The region of northern Borneo in South East Asia sits within a post-subduction setting formed by the recent termination of two sequential but opposed subduction systems. In this study we use seismic data from a recent temporary array deployment to image the crustal velocity structure beneath northern Borneo using a two-stage Bayesian trans-dimensional tomography scheme, in which period dependent phase velocity maps are first generated, and then used to build a 3-D shear wave model through a series of 1-D inversions. In the second stage, we also apply an Artificial Neural Network to solve the 1D inverse problem, which results in a smoother 3-D model compared to the TransD approach without sacrificing data fit. Our shear wave velocity model reveals a complex crustal structure. Under the Crocker Range, a heterogeneous velocity structure likely represents remnants of early Miocene subduction, including underthrust continental crust from subsequent continent-continent collision. In the east we observe high velocities that are interpreted to be igneous rocks in the crust generated by melting due to mid Miocene Celebes Sea subduction and later decompression melting as well as a low velocity zone that could represent underthrust sediment or duplexes from Celebes Sea subduction. A low velocity zone in the lower crust is present in a region of apparent crustal thinning. Our preferred explanation for this anomaly is remnant thermal upwelling within a failed rift that represents the on-shore continuation of the extension of the Sulu Sea, most likely caused by rollback of the Celebes Sea slab.

We deployed 27 ocean-bottom seismometers (OBSs) and five Macquarie Island stations between 2020 and 2021 along the Australian-Pacific plate boundary in the Macquarie Ridge Complex (MRC) of the Southern Ocean. From the waveforms recorded on successfully recovered OBSs and the island stations, including permanent station MCQ, we generated a 3-D S-wave velocity model for the crust and uppermost mantle using full-waveform ambient noise tomography. Distinct surface waves are identified via inter-station cross-correlation of the vertical components. The new S-wave velocity model reveals a pronounced increase in velocity across expected crustal and uppermost mantle depths. Relatively high S-wave velocities (>3.8 km/s at 7 km below sea level) are consistent with the presence of upper mantle rocks at relatively shallow depths spread out along the ridge. Overall, our findings provide initial insights on the sub-surface structure of the MRC, a complex tectonic setting of potential subduction initiation.

A document by Miriam Gauntlett. Click on the document to view its contents.

The 2021 Fagradalsfjall dike intrusion marked the initiation of a new era of volcanism on Iceland’s Reykjanes Peninsula. In this study, we present a large automatic catalog consisting of more than 80,000 earthquake hypocenters spanning the full period of the dike intrusion, which were derived from seismic data recorded by a dense network of seismic stations. The 9 – 10 km long dike exhibits a two-segment geometry of similar lengths. Linear regression on a relatively relocated subset of over 12,000 earthquakes revealed a strike of 029° with a standard deviation of 2° in the southern segment, and 046° with a standard deviation of 1° in the northern segment of the dike. A total of 97 detailed fault plane solutions from relative relocations of selected subsets of events provide new insight into the controls on faulting, showing almost exclusively right-lateral strike-slip/oblique-slip faulting associated with the dike intrusion, and a lack of left-lateral strike-slip fault motion. The alignment of fault planes is consistent with the orientation of pre-existing fractures, within uncertainty estimates. In light of these observations, we conclude that the likelihood of faulting being related to classical dike tip fracture of new rock ahead of the dike tip is low. Instead, our preferred explanation for the dominant controlling factor on the orientation of dike-related faulting is the extensive network of pre-existing fractures formed by the active transtensional plate boundary along the Reykjanes Peninsula.

Subduction termination remains poorly understood. Sabah, Malaysia (in northern Borneo) is a prime study location due to its subduction-rich history. We investigate the impact of subduction on Sabah’s mantle transition zone via P-to-S receiver function imaging. Receiver functions are computed using high resolution passive seismic data from the northern Borneo Orogeny Seismic Survey (nBOSS), MetMalaysia and KalNet arrays. Stacking of ~1800 receiver functions reveals considerable complexity, including a low amplitude 410 km discontinuity in north-western Sabah and two distinct discontinuities at depths ~675 km and ~735 km in eastern Sabah. Since 2019, three deep (>600 km) earthquakes have occurred near these discontinuities. We attribute these observations to subducted Proto-South China Sea slab material settling at the base of the mantle transition zone, introducing cool, hydrous oceanic material with a high-basalt fraction to the mantle, thus providing an interface for deep seismicity, and facilitating garnet phase transitions at 600-700 km depth.

Geological interpretations, earthquake source inversions and ground motion modelling, among other applications, require models that jointly resolve crustal and mantle structure. With the second generation of the Collaborative Seismic Earth Model (CSEM2), we present a global multi-resolution tomographic Earth model that serves this purpose. The model evolves through successive regional- and global-scale refinements. While the first generation aggregated regional models, with this study, we ensure consistency between all individual submodels, resulting in a model that accurately explains wave propagation across scales. Recent regional tomographic models were incorporated, comprising continental-scale inversions for Asia and Africa, as well as regional inversions for the Western US, Central Andes, Iran, and Southeast Asia. Across all regional refinements, over 793,000 unique source-receiver pairs contributed. Moreover, the long-wavelength Earth model (LOWE) introduces large-scale structures outside of pre-existing local refinements. A global full-waveform inversion over a total of 194 iterations with a minimum period of 50 s on a large data set of 2,423 earthquakes and over 6 million source-receiver pairs ensures that regional updates in the crust and uppermost mantle correctly translate into updates of deeper, global-scale structure. To test the performance of CSEM2, we evaluate waveform fits between observed and synthetic seismograms at 50 s for an independent data set on the global scale, and on the regional scale for lower periods. We show that we can accurately simulate waveforms within and across the regional refinements, maintaining the original resolution of the submodels embedded in the global framework.

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.

The Icelandic crust is a product of its unique tectonic setting, where the interaction of an ascending mantle plume and the mid-Atlantic Ridge has caused elevated mantle melting, which has accreted and cooled in the crust to form an oceanic plateau. Here, we investigate the strength, orientation and distribution of seismic anisotropy in the upper crust of the Northern Volcanic Zone using local earthquake shear wave splitting, with a view to understanding how the contemporary stress field may influence sub-wavelength structure and processes. This is achieved using a dataset comprising >50,000 earthquakes located in the top 10 km of the crust, recorded by up to 70 stations over a 9 year period. We find that anisotropy is largely confined to the top 3–4 km of the crust, with an average delay time of 0.10 ± 0.08 s and an average orientation of the fast axis of anisotropy of N15° ± 33°E, which closely matches the spreading direction of the Eurasian and North American plates (~N16°E). These results are consistent with the presence of rift-parallel cracks that gradually close with depth, the preferential opening of which is controlled by the regional stress field. Lateral variations in the strength of shear wave anisotropy reveal that regions with the highest concentrations of earthquakes have the highest SWA values (~10%), which reflects the presence of significant brittle deformation. Disruption of the orientation of the fast axis of anisotropy around Askja volcano can be related to local stress changes caused by underlying magmatic processes.

The post-subduction tectonic evolution of northern Borneo, which experienced two sequential subduction episodes of opposite polarity in the Neogene, is still widely debated with first-order questions such as whether the region has been in a state of compression or extension remaining unresolved. We use waveform data recorded from a dense seismic network in northern Borneo to investigate crustal thickness variations through the application of Virtual Deep Seismic Sounding (VDSS). The new results reveal an extensive area of thin crust in central and southeastern Sabah that appears to extend northeast into the Sulu Sea, where rifting initiated. We also compute local earthquake focal mechanisms, which suggest that extension is ongoing, though now dominated by orogen collapse in the NW. Together these results point to the pervasiveness of regional extension tectonics over the last 15-20 Myr and its role in the post-subduction cycle of plate tectonics in SE Asia.

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 - 150s. Based on >3,000h 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, 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-degree 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.