An analysis of very low frequency earthquakes reveals a fragment of an old tectonic plate moving under the North American plate at the Mendocino Triple Junction
https://www.ucdavis.edu/news/tiny-earthquakes-reveal-hidden-faults-under-northern-california
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Link to paper (Science): [https://www.science.org/doi/10.1126/science.aeb2407](https://www.science.org/doi/10.1126/science.aeb2407)