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Crustal thickness beneath Mt. Merapi and Mt. Merbabu, Central Java,
Indonesia, inferred from receiver function analysis.

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S. K. Suhardja¹, S. Widiyantoro²,³, J.-P. Métaxian⁴, N. Rawlinson⁵, M. Ramdhan⁶ and Agus Budi-Santoso⁷

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¹University of Pertamina, Jl. Teuku Nyak Arief, Simprug, Kebayoran Lama, Jakarta 12220, Indonesia

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²Global Geophysics Research Group, Faculty of Mining and Petroleum Engineering
Institute of Technology Bandung, Jalan Ganesa No. 10, Bandung 40132, Indonesia

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³Research Center for Disaster Mitigation, Institute of Technology Bandung, Jalan Ganesa No. 10, Bandung 40132, Indonesia

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⁴ISTerre, IRD R219, CNRS, Université de Savoie Mont Blanc, Le Bourget-du-Lac, France

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⁵Department of Earth Sciences – Bullard Labs, University of Cambridge, Cambridge CB30EZ, United Kingdom

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⁶Agency for Meteorology, Climatology and Geophysics, Jalan Angkasa I, No. 2, Kemayoran, Jakarta, Indonesia

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⁷Center for Volcanology and Geological Hazard Mitigation, Geological Agency, Jalan Diponegoro No. 57, Bandung, 40122, Indonesia

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SUMMARY

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In this study, we analysed 2708 receiver functions (RFs) using data recorded by 53 seismographic stations that surround- Mt. Merapi and Mt. Merbabu – two volcanos in Central Java - to map the boundary between Earth’s crust and upper mantle. We observe that a number of RFs from this new dataset have complex signals and do not exhibit typical RF
characteristics; in particular, where the converted Ps signal from the Moho discontinuity is
the clearest and strongest amplitude arrival following the P onset. This effect may be related
to complex shallow velocity structure due to the presence of magmatic rocks and sediments.
Further analysis of the RF results using the H-κ method suggests that Moho depth varies
between 27 to 32 km beneath the array, with no apparent correlation between crustal
thickness and surface topography-, as one might expect from Airy isostacy. For instance, the
Moho is quite shallow beneath Mt. Merapi (up to 27 km depth), despite its elevation of nearly
3 km. This may be a consequence of dynamic support from an active upper mantle coupled
with erosion and/or weakening of the lower crust due to the active volcanic plumbing system.
To the north of Mt. Merapi, the Moho is deeper (30-31 km depth) below Mt. Merbabu. Vp/Vs
ratio estimates from the H-κ method -are relatively high (~1.9) beneath the Mt. Merapi and
Kendeng Basin area, which may indicate the presence of a zone of hydrous and active partial
melting in the underlying crust. Lower Vp/Vs ratios (~1.7) are found beneath Mt. Merbabu,
which may be due to its relative lack of volcanic activity compared to Mt. Merapi.

Key words: Receiver Function, Mt. Merapi, Crustal Thickness, Indonesia

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42 INTRODUCTION

The island of Java is located at the southwestern edge of the Eurasian continent where the
Australian plate subducts beneath Sundaland. The tectonic setting of this area is dominated
by the Sunda Arc, which gives rise to frequent megathrust earthquakes and a large number of
active volcanoes. At present, the convergence rate between Australia and Indonesia is 67 mm
per year (Simons et al. 2007) and the dip angle of the slab increases from near-horizontal
inboard of the trench to very steep (70°-80°) from a depth of 50 km to the north of Java
(Koulakov et al. 2007). We estimate, based on the distribution of earthquakes in global
catalogues (Weston et al. 2019), that the depth of the slab beneath our study area is at about
51 100-120 km. Muller *et al.* (1997) estimated the age of the subducted plate beneath central Java to be about 80-100 Ma.

53 The arc magmatism and volcanic activity that characterises central Java are largely 54 dictated by the subduction setting. Overall there are two main volcanic arcs, the Southern 55 Mountain Arc (SMA) and Modern Volcanic Arc (MVA). Smyth *et al.* (2008) proposed that 56 from the middle Eocene (about 45 to 20 Ma), a volcanic arc formed in the southern coastal 57 region of the Island, which became what is now known as SMA. Clements *et al.* (2009) 58 postulated that subduction in this area ceased in the Cretaceous but then resumed in the late 59 Miocene and created the MVA, which is located about 50 km north of SMA. To the north of 60 the MVA, a large basin was formed on the edge of Sundaland and is named the Kendeng 61 zone (Fig. 1). The Merapi and Merbabu volcanoes, which are part of the MVA, are the focus 62 of this study.

63 Mt. Merapi is one of the most active volcanos in the world, with an eruption frequency of 64 between two to six years (Ratdomopurbo & Poupinet 2000). Eruptions from this volcano are 65 dominated by pyroclastic flows caused by lava dome collapse (Hidayati *et al.* 2008). Surono 66 *et al.* (2012) determined that a different eruption type, which is more explosive and of higher 67 magnitude like the 2010 eruption, occurs less frequently at 50-100 year intervals.

68 Camus *et al.* (2000) estimated that volcanism began at Mt. Merapi about 40,000 years 69 ago. The available radiocarbon data indicate almost continuous volcanic activity at Merapi 70 during the last 2000 years, during which it only experienced two periods of decreased activity 71 between 600-700 and 1200-1300 years B.P. (Gertisser & Keller 2003). Ratdomopurbo & 72 Poupinet (2000) studied seismicity in the vicinity of Merapi volcano and discovered an 73 aseismic zone within the cone of the volcano. They suggested that two magma reservoirs may 74 be present: A shallow reservoir at 1-2 km depth below the summit which may have a high- 75 hazard potential; and a larger and deeper one, most likely the main reservoir located below 5
76 km depth. Beauducel & Cornet (1999) used GPS and tilt data to conclude that the main 77 magma reservoir is located between 6 km and 9 km below the summit. More recently, high- 78 resolution gravity models show evidence of high-density bodies beneath the volcanic 79 summits of Merapi, Merbabu and Telemoyo, which can be interpreted as magma reservoirs 80 (Tiede et al. 2005).

81 A study by Widiyantoro et al. (2018) using local earthquake tomography with data from 82 the same network used in this study found three active areas with high Vp/Vs: the first is a 83 shallow zone interpreted to represent a region of intense fluid percolation, directly below the 84 summit of the volcano; the second is thought to be a pre-eruptive magma reservoir at 10 to 85 20 km depth below MSL that is several orders of magnitude larger than known erupted 86 magma volumes; and the third is a deep magma reservoir at 30 km depth which supplies the 87 main reservoir.

88 To date, there have been numerous geophysical studies that have undertaken seismic 89 velocity imaging in Central Java, including Mt. Merapi. Local earthquake tomography 90 models (Koulakov et al. 2007) exhibit high Vp/Vs ratios inside the Merapi Lawu Anomaly 91 (MLA) region, located to the north of Mt. Merapi, that may be related to the presence of 92 elevated fluids and partial melts. Wagner et al. (2007) also applied local earthquake 93 tomography in this region and found a very pronounced low velocity anomaly in the back-arc 94 crust north of the active volcano. They also found a low velocity anomaly in the upper mantle 95 which they interpret as the pathway of fluids and partially molten material.

96 Bohm et al. (2013) employed seismic attenuation tomography in central Java and 97 discovered a prominent zone of increased attenuation directly beneath and north of the 98 modern volcanic arc at depths down to 15 km. Their model also showed increased attenuation 99 outside the Kendeng Basin just beneath Mt. Merapi that goes down to a depth of 35 km with
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a southward dip. Elevated temperatures with possible partial melts and a magma chamber
beneath the Merapi volcano were proposed as part of the interpretation.

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Zulfakriza et al. (2014) carried out seismic ambient noise tomography using the MERAMEX eramex data set in central Java and found a band of low velocities centred between Mt Merapi and Mt Lawu that are located in the mid-crust at ~15 km depth and also concentrated in the upper 5 km of the crust. However, they found it difficult to interpret whether the low velocity zones relate to the presence of fluids and partial melt or sediments which fill the Kendeng zone.

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The work that is most relevant to this study are the crustal imaging results from the MERAMEX network by Wölbern (2016). By extracting receiver functions, similar to what is done in this study, they encountered a high degree of waveform complexity and reported an average crustal thickness of 34 km in central Java. Moreover, a shallower Moho at 30 km depth is seen beneath the Kendeng zone, which reflects the presence of ophiolitic basement in the center of the island related to the Meratus suture zone. A deeper Moho at 39 km depth is also seen to the north and west of the Kendeng zone, and is thought to be present due to crustal thickening by overthrusting and compressional deformation from a previous collision zone. Note that although part of the MERAMEX network spanned our study area, the DOMERAPI network has a much denser station coverage.

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DATA AND METHOD

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Illumination of lithospheric structure using seismic methods can provide important information for understanding regional tectonics; for example, if we can determine crustal thickness in a zone of extension, then we can place constraints on the amount of extension that has occurred. Crustal imaging can also provide insight into mantle properties by comparing predicted and observed topography for a known crustal thickness. In the case of

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P-wave receiver functions, when a teleseismic P wave encounters a boundary, mode
conversion to SV waves will occur, which will be recorded on the radial component of the
seismogram. These mode conversions can be exploited to determine the depth of interfaces
and the S-wave velocity of layers. When there is a velocity increase with depth across a
boundary, a positive RF pulse is produced, whereas a negative pulse is produced by a
decrease in S velocity with depth across a boundary. The vertical and radial components of
the early part of a teleseismic waveform (prior to direct S arrivals) are a function of the
source time function and P-wave arrivals; however, P-SV conversions should primarily
appear on the radial component. To isolate such signal, the RF is calculated by removing or
deconvolving the recorded vertical component on a seismogram from the radial component.
Typical RF responses show a strong pulse at the P wave arrival, with the next strongest pulse
likely being the Moho signal followed by its reverberations or multiples. Further details on
the computation of RFs can be found in Langston (1977), Owen et al. (1984) and Suhardja et
al. (2013).

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140 **Data**

In this study we use data recorded by the DOMERAPI temporary seismic network
installed in the neighbourhood of Merapi volcano. A total 53 broad-band seismometers, most
of them Guralp CMG-40Ts, were installed from October 2013 to mid-April 2015, with an
average inter-station distance of ~4 km. This has provided by far the densest coverage of
seismograph stations ever used on Merapi (Fig. 2). The goal of the seismic experiment was to
image the subsurface beneath Merapi in order to illuminate the deep magma source and
associated magmatic processes. This study is a collaborative effort between Universitas
Pertamina, Institut Teknologi Bandung, Institut de Recherche pour le Développement (IRD),
France, and the Agency for Meteorology, Climatology and Geophysics (BMKG).
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The first step in RF data processing is to obtain an event catalog from the International Seismological Centre (ISC) for earthquakes that occurred during the station deployment. For this study, we obtain more than 150 earthquakes with magnitude > 5.5 and epicentral distances between 30 to 100 degrees. The locations of the hypocenters and stations are shown in Fig. 3. Most of the good quality data come from events in Japan, Papua New Guinea and New Zealand. Next, we cut the waveforms at 20 s before and 60 s after the direct P arrival to ensure that all converted phases to a depth of 100 km are included. After that, using calculated back-azimuth information, horizontal component data were then rotated to radial and tangential components, where the radial direction is parallel to the great circle from the event to the station. Next, the vertical and radial components are rotated into P-SV components using incidence angle information estimated from the ak135 velocity model (Kennett et al. 1995). A rotation into P-SV components will theoretically increase the amplitude of the Ps wave on the SV component. However, this depends on how accurately we know the angle of incidence which in turn depends on how well we know the near surface velocities. This procedure should also minimize the direct P wave amplitude on the SV component, although in practice, we still see strong amplitudes on the P onset.

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To improve the quality of the waveform, a signal-to-noise ratio check was carried out by comparing the power in the seismic traces 20s before and after the predicted arrival time of the P wave. We only used seismograms with signal-to-noise ratios higher than two for both the P and SV components to ensure that only high-quality data are used in the deconvolution. Although we initially collected seismograms with high magnitude events, some seismograms don't show high quality data and the signal-to-noise ratio criteria removed 30-40% of all seismograms we initially considered. For example, at station ME29, from 150 events collected, 48 were removed due to high noise levels. An incoming P wave can have a very complicated shape due to the source time function, near surface reverberations at the source,

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multipathing along the propagation path as well as P wave multiples near the receiver. Each 175 Ps converted wave should also have the same shape as the incoming P wave. Thus, to isolate 176 the Ps waves and convert them to simple pulses, the P component is deconvolved from the 177 SV component. We performed the deconvolution process in the frequency domain by using 178 the water-level stabilization method and a low-pass Gaussian filter to remove high-frequency 179 noise (Langston 1977). The RF H(ω) is calculated using the following: 180

$$H(\omega) = \frac{R(\omega)Z^*(\omega)}{\max\{Z(\omega)Z^*(\omega), c \max\{Z(\omega)Z^*\}\}} G(\omega) \quad \dots \ (\text{eq. 1})$$

$$G(\omega) = \exp\left(-\frac{\omega^2}{4\alpha^2}\right)$$

where ω is angular frequency, Z(ω) is the Fourier transform of the P component waveform, 181 R(ω) is the transform of the SV component, and Z*(ω) is the complex conjugate of Z(ω). 182 G(ω) is a Gaussian filter that has zero phase distortion and a lack of sidelobes. The values of 183 α and c were chosen by trial and error, where we tried to make the RF as sharp as possible but 184 also tried to minimize noise. All RFs were computed using a water-level parameter c of 0.001 185 and a Gaussian smoothing parameter α of 3.5. A final visual check was also performed. Good 186 RFs are identified by having a sharp P wave signal with little energy arriving earlier. Low- 187 quality RFs tend to have anomalously high-amplitude signals at later times or very wide 188 sidelobes. We eliminated these data before further data processing. An example of RFs for 189 station ME29, plotted as a function of ray parameter, is shown in Fig. 4. Note that each RF is 190 stacked into each bin. Most of the RFs have similar signals with a peak at 0 s (the P wave) 191 followed by negative side lobes and several positive signals; the third positive signal (at 3.6 192 sec) appears to be much stronger, and we interpret it as a conversion from the Moho. 193 194

We also performed a stack of all the ME29 RFs in the depth domain. Starting with a 1-D 195 velocity model from seismic tomography (Ramdhan et al. 2019) and horizontal slowness 196
information, each RF can be interpolated into the depth domain to correct for moveout and then stacked. Fig. 5 shows a stacked RF in the depth domain with high amplitude at 0 km followed by a strong positive amplitude at 30 km depth interpreted as an arrival from the Moho. Strong signals are also seen at 12 s and 15 s in the time domain, which are likely PpPs and PsPs/PpSs multiples from the Moho but could also be conversions from deeper mantle discontinuities. As an additional stacking analysis, we also performed Nth-root stacking (Muirhead 1968) with N set to 2; this causes the final stack to have slightly spikier positive signal and suppresses some of the noise especially at later times (Fig. 5).

Station ME29 is located near Mt. Merapi, where a strong negative signal occurs between two positive signals. A detailed forward waveform model or RF inversion may be needed to better understand the cause of this feature. However, these signals might be related to the presence of low velocity layers at shallow and deeper crustal depths associated with magma chambers.

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211 Complex RF signal

Fig. 6 shows a collection of stacked RFs from stations to the north, east, south and west of Mt. Merapi, as well as one station located on the volcano itself. Typically, a stacked Receiver Function would show a strong positive and clear P wave signal, followed by the second strongest positive signal corresponding to a Ps conversion at the Moho. However, some stations show a much more complex pattern with a mix of positive and negative signals shortly after the incoming P wave, as seen on Figure 6. For example, station ME19 to the west and ME43 to the south show a strong positive signal after the P wave onset followed by a negative side lobe and a slightly stronger positive pulse (marked with a star) that we think is the Moho signal. High amplitude signal shortly after the P wave arrival may represent reverberations from boundaries at shallow depths (e.g. sedimentary or volcanic rock
222 layering), which may mask later arriving signal that we may wish to exploit. Another unusual
223 observation is that some stations do not have the Moho conversion as the strongest signal
224 after the P wave arrival. Note that RFs from the MERAMEX study of Wölbern & Rümpker
225 (2016) also feature high levels of waveform complexity, which may be attributed to
226 complexity of local structures. Suhardja et al. (2015) performed RFs near the subduction
227 zone in southwestern Mexico and found that the converted wave from the Moho could
228 essentially become undetectable as a result of dehydration process from the down-going slab.
229 and serpentinization reducing the velocity contrast and hence decreasing the amplitude of the
230 converted wave. However, the depth of the slab beneath our study area is estimated to be
231 about 120 km (Koulakov et al. 2007), which is deeper than the slab beneath Cascadia and
232 southwestern Mexico (~60-70 km depth). Nevertheless, both slab dehydration and
233 serpentinization of the uppermost mantle and strong shallow velocity anomalies may be
234 responsible for the complex RFs observed in our study area. Further insight into these
235 complexities could be achieved through receiver function inversion, which is the subject of a
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238 Delayed P wave arrival

239 Typically, the direct P wave, which is shown as the first positive spike in most RFs, arrives at
240 a similar time to ak135 predictions, although lateral heterogeneity in the crust and mantle,
241 and errors in earthquake origin time, contribute to the observed differences. However, we
242 noticed that the P-arrivals at some stations in the northern and eastern part of the study area
243 have a significantly delayed onset observed on the vertical component before deconvolution.
244 For example, station ME01, which is located in the northern part of the study area, has a P
245 onset that arrives almost one second after the predicted time. A similar late arrival can also be
246 observed at station ME 27 in the eastern part of the study area. Previous tomography results
247 (Koulakov et al. 2007; Wagner et al. 2007) have found a very low velocity anomaly in this
248 region, with a maximum amplitude of -30% at 5 km depth. This has been interpreted to be the
249 result of thick lava and sedimentary deposits in the Kendeng zone. In addition, tomography
250 studies reveal the presence of a low velocity zone extending all the way to the mantle, yet
251 without any active volcanism in the Kendeng zone. These observations lead them to propose
252 a cooling process which produced a rigid matrix filled with pockets of molten materials.

253 The delayed teleseismic P wave arrivals we have found are located at the most eastern
254 and northern regions of our study area and are coincident with the location of Kendeng Basin.
255 It is therefore likely that they are also caused by the same phenomena that produced the low
256 velocity zones in the previous studies i.e. sedimentary deposits and recent magmatic
257 intrusions. Teleseismic tomography may be able to provide further insight into these large
258 delay times, which will be the subject of a future study.

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260 H-κ method

261 Typical RFs exhibit a series of pulses that can be attributed to P to S converted waves from
262 interfaces in the subsurface beneath a seismic station. To convert these pulses to interface
263 depth beneath a station requires knowledge of the P and S velocities above the interface. Zhu
264 & Kanamori (2000) introduced a method (the H-κ method) that can minimize the ambiguity
265 due to the trade-off between depth and velocity in a flat and uniform layer. In most
266 applications, it involves a grid search across a range of crustal thicknesses and Vp/Vs ratios
267 to obtain theoretical arrival times of the Ps converted wave and its multiple. The method will
268 then sum all RFs at the times corresponding to the Ps arrival time as well as the arrival times
269 for the multiples for various choices of layer thickness (H) and Vp/Vs ratio. When the correct
270 depths and velocities are used, the summation amplitude should be large because it would
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positive converted wave at 3.3 s. Note that predicted times for the multiples are also plotted
in Fig. 7.

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The H-κ method has several benefits such as fast computation time and no requirement to pick arrival times. However, the main weakness of the method is that the theoretical arrival times for the converted waves assumes a single crustal layer with a flat Moho. As such, the method may fail to produce meaningful results in a range of cases, such as a dipping Moho in a subduction zone or when multiple layers featuring strong velocity contrasts across interfaces are present e.g. when sedimentary basins are present. We observed that some RFs in our study area do not show a strong Ps converted wave and instead feature a complex waveform. As noted earlier, this may be due to multiple layers of sediment and volcanics that have large impedance contrasts with the basement, such that the multiple reflections interfere with the Moho signal. We have 11 stations out of a total of 51 which failed to constrain realistic numbers for Moho depth and Vp/Vs ratio. In cases where Moho depth cannot be derived from the H-κ method, we estimate Moho depth directly from the stacked RF, which is migrated using the velocity model obtained from seismic tomography by Ramdhan *et al.* (2018), by picking the peak corresponding to the Ps phase. This method likely has greater uncertainty in the depth. We estimated the depth uncertainty from this method around ± 2-3 km based on a bootstrap method explained below.

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## RESULTS AND DISCUSSION

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The H-κ stacking method— was applied to all data collected by the DOMERAPI array. Altogether, 40 reliable Vp/Vs ratio measurements and 51 reliable crustal thickness measurements were obtained. The results of the RF analysis are shown in Figs. 9 and 10 as interpolated crustal thickness and Vp/Vs ratio maps respectively (see supplementary section for the contoured plots of S(H,K) and receiver functions from all available stations). The Zhu

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and Kanamori method (2000) provides a simple means of calculating uncertainties by
measuring the flatness of the peak in the plot of S(H,K). For stations where the H-κ method
does not work, we use the depth of the Moho Ps conversion obtained by using Vp/Vs ratios
of 1.75 and 1.85, which provides a bound on the range of plausible values. We also
performed a bootstrapping method (Efron and Tibshirani, 1991) to separately estimate
uncertainty for each station. We generated 100 random populations of receiver functions
from the total pool available for each station and computed the standard deviation (σ) of the
depth variations between the populations. The results of the RF analysis are listed in Table 1
and 2 along with the 2σ errors.

Fig. 9 shows a map of crustal thickness estimated from stacked RFs and the H-κ method.
The average crustal thickness is 29.4 km and ranges from 26.7 to 32.5 km. Crustal thickness
varies from about 32 km in the southern part of the study area, then gets thinner near Mt.
Merapi before thickening again beneath Mt. Merbabu (see Figure 9). Further north, it thins to
about 29 km thick. The other regions do not show a very distinct change, averaging around
29-30 km thick. Fig. 10 illustrates stacked RFs using a velocity model obtained from a prior
tomography study from southwest to northeast across Mt. Merapi. Most stacked RFs show
waveform complexity such that the Ps converted wave from the Moho is not the strongest
positive signal that follows the incoming P wave. This may be due to a number of factors,
such as a gradational or suppressed Moho, which in this case may be due to the existence of a
sizable magma reservoir at Moho depths, which was identified in a recent local earthquake
tomography study (Widiyantoro et al., 2018).

The average Vp/Vs ratio of the crust in this study is 1.80 (Fig. 11), which is slightly
higher than the global average of 1.78 (Christensen 1996; Chevrot & van der Hilst 2000).
However, there is a large range in values, from 1.70 to 1.99, with two regions having
abnormally high crustal Vp/Vs ratios. One of these regions is located close to the active
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Merapi volcano, where four stations located at the southwestern edge have Vp/Vs ratios of 1.85 or greater. To the north of these stations, near the Merbabu volcano, the Vp/Vs ratio is closer to average. The second region, located to the northeast of the network, shows a band of high Vp/Vs ratios ranging from 1.85 to 1.87.

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Average crustal Vp/Vs can be used to constrain the petrology and physical state of the crust. Christensen (1996) showed from laboratory experiments that Vp/Vs ratio does not significantly vary as a function of reasonable crustal temperatures (0-400° C) and pressure changes greater than 100-200 Mpa. The primary factors that control the Vp/Vs ratio in the crust are the presence of melt or fluids and changes in mineralogy. The relative abundance of quartz and plagioclase feldspar has a strong effect on Vp/Vs (Christensen, 1996): for felsic quartz-rich rocks such as granite, Vp/Vs is 1.71; intermediate rocks have a Vp/Vs ratio of near 1.78 and mafic plagioclase-rich rocks such as gabbro have a Vp/Vs ratio near 1.87. The average composition for continental crust is close to andesite or diorite (Anderson, 1989) and laboratory measurements by Carmichael (1982) confirmed that Vp/Vs for diorite at crustal pressures ranges from 1.75 to 1.79. In the case of partial melt, Hammond and Humphreys (2000) show that 2% partial melting can increase Vp/Vs ratio by as much as 10% in the uppermost mantle.

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The high Vp/Vs ratio regions could indicate a very mafic crust or the presence of high pore pressure fluids or partial melt. Thus, we suggest that the high Vp/Vs ratios we observe near Mt. Merapi are due to partial melt or high fluid content within the crust, although some mafic underplating of the crust could also contribute

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To better understand the crust beneath Mt. Merapi and Mt. Merbabu, we plot cross-sections which show elevation, crustal depth and Vp/Vs ratio along a line from south to north (see Figure 12). The location of the cross-section is marked on Fig. 9. Interestingly, Mt Merapi has thinner crust (by about 3 km) compared to Mt. Merbabu and a higher Vp/Vs ratio

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(1.9 vs 1.73). Recent local earthquake tomography results (Widiyantoro et al, 2018) which exploited data from the same network used in this study found three active areas with high Vp/Vs: the first is a shallow zone interpreted to represent a region of intense fluid percolation, directly below the summit of the volcano; the second is thought to be a pre-eruptive magma reservoir at 10 to 20 km depth below MSL that is several orders of magnitude larger than known erupted magma volumes; and the third is a deep magma reservoir at 30 km depth which supplies the main reservoir. We suggest that the zone of partial melting, magma reservoir and volcanic sediments may be responsible for the higher Vp/Vs ratio observed in this study. These results appear to support the observation that Mt. Merbabu is much less volcanically active than Mt. Merapi.

In Fig. 13, we plot station elevation as a function of crustal thickness derived from our analysis of a map of topography. In the case of Airy isostasy, the thickness of a crustal root has a linear relationship with excess topography, so a deeper Moho should be compensated with greater elevation.

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<figure><img src="image_1.png" /></figure>

<figure><img src="image_2.png" /></figure>

In our case, the scatterplot in Figure 13 appears to reveal no obvious correlation between elevation and crustal thickness. In attempt to quantify a trend, we compute the correlation coefficient, and find that $r^2=0.1292$. This implies a lack of correlation between crustal thickness and topography, and hence an Airy isostacy model is not applicable for our study.

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area. However, given the footprint of the volcanic edifices (a few 10s of km), this is perhaps unsurprising since the flexural strength of the lithosphere should be more than sufficient to support narrowly distributed loads of this sort. Isostatic equilibrium is instead maintained over distances of 100s of km (see Watts, 2001), which means that locally one should not expect a correlation between crustal thickness and elevation. .

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## CONCLUSIONS

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We have calculated the crustal thickness and bulk crustal Vp/Vs ratio beneath Mt. Merapi and Mt. Merbabu. RFs that feature a complex signal are likely influenced by near surface sedimentary layering and the presence of volcanic rocks. Crustal thickness does not vary strongly beneath the study region, but a shallower Moho is found beneath Mt. Merapi, along with a higher Vp/Vs ratio (~1.86) which points to a more active volcano with partial melting and a hydrous zone. These results agree with recent tomographic models -that shows a low velocity zone and multiple zones of higher Vp/Vs ratio beneath Mt. Merapi that extend from the near surface to the base of the crust. The northern part of our study area also shows higher Vp/Vs ratio that may be related to the Kendeng Basin. Future work will involve the inversion of receiver functions for depth dependent shear wave velocity beneath individual stations, which has the potential to better recover the true complexity of subsurface seismic structure beneath the two volcanoes. Further insight can also be gained from the strong variations in teleseismic arrival time residuals that have been observed across the network.

## ACKNOWLEDGEMENTS

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We are grateful to the French National Research Agency and Institut de recherche pour le développement for funding the DOMERAPI project. We would also like to express our gratitude to PVMBG as the main partner of the DOMERAPI project in Indonesia. This study
417 was supported in part by Direktorat Perguruan Tinggi (DIKTI), the Republic of Indonesia,
418 through a WCU research grant 2017/2018, and a 2018 research grant from Institut Teknologi
419 Bandung (ITB) awarded to SW. We used the Generic Mapping Tools (Wessel & Smith,
420 1998) to create figures presented in this study.

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423 References

424 Anderson, D. L., 1989. Theory of the Earth. Blackwell Scientific Publications, Boston, p. 366

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<figure><img src="image_3.png" /><figcaption>**Figure 1.** Simplified geological map of the study region from Smyth *et al.* (2008). The modern magmatic arc (Sunda Arc) is indicated by the darkest shading with young volcanoes numbered as, 1: Dieng volcanic complex; 2: Mt. Sundoro; 3: Mt. Sumbing; 4: Mt. Ungaran; 5: Mt. Telomoyo; 6: Mt. Merbabu; 7: Mt. Merapi; 8: Mt. Muria; 9: Mt. Lawu; 10: Mt. Lasem; 11: Mt. Wilis. The dotted line is the Muria-Progo lineament (Smyth *et al.* 2008); the white dashed line indicates the 0 mGal contour of the negative Bouguer gravity anomaly (min. -58 mGal) which is located in the thickest part of the Kendeng Basin.</figcaption></figure>

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<figure><img src="image_4.png" /></figure>

**Figure 2.** Location of seismometers installed in the vicinity of Mt. Merapi (red triangle) and Mt. Merbabu (blue triangle) as part of the DOMERAPI experiment. The small inset shows the location of the study area relative to southeast Asia.

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<figure><img src="image_5.png" /><figcaption>**Figure 3.** Teleseismic events used in this study. The DOMERAPI network is located in the centre of the projection.</figcaption></figure>

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<figure><img src="image_6.png" /><figcaption>**Figure 4.** An example of stacked RFs for station ME29 as a function of ray parameter. High amplitude pulses at 0 sec represent the direct P arrival. The vertical axis represents seconds after the P onset.</figcaption></figure>

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<figure><img src="image_7.png" /><figcaption>**Figure 5.** RF stack for all events at station ME29. The black line is the 2nd root stack used to improve the signal to noise ratio. The red line represents the original linear stack.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Stacked RFs at stations to the west (ME19), south (ME43), east (ME27), north (ME01) and on Mt. Merapi (ME25). Blue star denotes pick of the Ps conversion.</figcaption></figure>

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<figure><img src="image_9.png" /><figcaption>**Figure 7.** RF traces that have been binned with respect to ray parameter at station ME11. The predicted Ps, PpPs, and PpSs arrivals are represented using green, red and blue lines respectively. Note the relative change in times between the three arrivals as a function of ray parameter. The green line is the predicted Ps time assuming a crustal thickness of 28.1 km and a Vp/Vs ratio of 1.80. The red line is the predicted time for PpPs and the blue line is the predicted time for PpSs using the same crustal model. The right figure shows a stack of all available traces.</figcaption></figure>
Station ME11: Vp=6.40, VpVs=1.80+-0.03, H=28.09+-0.61

<figure><img src="image_10.png" /></figure>

**Figure 8.** Contours of stack amplitudes for station ME11 as a function of crustal thickness H and Vp/Vs ratio. The grid search calculates the stack amplitude of all available RFs for a range of possible crustal thicknesses (25-40 km) and Vp/Vs ratios (1.65-2.0). The final result is taken by choosing the highest amplitude from the contour, and uncertainty is calculated by measuring the flatness of the contour peak.
<figure><img src="image_11.png" /><figcaption>**Figure 9.** An interpolation of crustal thickness measurements using the H-κ method. The measured crustal thickness beneath individual stations is also given with units of km. Thinner crust is seen near Mt Merapi. The X-Y line denotes the location of the cross section shown in Fig. 11. The A-B line shows the location of the cross section shown in Figure 10a.</figcaption></figure>

Mt. Merapi is denoted by the amber triangle and Mt. Merbabu by the green triangle.
<figure><img src="image_12.png" /><figcaption>Figure 10. Plot of stacked RFs from south west to north east (line A-B at Fig 9). Horizontal axis is distance in km, vertical axis on the left is time (s), and vertical axis on the right is the estimated depth. Orange triangle shows the location of Mt Merapi. Red dashed line shows interpolated crustal thickness from this study.</figcaption></figure>

**Commented [1]:** I still wonder why the initial P-phase is not aligned at zero for all traces.
<figure><img src="image_13.png" /><figcaption>**Figure 11.** Individual measurements of the bulk crustal Vp/Vs ratio along with an interpolated map made using splines under tension. Mt. Merapi is denoted by the amber triangle and Mt. Merbabu by the green triangle.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Figure 12.** Top panel: Elevation cross section from south to north across Mt. Merapi (red triangle) and Mt. Merbabu (blue triangle) – see Figure 9 for the location of the X-Y section.

Middle panel: Interpolated crustal thickness estimate along the same line.

Bottom panel: Interpolated Vp/Vs ratio from H-K stacking.</figcaption></figure>

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<figure><img src="image_15.png" /><figcaption>**Figure 13.** Crustal thickness measurement vs elevation. Elevation data were taken from the National Oceanic and Atmospheric Administration website (www.noaa.gov).</figcaption></figure>
<table><thead><tr><th>Station name</th><th>Area name</th><th>Latitude (°)</th><th>Longitude (°)</th><th>Elevation (m)</th><th>Number of RFs</th><th>Thickness (km)</th><th>Uncertainty (km)</th><th>Vp/Vs ratio</th><th>Uncertainty (km)</th></tr></thead><tbody><tr><td>ME01</td><td>Daren</td><td>-7.29</td><td>110.4609</td><td>504</td><td>41</td><td>29.61</td><td>±</td><td>1.5</td><td>1.93</td><td>±</td><td>0.07</td></tr><tr><td>ME02</td><td>"Wates Barat"</td><td>-7.3523</td><td>110.7024</td><td>291</td><td>28</td><td>29.33</td><td>±</td><td>2.1</td><td>2.12</td><td>±</td><td>0.15</td></tr><tr><td>ME03</td><td>"Brangkongan"</td><td>-7.3449</td><td>110.5475</td><td>653</td><td>33</td><td>30.05</td><td>±</td><td>1.7</td><td>1.72</td><td>±</td><td>0.06</td></tr><tr><td>ME04</td><td>"Kebrok"</td><td>-7.3396</td><td>110.451</td><td>589</td><td>26</td><td>29.05</td><td>±</td><td>2.3</td><td>1.93</td><td>±</td><td>0.07</td></tr><tr><td>ME05</td><td>"Ngrancah"</td><td>-7.3407</td><td>110.3544</td><td>865</td><td>51</td><td>28.32</td><td>±</td><td>1.1</td><td>1.89</td><td>±</td><td>0.04</td></tr><tr><td>ME08</td><td>"Bumiayu"</td><td>-7.3714</td><td>110.4454</td><td>1052</td><td>47</td><td>31.2</td><td>±</td><td>1.9</td><td>1.85</td><td>±</td><td>0.04</td></tr><tr><td>ME09</td><td>"Bangongan"</td><td>-7.3798</td><td>110.4073</td><td>1311</td><td>45</td><td>28.22</td><td>±</td><td>0.8</td><td>1.92</td><td>±</td><td>0.08</td></tr><tr><td>ME10</td><td>"Nganggrung"</td><td>-7.4077</td><td>110.513</td><td>799</td><td>53</td><td>28.91</td><td>±</td><td>2.1</td><td>1.7</td><td>±</td><td>0.09</td></tr><tr><td>ME11</td><td>"Kumpulrejo"</td><td>-7.4165</td><td>110.4763</td><td>1083</td><td>52</td><td>28.28</td><td>±</td><td>1.3</td><td>1.79</td><td>±</td><td>0.05</td></tr><tr><td>ME12</td><td>"Kulian"</td><td>-7.4011</td><td>110.4524</td><td>1211</td><td>38</td><td>28</td><td>±</td><td>2.3</td><td>1.7</td><td>±</td><td>0.08</td></tr><tr><td>ME13</td><td>"Ngeyo"</td><td>-7.4148</td><td>110.4351</td><td>1674</td><td>47</td><td>28.91</td><td>±</td><td>3.2</td><td>1.7</td><td>±</td><td>0.12</td></tr><tr><td>ME14</td><td>"Banaran"</td><td>-7.4089</td><td>110.3927</td><td>1350</td><td>33</td><td>28</td><td>±</td><td>2.8</td><td>1.9</td><td>±</td><td>0.05</td></tr><tr><td>ME15</td><td>"Dawung"</td><td>-7.4532</td><td>110.5786</td><td>586</td><td>45</td><td>30.81</td><td>±</td><td>2.2</td><td>1.92</td><td>±</td><td>0.08</td></tr><tr><td>ME16</td><td>"Kembang"</td><td>-7.4569</td><td>110.5068</td><td>997</td><td>29</td><td>31.9</td><td>±</td><td>1.7</td><td>1.72</td><td>±</td><td>0.04</td></tr><tr><td>ME17</td><td>"Glagah"</td><td>-7.4459</td><td>110.476</td><td>1492</td><td>30</td><td>31.72</td><td>±</td><td>1.9</td><td>1.7</td><td>±</td><td>0.09</td></tr><tr><td>ME20</td><td>"Gunungsari"</td><td>-7.4721</td><td>110.4797</td><td>1434</td><td>47</td><td>31.03</td><td>±</td><td>1.8</td><td>1.7</td><td>±</td><td>0.11</td></tr><tr><td>ME21</td><td>"Sobleman"</td><td>-7.4707</td><td>110.3983</td><td>1400</td><td>49</td><td>30.02</td><td>±</td><td>2.1</td><td>1.9</td><td>±</td><td>0.07</td></tr><tr><td>ME22</td><td>"Tumangsari"</td><td>-7.4938</td><td>110.5243</td><td>842</td><td>52</td><td>28.5</td><td>±</td><td>1.5</td><td>1.7</td><td>±</td><td>0.08</td></tr><tr><td>ME23</td><td>"Genting"</td><td>-7.5068</td><td>110.484</td><td>1355</td><td>49</td><td>27.5</td><td>±</td><td>3.2</td><td>1.77</td><td>±</td><td>0.04</td></tr><tr><td>ME24</td><td>"Plalangan"</td><td>-7.5069</td><td>110.4548</td><td>1628</td><td>51</td><td>27.67</td><td>±</td><td>4.3</td><td>1.86</td><td>±</td><td>0.06</td></tr><tr><td>ME25</td><td>"Kajor"</td><td>-7.4976</td><td>110.4269</td><td>1321</td><td>44</td><td>29</td><td>±</td><td>4.7</td><td>1.79</td><td>±</td><td>0.05</td></tr><tr><td>ME26</td><td>"Wonolelo"</td><td>-7.4917</td><td>110.3899</td><td>1163</td><td>45</td><td>30.42</td><td>±</td><td>3.2</td><td>1.7</td><td>±</td><td>0.07</td></tr><tr><td>ME27</td><td>"Pulisen"</td><td>-7.5383</td><td>110.5958</td><td>467</td><td>49</td><td>28.52</td><td>±</td><td>1.8</td><td>1.82</td><td>±</td><td>0.07</td></tr><tr><td>ME28</td><td>"Pusung"</td><td>-7.536</td><td>110.5071</td><td>1062</td><td>54</td><td>27.23</td><td>±</td><td>2.1</td><td>1.8</td><td>±</td><td>0.05</td></tr><tr><td>ME29</td><td>"Lendong"</td><td>-7.5335</td><td>110.4793</td><td>1567</td><td>47</td><td>27.41</td><td>±</td><td>2.5</td><td>1.78</td><td>±</td><td>0.06</td></tr><tr><td>ME31</td><td>"Tempel"</td><td>-7.5322</td><td>110.3882</td><td>1000</td><td>19</td><td>29.06</td><td>±</td><td>1.2</td><td>1.9</td><td>±</td><td>0.12</td></tr><tr><td>ME33</td><td>"Garonan"</td><td>-7.5494</td><td>110.3183</td><td>534</td><td>18</td><td>27.67</td><td>±</td><td>2.1</td><td>1.7</td><td>±</td><td>0.09</td></tr><tr><td>ME34</td><td>"Candi"</td><td>-7.5624</td><td>110.4926</td><td>1132</td><td>45</td><td>27.86</td><td>±</td><td>2.2</td><td>1.84</td><td>±</td><td>0.03</td></tr><tr><td>ME36</td><td>"Kaliurang"</td><td>-7.5729</td><td>110.4055</td><td>1034</td><td>22</td><td>26.77</td><td>±</td><td>3.1</td><td>1.73</td><td>±</td><td>0.04</td></tr><tr><td>ME37</td><td>"Balerante"</td><td>-7.6029</td><td>110.4662</td><td>887</td><td>19</td><td>27.87</td><td>±</td><td>2.9</td><td>1.74</td><td>±</td><td>0.05</td></tr><tr><td>ME38</td><td>"Karangkendal"</td><td>-7.6223</td><td>110.4853</td><td>662</td><td>47</td><td>29.72</td><td>±</td><td>3.5</td><td>1.75</td><td>±</td><td>0.02</td></tr><tr><td>ME39</td><td>"Kemiri Kebo"</td><td>-7.6134</td><td>110.4054</td><td>717</td><td>22</td><td>27</td><td>±</td><td>2.9</td><td>1.75</td><td>±</td><td>0.08</td></tr><tr><td>ME40</td><td>"Randulanang"</td><td>-7.661</td><td>110.5555</td><td>331</td><td>22</td><td>32</td><td>±</td><td>2.6</td><td>1.83</td><td>±</td><td>0.06</td></tr><tr><td>ME41</td><td>Purwobinangun</td><td>-7.7024</td><td>110.465</td><td>287</td><td>45</td><td>29.67</td><td>±</td><td>3.4</td><td>1.78</td><td>±</td><td>0.05</td></tr><tr><td>ME42</td><td>"Bangunharjo"</td><td>-7.65405</td><td>110.35652</td><td>287</td><td>47</td><td>29.52</td><td>±</td><td>2.7</td><td>1.76</td><td>±</td><td>0.04</td></tr><tr><td>ME43</td><td>"Salakan"</td><td>-7.7349</td><td>110.4484</td><td>228</td><td>24</td><td>32</td><td>±</td><td>3.1</td><td>1.74</td><td>±</td><td>0.06</td></tr><tr><td>ME47</td><td>Gunung Andong</td><td>-7.39663</td><td>110.35875</td><td>989</td><td>24</td><td>28.63</td><td>±</td><td>2.1</td><td>1.71</td><td>±</td><td>0.07</td></tr><tr><td>ME49</td><td>"Tegal Randu"</td><td>-7.56907</td><td>110.3549</td><td>623</td><td>8</td><td>31.14</td><td>±</td><td>3.1</td><td>1.74</td><td>±</td><td>0.05</td></tr><tr><td>ME52</td><td>"Penggung"</td><td>-7.50483</td><td>110.58432</td><td>524</td><td>11</td><td>29.61</td><td>±</td><td>2.6</td><td>1.83</td><td>±</td><td>0.05</td></tr><tr><td>ME53</td><td>"Kenteng"</td><td>-7.39794</td><td>110.56211</td><td>0</td><td>13</td><td>30</td><td>±</td><td>1.4</td><td>1.93</td><td>±</td><td>0.08</td></tr></tbody></table>

651

652

653

654
<table><caption>Table 1. Crustal Thickness and Vp/Vs Ratio calculated from the H-κ method with 2σ uncertainty from a bootstrap method.</caption><thead><tr><th>Station name</th><th>Latitude</th><th>Longitude</th><th>Elevation (m)</th><th>Thickness (km)</th><th>±</th><th>Vp/Vs ratio</th><th>±</th><th></th></tr></thead><tbody><tr><td>ME01</td><td>-7.29</td><td>110.4609</td><td>504</td><td>29.61</td><td>±</td><td>1.5</td><td>1.93</td><td>0.07</td></tr><tr><td>ME02</td><td>-7.3523</td><td>110.7024</td><td>291</td><td>29.33</td><td>±</td><td>2.1</td><td>2.12</td><td>0.15</td></tr><tr><td>ME03</td><td>-7.3449</td><td>110.5475</td><td>653</td><td>30.05</td><td>±</td><td>1.7</td><td>1.72</td><td>0.06</td></tr><tr><td>ME04</td><td>-7.3396</td><td>110.451</td><td>589</td><td>29.05</td><td>±</td><td>2.3</td><td>1.93</td><td>0.07</td></tr><tr><td>ME05</td><td>-7.3407</td><td>110.3544</td><td>865</td><td>28.32</td><td>±</td><td>1.1</td><td>1.89</td><td>0.04</td></tr><tr><td>ME08</td><td>-7.3714</td><td>110.4454</td><td>1052</td><td>31.2</td><td>±</td><td>1.9</td><td>1.85</td><td>0.04</td></tr><tr><td>ME09</td><td>-7.3798</td><td>110.4073</td><td>1311</td><td>28.22</td><td>±</td><td>0.8</td><td>1.92</td><td>0.08</td></tr><tr><td>ME10</td><td>-7.4077</td><td>110.513</td><td>799</td><td>28.91</td><td>±</td><td>2.1</td><td>1.7</td><td>0.09</td></tr><tr><td>ME11</td><td>-7.4165</td><td>110.4763</td><td>1083</td><td>28.28</td><td>±</td><td>1.3</td><td>1.79</td><td>0.05</td></tr><tr><td>ME12</td><td>-7.4011</td><td>110.4524</td><td>1211</td><td>28</td><td>±</td><td>2.3</td><td>1.7</td><td>0.08</td></tr><tr><td>ME13</td><td>-7.4148</td><td>110.4351</td><td>1674</td><td>28.91</td><td>±</td><td>3.2</td><td>1.7</td><td>0.12</td></tr><tr><td>ME14</td><td>-7.4089</td><td>110.3927</td><td>1350</td><td>28</td><td>±</td><td>2.8</td><td>1.9</td><td>0.05</td></tr><tr><td>ME15</td><td>-7.4532</td><td>110.5786</td><td>586</td><td>30.81</td><td>±</td><td>2.3</td><td>1.92</td><td>0.08</td></tr><tr><td>ME16</td><td>-7.4569</td><td>110.5068</td><td>997</td><td>31.9</td><td>±</td><td>1.7</td><td>1.72</td><td>0.04</td></tr><tr><td>ME17</td><td>-7.4459</td><td>110.476</td><td>1492</td><td>31.72</td><td>±</td><td>1.9</td><td>1.7</td><td>0.09</td></tr><tr><td>ME20</td><td>-7.4721</td><td>110.4797</td><td>1434</td><td>31.03</td><td>±</td><td>1.8</td><td>1.7</td><td>0.11</td></tr><tr><td>ME21</td><td>-7.4707</td><td>110.3983</td><td>1400</td><td>30.02</td><td>±</td><td>2.1</td><td>1.9</td><td>0.07</td></tr><tr><td>ME22</td><td>-7.4938</td><td>110.5243</td><td>842</td><td>28.5</td><td>±</td><td>1.5</td><td>1.7</td><td>0.08</td></tr><tr><td>ME23</td><td>-7.5068</td><td>110.484</td><td>1355</td><td>27.5</td><td>±</td><td>3.2</td><td>1.77</td><td>0.04</td></tr><tr><td>ME24</td><td>-7.5069</td><td>110.4548</td><td>1628</td><td>27.67</td><td>±</td><td>4.3</td><td>1.86</td><td>0.06</td></tr><tr><td>ME25</td><td>-7.4976</td><td>110.4269</td><td>1321</td><td>29</td><td>±</td><td>4.7</td><td>1.79</td><td>0.05</td></tr><tr><td>ME26</td><td>-7.4917</td><td>110.3899</td><td>1163</td><td>30.42</td><td>±</td><td>3.2</td><td>1.7</td><td>0.07</td></tr><tr><td>ME27</td><td>-7.5383</td><td>110.5958</td><td>467</td><td>28.52</td><td>±</td><td>1.8</td><td>1.82</td><td>0.07</td></tr><tr><td>ME28</td><td>-7.536</td><td>110.5071</td><td>1062</td><td>27.23</td><td>±</td><td>2.1</td><td>1.8</td><td>0.05</td></tr><tr><td>ME29</td><td>-7.5335</td><td>110.4793</td><td>1567</td><td>27.41</td><td>±</td><td>2.5</td><td>1.78</td><td>0.06</td></tr><tr><td>ME31</td><td>-7.5322</td><td>110.3882</td><td>1000</td><td>29.06</td><td>±</td><td>1.2</td><td>1.9</td><td>0.12</td></tr><tr><td>ME33</td><td>-7.5494</td><td>110.3183</td><td>534</td><td>27.67</td><td>±</td><td>2.1</td><td>1.7</td><td>0.09</td></tr><tr><td>ME34</td><td>-7.5624</td><td>110.4926</td><td>1132</td><td>27.86</td><td>±</td><td>2.2</td><td>1.84</td><td>0.03</td></tr><tr><td>ME36</td><td>-7.5729</td><td>110.4055</td><td>1034</td><td>26.77</td><td>±</td><td>3.1</td><td>1.73</td><td>0.04</td></tr><tr><td>ME38</td><td>-7.6223</td><td>110.4853</td><td>662</td><td>29.72</td><td>±</td><td>3.5</td><td>1.75</td><td>0.02</td></tr><tr><td>ME41</td><td>-7.7024</td><td>110.465</td><td>287</td><td>29.67</td><td>±</td><td>3.4</td><td>1.78</td><td>0.05</td></tr><tr><td>ME42</td><td>-7.65405</td><td>110.3565</td><td>287</td><td>29.52</td><td>±</td><td>2.7</td><td>1.76</td><td>0.04</td></tr><tr><td>ME47</td><td>-7.39663</td><td>110.3588</td><td>989</td><td>28.63</td><td>±</td><td>2.1</td><td>1.71</td><td>0.07</td></tr><tr><td>ME49</td><td>-7.56907</td><td>110.3549</td><td>623</td><td>31.14</td><td>±</td><td>3.1</td><td>1.74</td><td>0.05</td></tr><tr><td>ME52</td><td>-7.50483</td><td>110.5843</td><td>524</td><td>29.61</td><td>±</td><td>2.6</td><td>1.83</td><td>0.05</td></tr><tr><td>ME53</td><td>-7.39794</td><td>110.5621</td><td>0</td><td>30</td><td>±</td><td>1.4</td><td>1.93</td><td>0.08</td></tr></tbody></table>

655

656

657

658

659

660

661

662
<table><thead><tr><td>Station Name</td><td>Area name</td><td>Latitude (°)</td><td>Longitude (°)</td><td>Elevation (m)</td><td>Number of RFs</td><td>Thickness (km)</td><td>Uncertainty (km)</td></tr></thead><tbody><tr><td>ME06</td><td>"Gandokan"</td><td>-7.3522</td><td>110.2119</td><td>496</td><td>13</td><td>30</td><td>2.4</td></tr><tr><td>ME07</td><td>"Randuacir"</td><td>-7.3763</td><td>110.4877</td><td>738</td><td>26</td><td>28.52</td><td>2.1</td></tr><tr><td>ME18</td><td>"Kragilan"</td><td>-7.44217</td><td>110.39342</td><td>1382</td><td>27</td><td>29.33</td><td>3.5</td></tr><tr><td>ME19</td><td>"Warangan"</td><td>-7.4402</td><td>110.3359</td><td>882</td><td>31</td><td>30.51</td><td>2.6</td></tr><tr><td>ME30</td><td>"Pos Babadan"</td><td>-7.5261</td><td>110.4107</td><td>1295</td><td>44</td><td>30.1</td><td>2.1</td></tr><tr><td>ME32</td><td>"Pos Gemer"</td><td>-7.5437</td><td>110.4093</td><td>1266</td><td>27</td><td>27.2</td><td>2.7</td></tr><tr><td>ME35</td><td>"Wonorejo"</td><td>-7.586</td><td>110.5095</td><td>788</td><td>19</td><td>28.21</td><td>1.5</td></tr><tr><td>ME44</td><td>"Candi"</td><td>-7.797</td><td>109.8938</td><td>45</td><td>12</td><td>33</td><td>3.7</td></tr><tr><td>ME45</td><td>"Watugajah"</td><td>-8.0097</td><td>110.3432</td><td>257</td><td>22</td><td>30.66</td><td>1.1</td></tr><tr><td>ME46</td><td>"Tileng"</td><td>-8.1646</td><td>110.7543</td><td>243</td><td>11</td><td>30</td><td>3.5</td></tr><tr><td>ME51</td><td>"Pager Jurang"</td><td>-7.57897</td><td>110.55963</td><td>549</td><td>5</td><td>29.3</td><td>4</td></tr></tbody></table>

663

664

<caption>Table 2. Crustal thickness estimated for stations which we have no independent control on Vp/Vs ratio. Uncertainty was calculated using a bootstrapping method.</caption>

665

666

667

668

669
Conflict of Interest and Authorship Conformation Form

Please check the following as appropriate:

✓ All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.

✓ This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.

✓ The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript

✓ The following authors have affiliations with organizations with direct or indirect financial interest in the subject matter discussed in the manuscript:

<table><thead><tr><td>Author's name</td><td>Affiliation</td></tr></thead><tbody><tr><td>Dr. Sandy Kumiawan Suhardja</td><td>Pertamina University</td></tr><tr><td>Prof. Sri Widiyantoro</td><td>Institut Teknologi Bandung</td></tr><tr><td>Dr. Metaxian Jean-Philippe</td><td>Université de Savoie Mont Blanc</td></tr><tr><td>Professor Nicholas Rawlinson</td><td>University of Cambridge</td></tr><tr><td>Dr. Mohamad Ramdhan</td><td>Indonesian Agency for Meteorology, Climatology and Geophysics</td></tr><tr><td>Dr. Agus Budi Santoso</td><td>Center for Volcanology and Geological Hazard Mitigation, Indonesia</td></tr></tbody></table>
Plots of RFs with respect to epicentral
distance/ray parameter and H-K stack
contour for all stations used in the
network
Station ME01: Vp = 6.5, Vp/Vs = 1.93 ± 0.07, H = 29 ± 1.5

<figure><img src="image_16.png" /></figure>
Station ME01

<figure><img src="image_17.png" /></figure>
Station ME02: Vp = 6.3, Vp/Vs = 2.12 ± 0.07, H = 29.3 ± 2.1

<figure><img src="image_18.png" /></figure>
Station ME02

<figure><img src="image_19.png" /></figure>
Station ME04: Vp = 6.5, Vp/Vs = 1.93 ± 0.07, H = 29.05 ± 2.3

<figure><img src="image_20.png" /></figure>
Station ME04

<figure><img src="image_21.png" /></figure>
Station ME05: Vp = 6.4, Vp/Vs = 1.89 ± 0.04, H = 28.3 ± 1.1

<figure><img src="image_22.png" /></figure>
ME05 Stack Bin

<figure><img src="image_23.png" /></figure>
Station ME08: Vp = 6.4, Vp/Vs = 1.85 ± 0.04, H = 31.2 ± 1.9

<figure><img src="image_24.png" /></figure>
ME08 Stack Bin

<figure><img src="image_25.png" /></figure>

Ray Parameter (s/km)
Station ME09: Vp = 6.5, Vp/Vs = 1.92 ± 0.08, H = 28.2 ± 0.8

<figure><img src="image_26.png" /></figure>
ME09 Stack Bin

<figure><img src="image_27.png" /></figure>
Station ME10: Vp = 6.3, Vp/Vs = 1.7 ± 0.02, H = 28.9 ± 2.1

<figure><img src="image_28.png" /></figure>
ME10 Stack Bin

<figure><img src="image_29.png" /></figure>

Ray Parameter (s/km)
Station ME11: Vp = 6.4, Vp/Vs = 1.8 ± 0.03, H = 28.3 ± 0.69

<figure><img src="image_30.png" /></figure>
ME11 Stack Bin

<figure><img src="image_31.png" /></figure>

Ray Parameter (s/km)
Station ME12: Vp = 6.5, Vp/Vs = 1.77 ± 0.03, H = 27.3 ± 0.67

<figure><img src="image_32.png" /></figure>
Station ME12

<figure><img src="image_33.png" /></figure>
Station ME13: Vp = 6.4, Vp/Vs = 1.7 ± 0.05, H = 28.9 ± 0.67

<figure><img src="image_34.png" /></figure>
ME13 Stack Bin

<figure><img src="image_35.png" /></figure>

Ray Parameter (s/km)
Station ME14: Vp = 6.4, Vp/Vs = 1.9 ± 0.05, H = 28 ± 0.88

<figure><img src="image_36.png" /></figure>
Station ME14

<figure><img src="image_37.png" /></figure>
Station ME15: Vp = 6.4, Vp/Vs = 1.92 ± 0.03, H = 30.8 ± 1.2

<figure><img src="image_38.png" /></figure>
ME15 Stack Bin

<figure><img src="image_39.png" /></figure>

Ray Parameter (s/km)
Station ME16: Vp = 6.4, Vp/Vs = 1.72 ± 0.03, H = 31.9 ± 0.81

<figure><img src="image_40.png" /></figure>
Station ME16

<figure><img src="image_41.png" /></figure>

Epicentral Distance (deg)
Station ME17: Vp = 6.4, Vp/Vs = 1.7 ± 0.03, H = 31.7 ± 0.81

<figure><img src="image_42.png" /></figure>
Station ME17

<figure><img src="image_43.png" /></figure>

Epicentral Distance (deg)
Station ME20: Vp = 6.3, Vp/Vs = 1.7 ± 0.02, H = 31 ± 0.67

<figure><img src="image_44.png" /></figure>
ME20 Stack Bin

<figure><img src="image_45.png" /></figure>

Ray Parameter (s/km)
Station ME21: Vp = 6.3, Vp/Vs = 1.9 ± 0.09, H = 30 ± 1.08

<figure><img src="image_46.png" /></figure>
ME21 Stack Bin

<figure><img src="image_47.png" /></figure>
Station ME22: Vp = 6.4, Vp/Vs = 1.7 ± 0.04, H = 28.5 ± 0.75

<figure><img src="image_48.png" /></figure>
ME22 Stack Bin

<figure><img src="image_49.png" /></figure>
Station ME23: Vp = 6.5, Vp/Vs = 1.77 ± 0.05, H = 27.5 ± 0.96

<figure><img src="image_50.png" /></figure>
ME23 Stack Bin

<figure><img src="image_51.png" /></figure>
Station ME24: Vp = 6.4, Vp/Vs = 1.86 ± 0.03, H = 27.7 ± 0.73

<figure><img src="image_52.png" /></figure>
ME24 Stack Bin

<figure><img src="image_53.png" /></figure>
Station ME25: Vp = 6.4, Vp/Vs = 1.79 ± 0.04, H = 29 ± 0.59

<figure><img src="image_54.png" /></figure>
ME25 Stack Bin

<figure><img src="image_55.png" /></figure>

Ray Parameter (s/km)
Station ME26: Vp = 6.4, Vp/Vs = 1.7 ± 0.04, H = 30.4 ± 0.59

<figure><img src="image_56.png" /></figure>
ME26 Stack Bin

<figure><img src="image_57.png" /></figure>
Station ME27: Vp = 6.4, Vp/Vs = 1.82 ± 0.03, H = 28.52 ± 1.3

<figure><img src="image_58.png" /></figure>
ME27 Stack Bin

<figure><img src="image_59.png" /></figure>
Station ME28: Vp = 6.4, Vp/Vs = 1.8 ± 0.02, H = 27.23 ± 0.57

<figure><img src="image_60.png" /></figure>
ME28 Stack Bin

<figure><img src="image_61.png" /></figure>
Station ME29: Vp = 6.4, Vp/Vs = 1.78 ± 0.02, H = 27.4 ± 0.47

<figure><img src="image_62.png" /></figure>
ME29 Stack Bin

<figure><img src="image_63.png" /></figure>
Station ME31: Vp = 6.4, Vp/Vs = 1.9 ± 0.05, H = 29 ± 0.47

<figure><img src="image_64.png" /></figure>
ME31 Stack

<figure><img src="image_65.png" /></figure>
Station ME33: Vp = 6.5, Vp/Vs = 1.7 ± 0.07, H = 27.7 ± 1.94

<figure><img src="image_66.png" /></figure>
Station ME33

<figure><img src="image_67.png" /></figure>
Station ME34: Vp = 6.4, Vp/Vs = 1.84 ± 0.06, H = 27.8 ± 1.05

<figure><img src="image_68.png" /></figure>
ME34 Stack Bin

<figure><img src="image_69.png" /></figure>
Station ME36: Vp = 6.5, Vp/Vs = 1.73 ± 0.04, H = 26.7 ± 1.35

<figure><img src="image_70.png" /></figure>
Station ME36

<figure><img src="image_71.png" /></figure>
Station ME37: Vp=6.50, VpVs=1.74+-0.05, H=27.87+-1.42

<figure><img src="image_72.png" /></figure>
Station ME37

<figure><img src="image_73.png" /></figure>
Station ME38: Vp = 6.4, Vp/Vs = 1.75 ± 0.02, H = 29.72 ± 0.79

<figure><img src="image_74.png" /></figure>
ME38 Stack Bin

<figure><img src="image_75.png" /></figure>
Station ME39: Vp=6.50, VpVs=1.75+-0.05, H=26.85+-1.34

<figure><img src="image_76.png" /></figure>
Station ME39

<figure><img src="image_77.png" /></figure>
Station ME40: Vp=6.50, VpVs=1.83+-0.04, H=32.00+-1.06

<figure><img src="image_78.png" /></figure>
Station ME40

<figure><img src="image_79.png" /></figure>
Station ME41: Vp = 6.4, Vp/Vs = 1.78 ± 0.02, H = 29.67 ± 0.7

<figure><img src="image_80.png" /></figure>
ME41 Stack Bin

<figure><img src="image_81.png" /></figure>
Station ME42: Vp = 6.4, Vp/Vs = 1.76 ± 0.02, H = 29.5 ± 0.97

<figure><img src="image_82.png" /></figure>
ME42 Stack Bin

<figure><img src="image_83.png" /></figure>

Ray Parameter (s/km)
Station ME43: Vp=6.50, VpVs=1.74+-0.05, H=32.56+-1.26

<figure><img src="image_84.png" /></figure>
Station ME43

<figure><img src="image_85.png" /></figure>
Station ME47: Vp = 6.4, Vp/Vs = 1.71 ± 0.04, H = 28.6 ± 1.29

<figure><img src="image_86.png" /></figure>
Station ME47

<figure><img src="image_87.png" /></figure>
Station ME49: Vp = 6.4, Vp/Vs = 1.74 ± 0.05, H = 31.14 ± 1.55

<figure><img src="image_88.png" /></figure>
Station ME49

<figure><img src="image_89.png" /></figure>
Station ME52: Vp = 6.4, Vp/Vs = 1.83 ± 0.04, H = 29.6 ± 1.07

<figure><img src="image_90.png" /></figure>
Station ME52

<figure><img src="image_91.png" /></figure>
Station ME53: Vp=6.50, VpVs=1.97+-0.06, H=28.94+-1.42

<figure><img src="image_92.png" /></figure>
Station ME53

<figure><img src="image_93.png" /></figure>
List of Stations that do not show good H-K method results. Crustal depth is measured by simple stack all RFs in depth domain using velocity input from tomography model (Ramdhan, 2019)
Station ME06

<figure><img src="image_94.png" /></figure>
Station ME07

<figure><img src="image_95.png" /></figure>
Station ME18

<figure><img src="image_96.png" /></figure>
Station ME19

<figure><img src="image_97.png" /></figure>
Station ME30

<figure><img src="image_98.png" /><figcaption>Time (s)</figcaption></figure>

Epicentral Distance (Deg)
Station ME32

<figure><img src="image_99.png" /></figure>
Station ME35

<figure><img src="image_100.png" /></figure>
Station ME45

<figure><img src="image_101.png" /></figure>
Station ME46

<figure><img src="image_102.png" /></figure>