We present the first continental‐scale seismic model of the lithosphere and underlying mantle beneath Southeast Asia obtained from adjoint waveform tomography (often referred to as full‐waveform inversion or FWI), using seismic data filtered at periods from 20 to 150 s. Based on >3,000 hr of analyzed waveform data gathered from ∼13,000 unique source‐receiver pairs, we image isotropic P‐wave velocity, radially anisotropic S‐wave velocity and density via an iterative non‐linear inversion that begins from a 1‐D reference model. At each iteration, the full 3‐D wavefield is determined through an anelastic Earth, accommodating effects of topography, bathymetry and ocean load. Our data selection aims to maximize sensitivity to deep structure by accounting for body wave arrivals separately. SASSY21, our final model after 87 iterations across seven period bands, is able to explain true‐amplitude data from events and receivers not included in the inversion. The trade‐off between inversion parameters is estimated through an analysis of the Hessian‐vector product. SASSY21 reveals detailed anomalies down to the mantle transition zone, including multiple subduction zones. The most prominent feature is the (Indo‐)Australian plate descending beneath Indonesia, which is imaged as one continuous slab along the 180° curvature of the Banda Arc. The tomography confirms the existence of a hole in the slab beneath Mount Tambora and locates a high S‐wave velocity zone beneath northern Borneo that may be associated with subduction termination in the mid‐late Miocene. A previously undiscovered feature beneath the east coast of Borneo is also revealed, which may be a signature of post‐subduction processes, delamination or underthrusting from the formation of Sulawesi.

Northern Borneo-broadly coextensive with the Malaysian state of Sabah-lies near the north-eastern edge of the present-day Sundaland block, in Southeast Asia (Figure 1). This block, bounded by the seismically active Sunda and Philippines subduction zones, represents the southern extent of the slow-moving (∼20 mm year −1 ) Eurasian plate (Simons et al., 1999;Argus et al., 2011). Like much of eastern Borneo, northern Borneo was accreted onto the eastern margin of Mesozoic Sundaland between the Late Cretaceous and the Early Miocene (Hall, 1996). Though it now exhibits the characteristics of an intraplate setting, there is evidence in the geological record to suggest that it has been host to two opposing subduction systems since the start of the Neogene, both now terminated (see Figure 2). It is widely thought that the proto-South China Sea was subducted beneath the north-west continental margin of northern Borneo-continuing north-east along what is now Palawan-during the Paleogene, before terminating in the Early Miocene with continent-continent collision between the Dangerous

Southeast Asia is one of the most complex tectonic regions on Earth and has an extensive history of destruction and loss of life associated with earthquakes, tsunamis, and volcanoes (e.g., the 2018 Palu earthquake, Socquet et al., 2019). The eastern region is located at the confluence of three main tectonic plates, and is further complicated by a system of microplates (see Figure 1), with multiple subduction zones that were activated during the Neogene (∼23-2 Myr, Hall, 2012). This complexity is largely driven by the Southeast Asia-Australia collision zone (e.g., Hall, 2011), where subduction along the Indonesian volcanic arc transitions to an arc-continent

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We present a new 3-D seismic structural model of the eastern Indonesian region and its surroundings from full-waveform inversion (FWI) that exploits seismic data filtered at periods between 15 -150 s. SASSY21 -a recent 3-D FWI tomographic model of Southeast Asia -is used as a starting model, and our study region is characterized by particularly good data coverage, which facilitates a more refined image. We use the spectral-element solver Salvus to determine the full 3-D wavefield, accounting for the fluid ocean explicitly by solving a coupled system of acoustic and elastic wave equations. This is computationally more expensive but allows seismic waves within the water layer to be simulated, which becomes important for periods [?] 20 s. We investigate path-dependent effects of surface elevation (topography and bathymetry) and the fluid ocean on synthetic waveforms, and compare our final model to the tomographic result obtained with the frequently used ocean loading approximation. Furthermore, we highlight some of the key features of our final model -SASSIER22 -after 34 L-BFGS iterations, which reveals detailed anomalies down to the mantle transition zone, including a convergent double-subduction zone along the southern segment of the Philippine Trench, which was not evident in the starting model. A more detailed illumination of the slab beneath the North Sulawesi Trench reveals a pronounced positive wavespeed anomaly down to 200 km depth, consistent with the maximum depth of seismicity, and a more diffuse but aseismic positive wavespeed anomaly that continues to the 410 km discontinuity.