Checkerboard test
Several “checkerboard tests” have been conducted to ascertain the resolution of the Rayleigh wave group velocity maps produced in the tomographic inversion for each period. These tests have been conducted using the configuration of ray paths from the source to the receiver that resulted from the seismometer deployments described above.
Figure 4 shows the results of checkerboard tests at different periods using a grid size of 0.25o × 0.25o, approximately 27.8 km × 27.8 km with a maximum velocity perturbation of 1 km/s. We selected values for the damping and smoothing parameters from a range of 500–5000 to obtain the optimum values and found the optimum values of 1800 and 1800, respectively. The results indicate that the southeastern portion of the research area is not as well resolved as the other areas, so we expect that the results of the tomographic inversion may not recover the details of Rayleigh wave group velocities in this area well.
Group velocity
The ray path distributions associated with the group velocity models resulting from our tomographic inversions for different periods are shown in Fig. 5. There are various numbers of ray paths at different periods, due to the decreasing number of available station pairs for long periods and the decreasing signal-to-noise ratio at short periods. Figure 4 shows that the ray paths exhibit pronounced bending due to the strong variations in group velocity. The ANT inversion results (Fig. 6) illustrate the variation of Rayleigh wave group velocity beneath the surface of the research area in the period range from 0.8 to 12 s. They show a strong contrast between group velocities of the Southern Mountains zone and those of the Kendeng and Rembang zones. The Southern Mountain zone, which is associated with intrusive Oligo–Miocene age volcanic rocks and middle Miocene carbonates, is dominated by relatively high group velocity (1.8–3.2 km/s). The Kendeng zone, which is composed of sedimentary basin fill, is dominated by very low group velocity (0.2–1 km/s). The Rembang zone, which is associated with modern volcanic and alluvial deposits and carbonate rocks, is dominated by medium group velocity (1–1.8 km/s), but its most northern part is associated with middle Miocene carbonates having higher Vs (1.8–2.5 km/s). Bali Island, which is associated with modern volcanics, was dominated by high group velocity, but the southwestern part of Bali, which is associated with modern volcanic and alluvial deposits, is dominated by low group velocity.
Inversion of group velocity curves
In this work, we used the Neighborhood Algorithm (NA) developed by Sambridge (1999a, b), to invert the ANT group velocity dispersion curves for Vs–depth profiles at each point in a regular grid covering the study area. Using a grid spacing of 0.125o, we took 469 sample points of group velocity results in the period range from 0.5 to 12.2 s in the research area. The dispersion curve at each of these points was inverted to obtain the profile of Vs as a function of depth.
Figure 7 shows two examples of dispersion curve inversions using the NA. Points 33, 139, and 226 are located at the Rembang, Kendeng, and Southern Mountains zones, respectively. The red curves indicate the 1D Vs–depth profiles corresponding to the minimum misfit among the models sampled by NA at each point. As expected, point 226 in the Southern Mountains zone has generally higher shear wave velocity than points 33 and 139 in the Rembang and Kendeng Zone, respectively.
Figure 8 shows the NA inversion results for shear wave velocity throughout the study area, which included the Southern Mountains, Kendeng, and Rembang zones (Fig. 1). The Southern Mountains Zone is dominated by high shear wave velocity. Velocities of 2.0–3.1 km/s are evident at depths as shallow as 0.8 km, and this indicates that the bedrock is fairly shallow in this zone. The tomography results show that the boundary zone between the Southern Mountains zone and Kendeng zone is clearly defined. There is a continuous low-velocity anomaly to the north that merges with the Kendeng zone in the depth range of 0.8–4.4 km in the southern part of the study area. The highest Vs are to the south of the arc of active volcanoes. In general, the deeper the higher velocity extends, the clearer it can be seen in the Vs maps.
The tomographic image of the Kendeng zone is dominated by a strong low-velocity anomaly. This result indicates that this area is an area with thick sedimentary basin fill. The lowest velocity can be seen in Ngawi, Lamongan, Madura Sea, and Bojonegoro, where velocities as low as 1 km/s extend to depths as great as 6.0 km. The location of the mud volcano disaster in Sidoarjo was associated with Vs <1.0 km/s ranging from 0.8 to 4.4 km depth.
The shear wave velocity structure of the Rembang Zone is characterized by moderate Vs on average, but it includes relatively high Vs in the southern part of this zone and middle part of Madura Island. These regions are areas of limestone excavation at the surface so that its appearance is that of high cliffs. In the Bojonegoro and Cepu regions, which are well known for exploitation of oil and gas, the results are dominated by low-Vs anomalies.
Cross-sections
Figure 9 shows the variation of Vs in W–E (West–East) cross-sectional images across the study area. Cross-section 1 transects the Rembang zone which has varying sediment thickness. The imaging results of the Vs structure beneath cross-section 1 shows low (Vs < 2.0 km/s) velocity throughout the upper crust, but particularly in its central portion, where Vs below 1.0 km/s extends to depths >10 km.
Cross-section 2 that cuts W–E through land to the Madura Sea is in the Kendeng zone. The Vs structure under cross-section 2 also exhibits low (Vs < 2.0 km/s) velocity throughout the upper crust. There is a pronounced low-velocity zone beneath the Sidoarjo mud volcano at 112.50o–112.75o longitude that extends from the surface to about 4 km depth which may be related to the source of the massive mud eruption that has been ongoing there since 2006.
Cross-section 3 is another W–E section in the Southern Mountains zone. This cross-section shows that this entire zone is underlain by relatively normal crustal velocities of 1.8–3.1 km/s. Cross-section 3 crosses several volcanoes, including Semeru, Argopuro, Raung, and Ijen mountains. On this cross-section, the existence of volcanoes seems correlated with relatively low velocity. This could be caused the age of these volcanic rocks is younger than that of the southern part of volcanoes.
The cross-sections in Fig. 10 are in the S–N (South–North) direction and cuts through the three different zones. These cross-sections show the boundary of the three zones—Rembang, Kendeng, and Southern Mountains—very clearly. Cross-sections 4–6 are all dominated by higher shear wave velocities (2–3.0 km/s) at the Southern, lower shear wave velocity (0.4–1.5 km/s) in the middle and intermediate (1.5–3 km/s) in the northern part of the cross-sections. Cross-sections 4 and 5 show that the Kendeng Basin depth is in the range of 10–11 km.
Comparison of results with geology and oil and gas fields
The comparison between imaging results of this research and of previous research (Fig. 11) shows correlations of the Vs structure with surface geology and oil and gas fields. In the Southern Mountains zone, which was dominated by high Vs, high Bouguer anomalies correlate with the presence of igneous and carbonate rocks. The very low average Vs in the upper crust of the Kendeng zone correlates well with low Bouguer anomalies and the presence of basins and thick sedimentation. The Madura Sea, which is also dominated by very low Vs, also correlates with very low Bouguer anomalies. The middle part of Madura Island, which is dominated by medium shear wave velocity, corresponds to a medium Bouguer anomaly. The low Vs in the Rembang and Kendeng zones also correlate well with the existence of oil and gas fields. Relatively high Vs beneath the southern part of Bali Island, on the other hand, correlate with a high Bouguer anomaly.