Searching for dynamic triggering on the central Queen Charlotte Fault following the July 28th 2021 M8.2 Chignik Earthquake using a dense short period OBS array

On July 28th 2021 at 10:15 PM Alaska Daylight Time, a M8.2 earthquake ruptured the subduction megathrust south of the Alaska peninsula, near Chignik, AK. At the time, an array of 54 short period ocean bottom seismometers (OBS) were deployed along the central section of the Queen Charlotte fault (QCF) to record airgun shots as part of the active source component of the TOQUE (Transform Obliquity along the QCF and Earthquake) study. Fortuitously, OBS were recording continuously through the time of the mainshock and for 4 days afterwards, providing a unique set of near-fault high-frequency seismic data, with seafloor observations surrounding the fault trace from ~55.3-56.3° N. The QCF represents the primary plate boundary between the Pacific and North American plates offshore British Columbia and SE Alaska and accommodates > 5 cm/yr of right-lateral strike-slip plate motion. The QCF has been the focus of several past studies on dynamic triggering, but never before using a local array. Large earthquakes that generate high-amplitude, long-period ground motion, and large dynamic stress (>1 kPa), have been found to trigger tectonic tremor along the southern QCF. It has also been proposed that the 2013 M7.5 Craig earthquake may have occurred during a period of enhanced dynamic triggering associated with the 2012 M7.8 Haida Gwaii earthquake, which generated high-amplitude ground motions enhanced by guided waves through the Queen Charlotte Trough along the southern portion of the margin. We will investigate the presence or absence of triggered activities along the QCF associated with the surface waves from the Chignik earthquake with the goal of characterizing the fine-scale details of dynamic triggering behavior. We will evaluate the potential of dynamic triggering by considering the surface wave incident angles, the different particle motion styles of Love and Rayleigh waves, and the local fault orientation and structure. Waveform recordings of the Chignik earthquake from a land broadband seismometer near the array suggest dominant long-period energy and high dynamic stresses of a few to several 10s of kPa. Our study may provide insights into the mechanical properties of the QCF fault zone, as well as the triggering susceptibility as governed by fault-plane geometry and local transpressive vs transtensional stress state.