Abstract
The Central and East Java region, which is part of the Sunda Arc, has an important role in producing destructive earthquakes and volcanic complexes in Indonesia as a result of the convergence between the Indo-Australian plate that subducts under the Eurasian plate. In this study, the 3-D seismic velocity structure (Vp, Vs, and Vp/Vs) of the crust and upper mantle was determined to reveal the presence of the subducted slab, volcanic sources, and seismogenic features in the Central and East Java region. We have manually re-picked P- and S-arrivals of 1,488 events from January 2009 to September 2017 recorded at 27 stations of the BMKG network. An iterative damped least-squares inversion method was applied to simultaneously calculate both hypocenter relocations and velocity structure beneath this high-risk region to a depth of 200 km. We then compare the tomographic results and seismicity to interpret structural features in the seismic zones. The subducted slab is dipping toward the north, imaged by the high-velocity regions with low Vp/Vs at depths of about 50–100 km. Low-velocity anomalies with high Vp/Vs above the slab at a depth of ∼100 km, imply the possible location of partial melting from slab dehydration. Fluids and melts are ascending to feed the volcanoes i. e., Merapi-Merbabu, Wilis, Pandan, Semeru, Bromo, and Ijen that also have similar low-velocity anomalies at 10–30 km depths, suggesting the presence of the sedimentary basin or magma reservoir. We also have redetermined the hypocenter location of the 2021 (Mw 6.1) Malang earthquake at 8.94oS, 112.45oE, with a depth of 59.7 km. The location error in the x, y, and z directions are 3.08, 6.39, and 11.91 km, respectively. This intraslab event with a thrusting mechanism is located in the high-velocity region and close to the intermediate-depth seismic clusters, which indicates the geometry of the oceanic slab. In the region of 1994 (Mw 7.8) Banyuwangi earthquake, we found a low-velocity anomaly at ∼ 50 km depth that might be associated with the presence of subducting seamount that is more hydrated than the surrounding slab. The slip over this subducting seamount caused the tsunamigenic earthquake.
Highlights
Plate subduction along the Java Trench plays a major role in producing destructive earthquakes and volcanic complexes for the region
We used several resolution tests, i.e., ray hit count (RHC), derivative weighted sum (DWS) (Toomey and Foulger, 1989), diagonal resolution element matrix (DRE) (Menke, 1989), and checkerboard resolution test (CRT), which is the most common way to examine the resolution of seismic tomography (Supplementary Figure S4 and Supplementary Figure S5)
In order to evaluate the resolution test more appropriately, we added Gaussian noise with a mean of −0.0401 and a standard deviation of 0.6149 s to synthetic data (Supplementary Figure S6 and Supplementary Figure S7). This synthetic data set of travel times was inverted for the velocity structure using the same number of iterations as the real inversion to examine how well the checkerboard pattern was recovered and is usually consistent with the higher values of DWS, RHC, and DRE
Summary
Plate subduction along the Java Trench plays a major role in producing destructive earthquakes and volcanic complexes for the region. To locate the subduction slab and estimate the magma plumbing system beneath the region, reliable subsurface information is necessary. The presence of cold subducted slab and hot magma inclusions in the crust and upper mantle is reflected in the strong contrast of seismic velocities, which have been observed in many subduction zones (e.g., Zhao et al, 1995; Nakajima et al, 2001; Hall and Spakman, 2002; Husen et al, 2003; Reyners et al, 2006; Tsuji et al, 2008; Zhao et al, 2012)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
