Neogene. A southern high was active relatively early, whereas further north there is evidence of young displacements during the Late Neogene. These are visible on the seabed above a high linked to the Kolaka Fault on land. Early basin-bounding faults are oriented NNW-SSE and record extension and strike-slip movements, like the sub-parallel Walanae Fault of South Sulawesi which can be traced offshore into extensional faults bounding the young and narrow Selayar Trough. Sediment in the basins came mainly from the north with contributions from both west and east. Carbonate deposits formed at the margins while deeper marine sediments were deposited in the axial parts of the gulf. An Early Pliocene unconformity can be mapped across the study area marking major uplift of Sulawesi and subsidence of Bone Gulf. This regional event caused major influx of clastic sediments from the north, development of a southward-flowing canyon system, and back-stepping and drowning of carbonates at the basin margins. Hydrocarbons are indicated by seeps, and Bone Gulf has potential sources, reservoirs and seals, but the complex faulting history is a risk.
Chapter
The stratigraphic record of volcanic arcs provides insights into their eruptive history, the formation of associated basins, and the character of the deep crust beneath them. Indian Ocean lithosphere was subducted continuously beneath Java from ca. 45 Ma, resulting in formation of a volcanic arc, although volcanic activity was not continuous for all of this period. The lower Cenozoic stratigraphic record on land in East Java provides an excellent opportunity to examine the complete eruptive history of a young, well-preserved volcanic arc from initiation to termination. The Southern Mountains Arc in Java was active from the middle Eocene (ca. 45 Ma) to the early Miocene (ca. 20 Ma), and its activity included significant acidic volcanism that was overlooked in previous studies of the area. In particular, quartz sandstones, previously considered to be terrigenous clastic sedimentary rocks derived from continental crust, are now known to be of volcanic origin. These deposits form part of the fill of the Kendeng Basin, a deep flexural basin that formed in the backarc area, north of the arc. Dating of zircons in the arc rocks indicates that the acidic character of the volcanism can be related to contamination of magmas by a fragment of Archean to Cambrian continental crust that lay beneath the arc. Activity in the Southern Mountains Arc ended in the early Miocene (ca. 20 Ma) with a phase of intense eruptions, including the Semilir event, which distributed ash over a wide area. Following the cessation of the early Cenozoic arc volcanism, there followed a period of volcanic quiescence. Subsequently arc volcanism resumed in the late Miocene (ca. 12–10 Ma) in the modern Sunda Arc, the axis of which lies 50 km north of the older arc.
Article
Earthquake moment tensors reflecting seven years of global seismic activity (2004–2010) are presented. The results are the product of the global centroid-moment-tensor (GCMT) project, which maintains and extends a catalog of global seismic moment tensors beginning with earthquakes in 1976. Starting with earthquakes in 2004, the GCMT analysis takes advantage of advances in the mapping of propagation characteristics of intermediate-period surface waves, and includes these waves in the moment-tensor inversions. This modification of the CMT algorithm makes possible the globally uniform determination of moment tensors for earthquakes as small as MW = 5.0. For the period 2004–2010, 13,017 new centroid-moment tensors are reported.