Offshore Seismic Signals of Deformation in the Shallow Cascadia Subduction Zone
Abstract
At the Cascadia subduction zone, evidence suggests that the shallow megathrust is largely locked offshore except near ~45.5°N, where subducting seamounts are thought to affect the coupling of the downgoing plate. Constraining the shallow extent of the seismogenic zone is important for earthquake and tsunami hazard. At other subduction zones, shallow slow slip events delineate the extent of shallow locking and are often accompanied by seismic signals including nonvolcanic tremor (NVT). There are no reports of shallow NVT signals from the 2011-2015 Cascadia Initiative ocean bottom seismometer (OBS) experiment, but in most locations, observations were limited to <2 years and seafloor OBS are susceptible to NVT-like signals from ocean currents. Alternatively, two broadband OBS spaced 20 km apart on the Ocean Observatories Initiative cabled observatory provide ~10 years of unanalyzed seismic data at ~45.5°N. These OBS are buried and collocated with current meters. Traditional network-based methods of tremor detection are not feasible with just two stations, so we are investigating alternate approaches to analyze the dataset. We first create a catalog of emergent signals that are consistent with the characteristics of NVT using a short-term average / long-term average detector. For two years from 2015-2017, we detect over 18,000 emergent signals lasting at least 10 s, with 5,500 recorded by both instruments. Preliminary analysis suggests that many, but not all, are T-phases. To separate T-phases from potential NVT signals, we are exploring classification methods based on waveform features, spectral content, arrival timing between the two stations, and consistency with cataloged earthquakes. We are also comparing long-term noise levels on both instruments to near-bottom ocean currents and regional wind speed hindcasts to identify times when high seismic amplitudes do not correlate with environmental noise sources, which may indicate a sustained tectonic noise source. Though it may be difficult to confidently identify shallow NVT without a larger network, our investigation informs the overall range of emergent seismic signals observed in the shallow Cascadia subduction zone and the utility of buried sensors and collocated current meters in searching for NVT.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2023
- Bibcode:
- 2023AGUFM.T53E0189K