On the effects of wrongly aligned seismogram components for shear wave splitting analysis

Abstract

Seismic anisotropy inside the Earth’s interior, especially in the upper and lowermost mantle, is commonly studied by measuring shear wave splitting. This is mostly done by the determination of the splitting parameters fast polarization direction and delay time as well as the splitting intensity. The applied techniques highly rely on the correct temporal alignment of the single component traces (vertical, North, East referred to as Z, N, E components) of an earthquake relative to each other. Mixing wrongly aligned recording components would result in misleading and wrong data representations, including the particle motions in both the ZNE and the ray (LQT) coordinate systems and waveforms in the LQT coordinate system. The main pitfall in this context is that start and end times of the single traces in general differ due to data storage details. Unfortunately, the code of the widely used MATLAB based shear wave splitting software package SplitLab contains an error which can cause a wrong relative temporal alignment of the input seismograms in some cases. This effect distorts under certain conditions splitting signals or simulates non-existing ones. We show examples and offer a remedy.