Tectonophysics 639 (2015) 23–33
Contents lists available at ScienceDirect
Tectonophysics
journal homepage: www.elsevier.com/locate/tecto
Volcano-tectonic control of Merapi's lava dome splitting: The November
2013 fracture observed from high resolution TerraSAR-X data
T.R. Walter a,⁎, J. Subandriyo b, S. Kirbani c, H. Bathke a,d, W. Suryanto c, N. Aisyah b, H. Darmawan c, P. Jousset a,
B.-G. Luehr a, T. Dahm a
a
Dept. Physics of Earth, GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany
Technical Research Center for Geological Hazard (BPPTKG), Jalan Cendana 15, Yogyakarta 55166, Indonesia
Laboratory of Geophysics, Department of Physics, Faculty of Mathematics and Natural Sciences, Universitas Gadjah Mada (UGM), Yogyakarta, Indonesia
d
King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia
b
c
a r t i c l e
i n f o
Article history:
Received 26 May 2014
Received in revised form 10 October 2014
Accepted 5 November 2014
Available online 14 November 2014
Keywords:
Merapi volcano
Dome stability
Tectonic setting
Satellite radar
Remote sensing
a b s t r a c t
Volcanism at active andesite–dacite volcanoes is often associated with the formation and collapse of circularshaped protrusions of extruded, highly viscous lava, the so-called domes, which are emplaced in the near summit
region. Growing domes may experience stable and instable structural phases, with a gradual transition in
between. Dome collapse and the break-off of instable blocks of viscous lava may lead to pyroclastic flows, one
of the most lethal hazards at stratovolcanoes. At Merapi volcano, Indonesia, a new dome started growing in
the summit region in the amphitheater formed in the course of the climactic 2010 eruption. Three years later,
the dome reached a height of approximately 100 m and diameters of 220 and 190 m with a plateau-like surface
area of 40,000 m2 approximately. On 18/11/2013, an explosion occurred without identified precursors, leaving a
major fracture cutting the complete dome structure. Based on high resolution TerraSAR-X satellite radar imagery,
we could identify the linear fracture, traceable over ~200 m in the long axis, up to 40 m wide. After geocoding of
the radar amplitude imagery, the fractures azimuthal trend could be compared to other structural lineaments, indicative of a significant NNW–SSE structural direction that has formed on Merapi volcano in the past. The Merapi
dome fractured in a NW–SE direction, and is consistent with the alignment of regional tectonic structures and of
anticipated directions of pyroclastic flows. The fracture may be part of a larger volcano-tectonic system and may
affect the dynamics and the stability of the Merapi dome.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Silicic domes are high viscous extrusions of magma at the summit of
dominantly dacitic-andesitic volcanoes. Silicic domes may destabilize
and collapse, representing the source region for destructive pyroclastic
flows (Calder et al., 2002). The growth and collapse of silicic domes
are thought to be controlled by the extrusion dynamics (Hale et al.,
2009a, 2009b), the underlying slope and surrounding morphology
(Walter et al., 2013), the brittle (cooled) crust stabilizing the extrusive
magma body and hence the exogenous and endogenous growth (Hale,
2008; Kaneko et al., 2002). Therefore, at growing domes a fragile balance exists between extrusion of viscous lava, cooling and degassing,
erosion and the mechanical instability of the dome. At occasions, the
stability of domes may be modified by external forcing, at Merapi possibly associated with heavy rain fall (Elsworth et al., 2007; Neuberg,
2000), or with large tectonic or regional earthquakes (Jousset et al.,
2013; Voight et al., 2000; Walter et al., 2007, 2008). Little observations
⁎ Corresponding author.
E-mail address: [email protected] (T.R. Walter).
http://dx.doi.org/10.1016/j.tecto.2014.11.007
0040-1951/© 2014 Elsevier B.V. All rights reserved.
and research have been described on the causes and effects that a sudden disruption of the plug might have on the dynamics of the system
(Melnik and Sparks, 2002). In particular, it is important to monitor the
disruption of the solidified outer crust of a dome, a key component in
the factors controlling the dome morphometry. Observations at Mount
St. Helens (USA) and at Soufriere Hills (Montserrat) showed that
disruption and partial removal of the dome may release the plug,
depressurize the conduit and even trigger vulcanian explosions (Carn
et al., 2004; Walter, 2011; Watts et al., 2002). At Merapi, this solidified
brittle crust of the dome has been fractured during an explosion in
November 2013, making this event an example of a sudden disruption
of a dome surface.
1.1. Merapi volcano setting and dome formation
Merapi volcano is located on Java Island, Indonesia (Fig. 1), and has
become one of the best studied volcanoes since the IAVCEI volcano decade program (Gertisser et al., 2012; Newhall, 1994; Voight et al., 2000).
Commonly observed activity at Merapi is dominated by gradual dome
growth, intermittently destroyed either by explosions or by collapse
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T.R. Walter et al. / Tectonophysics 639 (2015) 23–33
Fig. 1. Location and overview of Merapi volcano. A. Topographic map showing the location of Merapi and seismic stations mentioned in the text. An earthquake (red star) occurred just
15 min before the November 18, 2013 Merapi explosion. B. Shaded relief image of Merapi and adjacent volcanoes (red triangles). The volcanoes are aligned in direction NW–SE, as indicated by the rose diagram (azimuthal values). White boxes indicate satellite image frames. C. Radar amplitude maps of TerraSAR-X data recorded in ascending track. D. Radar amplitude
maps of TerraSAR-X data recorded in descending track.
(Ratdomopurbo and Poupinet, 2000) associated with pyroclastic flows.
Peaks of this dome growth and collapse activity occur in a frequency of
3–5 years (Newhall et al., 2000).
Merapi had major VEI 4 eruptions in 1872 and in 2010 (Andreastuti
et al., 2000; Surono et al., 2012), the latest one destroyed the summit
part and left a major amphitheater that is open to the south-southeast
(SSE). The SSE opening direction of the amphitheater is aligned with
the Gendol river, and has deepened a valley created by the 2006 eruption (Charbonnier et al., 2013; Saepuloh et al., 2011; Thouret et al.,
2010). This opening direction is responsible for the southwardly channelized pyroclastic flows in 2010 (Komorowski et al., 2013). The reason
why this profound valley has developed in this particular direction
remained unclear, although it is important for understanding primary
and secondary hazards at Merapi: Modeling of anticipated pyroclastic
flows suggests that this valley path is directing hazards into southerly
regions, i.e. the populated part (~1 Mio pop.) of the district Yogyakarta
(Charbonnier and Gertisser, 2009). Therefore, understanding of this azimuthal trend and close observation of the growing dome are essential.
1.2. Dome growth and recent explosions
The most recent dome has commenced to grow shortly after the
centennial 2010 eruption (Surono et al., 2012). The continuous growth
of the new dome has been relatively stable in the three years since 2010.
The growth could be well observed through camera systems installed
by a joint collaborative project between the volcano observatory
BPPTKG, the Gadjah Mada University UGM and the GFZ German Research Centre for Geosciences. This dome of viscous andesitic lavas
has reached over 100 m in height and 200 m in width (Fig. 2). The
growth was interrupted by a few small-scale explosions, some phreatic
in origin, reaching 1 km height at most (VEI 1), as those that occurred in
July 15, 2012, or July 22, 2013. These small scale explosions formed a
T.R. Walter et al. / Tectonophysics 639 (2015) 23–33
25
to this tectonic event, strong rainfall preceded the explosion days before; unfortunately no rain gauge measures are available to further
quantify this effect. A first inspection just after the Nov 18, 2013 explosion revealed that a major fracture had opened on the solidified crust of
the lava dome surface.
This work describes the geometry and relevance of this newly
formed fracture, based on remote sensing data compared to field photographs. Satellite images acquired by the German TerraSAR-X satellite
provide detailed views into the summit region. After analyzing the
dimension, azimuth and architecture of the new fractures on the
dome, a discussion is provided about the implications for the structural
splitting mechanics of domes and the general relevance of this structural architecture for the future instability of the Merapi dome and the
hazards associated with eruptions that may occur with little or no
precursors.
2. Data and methods
Fig. 2. A. Quickbird image from 9 September 2013 shows the dome, and a prominent lineament feature oriented NNW–SSE as indicated by red arrows. B. Photographs from
Gendol into the amphitheater of Merapi, showing the steep front of the lava dome and
its lower limit (dashed white line). Estimated lava dome thickness is 100 m.
number of craters located either on the summit of the dome itself, or in
the valley between the dome and the amphitheater. The two largest of
these craters (vents in Fig. 2A) form a linear feature oriented NW–SE,
which is relevant for interpreting the results of this work.
The vent location enclosed by the dome and the amphitheater
(Fig. 2) is the site of higher temperatures as detected by handheld infrared measures routinely performed by the volcano observatory BPPTKG
in 2012–2013. The same region is the site of most intense fumarole activity and also minor explosions. As this valley is a closed valley, i.e.
without outflow, intense rainfall is occasionally trapped to form short
lived lakes.
In Nov 18, 2013 (local date) a major explosion occurred, 2 km high,
with little juvenile material being ejected mainly to the west and southwest of the edifice. Monitoring cameras allowed a first inspection of the
Nov 18, 2013 eruption effects (Fig. 3). Local seismic networks, tilt and
EDM measurements did not show any significant precursors. However,
seismic recordings at a broadband station of UGM-GFZ (KALI) located
6 km to the south of the summit reveal the occurrence of a tectonic
earthquake with a magnitude Mw 4.7 near the south coast of Java
(Fig. 4). This tectonic event preceded the volcano explosion by
~ 15 min only, and was recorded also by the Meteorological Agencies
of Indonesia's seismic network (YOGI) at larger distances. Residents living on the slopes of Merapi heard a roar sound from Merapi. In addition
Visual inspections at Merapi are amongst the most reliable methods
to record changes in the morphology of the summit over time and infer
potential hazards in the course of the recent activity. However, the
eruption on 18 November 2013 occurred in the rainy season of Java
and the summit was often cloud covered. Therefore visual inspections
and visibility by fixed installed surveying cameras were limited,
allowing only clear views with several days' separation (Fig. 3). To overcome such limitations, in support of the GEO Supersites initiative
(http://supersites.earthobservations.org/), the GFZ Potsdam has
commenced ensuring the acquisition of synthetic aperture radar images
acquired by the German satellite TerraSAR-X. Data were regularly
acquired before and after the explosion, and available for processing
within few days after acquisition. The satellite has usually a revisit
cycle of 11 days and in November 2013, only one image once every
two cycles was recorded, leading to a temporal baseline of 22 days. Recordings of both, ascending and descending orbits, were available for
this study in tracks 58 and 96, respectively. The radar is an active illumination technique and its operation is independent of day and night, and,
due to the wavelength of the signal (X-band), it propagates through
thick clouds. For the purpose of monitoring volcanic domes, the
TerraSAR-X satellite was configured by us to record images in spotlight
mode, providing an unprecedented high spatial resolution of ~ 2 m of
the Merapi dome (Supplemental material).
The processing of the imagery was done in several steps. First, we
performed an interferometric (InSAR) and pixel offset (PO) processing;
second we structurally analyzed the image amplitudes to obtain detailed structural information. The co-registration of the SAR data has
been done by using a digital elevation model, which is essential for
high resolution radar data (Nitti et al., 2011). We account for the linear
Doppler frequency shift in azimuth during the radar acquisition in spotlight mode by following Jendryke et al. (2013). We calculated interferograms using the software package Doris (Kampes and Usai, 1999).
Differential interferograms were derived by using a digital elevation
model (ASTER GDEM 30m). The interferograms were filtered using a
Goldstein adaptive approach (Goldstein and Werner, 1998).
In addition, we investigated the amplitude changes of the SAR
images before and after the explosion. To this aim we first geocoded
the images by using the digital elevation model. Results allow a spatial
comparison of the dome and its changes at unprecedented detail. We
investigated the length of the fracture by overlaying the amplitude
imagery in a GIS platform (ArcGIS 10.x). The geocoded images were
colorized in the red and green channels, allowing us to create a composite raster map. In the RGB composite renderer, the blue channel was
left blank. Treatment of the SAR amplitude data as a multiband raster
dataset provides a comprehensive visualization of the geometric
changes that might have occurred.
The length, width and azimuth of identified lineaments were digitized and measured and compared to other lineaments evident on the
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T.R. Walter et al. / Tectonophysics 639 (2015) 23–33
Fig. 3. Photographs from fixed installed cameras into the amphitheater of Merapi allow comparison of images before and after the November 18, 2013 explosion. Note the appearance of a
fracture after the explosion. Camera position at Gendol shown in the first two rows, camera position at Deles shown in the lower two photographs.
Fig. 4. Broadband seismic record of two seismic stations, YOGI (in gray) and KALI (in
black). For location of the earthquake, the volcano and the seismic stations refer to
Fig. 1. Records are shown for the evening hours of November 17, 2013 (or early morning
on the November 18, local time). The first event corresponds with the occurrence of a M
4.7 south of Java earthquake and the second event is associated with the Nov 18 explosion.
edifice and the surrounding. Lineaments that are defined as
mapable features, are often linear features and foremost thought
to reflect subsurface phenomena (O'Leary et al., 1976). To this
aim we apply a Hough transform edge detection filter on the gray
scale TerraSAR-X amplitude imagery. The edges were then
vectorized and converted to straight lines in MATLAB, and further
analyzed in an ArcMap shape file. In order to account for the different lengths of the lineaments, we subdivided them into 2000 sublineaments for the ascending and descending data, respectively,
and we calculated their azimuth. All lineament azimuths were
then plotted into rose diagrams. To systematically analyze the direction of the lineaments statistically, i.e., we consider a large number of lines in the landscape that are alleged to be associated with
the hidden architecture of the rock basement (Hobbs, 1904). The
comparison of rose diagrams obtained from lineaments inside the
Merapi crater and from lineaments on the flanks of the edifice allows structural interpretation of the newly formed fracture paths.
Also we compare this structural trend to the alignment of other
volcano centers adjacent to Merapi volcano.
Results are shown in map-view, as amplitude images, and in rose diagrams, compared to time lapse and field photographs taken after the
explosion from the eastern crater rim.
T.R. Walter et al. / Tectonophysics 639 (2015) 23–33
3. Results
3.1. Dome splitting
We first analyze the radar amplitude maps that provide important
constraints on the lineaments and the volcano-tectonic control. The
geocoded amplitude imagery clearly depicts the crater open in form of
an amphitheater to the south-southeast (Fig. 5). Due to the depth of
the crater floor, shadowing causes significant data gaps (shadow regions are in black). Areas facing the satellite, in turn, appear brighter
in the amplitude images. Within the amphitheater itself, a steep flank
facing the satellite is geometrically distorted due to foreshortening
artifacts. Nevertheless, in-between these two steep flanks of the amphitheater, the dome of Merapi is clearly represented in both ascending
and descending images.
The TerraSAR-X images acquired before the 18 Nov, 2013 eruption
show the summit region of the dome as a relatively flat plane (Fig. 6A,
B). The amplitude variation of the dome surface appears bended, with
slightly higher reflectivity centered on the dome. A localized spherical
region in the center of the dome and a region just northwest of the
dome correspond to craters created by earlier smaller phreatic explosions (e.g., July 2012 and 2013; marked as ‘vent’ in Fig. 6).
Radar images after the 18th November 2013 eruption show that the
same dome is breached by an open fracture or fissure oriented NW–SE
(Fig. 6C, D). In fact, the fracture is defined by two main segments, each
being 90–95 m long. Moreover, a pronounced shadowing region appears in the radar images just around the northern fracture segment,
sandwiched between the dome and the amphitheater. This region represents the crater that widened by the Nov 18 explosion. The size of this
27
crater is about 80–100 m wide. The width of the fracture reaches up to
40 m.
A close-up composite map analysis in GIS (Fig. 6E, F) shows the trace
of the fractures in more clarity, as they appear as an almost perfectly
linear feature traceable over 200 m, split in a well-defined 91 m and
95 m segment in the north and south, respectively. The dome, with
diameters of 200 m by 190 m (or ~40,000 m2) is therefore split in two
parts. Only the southernmost steep flank of the dome does not show
the continuation of this fracture, allowing speculations on a change in
the local stress field of the dome structure at its southernmost flank.
Instead, the southern part of the dome shows abrupt composite map
changes aligned normal to the main NW–SE fractures. These fractures
have an orientation perpendicular to the aforementioned main structural features. This southern part of the dome appears distinct or even
detached from the main dome. The detachment of the southern part
of the dome and therefore might be related to gravitational instability
rather than tectonics. Accordingly a large block at the southern front
of the dome (Fig. 4B) is defined, which is structurally detached from
the main dome and potentially prone for collapse towards the valley.
3.2. Deformation absence
To test for any deformation that might have occurred during the Nov
18 explosion, we investigate both the phase difference (InSAR) and the
amplitude offsets of the TerraSAR-X data.
InSAR images show minor edifice-wide color changes (Fig. 7). The
interferograms' phase changes are generally below 1 color cycle, and
are probably associated to atmospheric contributions, because the
appearance and pattern alter in independent interferograms. Therefore,
Fig. 5. Radar imagery from TerraSAR-X acquired in spotlight mode (2 m resolution). Amplitude imagery clearly depicts the newly formed fracture trend localized on the dome of Merapi
volcano. A, B. Images before the explosion. C, D. Images after the explosion. Ascending track (left side) and descending track (right side). Brighter colors correspond to stronger backscattering, related to surface backscattering properties or geometric effects.
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T.R. Walter et al. / Tectonophysics 639 (2015) 23–33
Fig. 6. Close-up view to the lava dome of Merapi from radar imagery. A pronounced fracture is splitting the dome into two halves. E, F. Composite raster map images created in the RGB
bands for R = pre-explosive and G = post-explosive imagery (B — blank) depicts changes and lengths of newly formed fractures. The main fracture is defined by two sub-fractures with a
slightly different azimuth, each about 100 m long. The rose diagrams show the strike angles of the lineaments (white arrows in E and F) within the crater. Note the change in the southern
dome area.
InSAR data reveals no edifice-wide deformation. The only fringes
observed in the interferograms that might indicate a phase change
associated with deformation are found in the uppermost crest of
the Merapi amphitheater (Fig. 7C). In particular, the ascending interferogram reveals a phase change of two fringes, suggesting the
occurrence of local subsidence. As the fringes are confined and appear only in the very steep uppermost proportion of the amphitheater, we believe that they are related to a very shallow geologic
process, which may explain why no precursors had been recorded
by existing networks. Whether this subsidence is associated to magmatic processes, cooling of a conduit, or related to other mass movement in the upper edifice region cannot be ascertained from this
data alone.
If associated to tensile or relative opening, an extent of deformation
would be certainly evident in the radar images; however our analyses
of the radar interferograms and their azimuth and range offset do not
support the opening or slip along the fractures. Therefore an explosive
origin, probably a phreatic explosion, appears the most plausible,
T.R. Walter et al. / Tectonophysics 639 (2015) 23–33
29
Fig. 7. Interferograms from TerraSAR-X data showing co-explosive interferograms in ascending (track 96, left panels A, C) and in descending viewing geometry (track 58, right panels B, D).
Zoom shown in panels C and D. No edifice wide deformation could be identified. Local phase differences (fringes) nearby the western amphitheater rim might represent local displacement
(movement away from the satellite). No deformation on the dome surface could be identified due to decorrelation. For pixel offset calculations the reader is referred to the Supplementary
information.
lacking significant ground movements. The location of the extended
fracture is exactly at the same position where in earlier images small
phreatic explosion sites have been identified (‘vent’ in Fig. 6).
3.3. Lineaments
Lineament analysis reveals that the trend of the main dome splitting,
oriented NW–SE, can similarly be observed on the flanks of the volcano
edifice as well. Are these related to the dome fracture? Rose diagrams
show the exact azimuth of the new fracture, which is NW–SE (Fig. 6E,
F). The trace of the sidewalls of the amphitheater is also NW–SE, very
similar to the fracture inside the dome. The dominant azimuthal trend
of the valleys and lineaments outside the summit amphitheater of
Merapi is in general also NNW–SSE (Fig. 8), again comparable to the
aforementioned structures. This result was confirmed also by using
the Aster GDEM-slope map, indicating a dominantly NNW–SSE lineament directivity. The edifice-wide lineaments, however, show a more
scattered trend, and appear slightly rotated clockwise with respect to
the fracture that has formed in Nov 18, 2013 on the dome. For instance,
it can be observed that lineaments on the southeastern sector of Merapi
are more oriented N–S, while the lineaments on the northwestern sector of Merapi are preferably oriented NW–SE. Although being in general
similar to the fractures that split the dome on Nov 18, 2013, a curvature
of the lineaments around the volcano summit appears systematic and is
further discussed below.
A NNW–SSE azimuthal trend is also observed at even larger scale by
the trend line, connecting the adjacent volcanoes of Merapi, Merbabu,
Telemoyo and Ungaran (Fig. 1C), already described by Van Bemmelen
(1949). The fact that azimuthal trends of the volcano centers, the
volcano-tectonic lineaments, its amphitheater and the newly formed
fracture are similar, suggests a close genetic relationship as discussed
further below.
4. Discussion
Merapi volcano has experienced a significant eruption on 18th November 2013. According to the volcano observatory records at BPPTKG
and our results from InSAR, the explosion was neither preceded by
significant deformation seen within the EDM network, nor by unusual
seismicity (see also Supplemental material).
4.1. Dome splitting and instability
Radar images acquired by the TerraSAR-X satellite before and after
the Nov 18, 2013 eruption show a new fracture breaching the lava
dome and parts of the summit. This fracture is NNW–SSE aligned and
split the domain into two parts. In addition, another structural separation has developed on the steep frontal part of the dome. The latter
structural separation is also confirmed by a recent photograph (Fig. 9),
implying that a block facing the Gendol valley is structurally detached
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T.R. Walter et al. / Tectonophysics 639 (2015) 23–33
Fig. 8. Edifice wide lineament mapping. Geocoded TerraSAR-X amplitude imagery in spot light mode was used to identify lineaments in ascending and descending track, respectively. A.
Edge detection results converted to lineaments. B. To weight the lineaments according to their length, all lineaments were resampled to 2000 sub-lineaments with similar length. For each
sub-lineament the strike direction was measured; this azimuthal direction was plotted in a rose diagram. Rose diagrams are shown in full circle, consisting of 10° sector, 36 classes, frequency–azimuth type. Both viewing geometries, ascending and descending, depict a predominant NNW–SSE trend.
from the bulk rock mass and potentially unstable. The detached block is
clearly defined by a NE–SW oriented fracture 85 m long, most expressed
on the southwestern sector of the dome. The fracture is at a distance of
70 m to the steep southern slope, and is steeply inclined southward. For
a 100 m thick dome this implies a lava dome rock volume of
~300,000 m3 that is structurally detached and potentially prone for collapse (Figs. 6F, 9C). The complex fracture network at the frontal part of
the dome would mean that any future growth of the dome might be
hazardous and is difficult to predict. New magma could be channeled
along the newly formed fracture and fissures, causing the failure of a
major block.
4.2. The NNW–SSE trend
The NNW–SSE azimuth of the new fracture is similar to the direction
of the open amphitheater and the Gendol valley further south, as well as
to lineaments on the volcano flanks and to the alignment of prominent
volcanoes in the surrounding. In addition, the prominent Kukusan fault,
a major fault zone bypassing the Merapi summit on its eastern flank
(Fig. 1D), interpreted as the delimiting landslide detachment
(Gertisser et al., 2012), has a trend that is NNW–SSE on the SE sector
of the volcano, curving to a NNW–SSE trend on the NW sector of the
volcano edifice. Such curving fault trends on volcanoes have been
described by crustal faults that are upwardly propagating into a volcano
edifice (Wooller et al., 2009). In our conceptual understanding, Merapi
is underlain by a structurally weak zone oriented NNW–SSE, which
has influenced growth of adjacent volcanoes, development of valleys
and flank instabilities and which has now also influenced the development of a fracture splitting the lava dome in two parts (Fig. 10).
The position of the lava dome fracture, and exact alignment with
earlier explosion sites (July 2012 and 2013 vents), suggests that this
trend was already established before the Nov 18, 2013 explosion. A
question we need to answer is to which degree cooling of the lava
dome has contributed to this fracture trend. The formation of fractures
in a cooling viscous lava dome may be compared to crack formation in
cooling basalt columns. Since the new fractures at Merapi's dome are
highly linear and not influenced by the circular shape of the protrusive
and cooling lava, the inner stress field of the dome is very likely influenced from an external stress field.
4.3. Origin of the lava dome fracture
Pixel offset calculations do not show any significant movement neither in ascending nor in descending track (Supplemental material). This
suggests that the fracture may have formed without any lateral displacement. Our observations hence indicate that major movements
have not occurred at depth or within the dome itself, and that this explosion was possibly influenced by near surface processes. A possible
rainfall and a strong local earthquake, and explosion-driven due to sudden overpressure, possibly superheated vapor, contributed jointly as a
trigger of the phreatic eruption.
Volcanic lineaments have led to define the NNW–SSE trend along
Ungaran–Telemoyo–Merbabu–Merapi as well as a WNW–ESE trend
along Lawu–Merapi–Sumbing–Sondoro–Slamet (Van Bemmelen
(1949)). Our work reveals that the NNW–SSE structural trend is not
only present within the dome but also within the edifice itself and the
surrounding. As the amphitheater and breaching of the Gendol valley
have been initiated in 2006 already, this trend appears to be a more
sustained structural trend that may affect further developments and
hazards at Merapi. It is interesting to note that this general trend is
also parallel to the border of a deep crustal velocity anomaly observed
by means of seismic tomography (Luehr et al., 2013).
Summit displacements recorded from 1993–1997 revealed the
importance of structural discontinuities in that same direction
(Beauducel et al., 2000). These trends, therefore, suggest that the past
and future evolution of the Merapi volcano has to be considered with
respect to a major NNW–SSE volcanotectonic pathway. Such preferential paths may affect or even control the paths of magma (Anderson,
1951), cause lateral widening (Rubin, 1992), and may even lead to
flank failures (van Wyk de Vries and Francis, 1997). Recent gravity
T.R. Walter et al. / Tectonophysics 639 (2015) 23–33
31
if the underlying fault has a sinistral strike-slip component. At Merapi,
the fracture breaching the summit region is rotated counterclockwise
with respect to the Merapi–Merbabu line and with respect to the edifice
wide lineaments, which is compatible by an underlying fault that has a
sinistral component of strike-slip motion. Whether this fault is active as
a transtensional, transpressional or pure strike slip fault cannot be
discriminated from this data alone and requires further investigations.
4.4. External triggering
Sudden structural or dynamic changes at Merapi volcano are not
entirely new. Near summit structural discontinuities have been
described based on geodetic data (Beauducel et al., 2006). In 2006, following a deadly earthquake just 40 km to the south of the summit
(Walter et al., 2007, 2008), Merapi tripled its volcanic activity and the
direction of pyroclastic flows changed from western direction to southern direction through the Gendol valley abruptly (Bignami et al., 2013;
Ratdomopurbo et al., 2013). In the case of the November 2013 activity,
it cannot be concluded with certainty that the explosion was triggered
by the earthquake 15 min before; a possible causal relationship remains
to be studied. A permanent broadband station confirmed that no preexplosive seismicity was detected, although this data has to be taken
with care as the seismic instrument is located 6 km away from the summit (Fig. 3 and Supplemental material). Moreover, the influence of rain
onto the dome splitting has to be further evaluated. However, we speculate that the existing fracture trend and the presence of structural features seen on the dome already before the Nov 18, 2013 explosion,
imply that also rainwater might follow the same zones of weakness.
Therefore rain might have contributed or even caused the explosion
and finally the splitting of the dome. In this view, also following explosions of Merapi took place along the NNW–SSE lineament, such as in
March 10 and 27, 2014, both of which exceeding plume heights of
1500 m. Opening of the NNW–SSE lineaments may in future increase
the threat of further eruptions in southerly directions.
5. Conclusion
Fig. 9. Photograph from the east amphitheater headwall onto the dome of Merapi. A. Prior
to the explosion, photo taken in September 2011. Note the flat top of the dome. B, C, D.
After the Nov 18, 2013 explosion, photo taken in January 2014. Note the new explosion
crater and the deep fracture in the image. Close-up C shows the unstable block on the
dome front that formed in 18 Nov 2013. Close-up D shows older vent that has been developed earlier.
data and three dimensional density model simulations revealed the
presence of a density contrast oriented NNW–SSE along the Merapi–
Merbabu trend (Tiede et al., 2005). Possibly a larger intrusive body is
geologically connecting these volcanoes (Tiede et al., 2005). The nature
of this NNW–SSE directed lineament, however, remained unclear.
The interaction between tectonic faults and volcanoes has been
extensively investigated in analog experimental studies (Lagmay et al.,
2000; Mathieu and van Wyk de Vries, 2011; Mathieu et al., 2011). Volcanoes influenced by strike-slip, transtensional and transpressional
fault zones show a structural fault network that generally reflects the
underlining tectonic trend, but higher at elevated regions might geometrically differ from the regional trend. Faults that have a strike-slip
component lead to curvature in a sigmoidal pattern (Lagmay et al.,
2000; Mathieu and van Wyk de Vries, 2011). This develops into a summit graben that is parallel to the main horizontal stress, or oriented
obliquely to the actual fault trend (Mathieu and van Wyk de Vries,
2011). Orientation of the summit grabens is rotated counterclockwise
This study based on radar remote sensing shows the strength of high
resolution radar imagery in volcano observations. Moreover, it shows
that the style and architecture of domes at explosive volcanoes may
abruptly change within a short time. It shows that extrusive domes of
viscous lava at a volcanic summit can be influenced by the regional
fault lines. The explosion at Merapi on Nov 18, 2013 has basically broken
the dome into two main parts. The consequences of this structural
change for the future development, the growth and the collapse of the
dome, are at this stage difficult to foresee. Changes might be associated
with a different direction of magma, collapse, and more intensely
expressed NNW–SSE pathway down into the Gendol valley and towards populated regions. A link with a deep seated NNW–SSE tectonic
trend is postulated.
Acknowledgments
This work benefited from very detailed and constructive reviews by
Maurizio Battaglia and Mario Mattia, as well as by the editor Rob Govers.
This work is financially supported by the Helmholtz Alliance EDA. Satellite radar imagery was provided by the German Aerospace Centre, special thanks go to Ursula Marschalk and Achim Roth for support and data
handling. This work is a contribution to the new technologies grant
from GFZ.
Appendix A. Supplementary data
Supplementary data associated with this article can be found in the
online version, at http://dx.doi.org/10.1016/j.tecto.2014.11.007. These
32
T.R. Walter et al. / Tectonophysics 639 (2015) 23–33
Fig. 10. Conceptual model of the Merapi summit. A. A NNW–SSE directed fracture zone responsible for the alignment of Merapi and adjacent volcanoes (the Merapi–Merbabu trend) is
causing fractures at the Merapi summit dome. B. During the Nov 18, 2013 explosion, the dome splitted in two halves, the fracture trend is NW–SE. C. Left lateral motion along the
Merapi–Merbabu trend may explain the structures seen on Merapi and the counterclockwise rotation of the 18 No 2013 lava dome fracture. C. Modified from Mathieu et al. (2011).
See text for details.
data include Google maps of the most important areas described in this
article.
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