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Understanding Tremor: Insights from an 84-Sensor Array Study in Seattle

This presentation delves into the initial findings of a new 84-element array aimed at understanding seismic tremor. It explores the concept of tremor, its implications for earthquakes, and the innovative methods employed in the study. Key topics include the performance of the pilot array, analysis of geophysical noises, and the potential relationship between tremor and the megathrust earthquakes that occur in the locked zone. By contrast with traditional methods that only capture the strongest tremor, the study reveals a more detailed, patchy source region of tremors, enhancing our understanding of seismic activity.

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Understanding Tremor: Insights from an 84-Sensor Array Study in Seattle

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Presentation Transcript

  1. What does tremor really look like? Initial results from an 84-element arraywith help from Seattle mafiaSweet , Ghosh, Creager, Houston, Pratt, …

  2. Talk outline • Background • 2008 Pilot Array • Some irritating noises • Geophysical noises • Next step - Array of Arrays

  3. State of affairs • What is tremor? • we don’t know physical process • Can it tell us about updip great earthquakes? • Maybe defines edge of locked zone • Maybe loads megathrust at an unsteady rate

  4. ETS Every 14 months BS Array * Creager & Wech, U33A-0022 and 0026

  5. UW dense array pilot study 84 sensors in a km2, Caught 8 days with light tremor in March, then 17 days of heavy tremor passing directly underneath this May 1 km Time (days)

  6. ~5Hz persistent sources for 8 days in March storm tremor LFEs storm ? tremor Near and north Just daytime Downtown Seattle buzzer

  7. Boat noise Near and north Theoretical optimum Narrowband vibration Shallow angle of incidence Migrating direction Time progression on regional seismometers

  8. Sensitivity compared to envelope method BS Array recorded in March most of a small, weak episode under Bremerton, some distance away. Detected 2180 minutes vs 539 minutes of tremor. Like earthquakes, no apparent minimum size. Storms Sweet, U33A-0024

  9. Checking location thrice Beam (rainbow) and envelope (green stars) S-P time La Rocca, Science, in revision

  10. Movie of 14 hours of tremor Pink circles - peak of beam slownesses Rainbowbackground - entire beam Green stars - envelope location estimates Each mark shrinks then disappears with 30 minutes

  11. Tremor more irregular than previously mapped?Tremor looks pointillist, more continuous, no evidence of large source volume so far. Abhi Ghosh, last week

  12. EarthScope • Array of Arrays experiment • Volunteers? Contact • Justin Sweet • Abhijit Ghosh • Ken Creager • Heidi Houston • Summer 2009 • Summer 2010 • Pitch for more grad student applications

  13. The end • Main points • Arrays give detailed recording of tremor • Traditional methods only recorded strongest tremor • Tremor source region looks patchy, pointillist

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