What does tremor really look like? Initial results from an 84-element array with help from Seattle mafia Sweet, Ghosh, Creager, Houston, Pratt, …

Talk outline BackgroundBackground 2008 Pilot Array2008 Pilot Array Some irritating noisesSome irritating noises Geophysical noisesGeophysical noises Next step - Array of ArraysNext step - Array of Arrays

State of affairs What is tremor? What is tremor? –we don’t know physical process Can it tell us about updip great earthquakes? 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 Creager & Wech, U33A-0022 and 0026 * BS Array

5 84 sensors in a km 2, Caught 8 days with light tremor in March, then 17 days of heavy tremor passing directly underneath this May Time (days) UW dense array pilot study 1 km

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

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

8 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 Sensitivity compared to envelope method Sweet, U33A-0024

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

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

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

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