Seismic and geodetic constraints on Cascadia slow slip

[1] Automatically detected and located tremor epicenters from episodic tremor and slip (ETS) episodes in northern Cascadia provide a high-resolution map of Washington's slow slip region. Thousands of epicenters from the past four ETS events from 2004 to 2008 provide detailed map-view constraints that correlate with geodetic estimates of the simultaneous slow slip. Each of these ETS events exhibits remarkable similarity in the timing and geographic distribution of tremor density and geodetically inferred slip. Analysis of the latest 15-month inter-ETS period also reveals ageodetic tremor activity similar both in duration and extent to ETS tremor. Epicenters from both ETS and inter-ETS tremor are bounded between the 30- and 45-km plate interface depth contours and locate approximately 75 km east of previous estimates of the locked portion of the subducting Juan de Fuca plate. Inter-ETS tremor overlaps but is generally downdip of ETS tremor and does not yet correlate with geodetically observed slip, but this is likely because the slip is below current GPS detection levels. Based on the tremor and slip correlation and the tremor-duration and slip magnitude relationship, we suggest that the well-resolved, sharp updip edge of tremor epicenters reflects a change in plate interface coupling properties. The region updip of this boundary may accumulate stress with the potential for coseismic shear failure during a megathrust earthquake. Alternatively, plate convergence in this region could be accommodated by continuous slow slip with no detectable tremor or by slow slip events with sufficiently long recurrence intervals that none have been detected during the past 10 years of GPS observations.

[1]  Douglas S. Wilson.  Confidence intervals for motion and deformation of the Juan de Fuca Plate , 1993 .

[2]  K. Creager,et al.  Cascadia tremor polarization evidence for plate interface slip , 2007 .

[3]  Aaron G. Wech,et al.  Automated detection and location of Cascadia tremor , 2008 .

[4]  P. Lomdahl,et al.  Nonvolcanic Deep Tremor Associated with Subduction in Southwest Japan Kazushige Obara , 2008 .

[5]  G. Beroza,et al.  Low-frequency earthquakes in Shikoku, Japan, and their relationship to episodic tremor and slip , 2006, Nature.

[6]  S. Sekine,et al.  Slow Earthquakes Coincident with Episodic Tremors and Slow Slip Events , 2007, Science.

[7]  Slow Earthquakes , 1999, Science.

[8]  A. Kato,et al.  High Pore Fluid Pressure May Cause Silent Slip in the Nankai Trough , 2004, Science.

[9]  W. Szeliga,et al.  GPS constraints on 34 slow slip events within the Cascadia subduction zone, 1997–2005 , 2008 .

[10]  G. Beroza,et al.  Bridging the gap between seismically and geodetically detected slow earthquakes , 2008 .

[11]  H. Kao,et al.  A wide depth distribution of seismic tremors along the northern Cascadia margin , 2005, Nature.

[12]  S. Mazzotti,et al.  Variability of Near-Term Probability for the Next Great Earthquake on the Cascadia Subduction Zone , 2004 .

[13]  Robert W. King,et al.  Fault locking, block rotation and crustal deformation in the Pacific Northwest , 2007 .

[14]  Herb Dragert,et al.  Automatic detection and characterization of seismic tremors in northern Cascadia , 2007 .

[15]  K. Obara,et al.  Deep low‐frequency tremors as a proxy for slip monitoring at plate interface , 2008 .

[16]  S. Malone,et al.  Temporal and spatial occurrence of deep non‐volcanic tremor: From Washington to northern California , 2005 .

[17]  S. Malone,et al.  Cascadia Tremor Located Near Plate Interface Constrained by S Minus P Wave Times , 2009, Science.

[18]  Kelin Wang,et al.  Three‐dimensional dislocation model for great earthquakes of the Cascadia Subduction Zone , 1997 .

[19]  G. Beroza,et al.  Non-volcanic tremor and low-frequency earthquake swarms , 2007, Nature.

[20]  G. Beroza,et al.  A scaling law for slow earthquakes , 2007, Nature.

[21]  Kazushige Obara,et al.  Nonvolcanic Deep Tremor Associated with Subduction in Southwest Japan , 2002, Science.

[22]  Richard M. Allen,et al.  Segmentation in episodic tremor and slip all along Cascadia , 2006 .

[23]  Tim Melbourne,et al.  Periodic Slow Earthquakes from the Cascadia Subduction Zone , 2002, Science.

[24]  P. A. McCrory,et al.  Depth to the Juan De Fuca slab beneath the Cascadia subduction margin - a 3-D model for sorting earthquakes , 2004 .

[25]  H. Dragert,et al.  Episodic Tremor and Slip on the Cascadia Subduction Zone: The Chatter of Silent Slip , 2003, Science.

[26]  A. C. Aguiar,et al.  Moment release rate of Cascadia tremor constrained by GPS , 2009 .

[27]  Kelin Wang,et al.  A Silent Slip Event on the Deeper Cascadia Subduction Interface , 2001, Science.

[28]  G. Beroza,et al.  Mechanism of deep low frequency earthquakes: Further evidence that deep non‐volcanic tremor is generated by shear slip on the plate interface , 2007 .

[29]  C. Goldfinger,et al.  HOLOCENE EARTHQUAKE RECORDS FROM THE CASCADIA SUBDUCTION ZONE AND NORTHERN SAN ANDREAS FAULT BASED ON PRECISE DATING OF OFFSHORE TURBIDITES , 2003 .

[30]  Kenji Satake,et al.  Fault slip and seismic moment of the 1700 Cascadia earthquake inferred from Japanese tsunami descriptions , 2003 .