Enhanced detection and estimation of regional S-phases using the 3-component ARCES array

Abstract

In September 2014, the ARCES array in Northern Norway was upgraded to consist of 3-component instruments at each of its 25 sites. It is thereby the first array station of the International Monitoring System to be fully 3-component. S-phases are of paramount importance for the detection and location of seismic events at regional distances, especially given sparse station coverage, and it is important that these phases are both detected confidently and attributed accurate slowness estimates. The slowness estimates both identify the phase and provide phase association algorithms with the information necessary to form a high-quality seismic event hypothesis. Prior to September 2014, ARCES had 3-component sensors at four sites only. While these 3-component seismometers were highly beneficial for the detection of regional S-phases, often using incoherent beams, it was often more reliable to perform f-k analysis on the 25 vertical sensors rather than the small number of available horizontal waveforms. In addition to providing an increased signal-to-noise ratio, signals on the horizontal traces are more coherent for the S-phases and display a high measure of semblance over the array. This improves the stability of f-k analysis, since the estimates are less susceptible to the effects of scattering and incoherence. The improved coherence of S-phases across the array aperture on the horizontal components provides the basis for superior secondary phase detection capability using F-detectors and other coherence-based algorithms. Analysis of regional signal coda on the 3-component array indicates different characteristics of the apparent slowness on the different components. The 27 January 2018 earthquake on Novaya Zemlya was the first seismic event close to the former Soviet nuclear test-site to be recorded using the upgraded 3-component ARCES array. We examine the evolution of the slowness pattern for on the different components of the wavetrain and speculate that a continuous f-k detector on three components may be able to detect small seismic events in the eastern Barents Sea with increased robustness and sensitivity. We review the development of the instrumentation at ARCES and explain the motivation behind the hybrid response sensors currently deployed.