Cascadia Tremor Located Near Plate Interface Constrained by S Minus P Wave Times

Nonvolcanic tremor is difficult to locate because it does not produce impulsive phases identifiable
across a seismic network. An alternative approach to identifying specific phases is to measure
the lag between the S and P waves. We cross-correlate vertical and horizontal seismograms to
reveal signals common to both, but with the horizontal delayed with respect to the vertical. This
lagged correlation represents the time interval between vertical compressional waves and
horizontal shear waves. Measurements of this interval, combined with location techniques, resolve
the depth of tremor sources within T2 kilometers. For recent Cascadia tremor, the sources
locate near or on the subducting slab interface. Strong correlations and steady S-P time differences
imply that tremor consists of radiation from repeating sources.

1. K. Obara, Science 296, 1679 (2002).
2. G. Rogers, H. Dragert, Science 300, 1942 (2003).
3. K. Obara, H. Hirose, Tectonophysics 417, 33
(2006).
4. J. L. Rubinstein et al., Nature 448, 579 (2007).
5. J. Gomberg et al., Science 319, 173 (2008).
6. M. Miyazawa, J. Mori, Geophys. Res. Lett. 32, L10307
(2005).
7. J. L. Rubinstein, M. La Rocca, J. E. Vidale, K. C. Creager,
A. G. Wech, Science 319, 186 (2008).
8. D. R. Shelly, G. C. Beroza, S. Ide, S. Nakamula, Nature
442, 188 (2006).
9. D. R. Shelly, G. C. Beroza, S. Ide, Nature 446, 305
(2007).
10. Shelly D. R., G. C. Beroza, S. Ide, Geochem. Geophys.
Geosys. 8 (2007).
11. H. Kao et al., Nature 436, 841 (2005).
12. W. McCausland, S. Malone, D. Johnson, Geophys. Res. Lett.
32, L24311 (2005).
13. Wech A. G., K. C. Creager, Geophys. Res. Lett. 34,
L22306 (2007).
14. A. G. Wech, K. C. Creager, Geophys. Res. Lett. 35,
L20302 (2008).
15. H. Kao, S.-J. Shan, Geophys. J. Int. 157, 589 (2004).
16. H. Kao et al., J. Geophys. Res. 111, B03309 (2006).
17. M. La Rocca et al., Bull. Seismol. Soc. Am. 98, 620
(2008).
18. M. La Rocca et al., Geophys. Res. Lett. 32, L21319
(2005).
19. R. S. Crosson, J. Geophys. Res. 81, 3047 (1976).
20. L. A. Preston, K. C. Creager, R. S. Crosson, T. M. Brocher,
A. M. Trehu, Science 302, 1197 (2003).
21. K. Ramachandran, R. D. Hyndman, T. M. Brocher,
J. Geophys. Res. 111, B12301 (2006).
22. T. I. Melbourne, W. M. Szeliga, M. M. Miller,
V. M. Santillan, Geophys. Res. Lett. 32, L04301 (2005).
23. Ide S., D. R. Shelly, G. C. Beroza, Geophys. Res. Lett. 34,
L03308 (2007).
24. Istituto Nazionale di Geofisica e Vulcanologia and
IRIS/PASSCAL provided instruments for the arrays; other
seismograms were provided by the Pacific Northwest
Seismic Network. Discussions with T. Pratt, J. Gomberg,
H. Houston, A. Ghosh, E. Del Pezzo and comments from two
anonymous reviewers improved this manuscript. Funding
was provided by INGV, NSF and the U.S. Geological Survey.