Estimation of coda Q in the mantle and characteristics of regional S-wave envelope

摘要

Coda envelope reflects heterogeneity in the Earth medium. Examining seismogram envelopes of regional earthquakes over the world in 1–20 s periods for a wide lapse time range up to 2000 s, we classified them focusing on the envelope characteristics around ScS arrivals. We found that coda decay gradient decreases after ScS arrival irrespective of focal depths and epicentral distances on a semi-logarithmic scale plot. Amplitude offset behavior observed in seismic envelopes is an evidence that scattered waves of ScS waves sometimes dominate over scattered S waves and scattered surface waves. Such an offset associated with ScS arrival is not clear for shallow events (<50 km). Further, the change of coda decay gradient around ScS arrival decreases as the period becomes shorter and the focus shallower. Inverse of coda Q values, which is estimated by applying the single-isotropic scattering model, are 0.11 × 10?2～0.45 × 10?2 and 0.33 × 10?3～3.90 × 10?3 at 4 s, 0.32 × 10?2～0.99 × 10?2 and 0.13 × 10?3～2.60 × 10?3 at 10 s, 0.53 × 10?2～1.89 × 10?2 and 0.69 × 10?3～2.20 × 10?3 and at 15 s for before ScS and after ScS arrival, respectively. These changes in coda decay gradient and offsets in coda level enable us to measure the medium heterogeneity of the mantle.