No Tremor Accompanying Subduction Zone Slow-Slip Events in the Lower Cook Inlet, Alaska

Slow-slip events (SSEs) have been reported in the Cook Inlet segment of the Alaska–Aleutian subduction zone. Tectonic tremor, a repeating low-frequency signal, has been observed in the subduction zone in the Upper Cook Inlet (UCI) slow-slip zone, but thus far no studies have identified tremor in the Lower Cook Inlet (LCI) region. This study addresses whether or not tremor occurred in the LCI from 2015 to 2017, a time window that includes multiple SSEs. Two methods are used to search for tremor within continuous data from 34 broadband stations. The first approach utilizes envelope cross-correlations across an array of stations to identify clusters of seismic events. The second method tracks relative changes in the frequency content of seismic records over time at a single station. Neither approach provides evidence of events that can conclusively be identified as tectonic tremor. Subsequently, we consider whether tremor in the LCI might be masked by noise and basin effects by simulating seismograms of tremor-like signals. Stations within the basin have high noise levels that may interfere with visual verification of tremor, but methods that leverage cross correlations should identify tremor. The apparent absence of tremor in the LCI during SSEs supports the hypothesis that fundamental differences in subduction zone geometry, thermal state, sediment load, or other properties between the LCI, where the Pacific plate subducts, and the UCI, where the Yakutat slab is found, play a role in allowing or inhibiting tremor. This work underscores emerging evidence that coupled episodic tremor and slip is not globally ubiquitous.