Spatial variations of the effective elastic thickness and internal load fraction in the Cascadia subduction zone

Abstract

SUMMARYThe effective elastic thickness (Te) can reflect lateral change of rheological properties from subducting plate to overlying plate, and aid in understanding the relationship between geodynamic evolution and regional tectonic response to accumulated forces by plate convergence. The internal load fraction (F), calculated simultaneously with Te, is an index for subsurface mass distribution. Here we introduce high-resolution maps of Te and F in the Cascadia subduction zone by analysing the coherence between gravity anomaly and topography using a fan wavelet method. The general Te pattern correlates well with the overall tectonic characteristics. The diverse Te pattern along the forearc zone offshore North America indicates different mechanical coupling states of the slabs between the northern and southern parts. The central subducting plate is locally decoupled from the overlying plate due to a group of subducted seamounts. The F pattern shows intriguing correlations with the full locked zone (high F) and the location of episodic tremor and slip (low F) along the forearc area. F may also be a good indicator for the location of a fluid/melt upwelling in the volcanic arc area. Seismicity is more likely to occur in areas of low Te or steep Te gradient. The long-term mechanical state of lithosphere will affect or even control the temporal variations of plates during multiple seismic cycles.