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A wide depth distribution of seismic tremors along the northern Cascadia margin


The Cascadia subduction zone is thought to be capable of generating major earthquakes with moment magnitude as large as Mw = 9 at an interval of several hundred years1,2,3. The seismogenic portion of the plate interface is mostly offshore and is currently locked, as inferred from geodetic data4,5,6. However, episodic surface displacements—in the direction opposite to the long-term deformation motions caused by relative plate convergence across a locked interface—are observed about every 14 months with an unusual tremor-like seismic signature7,8,9. Here we show that these tremors are distributed over a depth range exceeding 40 km within a limited horizontal band. Many occurred within or close to the strong seismic reflectors above the plate interface where local earthquakes are absent, suggesting that the seismogenic process for tremors is fluid-related. The observed depth range implies that tremors could be associated with the variation of stress field induced by a transient slip along the deeper portion of the Cascadia interface or, alternatively, that episodic slip is more diffuse than originally suggested.

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Figure 1: Maps showing the distribution of seismograph stations (a) and the gradual migration path of seismic tremors (b).
Figure 2: Map and profiles showing two seismic tremors occurring at different locations and depths.
Figure 3: Two cross-sections showing the spatial distribution of seismic tremors with respect to background seismicity, regional tomography, and strong reflectors identified from seismic reflection surveys.
Figure 4: A comparison of frequency spectra between local earthquakes and ETS tremors in northern Cascadia.


  1. 1

    Atwater, B. F. Evidence for great Holocene earthquakes along the outer coast of Washington state. Science 236, 942–944 (1987)

    ADS  CAS  Article  Google Scholar 

  2. 2

    Atwater, B. F. & Hemphill-Haley, E. Recurrence intervals for great earthquakes of the past 3500 years at northeastern Willapa Bay, Washington. US Geol. Surv. Prof. Pap. 1576, 1–108 (1997)

    Google Scholar 

  3. 3

    Satake, K., Wang, K. & Atwater, B. F. Fault slip and seismic moment of the 1700 Cascadia earthquake inferred from Japanese tsunami descriptions. J. Geophys. Res. 108, doi:10.1029/2003JB002521 (2003)

  4. 4

    Savage, J. C. & Lisowski, M. Strain measurements and the potential for a great subduction earthquake off the coast of Washington. Science 252, 101–103 (1991)

    ADS  CAS  Article  Google Scholar 

  5. 5

    Dragert, H. & Hyndman, R. D. Continuous GPS monitoring of elastic strain in the northern Cascadia subduction zone. Geophys. Res. Lett. 22, 755–758 (1995)

    ADS  Article  Google Scholar 

  6. 6

    Wang, K., Wells, R., Mazzotti, S., Hyndman, R. D. & Sagiya, T. A revised dislocation model of interseismic deformation of the Cascadia subduction zone. J. Geophys. Res. 108, 2026, doi:10.1029/2001JB001227 (2003)

    ADS  Google Scholar 

  7. 7

    Dragert, H., Wang, K. & James, T. S. A silent slip event on the deeper Cascadia subduction interface. Science 292, 1525–1528 (2001)

    ADS  CAS  Article  Google Scholar 

  8. 8

    Miller, M. M., Melbourne, T., Johnson, D. J. & Sumner, W. Q. Periodic slow earthquakes from the Cascadia subduction zone. Science 295, 2423 (2002)

    CAS  Article  Google Scholar 

  9. 9

    Rogers, G. & Dragert, H. Episodic tremor and slip on the Cascadia subduction zone: The chatter of silent slip. Science 300, 1942–1943 (2003)

    ADS  CAS  Article  Google Scholar 

  10. 10

    Kao, H. & Shan, S.-J. The source-scanning algorithm: mapping the distribution of seismic sources in time and space. Geophys. J. Int. 157, 589–594 (2004)

    ADS  Article  Google Scholar 

  11. 11

    Ramachandran, K., Dosso, S. E., Spence, G. D., Hyndman, R. D. & Brocher, T. M. Forearc structure beneath southwestern British Columbia: A three-dimensional tomographic velocity model. J. Geophys. Res. 110, B02303, doi: 10.1029/2004JB003258 (2005)

    ADS  Article  Google Scholar 

  12. 12

    Calvert, A. J. Seismic reflection imaging of two megathrust shear zones in the northern Cascadia subduction zone. Nature 428, 163–167 (2004)

    ADS  CAS  Article  Google Scholar 

  13. 13

    Nedimovic, M. R., Hyndman, R. D., Ramachandran, K. & Spence, G. D. Reflection signature of seismic and aseismic slip on the northern Cascadia subduction interface. Nature 424, 416–420 (2003)

    ADS  CAS  Article  Google Scholar 

  14. 14

    Calvert, A. J. & Clowes, R. M. Deep, high-amplitude reflections from a major shear zone above the subducting Juan de Fuca plate. Geology 18, 1091–1094 (1990)

    ADS  Article  Google Scholar 

  15. 15

    Hyndman, R. D. Dipping seismic reflectors, electrically conductive zones, and trapped water in the crust over a subducting plate. J. Geophys. Res. 93, 13391–13405 (1988)

    ADS  Article  Google Scholar 

  16. 16

    Clowes, R. M. et al. LITHOPROBE-southern Vancouver island: Cenozoic subduction complex imaged by deep seismic reflections. Can. J. Earth Sci. 24, 31–51 (1987)

    ADS  Article  Google Scholar 

  17. 17

    Cassidy, J. F. & Ellis, R. M. S wave velocity structure of the northern Cascadia subduction zone. J. Geophys. Res. 98, 4407–4421 (1993)

    ADS  Article  Google Scholar 

  18. 18

    Peacock, S. M. Fluid processes in subduction zones. Science 248, 329–337 (1990)

    ADS  CAS  Article  Google Scholar 

  19. 19

    Wang, K. & Hyndman, R. D. Challenges in defining seismogenic zone using geodetic and structural observations. Eos (AGU Fall Meeting Suppl.) 85, abstr. S43D–05 (2004).

  20. 20

    Adam, J., Klaeschen, D., Kukowski, N. & Flueh, E. Upward delamination of Cascadia Basin sediment infill with landward frontal accretion thrusting caused by rapid glacial age material flux. Tectonics 23, TC3009, doi:10.1029/2002TC001475 (2004)

    ADS  Article  Google Scholar 

  21. 21

    von Huene, R., Ranero, C. R. & Vannucchi, P. Generic model of subduction erosion. Geology 32, 913–916 (2004)

    ADS  Article  Google Scholar 

  22. 22

    Hyndman, R. D., Mazzotti, S., Weichert, D. & Rogers, G. C. Frequency of large crustal earthquakes in Puget Sound-Southern Georgia Strait predicted from geodetic and geological deformation rates. J. Geophys. Res. 108, 2033, doi:10.1029/2001JB001710 (2003)

    ADS  Article  Google Scholar 

  23. 23

    Szeliga, W., Melbourne, T. I., Miller, M. M. & Santillan, V. M. Southern Cascadia episodic slow earthquakes. Geophy. Res. Lett. 31, L16602, doi:10.1029/2004GL020824 (2004)

    ADS  Article  Google Scholar 

  24. 24

    Broan, K. M., DeShon, H., Tryon, M., Dorman, L. & Schwartz, S. Transient fluid pulsing and noise in the Costa Rican subduction zone: Nearly silent slip events? Eos (AGU Fall Meet. Suppl.) 84, abstr. T41E–06 (2003).

  25. 25

    Obara, K. Nonvolcanic deep tremor associated with subduction in southwest Japan. Science 296, 1679–1681 (2002)

    ADS  CAS  Article  Google Scholar 

  26. 26

    Katsumata, A. & Kamara, N. Low-frequency continuous tremor around the Moho discontinuity away from volcanoes in the southwest Japan. Geophys. Res. Lett. 30, 1020, doi:10.1029/2002GL015981 (2003)

    ADS  Article  Google Scholar 

  27. 27

    Linde, A. T. & Silver, P. G. Elevation changes and the great 1960 Chilean earthquake: Support for aseismic slip. Geophys. Res. Lett. 16, 1305–1308 (1989)

    ADS  Article  Google Scholar 

  28. 28

    Thatcher, W. Seismic triggering and earthquake prediction. Nature 299, 12–13 (1982)

    ADS  Article  Google Scholar 

  29. 29

    Shimazaki, K. & Nakata, T. Time predictable recurrence model for large earthquakes. Geophys. Res. Lett. 7, 279–282 (1980)

    ADS  Article  Google Scholar 

  30. 30

    Mazzotti, S. & Adams, J. Variability of near-term probability for the next great earthquake on the Cascadia subduction zone. Bull. Seismol. Soc. Am. 94, 1954–1959 (2004)

    Article  Google Scholar 

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We thank I. Al-Khoubbi, T. Christie, T. Claydon and R. Hall for their efforts in deploying and maintaining the seismic network; A. Bird, T. Mulder and W. Bentkowski for their assistance in seismic data processing; R. Baldwin for his software support; A. Calvert, R. Hyndman and K. Wang for stimulating discussion; and the POLARIS consortium for providing data from the BC POLARIS array. Most SSA computation is done on the Mercury cluster of the University of Victoria. This work is supported in part by a USGS NEHRP research grant.

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Correspondence to Honn Kao.

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Reprints and permissions information is available at The authors declare no competing financial interests.

Supplementary information

Supplementary Notes.

This file contains Supplementary Methods, Supplementary Table S1 (a list of tremor parameters) and Supplementary Figures S1–S4. (DOC 2633 kb)

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Kao, H., Shan, SJ., Dragert, H. et al. A wide depth distribution of seismic tremors along the northern Cascadia margin. Nature 436, 841–844 (2005).

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