Earthq Sci (2019)32: 12–25 12 doi: 10.29382/eqs-2019-0012-2

Double-difference tomography of P- and S-wave velocity structure beneath the western part of , *

Shindy Rosalia1,* Sri Widiyantoro1 Andri Dian Nugraha1 Pepen Supendi1,2

1 Faculty of Mining and Petroleum Engineering, Institute of Technology , Bandung 40132, Indonesia 2 Meteorological, Climatological, and Geophysical Agency (BMKG), Bandung 40161, Indonesia

Abstract in the western part of the Sunda 1 Introduction Arc has a relatively high seismicity due to subduction activity and faults. In this study, double-difference tomography was The western part of Java is a tectonically active zone used to obtain the 3D velocity tomograms of P and S waves between the oblique subduction of the Indo-Australian beneath the western part of Java. To infer the geometry of the plate beneath and the perpendicular subduction of structure beneath the study area, precise earthquake hypo- the same plate beneath Java (Malod et al., 1995). The center determination was first performed before tomographic oceanic lithosphere is continually being subducted north- imaging. For this, earthquake waveform data were extracted ward beneath the Sunda volcanic arc at a rate of 6–7 cm/ from the regional Meteorological, Climatological, Geophy- year (Hamilton, 1979). The change in the subduction orien- sical Agency (BMKG) network of Indonesia from South tation affects the deformation, e.g., the fault structure and Sumatra to Central Java. The P and S arrival times for about the seismic activity in the western part of Java. This tect- 1,000 events in the period April 2009 to July 2016 were onic activity is also related to the development of an active selected, the key features being events of magnitude > 3, volcanic arc in the southern part of West Java which has azimuthal gap < 210° and number of phases > 8. A nonlinear the highest volcanic density in Java Island (Setijadji, method using the oct-tree sampling algorithm from the 2010). Subduction beneath Java began in the Eocene, NonLinLoc program was employed to determine the ear- but an older Paleogene arc ceased activity in the Early thquake hypocenters. The hypocenter locations were then Miocene and volcanic activity resumed in the Late relocated using double-difference tomography (tomoDD). A Miocene producing a younger arc to the north of the older significant reduction of travel-time (root mean square basis) arc, which continues to the present day (Cottam et al., and a better clustering of earthquakes were achieved which 2010). correlated well with the geological structure in West Java. The structure beneath the Sunda arc, including West Double-difference tomography was found to give a clear Java and its surroundings, has been investigated by e.g. velocity structure, especially beneath the volcanic arc area, Widiyantoro and van der Hilst (1996) using a global data i.e., under Mt Anak Krakatau, Mt Salak and the mountains set, and Widiyantoro et al. (2011) using a non-linear complex in the southern part of West Java. Low velocity approach. Other studies include hypocenter relocation and anomalies for the P and S waves as well as the vP/vS ratio 3D seismic velocity imaging for West Java (Sakti et al., below the volcanoes indicated possible partial melting of the 2012), seismic tomographic inversion beneath Mt Guntur, upper mantle which ascended from the subducted slab West Java (Nugraha et al., 2013), hypocenter beneath the volcanic arc. determination using nonlinear methods in West Java (Rosalia et al., 2017), attenuation tomography beneath the Keywords: West Java; P- and S-wave velocity structures; Sunda Strait between Sumatra and West Java (Anshori et double-difference tomography al., 2017), hypocenter relocation in Central Java (Ramdhan et al., 2017b) and seismic travel-time tomography beneath

Merapi Volcano and it surroundings (Ramdhan et al., * Received 23 February 2018; accepted in revised form 18 2017a). The use of a global data set cannot image shallow September 2018; published 2 September 2019. structures in detail because of the limited data coverage * Corresponding author. e-mail: [email protected] © The Seismological Society of China and Institute of Geophysics, and the grid size employed, which is rather coarse for China Earthquake Administration 2019 relatively small structures. Meanwhile, a previous study

Earthq Sci (2019)32: 12–25 13 used local data but did not use the S phase arrival time data in total 1,305 events recorded within the above period used (Sakti et al., 2012). in the hypocenter determination. In this study, we emp- In this study, the tomographic method was used to loyed a nonlinear hypocenter determination method using obtain the 3D seismic velocity structures in order to image the NonLinLoc program (Lomax et al., 2009). The events the subsurface and to gain a better understanding of the for the double difference tomography (tomoDD) were tectonic processes and the development of the volcanic arc selected based on the following criteria: (i) azimuthal gap in the study area. For this, a different set of data and a < 210°, (ii) magnitude > 3.0, (iii) number of phases > 8, different tomographic method were compared to previous and (iv) shifting between BMKG and NonLinLoc locations studies. In addition, the S phase arrival times were used to < 50 km. Based on the selection criteria, there were 1,097 improve the characterization of the velocity structures of earthquake events with 16,245 P and 8,207 S phases. We the various tectonic regimes in the study area. also used a Wadati diagram to check the quality of selection and to obtain a vP/vS ratio model to be used as an initial model upon inversion (Figure 3). 2 Data 3 Method The P- and S-wave arrival times of waveform data recorded at 33 stations of Meteorological, Climatological, In this study, a tomoDD algorithm developed by Zhang Geophysical Agency network of Indonesia (BMKG) from and Thurber (2003) and based on the double-difference April 2009 to July 2016 and additional P- and S- wave algorithm of Waldhauser and Ellsworth (2000) was used. arrival time data from an independent study (Supendi and The tomoDD method uses the combination of cross- Nugraha, 2016) were selected. Figure 1 shows the correlation data, differential data and absolute data to distribution of the stations ranged from South Sumatra to calculate simultaneously the hypocenter relocation and Central Java with the number of dominant phases being velocity model. In this study, tomoDD was applied using recorded in the southern part of West Java. An example of the absolute catalog and the differential catalog data only, the arrival time selection is shown in Figure 2. There were without using a cross-correlation waveform. The differ- ential data were obtained from the clustering process by

95° 100° 105° 110° 115° 120° 125° 130° 135° 140° input of the absolute catalog data of P- and S-wave arrival

5° times into the ph2dt program, which is part of the hypoDD algorithm (Waldhauser and Ellsworth, 2000). These 0° differential catalog data were used to ensure the stability −5° of the least-squares solutions and to optimize the −10° relationship between two earthquake events. The adjacent seismic events were collected in one group. The number −4° MDSI of seismic events that are clustered depends on the KLSI KLI MNAI LWLI grouping parameters used. The best quality is achieved by −5° BLSI Java Sea

EGSI making as many group pairs of events as possible (Boyle KASI SBJI −6° TNGI UWJI CGJI DBJI CBJI et al., 2007). LEM JCJI TGJI CTJI SMRI −7° KPJI BJI NGJI The initial velocity model used in this study was a 1D SKJI CNJI CMJI YOGI WOJI P-wave velocity model generated by Sakti et al. (2012) Indo-Australia subductionCISI zoneCLJI SCJI −8° UGM PCJI using the VELEST program (Kissling et al., 1995) to −9° update the AK135 velocity model of Kennett et al. (1995); Indian Ocean see Figure 4. The S-wave initial velocity model was −10° XMIS constructed using a vP/vS value of 1.76 obtained from the −11° Wadati diagram shown in Figure 3. For horizontal model 102° 103° 104° 105° 106° 107° 108° 109° 110° 111° parameterization, a grid node size of 30 km × 30 km was 0 200 400 600 800 1000 1200 1400 1600 used for the area with dense seismic rays and for the No. of phases Trench Volcano station network, and 60 km × 60 km elsewhere. For ver- Fault Station tical parameterization, a grid node size of 10 km was used Figure 1 Location of the study area and distribution of and increasing with depth as shown in Figure 5. BMKG stations (reverse triangles). The stations are color- In the tomoDD inversion process, there are several coded to show the number of phases recorded by each station important parameters that need to be determined: the wei-

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102° 103° 104° 105° 106° 107° 108° 109° 110° 111° −4°

−5°

−6° TGJI −7° WOJI Indo-Australia subduction zone Tasikmalaya CLJI −8° UGN Earthquake, Mw7 −9°

−10°

−11°

Tasikmalaya earthquake 02 September 2009 2009 SEP 02 (245) 07h54 m 42.898 s 2E6 IA: CLJI;: BHE. : BHZ Z Pd S 0 2E6 IA: CLJI;: BHN. : BHN 0 ts−tp 24.6 s CLJI 0 2E6 IA: CLJI;: BHE. : BHE 90 0

IA: TGJI;: BHZ. : BHZ Z 5E6 Pd S 0

5E6 IA: TGJI;: BHN. : BHN 0 ts−tp 27.2 s TGJI 0 5E6 IA: TGJI;: BHE. : BHE 90 0

1E6 IA: UGM;: BHZ. : BHZ Z Pd0 S 1E6 UGM 1E6 IA: UGM;: BHN. : BHN 0 ts−tp 36.08 s 1E6 1E6 IA: UGM;: BHE. : BHE 90 1E6

5E5 IA: WOJI;: BHZ. : BHZ Z Pd S 5E5 5E5 IA: WOJI;: BHN. : BHN 0 ts−tp 45.65 s WOJI 5E5 5E5 IA: WOJI;: BHE. : BHE 90 5E5 07 h 55 min 07 h 56 min 07 h 57 min 07 h 58 min Time Figure 2 Example of P- and S-wave arrival time selection for 3-component waveforms of the Tasikmalaya earthquake (2009-09-02) with MW 7 recorded by the BMKG network. The location of the earthquake and the station are shown in the top figure, blue and red lines depict P and S arrival times, respectively

Wadati diagram Velocity (km/s) 200 0 2 4 6 8 10 0 180

160 50 140

120 100

100 y=1.7606x−0.3183 150 80

60 S-wave travel time (s)

Depth (km) 200 40

20 250 0 0 10 20 30 40 50 60 70 80 90 100 110 300 P-wave travel time (s) Figure 3 Wadati diagram for all P and S phases extracted 350 vP (AK135) vS (AK135) from the BMKG data that gave a vP/vS value used as an initial v (Sakti et al., 2012) v (Sakti et al., 2012) velocity model in double-difference tomographic imaging P S Figure 4 One-dimensional reference model of P- and S-wave ghts for each phase, the weighting scheme for the absolute velocities used in this study (taken from Sakti et al., 2012). and differential catalog data, and damping. The phase Navy blue and red represent vP and vS, respectively, (from weights used in this study were 1 for the P phase structures Sakti et al., 2012) and light blue and orange represent the and 0.75 for the S phase structures. The weight of the P AK135 vP and vS models, respectively (from Kennett et al., phase was greater than that of the S phase, because the 1995)

Earthq Sci (2019)32: 12–25 15

100° 102° 104° 106° 108° 110° 112° −2° −2° −3° −3° −4° −4°

−5° −5°

−6° −6°

−7° −7° −8° −8° −9° −9° −10° −10° −11°

−12° −11° (a) −13° −12° (b) −13°

0 100 200 300 400 500 600 0 Depth (km) 50 100 150 200 250 300 350 Depth (km) 400 450 500 550 600 (c)

100° 102° 104° 106° 108° 110° 112° Figure 5 Grid nodes parameterization used in the tomographic processing (a). Horizontal distance between nodes is 30 km for the area within the dense station network and the dense seismic ray paths and 60 km elsewhere (b). Vertical distance between nodes increases with depth starting with a 10 km interval for the uppermost layer (c). Blue lines depict ray paths from earthquakes (red circles) to stations determination of the S arrival time is more difficult and were 1 and 10, respectively. Assessment of the optimal has a higher uncertainty than that for the P arrival times. damping parameter was made based on the condition For the weightings of the catalog and differential data, number (CND) and the trade-off curve. The CND value Zhang and Thurber (2003) proposed to give a greater wei- was the ratio of the largest to the smallest eigenvalues. ght to the absolute data at the beginning of the inversion to Following the study of Waldhauser and Ellsworth (2000), obtain the velocity structure globally and at the end of the the CND should be between 40 to 80. Trade-off curves inversion a greater weight for differential data should be were also constructed between the data and model vari- employed to obtain the velocity structure in more detail. In ance, and the model variance was computed for various this study, the following weighting schemes were used: for damping values. A damping value of 125 was chosen for the 1st scheme, at the beginning of the inversion the all schemes given that the CND values were between 40 weights of the absolute catalog and differential data were and 80 and this damping corresponds to a change in the 10 and 1, respectively; for the 2nd scheme, an equal gradient of the trade-off curves (Figure 6). weighting of the absolute and differential data was used, The resolution of the inversion result was tested using i.e., 1 for the absolute and differential data; and for the 3rd the checkerboard resolution test along with the derivative iteration, the weights of the absolute and differential data weight sum (DWS). The result of the checkerboard reso-

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Damping analysis for 1st scheme Damping analysis for 2nd scheme 2.4 2.05 500, CND 12 (a) (b) 2.3 500, CND 11 2.00 ) ) 250, CND 25 2 250, CND 27 2 s 2.2 s −5 −5 1.95 200, CND 36 2.1 200, CND 37 150, CND 53 150, CND 52 1.90 125, CND 66 2.0 125, CND 67 Data variance (10 Data variance (10 100, CND 92 1.86 100, CND 90 1.9 50, CND 233 50, CND 235 50, CND 1202 5, CND 1204 1.8 1.80 1.01.5 2.0 2.5 3.0 3.5 4.0 4.5 4.5 5.0 5.5 6.0 6.5 7.0 7.5 Model variance (km/s)2 ×10−4 Model variance (km/s)2 ×10−4

Damping analysis for 3rd scheme 5.4 (c) 500, CND 12 5.3 250, CND 27 200, CND 36 150, CND 53 ) 2 5.2 125, CND 67 s −5 5.1 100, CND 90 5.0

Data variance (10 4.9

4.8 50, CND 255 5, CND 1207 4.7 5 6 7 8 Model variance (km/s)2 ×10−4 Figure 6 Damping analysis to decide the damping value used in the inversions for the 1st (a), 2nd (b) and 3rd (c) weighting schemes. The curves show the trade-offs between the various damping factors and the CNDs values. A damping factor of 125 was used for each weighting scheme lution test is shown in Figure 7, where the horizontal sec- wave arrival times was less than that for the P-wave arrival tions show good resolution starting at a depth of 10 km to times. 80 km. In particular, good resolution was achieved in the area along Java Island, where the majority of seismic rays 4 Results and discussion pass through this area. Resolution diminished toward the trench due to lack of ray path coverage as most stations The results for hypocenter determination using the were located on Java Island. The seismic rays passed mai- NonLinLoc program show smaller travel-time residuals nly through the front arc region so that the slab geometry compared to those based on the Geiger method, which is was not well resolved. usually used for routine hypocenter determination in The DWS values, which were plotted on a logarith- Indonesia (Figure 9). The nonlinear global search methods mic scale, are presented in Figure 8. High DWS values for performed for hypocenter determination allow us to the P- and S-wave data are indicated by the dark colors. calculate complete probabilistic locations, so that we can The DWS results look reasonably good for depths from avoid hypocenter solutions trapped in local minima. The 10 km to 60 km. The inferences of the DWS results present hypocenter determination results were used as an correspond well to the results of the checkerboard test, in input in the tomoDD process and this produced even which good recovery is indicated by log (DWS) > 3.25 for smaller travel-time residuals compared to that for the the P-wave structures and > 3.00 for the S-wave structures. NonLinLoc program (Figure 9).

For the vP/vS tomogram the resolution test results based on In this section, the tomoDD results are presented in the S-wave data were used, given that the number of S- terms of velocity perturbations relative to the initial

Earthq Sci (2019)32: 12–25 17

Initial model vP checkerboard test vS checkerboard test Depth 0 km Depth 0 km Depth 0 km −5° −5° −5° −6° −6° −6° −7° −7° −7° −8° −8° −8° −9° −9° −9° −10° −10° −10° −11° −11° −11° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° Depth 10 km Depth 10 km Depth 10 km −5° −5° −5° −6° −6° −6° −7° −7° −7° −8° −8° −8° −9° −9° −9° −10° −10° −10° −11° −11° −11° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° Depth 20 km Depth 20 km Depth 20 km −5° −5° −5° −6° −6° −6° −7° −7° −7° −8° −8° −8° −9° −9° −9° −10° −10° −10° −11° −11° −11° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° Depth 30 km Depth 30 km Depth 30 km −5° −5° −5° −6° −6° −6° −7° −7° −7° −8° −8° −8° −9° −9° −9° −10° −10° −10° −11° −11° −11° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° Depth 40 km Depth 40 km Depth 40 km −5° −5° −5° −6° −6° −6° −7° −7° −7° −8° −8° −8° −9° −9° −9° −10° −10° −10° −11° −11° −11° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° Depth 50 km Depth 50 km Depth 50 km −5° −5° −5° −6° −6° −6° −7° −7° −7° −8° −8° −8° −9° −9° −9° −10° −10° −10° −11° −11° −11° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110°

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Initial model vP checkerboard test vS checkerboard test Depth 60 km Depth 60 km Depth 60 km −5° −5° −5° −6° −6° −6° −7° −7° −7° −8° −8° −8° −9° −9° −9° −10° −10° −10° −11° −11° −11° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° Depth 80 km Depth 80 km Depth 80 km −5° −5° −5° −6° −6° −6° −7° −7° −7° −8° −8° −8° −9° −9° −9° −10° −10° −10° −11° −11° −11° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110°

−15 0 15 −15 0 15 −15 0 15 Perturbation (%) v perturbation (%) v perturbation (%) P S Figure 7 The results of checkerboard tests for P and S waves at depths down to 80 km beneath the study area. Blue and red depict positive and negative velocity perturbations, respectively

DWS of vP DWS of vS Depth 0 km Depth 0 km −5° −5°

−6° −6°

−7° −7°

−8° −8°

−9° −9°

−10° −10°

−11° −11° 102° 103° 104° 105° 106° 107° 108° 109° 110° 102° 103° 104° 105° 106° 107° 108° 109° 110° Depth 10 km Depth 10 km −5° −5°

−6° −6°

−7° −7°

−8° −8°

−9° −9°

−10° −10°

−11° −11° 102° 103° 104° 105° 106° 107° 108° 109° 110° 102° 103° 104° 105° 106° 107° 108° 109° 110° Depth 20 km Depth 20 km −5° −5°

−6° −6°

−7° −7°

−8° −8°

−9° −9°

−10° −10°

−11° −11° 102° 103° 104° 105° 106° 107° 108° 109° 110° 102° 103° 104° 105° 106° 107° 108° 109° 110°

Earthq Sci (2019)32: 12–25 19

DWS of vP DWS of vS Depth 30 km Depth 30 km −5° −5°

−6° −6°

−7° −7°

−8° −8°

−9° −9°

−10° −10°

−11° −11° 102° 103° 104° 105° 106° 107° 108° 109° 110° 102° 103° 104° 105° 106° 107° 108° 109° 110° Depth 40 km Depth 40 km −5° −5°

−6° −6°

−7° −7°

−8° −8°

−9° −9°

−10° −10°

−11° −11° 102° 103° 104° 105° 106° 107° 108° 109° 110° 102° 103° 104° 105° 106° 107° 108° 109° 110° Depth 50 km Depth 50 km −5° −5°

−6° −6°

−7° −7°

−8° −8°

−9° −9°

−10° −10°

−11° −11° 102° 103° 104° 105° 106° 107° 108° 109° 110° 102° 103° 104° 105° 106° 107° 108° 109° 110° Depth 60 km Depth 60 km −5° −5°

−6° −6°

−7° −7°

−8° −8°

−9° −9°

−10° −10°

−11° −11° 102° 103° 104° 105° 106° 107° 108° 109° 110° 102° 103° 104° 105° 106° 107° 108° 109° 110° Depth 80 km Depth 80 km −5° −5°

−6° −6°

−7° −7°

−8° −8°

−9° −9°

−10° −10°

−11° −11° 102° 103° 104° 105° 106° 107° 108° 109° 110° 102° 103° 104° 105° 106° 107° 108° 109° 110°

0 1 2 3 4 5 Log DWS Figure 8 The derivative weight sum (DWS), i.e., a weighted measure of the total length of the seismic ray through the nodes, of the real data inversion of P and S waves at depths down to 80 km

20 Earthq Sci (2019)32: 12–25 velocity model for the P and S waves, while the vP/vS start from the surface to a 40 km depth in the southern ratios are plotted as absolute values (Figure 10). The blue mountains of West Java (Mts Guntur, Papandayan, Galu- color scale shows high velocity anomalies for the P and S nggung, Malabar, Wayang Windu and Patuha) and from a waves, while the red color scale shows low anomalies. For 20–40 km depth below the Mt Salak complex. Meanwhile, the vP/vS scale, the red color indicates a high vP/vS value under Mt Anak Krakatau, the negative anomaly was while the blue color indicates a low vP/vS value. Horiz- observed at a depth of 10 km and this is continuous to a ontal sections are displayed for a depth of 0 to 80 km, depth of 20 km. The anomaly at a depth of 10 km is in while three vertical sections are displayed across West good agreement with a local earthquake tomography study Java through A-A', B-B' and C-C' (Figure 11). The A-A’ of Jaxybulatov et al. (2011) given the additional data cross-section passes through Mt Anak Krakatau, B-B' coverage used in this study. In addition, there is also a through the Mt Salak complex and C-C’ through the moun- negative anomaly found in the Sumatran fault. For the tains complex in the southern part of West Java. positive anomaly which occurred in the southern part, this Interpretation of the tomograms is only conducted for may reflect the presence of rigid and dense rocks such as areas with a good checkerboard recovery or DWS. The P- tectonic slabs or basements. and S-wave tomograms in general agree well. Based on Generally, in the vertical sections of vP, vS and vP/vS previous results (Koulakov et al., 2007; Nakajima et al., (Figure 11) the subducted slab of the Indo-Australian plate 2001a; Tsuji et al., 2008), a negative anomaly may indicate is poorly imaged, although in some regions such as in the weak zones of a fault or thermal structures derived from B-B’ vertical section of vS, the slab is imaged as a positive partial melting of tectonic plates. In the resulting tomo- anomaly in the depth interval of 80–120 km. Also, from grams (Figure 10), a negative anomaly pattern is seen in the vertical sections of the vP and vS tomograms across the middle part of the area which corresponds to the volc- three regions in western Java (Figure 11), there are, anic arc with active volcanoes. These negative anomalies overall, negative anomalies for P and S wave tomograms

Travel-time residuals of Geiger’s Method Travel-time residuals of NLLoc 12 12 (a) (b) 10 10 ) ) 3 3 8 8

6 6

4 4 Number of phase (10 Number of phase (10 2 2

0 0 −10 −8 −6 −4 −2 0 2 4 6 8 10 −10 −8 −6 −4 −2 0 2 4 6 8 10 Travel-time residual (s) Travel-time residual (s)

Travel-time residuals of TomoDD 12 (c) 10 ) 3 8

6

4 Number of phase (10 2

0 −10 −8 −6 −4 −2 0 2 4 6 8 10 Travel-time residual (s) Figure 9 Histograms of the travel-time residuals for the Geiger method (a), NLLoc (b) and tomoDD (c). The NLLoc results give significantly better travel-time residuals than the Geiger method; and the tomoDD results provide better travel-time residuals than that of the NLLoc method

Earthq Sci (2019)32: 12–25 21

v v v /v Depth 0 km P Depth 0 km S Depth 0 km P S −5° −5° −5° −6° −6° −6° −7° −7° −7° −8° −8° −8° −9° −9° −9° −10° −10° −10° −11° −11° −11° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° Depth 10 km Depth 10 km Depth 10 km −5° −5° −5° −6° −6° −6° −7° −7° −7° −8° −8° −8° −9° −9° −9° −10° −10° −10° −11° −11° −11° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° Depth 20 km Depth 20 km Depth 20 km −5° −5° −5° −6° −6° −6° −7° −7° −7° −8° −8° −8° −9° −9° −9° −10° −10° −10° −11° −11° −11° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° Depth 30 km Depth 30 km Depth 30 km −5° −5° −5° −6° −6° −6° −7° −7° −7° −8° −8° −8° −9° −9° −9° −10° −10° −10° −11° −11° −11° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° Depth 40 km Depth 40 km Depth 40 km −5° −5° −5° −6° −6° −6° −7° −7° −7° −8° −8° −8° −9° −9° −9° −10° −10° −10° −11° −11° −11° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° Depth 50 km Depth 50 km Depth 50 km −5° −5° −5° −6° −6° −6° −7° −7° −7° −8° −8° −8° −9° −9° −9° −10° −10° −10° −11° −11° −11° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110°

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vP vS vP/vS Depth 60 km Depth 60 km Depth 60 km −5° −5° −5° −6° −6° −6° −7° −7° −7° −8° −8° −8° −9° −9° −9° −10° −10° −10° −11° −11° −11° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° Depth 80 km Depth 80 km Depth 80 km −5° −5° −5° −6° −6° −6° −7° −7° −7° −8° −8° −8° −9° −9° −9° −10° −10° −10° −11° −11° −11° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110° 102°103°104°105°106°107°108°109°110°

−15 −10 −5 0 5 10 15 −15 −10 −5 0 5 10 15 1.4 1.5 1.6 1.7 1.8 1.9 2.0 v Perturbation (%) v Perturbation (%) v /v (absolute) P S P S

Figure 10 Horizontal slices for vP, vS and vP/vS resulting from the double-difference tomographic inversions at depths from 0 to 80 km below active volcanoes. While for the vertical section of respectively. vP/vS it can be observed that there is a high vP/vS ratio of With regard to the seismicity resulting from the event 1.8 to 1.98 below the volcanoes. Based on previous studies relocation for profile A in Figure 11, this shows the occurr-

(Nakajima et al. (2001b); Koulakov et al., 2007), low vP ence of a few deep earthquakes compared to profiles B and and vS values with high vP/vS values of about 1.8–1.9 C. The area in profile A is well known as a seismic gap indicate the presence of material with high fluid content or zone. This area is the transition zone from the oblique thermal structures in the form of a partial melt below a subduction beneath Sumatra to the perpendicular subd- volcano. The thermal structural feature is considered to uction beneath Java. In Figure 12, it can be seen that the originate from a tectonic slab at a depth of 80–120 km slab beneath West Java has a steep subduction angle with which then rose and finally resulted in an active volcano an initial subduction angle of 10°–30° close to the surface, series, i.e., Mt. Anak Krakatau in the Sunda Strait, Mt. which becomes steeper, ~70°, at a depth of 100 km. This Salak in the southern part of West Java and the Southern finding is in good agreement with previous studies Mountain Complex of West Java. There is a high velocity (Cottam et al., 2010; Widiyantoro et al., 2011). An anomaly at shallower depths around Mt. Anak Krakatau interpretation of the tomograms in terms of the velocity (Figure 11, profile A) which may be interpreted as a structure beneath West Java is outlined in Figure 12. granitic basement (Jaxybulatov et al., 2011). In Figure 11, profile B, we observed low vP, low vS and low vP/vS just 5 Concluding remarks beneath Mt. Salak. Based on the results of Nakajima et al. (2001b) and Zhang et al. (2004), these anomalies may The 3D vP, vS and vP/vS structures beneath West Java indicate the presence of inclusions filled with water and have been imaged successfully using tomoDD. Based on with a large aspect ratio or may indicate an area with high the results, the tomographic method is sufficiently water content. We could not explain the anomalies beneath powerful to image the subsurface structure beneath, e.g., a the northern volcanoes such as Mts. Tangkuban, Perahu volcanic arc. The velocity structure below the volcanic arc and Burangrang because the locations of these volcanoes of West Java is, in general, characterized by low vP were in regions that corresponded to the resolution limit of anomalies, low vS anomalies and high vP/vS ratios. These the tomography as indicated by lack of checkerboard anomalies are associated with areas of high fluid content recovery (Figure 7) and low DWS value (Figure 8), due to partial melting of the upper mantle.

Earthq Sci (2019)32: 12–25 23

−5° −6° −7° −8° −9° A −10° B C −11° P-wave tomongraphy depth 10 km 102°103°104°105°106°107°108°109°110°

vP Anak KrakatauvS Anak KrakatauvP/vS Anak Krakatau (a) 0 0 0 M3 40 partial M4 melting 40 40 80 80 80 M5 120 120 120 M6 M7 160 160 160 200 200 200 Dopth (km) Dopth (km) Dopth (km) 240 240 240 280 280 280 0 100 200 300 400 500 0 100 200 300 400 500 0 100 200 300 400 500 Distance (km) Distance (km) Distance (km)

(b) vP Mt.SalakvS Mt.SalakvP/vS Mt.Salak 0 0 0 M3 40 Thermal 40 40 M4 80 fluid 80 80 M5 120 120 120 M6 160 160 160 M7 200 200 200 Dopth (km) Dopth (km) Dopth (km) 240 240 240 280 280 280 0 100 200 300 400 500 0 100 200 300 400 500 0 100 200 300 400 500 Distance (km) Distance (km) Distance (km)

vP South Mt. complexvS South Mt. complexvP/vS South Mt. complex (c) 0 partial 0 0 M3 40 melting 40 40 M4 80 80 80 M5 120 120 120 M6 160 160 160 M7 200 200 200 Dopth (km) Dopth (km) Dopth (km) 240 240 240 280 280 280 0 100 200 300 400 500 0 100 200 300 400 500 0 100 200 300 400 500 Distance (km) Distance (km) Distance (km)

−15−10−5 0 5 10 15 −15−10−5 0 5 10 15 1.4 1.5 1.61.7 1.8 1.92.0 v v v /v P Perturbation (%) S Perturbation (%) P S (absolute) Figure 11 Top figure shows the location of the vertical slice profile and bottom figure shows its corresponding vertical slice of vP, vs, and vP/vs through A (a), B(b), and C(c)

South mountain C Trench compiex C′ 0 Backthrust 20 Average Moho Discontinuity M3 (Wölbern and Rümpker, 2016) 40 M4 60 Partial M5 80 melting 100 M6 120 Indo-Australia slab M7 140 vP/vS depth 30 km 160 −5° 180 −6° C′ Depth (km) 200 −7° 220 −8° 240 260 −9° −10° 280 C 300 −11° 0 50 100 150 200 250 300 350 400 450 500 550 102° 104° 106° 108° 110° Distance (km) 1.4 1.5 1.6 1.7 1.8 1.9 2.0 v /v (absolute) P S Figure 12 Interpretation of structures beneath West Java. Blue line represents slab model 1.0 (Hayes et al., 2012), white dashed line shows the good resolution area based on S-wave checkerboard results and DWS values, and black dashed line depicts our interpretation of the subducted slab based on earthquake hypocenters resulting from tomoDD (black dots). The dashed yellow line depicts the average depth of the Moho discontinuity, i.e., 34 km, taken from Wölbern and Rümpker, (2016). Note that the angle of subduction is steeper than that from the slab model 1.0

24 Earthq Sci (2019)32: 12–25

Acknowledgments review Koulakov I, Bohm M, Asch G, Lühr B-G, Manzanares A, Brotopuspito KS, Fauzi P, Purbawinata Ma, Puspito NT, We are grateful to the Meteorology, Climatology and Ratdomopurbo A, Kopp H, Rabbel W and Shevkunova E (2 007) Geophysics Agency of Indonesia (BMKG) for the wave- P and S Velocity structure of the crust and the upper mantle form and catalog data used in this study and the Direct- beneath central Java from local tomography inversion. J Geophys orate General of Resources for Science Technology and Res 112(B8): 1–19 the Higher Education of the Republic of Indonesia for Lomax A, Michelini A and Curtis A (2 009) Earthquake location, granting a PMDSU scholarship to SR. We also thank the direct, global-search methods. In: Meyers R (eds) Encyclopedia reviewers for their constructive comments. Figures 1, 5, 6, of Complexity and Systems Science. Springer, New York 7, 9, 10 and 11 were produced using GMT Software Malod JA, Karta K, Beslier OM and Zen Jr MT (1 995) From normal to oblique subduction: Tectonic relationships between Java and (Wessel and Smith, 1998). Topography and bathymetry Sumatra J Southeast Asian Earth Sci 12: 85–93 data were taken from SRTM 30 Plus (Becker et al., 2009). Nakajima J, Matsuzawa T, Hasegawa A and Zhao D (2 001a) Three- The plate boundary diagram was adapted from Bird dimensional structure of vP, vS and vP/vS beneath northeastern (2003). Japan: Implications for arc magmatism and fluids. J Geophys Res

Solid Earth 106 https://doi.org/10.1029/2 000JB000 008

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