Analysis of teleseismic shear-wave splitting in the post-subduction environment of Sabah, Borneo

Abstract:

The termination phase of the subduction cycle, where subduction ceases and both plates adjust to a changing stress environment, is an important part of the long-term tectonic cycle. Understanding the geodynamic processes that are manifest in such settings is an integral part of understanding the subduction cycle as a whole. It has been proposed that Sabah, a Malaysian state in the north of Borneo, is flanked by two such extinct subduction margins, creating an exceptional natural laboratory for the study of plate tectonics. In particular, constraining upper mantle anisotropy using teleseismic shear-wave splitting analysis is crucial for discriminating between mantle flow patterns made by slab detachment, lithospheric delamination, or gravitational instability, all of which have been proposed to account for the geological surface features of Sabah.

Historically, Sabah (and much of Borneo) has seen little seismic instrumentation, resulting in notable gaps in global tomographic and mantle anisotropy studies. From March 2018 to January 2020 (22 months), 46 broadband seismometers were deployed across Sabah as part of the northern Borneo Orogeny Seismic Survey (nBOSS), with an average station spacing of 40 km. This dataset has been supplemented by 26 permanent seismometers operated by the Malaysian Meteorological Department, the vast majority of which were installed in 2017 as part of the response to the M6.0 2015 Ranau earthquake.

In this study, we analyse core-refracted phases (SKS, SKKS and PKS) from 171 earthquakes with magnitudes > 5.7, spanning the period from March 2018 to January 2020 at 71 stations across Sabah. We observe variations in the strength and orientation of the anisotropic signal over short distance scales, suggesting strong, shallow controls on anisotropy. Attempts to fit a simple, one-layer model to the splitting measurements suggest a more complex model of anisotropy is required to explain the observations.

Further, we measure teleseismic shear-wave splitting at two permanent stations that have been operated since 2006 to further constrain the anisotropic model through analysis of splitting as a function of back-azimuth.