Tremor asperities in the transition zone control evolution of slow earthquakes

[1] Slow earthquakes, characterized by slow slip and associated seismic radiation called non-volcanic tremor, have been observed in major subduction zones worldwide. They constitute an important mode of stress release for the fault’s transition zone, which lies directly downdip of the locked segment, the nucleation zone of large damaging earthquakes. However, the depth of tremor in Cascadia, and the factors governing tremor generation and rupture propagation during slow quakes remain enigmatic. Here, we develop a novel multibeam-backprojection (MBBP) method to detect and locate tremor using multiple mini seismic arrays. We apply this technique to image tremor activity during an entire ETS-cycle including a large episodic tremor and slip (ETS) event in Cascadia with unprecedented resolution. Our results suggest that the majority of the tremor is occurring near the plate interface. We observe strongly heterogeneous tremor distribution with patches in the transition zone that experience repeated tremor episodes and produce most of the tremor. The patches, tens of kilometers in dimension, behave like asperities on the fault plane. During the large ETS event, rupture propagation velocity varies at least by a factor of five, and seems to be modulated by these tremor asperities. These observations support a model in which the transition zone is heterogeneous and consists of patches of asperities with surrounding regions slipping aseismically. The asperities fail quasi-periodically releasing stress and appear to regulate rupture propagation and tremor generation during slow earthquakes. This study presents new observations revealing the tectonic characteristics of the transition zone controlling the generation and evolution of slow earthquakes.