Hunting for offshore very low frequency earthquakes (VLFE) in the Cascadia Subduction Zone using ocean-bottom seismometers

Abstract:

Very low frequency earthquake (VLFE) is a type of seismic event in the slow earthquake family. They can be larger than Mw 4.0, and therefore release significant amount of seismic moment along tectonic plates compared to its counterparts like, tremor and low frequency earthquakes (Ghosh et al., 2015). They are rich in 0.02-0.05 Hz energy and depleted in higher frequencies compared to local earthquakes of comparable magnitude. Study of very low frequency earthquakes help us in better understanding the physics involved in slow earthquakes, stress imparted to the seismogenic zone, and possible triggering of a regular fast earthquake, which in the Cascadia subduction zone, can potentially produce tsunami. Deep VLFEs along the Cascadia subduction zone under northern Washington state, have shown complex behaviors with respect to episodic tremor and slip (ETS) events (Hutchison and Ghosh, 2016; Ghosh et al., 2015). Interestingly, they occur near-continuously in the ETS region (Hutchison and Ghosh, 2018). Offshore VLFEs, however, still remain elusive. Here, we have used ocean-bottom seismometers deployed for the Cascadia Initiative project, and applied tilt, compliance and rotation corrections based on the surface wave arrivals of teleseismic earthquakes. Applying a centroid moment tensor inversion technique (Ghosh et al., 2015), we are able to identify a number of VLFE-like signals offshore Washington state in the Cascadia subduction . We are currently running tests on the signals to confirm their identity. If confirmed, these VLFEs may be one of the rare evidences of slow earthquakes offshore Cascadia, and have important implications on fault frictional properties, and seismic hazard in this area, which is devoid of damaging great earthquake in more than 300 years. The presence of offshore VLFEs along the Cascadia subduction zone will indicate a complex frictional behavior of the subduction fault and stress transfer mechanisms. Hence, it is imperative to understand how they relate to other seismic events, both slow and fast earthquakes, spatiotemporally.