(a) Map showing the example earthquake that occurred in Sichuan (yellow pentagon) and selected stations (red and blue triangles) recording the earthquake. The ray paths propagate westward (red) and northeastward (blue) respectively are also shown. (b) Vertical component seismograms of the stations on the westward paths. (c) Vertical component seismograms of the stations on the northeastward paths. The waveforms are sorted by epicentral distance. The onset of the Pn, Sn and Lg phases are marked on the seismograms as vertical bars. (d) Amplitude spectra of Lg wave (blue), pre-phase noise (red) and pre-event noise (green) recorded at station QML, one of the western stations. (e) The same as (d) but for station LIF, one of the northeastern stations.