Seismic signature of subduction termination from teleseismic P- and S-wave arrival-time tomography : the case of northern Borneo

Acknowledgments S.P. acknowledges support from the Natural Environmental Research Council (NERC) Grant NE/R013500/1 and from the European Union's Horizon 2020 Research and Innovation Program under Marie Skłodowska-Curie Grant Agreement 790203. We thank the TanDEM-X Science Communication Team (German Aerospace Center (DLR) e.V.) for providing TanDEM topographic data. We thank the NERC Geophysical Equipment Facility for loan 1038 and seismometers loaned by the University of Cambridge and Aberdeen. We would like to thank Zhong-Hai Li and an anonymous reviewer for their constructive feedback during the review process.Peer reviewedPostprin

Quantifying deformation in North Borneo with GPS

The existence of intra-plate deformation of the Sundaland platelet along its eastern edge in North Borneo, South-East Asia, makes it an interesting area that still is relatively understudied. In addition, the motion of the coastal area of North-West Borneo is directed toward a frontal fold-and-thrust belt and has been fueling a long debate on the possible geophysical sources behind it. At present this fold-and-thrust belt is not generating significant seismic activity and may also not be entirely active due to a decreasing shelfal extension from south to north. Two sets of Global Positioning System (GPS) data have been used in this study; the first covering a time period from 1999 until 2004 (ending just before the Giant Sumatra–Andaman earthquake) to determine the continuous Sundaland tectonic plate motion, and the second from 2009 until 2011 to investigate the current deformations of North Borneo. Both absolute and relative positioning methods were carried out to investigate horizontal and vertical displacements. Analysis of the GPS results indicates a clear trend of extension along coastal regions of Sarawak and Brunei in North Borneo. On the contrary strain rate tensors in Sabah reveal that only insignificant and inconsistent extension and compression occurs throughout North-West Borneo. Moreover, station velocities and rotation rate tensors on the northern part of North Borneo suggest a clockwise (micro-block) rotation. The first analysis of vertical displacements recorded by GPS in North-West Borneo points to low subsidence rates along the western coastal regions of Sabah and inconsistent trends between the Crocker and Trusmadi mountain ranges. These results have not been able to either confirm or reject the hypothesis that gravity sliding is the main driving force behind the local motions in North Borneo. The ongoing Sundaland–Philippine Sea plate convergence may also still play an active role in the present-day deformation (crustal shortening) in North Borneo and the possible clockwise rotation of the northern part of North Borneo as a micro-block. However, more observations need to be collected to determine if the northern part of North Borneo indeed is (slowly) moving independently

Effect of long-distance earthquake from Philippines and Sulawesi to Sabah region

Sabah is known for its active earthquake activities, especially in Ranau, Kudat and Lahad Datu areas. The effects of local earthquake can reach M W 6.0. Furthermore, Sabah was also hit by earthquakes from neighbouring countries such as Sulawesi and Philippines. These countries produce highly active earthquakes that can reach as high as M W 8.6. The increase in the frequency of earthquakes is one of the concerns of the Sabah government for the safety of its people because most people live in concentrated areas near the coast. This study shows the effects of major earthquakes from the Philippines and Sulawesi which have been recorded between 1900 to 2020 and analyzed in terms of peak ground acceleration (PGA). The eastern region of Sabah is adopted in the analysis for the effect of long-distance earthquakes, as these areas are close to both countries. The analysis uses standard seismic hazard assessment procedure with compilation magnitudes greater than M W 5.0. In the final analysis, it is shown that the effects of large earthquakes from both countries are relatively small compared to the effects of local earthquakes

Post-Subduction Tectonics of Sabah, Northern Borneo, Inferred From Surface Wave Tomography

Abstract: We use two‐plane‐wave tomography with a dense network of seismic stations across Sabah, northern Borneo, to image the shear wave velocity structure of the crust and upper mantle. Our model is used to estimate crustal thickness and the depth of the lithosphere‐asthenosphere boundary (LAB) beneath the region. Calculated crustal thickness ranges between 25 and 55 km and suggests extension in a NW‐SE direction, presumably due to back‐arc processes associated with subduction of the Celebes Sea. We estimate the β‐factor to be 1.3–2, well below the initiation of seafloor spreading. The LAB is, on average, at a depth of 100 km, which is inconsistent with models that ascribe Neogene uplift to wholescale removal of the mantle lithosphere. Instead, beneath a region of Plio‐Pleistocene volcanism in the southeast, we image a region 50–100 km across where the lithosphere has thinned to <50 km, supporting recent suggestions of lower lithospheric removal through a Rayleigh‐Taylor instability

Post-Subduction Tectonics of Sabah, Northern Borneo, Inferred From Surface Wave Tomography

Abstract: We use two‐plane‐wave tomography with a dense network of seismic stations across Sabah, northern Borneo, to image the shear wave velocity structure of the crust and upper mantle. Our model is used to estimate crustal thickness and the depth of the lithosphere‐asthenosphere boundary (LAB) beneath the region. Calculated crustal thickness ranges between 25 and 55 km and suggests extension in a NW‐SE direction, presumably due to back‐arc processes associated with subduction of the Celebes Sea. We estimate the β‐factor to be 1.3–2, well below the initiation of seafloor spreading. The LAB is, on average, at a depth of 100 km, which is inconsistent with models that ascribe Neogene uplift to wholescale removal of the mantle lithosphere. Instead, beneath a region of Plio‐Pleistocene volcanism in the southeast, we image a region 50–100 km across where the lithosphere has thinned to <50 km, supporting recent suggestions of lower lithospheric removal through a Rayleigh‐Taylor instability

Seismic hazard analysis for east Malaysia; based on a proposed ground motion prediction equation

The seismic hazard analysis requires an estimation of ground motion intensity where the process needs to use a compatible ground motion prediction equation or GMPE, which provides ground acceleration estimates in a function of earthquake magnitude and distance. Hence, the effect of current equation often does not accurately represent the earthquake condition in East Malaysia region. In this study, the characteristics of low-to-moderate databases were used and derived by regression analysis in terms of horizontal peak ground acceleration (PGA). The appropriate GMPE design for East Malaysia is based on the ground motion records compiled from strike slip earthquakes that occurred within 10 to 1,350 km. This earthquake data is based on actual data recorded at a broad range of magnitude levels within a wide range of distances. The new equation is used to predict the PGA value throughout East Malaysia by probabilistic method. PSHA is a method to analyse seismic hazard assessment using probability concept by considering the uncertainties of the size, location and rate of occurrence of earthquake and the variation of ground motion characteristics. The four well-known existing attenuation functions are evaluated with current equation to highlight their limitations in magnitude and distance. With a more complete collection of earthquake databases, GMPE has become more reliable. The GMPE of peak ground acceleration for low-to-moderate earthquake at long distance was found to be logarithmically distributed. The equation provides ease in both implementation and interpretation of physical parameters with a comparable standard deviation

Collision-Induced Subduction Polarity Reversal Explains the Crustal Structure of Northern Borneo: New Results From Virtual Deep Seismic Sounding (VDSS)

Abstract: Subduction polarity reversal (SPR) is a key subduction initiation mechanism often associated with arc‐continent collision zones. Northern Borneo has long been recognized as a location where sequential but opposing subduction zones were present in the Miocene, but has not been examined in the context of SPR. Here, we exploit teleseismic data from northern Borneo to investigate crustal thickness variations using Virtual Deep Seismic Sounding (VDSS). Our results reveal a thick crustal root beneath the Crocker Range and an area of relatively thin crust in the southeast, which appears to extend northeast into the Sulu Sea, where back‐arc rifting behind the younger subduction zone developed. Overall, our findings are consistent with predictions from numerical models of SPR involving arc‐continent collision, but with several important differences—including a substantial mountain range and more limited back‐arc rifting that can be attributed to northern Borneo being an example of SPR involving continent‐continent collision

Morphostructural analysis of onshore and offshore sabah using Digital Elevation Model

International audienc

The Signature of Lithospheric Anisotropy at Post-Subduction Continental Margins: New Insight From XKS Splitting Analysis in Northern Borneo

The relative paucity of recent post‐subduction environments globally has meant that, so far, little is known about tectonic processes that occur during and after subduction termination, as previously convergent tectonic plates adjust to the new stress regime. The region of Southeast Asia that now encompasses northern Borneo has been host to two sequential episodes of subduction—both now terminated—since the mid‐Paleogene. It is expected that these processes will have left signatures in the fabric of the upper mantle, which are manifest in the form of seismic anisotropy. We investigate the evidence for, and alignment of, anisotropic fabrics by measuring the splitting of a family of teleseismic shear phases. These observations provide a measure of the orientation of the effective anisotropic elastic tensor, in the form of the orientation of the fast shear‐wave polarization, ϕ, and add constraints on the strength of the anisotropic fabric, in the form of the delay time, δt. We observe two principal trends across northern Borneo that appear to be confined to the lithosphere. These patterns are likely related to tectonic processes associated with subduction, continental collision, and oceanic basin formation, events that can exert primary influence on the formation of post‐subduction settings