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Searching for Low-Amplitude Shallow Tectonic Tremor in Cascadia Using Buried Ocean Bottom Seismometers

Description: 

Tectonic tremor is an indicator of slow slip in the shallow portions of subduction zones, which typically lie offshore. Constraining the extent of shallow tremor is important for earthquake and tsunami hazard assessment, but offshore tremor detection is complicated by emergent oceanic signals such as T-phases, ship noise, and tremor-like signals from bottom currents. In the Cascadia subduction zone, shallow tremor has not been conclusively observed. However, offshore seismic data is limited and noisy, and previous investigations have only applied traditional land-based network methods. We explore new techniques to detect low-amplitude tremor signals using a limited ocean bottom seismometer (OBS) network. We analyze seismic data from 2014-2023 for two broadband OBS spaced 20 km apart near the deformation front at ~45.5°N on the Ocean Observatories Initiative Regional Cabled Array. The OBS are buried and collocated with bottom current meters. We find no relationship between seismic noise and bottom current speeds at either OBS, suggesting that the burial of the instruments successfully eliminates current-generated noise. We do find that background noise on both OBS is linearly related to wind speeds. We use this relationship to flag time periods with increased amplitudes in the frequency band of interest (3-10 Hz for tectonic tremor) that cannot be attributed to heightened wind speeds. Within flagged periods, we identify individual emergent signals as tremor candidates. For a test window from July 2018-July 2019, we find > 1,000 candidate 300-s detections present on both stations within flagged periods, most of which are visually assessed as T-phases. We will extend this approach to the full time period and systematically classify all candidate signals as T-phases, ship noise, or potential tremor on the basis of signal characteristics, hydrophone recordings, and comparison to regional earthquake catalogs. In addition to investigating tremor in shallow Cascadia, this will produce a labeled dataset of emergent oceanic seismic signals that can be used for detection at other offshore sites and with future networks.

Session: Marine Seismoacoustics - I

Type: Oral

Date: 5/1/2024

Presentation Time: 05:30 PM (local time)

Presenting Author: Zoe

Student Presenter: Yes

Invited Presentation: 

Authors