LINKING KEY TECTONIC FEATURES OF JUAN DE FUCA PLATE TO OBSERVED SEISMICITY IN CASCADIA SUBDUCTION ZONE

Seismicity pattern of the Cascadia subduction zone (CSZ) is quite unique. Higher amounts of earthquakes are observed in the northern part of the margin (i.e., Washington) as opposed to little or almost no observed earthquakes in the southern region (i.e., Oregon). Correlation between this atypical nature of seismicity and the current tectonic features of CSZ is still poorly understood. Since, Juan de Fuca (JdF) is the largest and major oceanic plate subducting in CSZ, in this study, we used multiple geophysical datasets (namely gravity, magnetic, seismic) to map and better understand the tectonic features of the JdF plate. The objective of this study is to identify any possible correlation between Wadati-Benioff zone earthquakes of CSZ and mapped tectonic features of the JdF plate.

First, we mapped the propagator wakes of the JdF plate from literature and cross-checked it with publicly available magnetic dataset. Literature suggests these propagator wakes have higher crustal density; however, our 2-D and 2.75-D gravity modeling indicate the opposite. In our modeling propagator wakes show lower crustal density, which we think represent structural weakness in the form of faults and fractures. This hypothesis agrees with the formation mechanism of these propagator wakes. Secondly, we have mapped several lineaments on JdF plate from magnetic dataset. These lineaments correlate to lower crustal density blocks of our plate-scale 2-D and smaller scale 2.75-D models which demonstrate the structurally weak nature of these features. Thirdly, we have mapped exposed and buried seamounts from bathymetry and gravity datasets. We think there is a spatial correlation between our mapped lineaments and seamounts suggesting the seamounts may have formed from mantle materials passing through the structurally weak lineaments. Finally, we have separated earthquakes from Wadati-Benioff zone of CSZ. After performing a regression analysis on the clusters of earthquakes, clear spatial correlation can be observed between the subduction zone earthquakes and the mapped tectonic features. We think the subduction of structurally weak propagator wakes and mapped lineaments with the seamounts on JdF plate may lead to higher amount of observed seismicity in the Northern part of CSZ.