TOMOGRAPHIC IMAGING OF 3D SEISMIC VELOCITY STRUCTURE AND ANISOTROPY BENEATH CENTRAL AND EAST JAVA REGION

• Main Author: Muttaqy, Faiz
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/66887
• Institution: Institut Teknologi Bandung

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. Using these data, 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 to 100 km. Lowvelocity 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 a subducting seamount that is more hydrated than the surrounding slab. The slip over this subducting seamount caused the tsunamigenic earthquake. In parallel, we have investigated shear wave splitting using seismograms from 30 broadband stations of BMKG within the time period of 2009-2020 using local S phases. We constrained observed results to the criteria of a crustal anisotropy study, i.e. event depths less than 30 km with radius 150 km from each recorded station. We compared our results with the regional strain rate derived from GPSii observations in Java, then divided the analysis into three regions, i.e. (A) Southern part of Java, (B) Northern part of Java, and (C) Easternmost Java and Bali region. Region A has predominant NE-SW direction of the fast polarization and experiences a relatively small strain rate except for the region near Yogyakarta where the average fast directions are parallel to the strike of the Opak fault, suggesting a structural-induced anisotropy. In region B, the fast directions are mainly parallel to the principal compressional axis of strain rate, suggesting stress-induced anisotropy. Meanwhile in region C, the fast polarizations are parallel and subparallel to the predominant principal strain at Madura and Bali, suggesting stress-induced anisotropy caused by cracks and microcracks as a response to the local tectonic stress. Moreover, in the area between East Java and Bali, our results exhibit fast directions almost perpendicular to the principal axis, suggesting that the presence of water inclusion in the cracks may cause structuralinduced anisotropy. We then extend the shear wave splitting measurement using local S-phases of 30- 300 km depth earthquakes from 38 BMKG stations within the period of 2009- 2020 to determine lithospheric anisotropy structure beneath the Central and East Java region, Indonesia. A total of 2,571 measurements were obtained and show a pattern of fast directions that are generally trench-parallel measured at stations located close to the trench in the forearc region, while the transition zone from trench-parallel to trench-perpendicular ???? at stations located further from the trench. The trench-perpendicular or sub-parallel ???? suggests mantle flow direction that is well correlated with the direction of plate motions due to the simple lattice preferred orientation (LPO) mechanism coming from mantle material. Meanwhile, the trench-parallel ???? indicates some possible causes of anisotropy: (1) preferred orientation of the mineral olivine due to the presence of water in the mantle, (2) shape preferred orientation of partial melting. The 2-D delay time tomography indicates a high strength of anisotropy (> 0.015 s/km) at the region near a subducted seamount that is responsible for the 1994 Java earthquake. Interaction between subducting seamount and overriding plate suggests a combination of stress and structural effect to the anisotropy with the SPO mechanism.