New Insights into the Active Tectonic Processes of the Indonesia-Australia-New Guinea Collision Zone

Abstract

The Indonesia-Australia-New Guinea collision zone (IANGCZ) comprises a complex system of tectonic blocks whose relative motion accommodates convergence of the Sunda Block, Pacific, Australian and Philippine Sea plates. This PhD project combines multiple datasets; to analyse the IANGCZ at a range of spatial-scales. The broad-scale kinematic model developed represents the first unified quantitative analysis of the IANGCZ and provides a coherent kinematic description of the first-order kinematic characteristics associated with the large-scale tectonic structures of the IANGCZ. The model is based on simultaneous inversion of 492 earthquake slip vectors and 267 GPS velocities and quantifies tectonic block movements spanning the Sunda-Banda Arc, Western New Guinea and Papua New Guinea. The block model comprises 23 blocks, for which we estimate the rotation rates and block boundary slip rates. The model shows that the Cenderawasih Bay was likely formed by a combination of both rotation of the Bird's Head Block and SW convergence along the Lowlands fault. Our estimated relative slip vectors across the New Guinea Fold-and-Thrust Belt indicate a transition in the tectonic regime of the block boundary, implying the combined effect of multiple driving mechanisms. Refining the broad-scale model, the regional-scale Java-Timor subduction-collision transition zone is investigated, to reveal the spatial variability of fault coupling and the potential of the earthquake hazards in the region. The estimated locking pattern of the forearc megathrust indicates two highly coupled areas. With the highest coupling zone being the western end of the Flores back-arc thrust concentrated north of Lombok - this being the source of strain accumulation released in the 2018 Lombok earthquake sequence. The tectonic character of the western end of Flores back-arc thrust have been widely debated, the sequence of earthquakes that struck north of Lombok in 2018 gives us an opportunity to better understand this controversial structure. We investigate the 2018 sequence of four deadly (Mw 6.2 to 6.9) earthquakes that occured north of Lombok, on 28 July, 5 August, and 19 August, between the Flores back-arc thrust and the Rinjani-Samalas volcanic complex. The fault geometries and slip distributions of the three mainshocks are modelled by inverting the InSAR measurements, based on rectangular dislocations embedded in a multi-layered elastic half-space. The relocated aftershocks were clustered using an unsupervised learning method to identify their orgins. We use an InSAR time-series to investigate the time-dependent post-seismic deformation in the two years following the earthquake sequence, based on the afterslip-only model and the combined model that simulates the viscoelastic relaxation and afterslip simultaneously. The estimated volumetric strain change indicates the pressure change in the magma conduit at different periods. To quantify the influence of the earthquake sequence on the spatiotemporal deformation pattern of the volcano, we extended our InSAR time-series range forward to the year 2014, just prior to the two eruptions that occurred in 2015 and 2016, and perform the Principal Component Analysis to investigate the time-dependent inflation and deflation signals. The modelled time-dependent volume and location change of the volcano pressure source reveals how changes in the magma body and magma movement may have been influenced by the 2018 Lombok earthquake sequence. The local-scale source modelling study for the 2018 Lombok earthquake sequence reveals the activation of the west-end segment of the Flores back-arc thrust, and the interaction between this thrust and Rinjani volcano. All the results derived from this project have implications for better investigating the characteristics of active structures located at this highly tectonically complicated oblique convergent plate boundary zone and quantifying the seismic hazard posed by these structures.