Double-Difference Tomography of P- and S-Wave Velocity Structure Beneath the Western Part of Java, Indonesia*
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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 Java, Indonesia* Shindy Rosalia1,* Sri Widiyantoro1 Andri Dian Nugraha1 Pepen Supendi1,2 1 Faculty of Mining and Petroleum Engineering, Institute of Technology Bandung, Bandung 40132, Indonesia 2 Meteorological, Climatological, and Geophysical Agency (BMKG), Bandung 40161, Indonesia Abstract West Java 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 Sumatra 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- 14 Earthq Sci (2019)32: 12–25 102° 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.