Widespread Very Low Frequency Earthquakes (VLFEs) Activity Offshore Cascadia

A significant amount of stress in the Cascadia subduction zone (CSZ) is released by different forms of slow earthquakes including tremors, low-frequency earthquakes, and very low-frequency earthquakes (VLFEs)-all occurring onshore at the deeper part of the transition zone. Their activity offshore, however, remains elusive. We discover widespread occurrence of discrete VLFEs offshore Cascadia using ocean-bottom seismometers. Barring occasional regular fast earthquakes, VLFEs are the only seismic stress indicators offshore. Using centroid moment tensor inversion and matched filtering, we detect sequences of 12 distinct families of VLFEs. The VLFEs in northern CSZ have focal mechanisms consistent with the area's subduction zone deformation. Interestingly, the VLFEs in the southern part of CSZ show strike-slip faulting, attributed to offshore faults, sediment consolidation, subduction bending, and transpressional regimes created by complex plate tectonics. It challenges a canonical view of the seismogenic zone in Cascadia characterized by a frictionally homogeneous fault segment producing only regular fast earthquakes.

Automated summary

Using ocean-bottom seismometers, we have discovered widespread occurrence of discrete very low-frequency earthquakes (VLFEs) offshore Cascadia. The VLFEs in the northern part of Cascadia have focal mechanisms consistent with subduction zone deformation. In the southern part, the VLFEs show characteristics of strike-slip faulting, possibly due to complex plate tectonics.