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What does tremor really look like? Initial results

from an 84-element arraywith help from Seattle mafia

Sweet , Ghosh, Creager, Houston, Pratt, …

Talk outlineTalk outline

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

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State of affairsState of affairs

• What is tremor?What is tremor?– we don’t know physical processwe don’t know physical process

• Can it tell us about updip great Can it tell us about updip great earthquakes?earthquakes?– Maybe defines edge of locked zoneMaybe defines edge of locked zone– Maybe loads megathrust at an unsteady rateMaybe loads megathrust at an unsteady rate

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4

ETSEvery

14 months

Creager & Wech, U33A-0022 and 0026

*

BS Array

5

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84 sensors in a km2,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

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1 km

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Near and north

Just daytime

Downtown Seattle buzzer

?

~5Hz persistent sources for 8 days in March

trem

or

LF

Es

stor

m

stor

m

trem

or

7

Near and north

Narrowband vibrationShallow angle of incidenceMigrating directionTime progression on regional seismometers

Boat noise

Theoretical optimum

8

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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

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La Rocca, Science, in revision

Checking location thriceS-P time Beam (rainbow)

and envelope (green stars)

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Movie of 14 hours of tremor

Pink circles - peak of beam slownessesRainbow background - entire beamGreen stars - envelope location estimatesEach mark shrinks then disappears with 30 minutes

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11Abhi Ghosh, last week

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Tremor more irregular than previously mapped?Tremor looks pointillist, more continuous, no evidence of large source

volume so far.

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EarthScope

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• Array of Arrays experiment

• Volunteers?Contact– Justin Sweet– Abhijit Ghosh– Ken Creager– Heidi Houston

• Summer 2009• Summer 2010• Pitch for more

grad student applications

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The end• Main points

– Arrays give detailed recording of tremor

– Traditional methods only recorded strongest tremor

– Tremor source region looks patchy, pointillist