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Very low frequency earthquakes (VLFEs) offshore Cascadia – a new window to megathrust slip and frictional behavior

Abhijit Ghosh, & Kuntal Chaudhuri

Submitted September 11, 2022, SCEC Contribution #12320, 2022 SCEC Annual Meeting Poster #047

Cascadia megathrust produces M9 earthquakes, the last one being more than 300 years ago. Little is known about the slip behavior of the megathrust and its variability as the seismogenic zone of the fault lies offshore with minimal recorded seismic activity. Slow earthquakes are abundant onshore Cascadia, at the base of the seismogenic zone. They, however, so far remain elusive offshore Cascadia. We discover very low frequency earthquakes (VLFEs) offshore Cascadia using ocean bottom seismometers (OBSs). We find that VLFEs are widespread along the Cascadia margin and occur frequently, providing evidence of slow earthquakes offshore Cascadia [Chaudhuri and Ghosh, GRL, 2022]. We have analyzed 6 months of seismic data so far using a grid-search moment tensor inversion algorithm to detect 12 distinct VLFEs. Each VLFE are active during this time period producing hundreds of similar events. Interestingly, a cluster of VLFEs offshore Oregon is located adjacent to a cluster of regular fast earthquakes and a slow slip patch reported near the coastline. This suggests a possible interaction between slow and fast slip in this area. Moreover, focal mechanisms of VLFEs indicate that strike-slip faulting offshore Cascadia may play an important role in accommodating deformation during interseismic time period. Discovery of VLFEs offshore Cascadia opens up a new avenue of studying megathrust slip behaviors, its relationship to seismic hazards and physics of show earthquakes.

Key Words
VLFE, very low frequency earthquake, slow earthquakes, ETS, Cascadia, subduction zone, offshore, seismogenic zone, friction

Ghosh, A., & Chaudhuri, K. (2022, 09). Very low frequency earthquakes (VLFEs) offshore Cascadia – a new window to megathrust slip and frictional behavior. Poster Presentation at 2022 SCEC Annual Meeting.

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