CRUSTAL STRUCTURE AND SEISMIC ANISOTROPY IN THE SUNDA-BANDA ARC TRANSITION ZONE

• Main Author: (NIM : 32412301), SYUHADA
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/24464
• Institution: Institut Teknologi Bandung

Crustal structure and seismic anisotropy has been investigated along Sunda-Banda arc transition zone using two seismological methods; receiver function and shear wave splitting. The aim of this research is to provide better knowledge about crustal and mantle deformation related to the change in tectonic regime from subduction to collision. Receiver functions estimate crustal thickness, Vp/Vs ratios and S-wave velocity structure. The results show crustal thickness of 34–37 km in Timor Island, which is consistent with the previous works. The thick crust (> 30 km) is also found beneath Sumba and Flores Islands, which might be related to the arc-continent collision causing the thickened crust. In Timor and Sumba Islands, high Vp/Vs ratio (> 1.84) with low velocity zone is observed that might be associated with the presence of mafic and ultramafic materials and fluid filled fracture zone.The high Vp/Vs ratio which is also observed at Sumbawa and Flores volcanic Islands might be an indication of partial melt related to the upwelling of hot asthenosphere material through the subducted slab. Futhermore, shear wave splitting has been utilised to investigate seismic anisotropy over varied geological regimes of the Sunda-Banda arc transition zone. Crustal anisotropy analysis, derived from high-quality splitting measurements of 262 event-station pairs, reveals that the orientations of fast polarisation for the stations located in the subduction regime are generally parallel or sub-parallel to the directions of the principal compressional strain-rate axes with a lack of dependency of delay time &#948;t on increasing depth, suggesting the influence of stress induced anisotropy on the upper crust. On the other hand, the average fast polarisations show more scattered for the stations located around Sumba Island and in the collision regime, implying a mix of anisotropy causes. Thus, anisotropy in this region is not only controlled by preferentially aligned cracks due to tectonic stress, but also by preferential mineral alignment and macro-scale faults associated with the regional tectonic deformation. Further analysis is also conducted to search possible temporal variations of splitting parameters associated with the stress changes excited by large earthquakes. The association between variation in splitting parameters and earthquake activity observed in this study might provide useful information about accumulation of stress before large events, and thus might be considered as an earthquake-forecasting tool in the future. <br /> <br /> Shear wave splitting analysis to characterise the sub-crustal anisotropy has been also performed to provide better knowledge about lithospheric deformation and mantle flow in this area. The splitting measurements show lateral and depth variation in the fast orientations of the S-waves in this area. Tha fast polarisation show both parallel and perpendicular to the absolute plate motion. The delay time on each station ranges between ~0.14–0.30 s, with percent anisotropy in the range of 0.75–1.40%. The high percent anisotropies are recorded for the stations located in Timor and Sumba Islands. The &#960;/2 periodical pattern of delay time as function of initial polarisation is observed for the DBNI station indicating the possibility of the presence of two layers of anisotropy. This pattern could be connected to the presence of strong anisotropic material in the upper mantle. The rotational pattern of fast polarisation is observed beneath Flores Island, which might be related to the fossil anisotropy, insertion of continental material beneath Flores, lattice preferred orientation on the partial melt or shape preferred orientation due to partial melting filling the cracks during the collision process. The high anisotropy values with the complex pattern of fast direction found for Sumba and Timor Islands indicate heterogeneity of local geological structure due to the collision process between Australian continent and Banda arc.