[S03-4] Imaging of heterogeneities in the Earth with seismic scattered waves and ambient noise IV

[S03-4-02] Illuminating the Cascadia forearc and Mendocino Triple Junction system from seismic interferometry

^○Benoit Tauzin¹, Thanh Son Pham², Hrvoje Tkalcic² (1.Laboratoire de Geologie de Lyon : Terre, Planetes, Environnement, Universite de Lyon, 2.Research School of Earth Sciences, Australian National University)

We use autocorrelation to extract the reflection response from the coda of transmitted P-waves from distant earthquakes recorded at dense seismic arrays in the western US (Cascadia 93 and FAME Mendocino experiments). This results in two 300 km-long reflectivity profiles across the forearc and arc of the Cascadia subduction zone in Oregon and northern California. The results show good agreement with prior knowledge of the forearc based on inverse scattering and multi-mode conversion imaging, including the presence of a dipping low-velocity zone (LVZ) from 20 to 40 km depth near the plate interface. From observed data and numerical modeling with spectral element methods, we investigate how the quality of the reflection response obtained from P-coda autocorrelation is influenced by the distribution of earthquakes and the complexity of the transmitted wavefields. We also illuminate more precisely the uppermost 100 km of the mantle in the transitional domain from subduction to transform regimes in the Mendocino Triple Junction system. We find that the LVZ extends from the subduction regime at North to the transform regime at South as far as the Clear Lake volcanic field (38.97 degree North; 122.77 degree West). We speculate the existence of a portion of the Gorda plate near 60 km depth in the transform domain below this same volcanic field. Our results raise questions on the nature of the LVZs, their relationship to non-volcanic tremors in the subduction and transform regimes of the Mendocino and San Andreas regions, and on the presence of remnants of fossil slabs attached to the Pacific plate underneath the edge of the North American plate.