Imaging Complexity in Slow Earthquakes in Cascadia Using Multiple Seismic Arrays

Slow earthquakes, characterized by slow slip and associated seismic radiation called non-volcanic tremor, have been observed in major subduction zones worldwide. They are an important mode of stress release for the fault's transition zone, which lies directly down-dip of the locked segment, the nucleation zone of large damaging earthquakes. However, the depth of the tremor in Cascadia, and the factors governing tremor generation and rupture propagation during slow quakes remain enigmatic. Here, we develop a multi-beam-backprojection (MBBP) method to detect and locate tremor using multiple seismic arrays. We apply this technique to image a complete episodic tremor and slip (ETS) cycle in Cascadia with unprecedented resolution. The 15 months of tremor activity we analyzed so far includes a large ETS event in August 2010 and an inter-ETS time period preceding it. Our results suggest that the majority of the tremor is occurring near the plate interface. The transition zone producing tremor in this region appears to be strongly heterogeneous and composed of several distinct patches that control tremor generation and rupture propagation during slow quakes. There are three patches down-dip in the tremor zone that repeat 10-15 times during this time period, and host most of the tremor activity during the inter-ETS period. In contrast, the patches up-dip in the tremor zone break only during the ETS event. Along-strike rupture propagation velocity varies by at least a factor of 5, from a few km/day to 20 km/day (Figure 1), and appears to be controlled by the patches. In addition, we saw a range of tremor propagation velocities over shorter time scales indicating complexity in slip propagation. For example, we observe tremor streaks [Ghosh et al., 2010, G-cubed] propagating rapidly at a velocity of ~100 km/hour, and slower up-dip propagation at 1 km/hour in the same ETS event. These observations give new clues to the underlying physics of slow earthquakes, and the tectonic behavior of the transition zone.