Recent ETS results in NW Washington

Tremor Locations and Migration during last four ETS eventsLocked Zone relative to Tremor LocationInter ETS Tremor Swarms:

Adds up to nearly as much as ETS tremorGutenberg-Richter Power-Law Distribution N = A -0.6 = a10-0.9Mw

L = A 1/n n = 2-3

Tremor depths beneath arrays are near plate interface based on S-P times from cross correlation

100 LFEs within 100 m of each other are on plate interfaceDown-dip limit of tremor coincides with down-dip limit of low-velocity zone

below plate interface (CAFE result)

Creager, Wech, Vidale, Sweet, Ghosh, Melbourne, Houston and Abers

Tremor Migration for

four ETS events

Wech, Creager and Melbourne, JGR, submitted

Tremor and Slip summed over 4 ETS events

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2004, 2005, 2007 ETS

• Sharp western boundary to tremor

• Consistently 75 km from tremor to 40% locking

• Crustal earthquakes stop at eastern tremor boundary

• Tremor is most active over water

ETS vs inter-ETS tremor

Wech, Creager and Melbourne; JGR, submitted

Gutenberg-Richter Power-Law Distributionof Tremor Swarms

N = A * -0.6

log10N=a10-bMw

b=0.9

if moment proportional to

duration

Creagerand Wech

Tremor Swarm Duration vs Length

Slope=1/2

Slope=1/3

Slope=1

Creager and Wech

An hour of LFEs

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LFEs locate ~20km east of BS array at depth of ~47km, which is the depth to the plate interface at that location.

Individual LFE S-P times vary by no more than 0.03s, suggesting they all locate within several hundred meters of each other.

La Rocca, Science, next week

Checking location by cross-correlation of horizontal with vertical across 5-minute windows

S-P timeZero lag

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La Rocca, Science, 2009

Checking location thriceS-P time Beam (rainbow)

and envelope (green stars)

LaRocca et al., Science, 2009

Schematic interpretation of receiver function results.

Abers, MacKenzie, Rondenay, Zhang, Wech, Creager, submitted, Geology

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ETSEvery15

months

Creager & Wech

*

BS Array

Inter ETS Tremor

Feb 2007 to April 2008

some patches repeat up to 5

times

Creager and Wech

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84 sensors in a km2,Caught 8 days with light tremor last March, then 17 days of heavy tremor passing directly underneath last May

Time (days)

UW dense array pilot study

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1 km