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Analysis of the Seismic Noise Using PPSD for Non-Volcanic Tremors in Cascadia Prior the Onset of SSEs

Description: 

Non-Volcanic Tremors (NVT) are a low-amplitude seismic signal, with a dominant frequency content of 1-10 Hz, lasting a few hours to a few days that are commonly observed in subduction zones. NVT contains predominantly S-wave energy, and are thought to emanate from locations on or near the plate interface. NVT often coincides spatially and temporally with geodetically identified Slow Slip Events (SSEs) and is recognized as the seismic manifestation of deep slow slip events. Since 2009, the Pacific Northwest Seismic Network (PNSN) detects, locates, and catalogs tectonic tremor across the Cascadia Subduction Zone (CSZ) resulting in a catalog of ~500,000 tremors that can be used to determine when and where SSEs occur. In this work we explore the character of seismic noise near the onset of known SSEs. Our goal is to determine whether the frequency content or amplitude of noise immediately prior to SSEs is systematically different than background noise levels prior to the first arriving tremor event.

To do this, we calculate the Probabilistic Power Spectral Densities (PPSD) at frequencies between 1 and 10 Hz. We calculate noise levels for both day-long time periods over the course of a year and for shorter hourly intervals since noise can have strong diurnal variations in character. In this manner, we are able to track the noise behavior throughout the course of the year and assign probabilistic values to the noise amplitude across the frequency band of interest. We then use the same process to determine the PPSD for the time period immediately prior to known SSEs from the tremor catalog. Preliminary results show that noise levels begin to grow prior to the SSE onset. For one event we analyzed in detail, noise levels were at or exceeded the 95th percentile in the 8 hours before the SSE. We plan to extend this analysis to multiple SSEs occurring along the margin but this initial result suggests that SSEs may have an extended nucleation process. The changes in noise character could be caused by smaller amplitude and/or less-frequent tremor that could signal precursory slip.

Session: Constraining Seismic Hazard in the Cascadia Subduction Zone [Poster]

Type: Poster

Date: 4/20/2023

Presentation Time: 08:00 AM (local time)

Presenting Author: Lois Papin

Student Presenter: No

Invited Presentation: 

Authors