Creager, Wech, Vidale, Melbourne Cycles within cycles: Repetitive tremor of and between Cascadia 14-month ETS episodes Creager, Wech, Vidale, Melbourne

Outline ETS InterETS ETS events repeat every 142 months Look at similarities/differences among last 4 ETS episodes Compare tremor and geodetic slip There is a slip deficit, even in the region with the most slip Sharp up-dip boundary is 75 km from inferred locked zone! InterETS InterETS tremor contributes about 45% of tremor Accounts for some of the slip deficit Down dip from ETS tremor Events with duration 1 - 200 hours follow b-value distribution; b=0.9 Repeating events

Tremor Migration for four ETS events Wech, Creager and Melbourne, JGR, submitted

Tremor vs Slip Wech, Creager and Melbourne; JGR, submitted

Summed Tremor and Slip, 2004, 2005, 2007, 2008 Wech, Creager and Melbourne; JGR, submitted

Tremor from 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

Tremors relative to Japan locked zone? Tremor from Obara, 2002 Slip of 1944 and 1946 thrust events from Sagiya and Thatcher, 1999 Coseismic slip (1 m contours) of 1944 and 1946 thurst earthquakes from geodetic data and tsnuami modeling by Sagiya and Thatcher, 1999, non-volcanic tremor is from Obara, 2002.

ETS vs interETS tremor Wech, Creager and Melbourne; JGR, submitted

ETS vs interETS Tremor

Power-Law Distribution of Tremor Swarms N = A * -0.6 log10N= a 10-bMw b=0.9 if moment proportional to duration

Summary ETS InterETS ETS events repeat every 142 months 2004 and 2005 ETS migrate updip, then bifurcate 2007 and 2008 migrate north along strike; late activity in south All four match geodetic slip pattern There is a slip deficit, even in the region with the most slip Sharp up-dip boundary is 75 km from inferred locked zone InterETS InterETS tremor contributes about 45% of tremor This accounts for some of the slip deficit Locates down dip from ETS tremor Events with duration 1 - 200 hours follow b-value distribution; b=0.9 Events of all sizes repeat

N = A * Duration-0.6 Tremor swarms follow power-law distribution

If moment proportional to duration: log10N=a 10-bMw b=0.9

Three-dimensional view of the top of the subducting Juan de Fuca plate (light grey), northward migration of deep non-volcanic tremor from January 14, 2007 to January 31, 2007 (colored dots), area of the plate boundary that slipped about 2 cm during this time, and estimated Three-dimensional view of western Washington and Vancouver Island showing the plate interface between the Juan de Fuca and North American Plates (light grey surface), the Moho of the subducting plate (black surface), locations of tremor during the January, 2007 Episodic Tremor and Slip event color coded by tremor date, region of geodetically inferred slow slip (dark grey shading) and contours indicating where the plate interface is inferred to be 20%, 40% and 60% locked (blue lines). Most of the relative plate motion in the shaded area is accommodated by similar slip events which repeat every 14 months. Slip to the west of the blue lines occurs during great earthquakes such as the M 9 Cascadia Megathrust Earthquake in 1700. Tremor is from Wech and Creager, GRL, 2008; Slip from Wech et al.,submitted; Locked zone from McCaffrey et al., GJI, 2007. Wech, A. G., and K. C. Creager, 2008, Automatic detection and location of Cascadia tremor, Geophys. Res. Lett., 34, doi:10.1029/2008GL035458, 1-5. Wech, A. G., K. C. Creager and T. I. Melbourne, 2008, Seismic and geodetic constraints on Cascadia slow slip, J. Geophys. Res., 2008, submitted. McCaffrey, R., et al., 2007, Fault locking, block rotation and crustal deformation in the Pacific Northwest, Geophys. J. Int., 169, 1315-1340.

Four ETS and One Inter ETS

Two kinds of quakes? Ide et al., Nature, 2007 LFE (red), VLF (orange), and SSE (green) occur in the Nankai trough while ETS (light blue) occur in the Cascadia subduction zone. Purple circles are silent earthquakes. Black symbols are slow events. a, Slow slip in Italy, representing a typical event (circle) and proposed scaling (line). b, VLF earthquakes in the accretionary prism of the Nankai trough. c, Slow slip and creep in the San Andreas Fault. d, Slow slip beneath Kilauea volcano. e, Afterslip of the 1992 Sanriku earthquake. Ide et al., Nature, 2007

Inter ETS Tremor Feb 2007-April 2008