Yuji Itoh , Anne Socquet, Mathilde Radiguet

In Cascadia, the concomitance of slow slip events (SSE) and tremors during Episodic Tremor and Slip (ETS) episodes is well documented. Brittle tremor patches embedded in the ductile background deforming aseismically is the most common sketch for the fault structure, but whether tremor patches impact the SSE process is under debate. This study focuses on the initiation stage of major Cascadia ETSs. So far, few observational constraints exist on the details of ETS initiation because spatiotemporal SSE inversions usually oversmooth their temporal evolution. Scrutinizing tremors and SSE over a short period at the beginning of major ETS events gives us insights into their mechanical relationship. We directly retrieve the temporal evolution of SSE moment by stacking sub-daily Global Navigation Satellite System time series at multiple sites, without slip inversions. Comparison of the GNSS stack with tremor activity demonstrates that SSE moment release accelerates drastically $\sim$1 day after the onset of vigorous tremor activity. We propose that heterogeneous interface strength limits the growth of SSE and that unruptured tremor patches may strengthen the fault. This scenario suggests that seeds of SSE grow more efficiently with the macroscopic weakening of the interface through the rupture of tremor patches. In that scenario, isolated tremor bursts lacking SSE signal would mark failed and aborted initiation due to an under-stressed interface. When the SSE moment release accelerates, the tremor area expands more rapidly, suggesting that the growth of the ETS occurs through a feedback mechanism between slip and tremor once the SSE is well developed.