Modelling the response of seismic wavefields to sharp lateral variations in crustal discontinuities is essential for seismic tomography application and path effects correction in earthquake source characterization. This is particularly relevant when wavefields cross back-arc oceanic basins, i.e. mixed continental-oceanic settings. High-frequency (>0.05 Hz) seismic waves resonate and get absorbed across these settings due to a shallow Moho, crustal heterogeneities, and energy leakage. Here, we provide the first high-frequency wave-equation model of full seismograms propagating through realistic 3D back-arc basins. Inversion by parameters trial based on correlation analyses identifies P-, S- and coda-wave as attributes able to estimate jointly 3D Moho variations, sediment thickness, and earthquake source characteristics using data from a single regional earthquake. We use as data waveforms produced by the Accumoli earthquake (Central Italy, 2016), propagating across the Southern Tyrrhenian basin and recorded across Southern Italy. The best model comprises a deep Moho ($$\sim$$ ∼ 18 km) in the middle of the basin and a crustal pinch with the continental crust in Sicily. The deep Moho corresponds to the Issel Bridge, a portion of continental crust trapped between the Vavilov and Marsili volcanic centres. The Accumoli earthquake is optimally described at a depth of 7.3 km using a boxcar with rise time of 6 s. Our results show that the early S-wave coda comprises trapped and reverberating phases sensitive to crustal interfaces. Forward modelling these waves is computationally expensive; however, adding these attributes to tomographic procedures allows modelling both source and structural parameters across oceanic basins.
<p>Lateral variations in crustal structure may affect the propagation of Lg phases, guided waves that propagate efficiently only in the continental crust. Seismic paths crossing continental-oceanic transitions are characterized by Lg blockage due to the drastic decrease in crustal thickness. Here, we investigate the effects of crustal thinning on wave propagation in the Tyrrhenian basin using radiative transfer theory. We first model regional coda envelopes (600-800km) using the software tool Radiative3D (Sanborn & Cormier 2018, <em>GJI</em>). It allows to synthesize seismograms envelopes produced by earthquakes by propagating energy packets through a deterministic structure, taking into account the crustal layers, including Moho transition depth, and parameters describing the medium heterogeneities. &#160;Then, we approach the complex problem of meshing, including measured Moho depths, for simulations based on spectral elements (Komatitsch D. et al., 2012, SPECFEM3D,<em> Computational Infrastructure for Geodynamics</em>) and finite differences methods (Maeda et al., 2017, OpenSWPC). The results aim at understanding complex wave attenuation and leakage in the mantle, for future implementations into the Multi-Resolution Attenuation Tomography code (MuRAT &#8211; De Siena et al. 2014,<em> JVGR</em>)</p>
<p>When seismic information is used to map Earth structures, a primary challenge is modelling the response of seismic wavefields to strong lateral variations in medium properties. These variations are especially relevant across oceanic basins with mixed continental-oceanic crust and including magmatic systems. These highly-scattering and absorption media produce stochastic signatures that are hard to separate from complex coherent reverberations due to shallow Moho. The discrimination between these two effects is fundamental for improving full-waveform techniques when imaging oceanic basins at regional and global scales. Here, we present a joint tomographic and modelling approach focusing on the ~1 Hz frequency band, where seismic scattering and attenuation mechanisms are predominantly resonant. Firstly, we image late-time coda attenuation as a marker of seismic absorption across the Italian peninsula and the Tyrrhenian Sea. Regional-scale data provide the ideal benchmark to explore the potential of attenuation imaging using radiative-transfer-derived sensitivity kernels in a mixed continental-oceanic crust. Then, we explore the response of seismic wavefield to structural variations combining coda-attenuation imaging with simulations based on radiative transfer and wave-equation modelling. The results provide evidence of intra-crustal reverberations and energy leakage in the mantle, finally being able to map Moho depths with regional earthquakes. This work is an ideal forward model of seismic wavefields recorded across the oceanic crust for future full-waveform inversions and imaging of crustal discontinuities.</p>
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