STRUCTURE AND SEISMICITY ALONG CASCADIA

There is clear evidence for segmentation in structure and processes along the length of the Cascadia subduction zone. Using seismic data from permanent networks, the Earthscope Transportable Array, and two Flexible Array deployments – the FACES and Mendocino experiments – we study the seismicity, seismic tremor, lithospheric and mantle structure beneath the Pacific Northwest. Teleseismic body wave imaging of P- and S-velocity structure constraints the large-scale mantle structures in the region. We image the subducting Gorda and Juan de Fuca slabs diving into the mantle. The slab shows significant variations in the maximum depth extent and also in the amplitude of the associated high velocity anomaly along its length. The strongest velocity anomaly is at the southern end where we also image mantle flow around the termination of the slab. Beneath northern Oregon the slab anomaly is the weakest which may be related to the presence of low velocity mantle material that is in turn linked to the Yellowstone upwelling. Ambient Noise Tomography is used to image the lithospheric S-velocity structure in greater detail. The top of the subducting oceanic lithosphere is imaged with varying depth along the length of Cascadia. The strength of the high-velocity anomaly associated with the oceanic lithosphere varies in a similar way to that observed at the larger scale. Again there is a relatively weak slab signal beneath northern Oregon. Seismic tremor episodes are also detected and located along the Cascadia forearc. The recurrence interval of the episodes falls into three broad zones which correlate with terrain boundaries. Within these zones individual tremor events have been mapped in time and space. The location of the tremor is strongly anticorrelated with the locations of earthquakes suggesting that the tremor is accommodating deformation in some regions while earthquakes accommodate the deformation in others. Finally we compare the spatial variation in these structures and arc segmentation with volcanic activity.