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 measuring shear wave splitting. This is mostly done by the determination of the splitting parameters the fast polarization direction and the time delay as well as the splitting intensity. The applied techniques highly relay on the correct temporal alignment of the single traces (vertical, north, east or Z, N, E components, respectively) 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, especially the code of the widely used MATLAB based shear wave splitting software package SplitLab contains an error source causing a wrong relative temporal alignment of the input seismograms. This effect distorts splitting signals or simulates non-existing ones. We show examples and offer a remedy.