Abstract
Shallow earthquakes occurring in the southern part of the Tyrrhenian basin excite a long‐duration low‐frequency coda that propagates efficiently along paths crossing the Tyrrhenian Sea. Seismograms recorded at regional distances in the Italian peninsula are analysed in the frequency‐time domain and compared with synthetics computed in simple vertically varying media. Based on results of our modeling, we interpret the coda as short‐period (5–10 s) suboceanic Rayleigh waves. Their large amplitude and long duration, that we emphasize here, are explained as due to the influence of the Tyrrhenian water layer over a fast seafloor with little S‐wave attenuation (QS > 300). A satisfactory fit of amplitude and duration of the observed waveforms is obtained assuming a young oceanic model with an average water column of 1 to 2 km and relatively high S‐wave velocity (2.5 km/s) in the shallow crust.
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