Surface Waves

Surface waves are the most prominent phases recorded at teleseismic distances at periods longer than 30 s, especially from shallow-focus earthquakes. Two types of surface waves are observed, distinguished by their polarization during propagation through the Earth: Love (SH) and Rayleigh (P–SV) waves, recorded on the transverse and vertical/longitudinal components, respectively. Surface wave arrivals are denoted by the orbit number (e.g. No = 1 for minor-arc L1 or R1 waves), a proxy for the number of times the wave circles around the Earth (Nc = [No – 1]/2 for odd No, No/2 otherwise). The wave trains excited by large mega-thrust earthquakes (Mw ≥ 7.5) circle the Earth multiple times (Nc ≥ 1) for many hours and manifest as discernible higher-orbit arrivals (e.g. L3–L5, R3–R5). Generation and propagation of surface waves can also be classified based on the properties of the corresponding normal modes. Fundamental-mode surface wave trains are excited more strongly by shallow and intermediate-depth earthquakes (h < 250 km) and appear well separated from other phases at teleseismic distances (> 30◦). Higher-mode or overtone vibrations are excited by deeper earthquakes and appear as faster propagating, compact wave packets that contribute to the long-period body waveforms. Characterizing surface waves and overtones is critical for the construction of elastic reference Earth models

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