Lateral variations in compressional/shear velocities at the base of the mantle

Science

Volumn 284, Issue 5411, 1999, Pages 120-125

  Wysession, Michael E ^a   Langenhorst, Amy ^a   Fouch, Matthew J ^a   Fischer, Karen M ^a   Al Eqabi, Ghassan I ^a   Shore, Patrick J ^a   Clarke, Timothy J ^a  

^a NONE

Author keywords

[No Author keywords available]

Indexed keywords

MANTLE; MINERALOGY; ROCK MICROSTRUCTURE; SEISMIC VELOCITY; SHEAR STRESS;

ARTICLE; COMPRESSION; GEOLOGY; PRIORITY JOURNAL; SHEAR STRESS; TOPOGRAPHY;

EID: 0033515819     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.284.5411.120     Document Type: Article

References (56)

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11. J.-M. Kendall and P. G. Silver, in (5), pp. 97-118.
12. Relative to SV, observed SH velocities have been either fast (24) [L. P. Vinnik, V. Farra, B. Romanowicz, Bull. Seismol. Soc. Am. 79, 1542 (1989); L. Vinnik, B. Romanowicz, Y. Le Stunff, L. Makeyeva, Geophys. Res. Lett. 22, 1657 (1995)], slow [J. Pulliam and M. K. Sen, Geophys. J. Int. 135, 113 (1998)], the same (9), or variable at small to intermediate spatial scales (24) [L. Vinnik, L. Bregér, B. Romanowicz, Nature 393, 564 (1998); S. A. Russell, T. Lay, E. J. Garnero, ibid. 396, 255 (1999)].
13. Relative to SV, observed SH velocities have been either fast (24) [L. P. Vinnik, V. Farra, B. Romanowicz, Bull. Seismol. Soc. Am. 79, 1542 (1989); L. Vinnik, B. Romanowicz, Y. Le Stunff, L. Makeyeva, Geophys. Res. Lett. 22, 1657 (1995)], slow [J. Pulliam and M. K. Sen, Geophys. J. Int. 135, 113 (1998)], the same (9), or variable at small to intermediate spatial scales (24) [L. Vinnik, L. Bregér, B. Romanowicz, Nature 393, 564 (1998); S. A. Russell, T. Lay, E. J. Garnero, ibid. 396, 255 (1999)].
14. Relative to SV, observed SH velocities have been either fast (24) [L. P. Vinnik, V. Farra, B. Romanowicz, Bull. Seismol. Soc. Am. 79, 1542 (1989); L. Vinnik, B. Romanowicz, Y. Le Stunff, L. Makeyeva, Geophys. Res. Lett. 22, 1657 (1995)], slow [J. Pulliam and M. K. Sen, Geophys. J. Int. 135, 113 (1998)], the same (9), or variable at small to intermediate spatial scales (24) [L. Vinnik, L. Bregér, B. Romanowicz, Nature 393, 564 (1998); S. A. Russell, T. Lay, E. J. Garnero, ibid. 396, 255 (1999)].
15. Relative to SV, observed SH velocities have been either fast (24) [L. P. Vinnik, V. Farra, B. Romanowicz, Bull. Seismol. Soc. Am. 79, 1542 (1989); L. Vinnik, B. Romanowicz, Y. Le Stunff, L. Makeyeva, Geophys. Res. Lett. 22, 1657 (1995)], slow [J. Pulliam and M. K. Sen, Geophys. J. Int. 135, 113 (1998)], the same (9), or variable at small to intermediate spatial scales (24) [L. Vinnik, L. Bregér, B. Romanowicz, Nature 393, 564 (1998); S. A. Russell, T. Lay, E. J. Garnero, ibid. 396, 255 (1999)].
16. Relative to SV, observed SH velocities have been either fast (24) [L. P. Vinnik, V. Farra, B. Romanowicz, Bull. Seismol. Soc. Am. 79, 1542 (1989); L. Vinnik, B. Romanowicz, Y. Le Stunff, L. Makeyeva, Geophys. Res. Lett. 22, 1657 (1995)], slow [J. Pulliam and M. K. Sen, Geophys. J. Int. 135, 113 (1998)], the same (9), or variable at small to intermediate spatial scales (24) [L. Vinnik, L. Bregér, B. Romanowicz, Nature 393, 564 (1998); S. A. Russell, T. Lay, E. J. Garnero, ibid. 396, 255 (1999)].
17. R. W. Valenzuela and M. E. Wysession, in (5), pp. 57-71.
18. The MOMA Broadband Deployment involved the temporary installation of 18 broadband-frequency seismic sensors in a linear array between permanent stations CCM (Cathedral Caves, MO) and HRV (Harvard, MA). Instruments were borrowed from the PASSCAL program of the Incorporated Research Institutions for Seismology (IRIS), and data were recorded between January 1995 and April 1996. The location was chosen because it records on average more than half (53%) of large global earthquakes in the range 100° to 140°, and as such it is ideal for the detection of core-diffracted waves [M. E. Wysession et al., Eos 77, 477 (1996)]. MOMA was unusual for seismic arrays in that it was largely designed for the purpose of examining structure at the base of the mantle.
19. M. E. Wysession, E. A. Okal, C. R. Bina, J. Geophys. Res. 97, 8749 (1992); G. S. Robertson and J. H. Woodhouse, ibid. 101, 20041 (1996); H. Bolton and G. Masters, Eos 77, F697 (1996).
20. M. E. Wysession, E. A. Okal, C. R. Bina, J. Geophys. Res. 97, 8749 (1992); G. S. Robertson and J. H. Woodhouse, ibid. 101, 20041 (1996); H. Bolton and G. Masters, Eos 77, F697 (1996).
21. M. E. Wysession, E. A. Okal, C. R. Bina, J. Geophys. Res. 97, 8749 (1992); G. S. Robertson and J. H. Woodhouse, ibid. 101, 20041 (1996); H. Bolton and G. Masters, Eos 77, F697 (1996).
22. s ratios across the bottom 250 km of the mantle are 1.888 to 1.878 for the Preliminary Reference Earth Model (PREM) (25) and 1.875 to 1.876 for the lasp91 model (B. L. N. Kennett and E. R. Engdahl, Ceophys. J. Int. 105, 429 (1991)].
23. For P-velocity models with slower than average D″ velocities beneath the northernmost Pacific and Alaska, see (3); H. Inoue, Y. Fukao, K. Tanabe, Y. Ogata, Phys. Earth Planet. Inter. 59, 294 (1990); R. J. Pulliam, D. W. Vasco, L. R. Johnson, J. Geophys. Res. 98, 699 (1993); M. E. Wysession, Nature 382, 244 (1996). For S-velocity models with faster than average D″ velocities beneath the northernmost Pacific and Alaska, see (4); T. Tanimoto, Geophys. J. Int. 100, 327 (1990); W. Su, R. L. Woodward, A. M. Dziewonski J. Geophys. Res. 99, 6945 (1994); G. Masters, S. Johnson, G. Laske, H. Bolton, Philos. Trans. R. Soc. London Ser. A 354, 1385 (1996); X.-D. Li and B. Romanowicz, J. Geophys. Res. 101, 22245 (1996); M. Sylvander and A. Souriau, Phys. Earth Planet, Inter. 94, 1 (1996); A. M. Dziewonski, G. Ekström, X.-F. Liu, in Monitoring a Comprehensive Test Ban Treaty (Kluwer Academic, Dordrecht, Netherlands, 1996), pp. 521-550; B. Y. Kuo and K. Y. Wu, J. Geophys. Res. 102, 11775 (1997).
24. For P-velocity models with slower than average D″ velocities beneath the northernmost Pacific and Alaska, see (3); H. Inoue, Y. Fukao, K. Tanabe, Y. Ogata, Phys. Earth Planet. Inter. 59, 294 (1990); R. J. Pulliam, D. W. Vasco, L. R. Johnson, J. Geophys. Res. 98, 699 (1993); M. E. Wysession, Nature 382, 244 (1996). For S-velocity models with faster than average D″ velocities beneath the northernmost Pacific and Alaska, see (4); T. Tanimoto, Geophys. J. Int. 100, 327 (1990); W. Su, R. L. Woodward, A. M. Dziewonski J. Geophys. Res. 99, 6945 (1994); G. Masters, S. Johnson, G. Laske, H. Bolton, Philos. Trans. R. Soc. London Ser. A 354, 1385 (1996); X.-D. Li and B. Romanowicz, J. Geophys. Res. 101, 22245 (1996); M. Sylvander and A. Souriau, Phys. Earth Planet, Inter. 94, 1 (1996); A. M. Dziewonski, G. Ekström, X.-F. Liu, in Monitoring a Comprehensive Test Ban Treaty (Kluwer Academic, Dordrecht, Netherlands, 1996), pp. 521-550; B. Y. Kuo and K. Y. Wu, J. Geophys. Res. 102, 11775 (1997).
25. For P-velocity models with slower than average D″ velocities beneath the northernmost Pacific and Alaska, see (3); H. Inoue, Y. Fukao, K. Tanabe, Y. Ogata, Phys. Earth Planet. Inter. 59, 294 (1990); R. J. Pulliam, D. W. Vasco, L. R. Johnson, J. Geophys. Res. 98, 699 (1993); M. E. Wysession, Nature 382, 244 (1996). For S-velocity models with faster than average D″ velocities beneath the northernmost Pacific and Alaska, see (4); T. Tanimoto, Geophys. J. Int. 100, 327 (1990); W. Su, R. L. Woodward, A. M. Dziewonski J. Geophys. Res. 99, 6945 (1994); G. Masters, S. Johnson, G. Laske, H. Bolton, Philos. Trans. R. Soc. London Ser. A 354, 1385 (1996); X.-D. Li and B. Romanowicz, J. Geophys. Res. 101, 22245 (1996); M. Sylvander and A. Souriau, Phys. Earth Planet, Inter. 94, 1 (1996); A. M. Dziewonski, G. Ekström, X.-F. Liu, in Monitoring a Comprehensive Test Ban Treaty (Kluwer Academic, Dordrecht, Netherlands, 1996), pp. 521-550; B. Y. Kuo and K. Y. Wu, J. Geophys. Res. 102, 11775 (1997).
26. For P-velocity models with slower than average D″ velocities beneath the northernmost Pacific and Alaska, see (3); H. Inoue, Y. Fukao, K. Tanabe, Y. Ogata, Phys. Earth Planet. Inter. 59, 294 (1990); R. J. Pulliam, D. W. Vasco, L. R. Johnson, J. Geophys. Res. 98, 699 (1993); M. E. Wysession, Nature 382, 244 (1996). For S-velocity models with faster than average D″ velocities beneath the northernmost Pacific and Alaska, see (4); T. Tanimoto, Geophys. J. Int. 100, 327 (1990); W. Su, R. L. Woodward, A. M. Dziewonski J. Geophys. Res. 99, 6945 (1994); G. Masters, S. Johnson, G. Laske, H. Bolton, Philos. Trans. R. Soc. London Ser. A 354, 1385 (1996); X.-D. Li and B. Romanowicz, J. Geophys. Res. 101, 22245 (1996); M. Sylvander and A. Souriau, Phys. Earth Planet, Inter. 94, 1 (1996); A. M. Dziewonski, G. Ekström, X.-F. Liu, in Monitoring a Comprehensive Test Ban Treaty (Kluwer Academic, Dordrecht, Netherlands, 1996), pp. 521-550; B. Y. Kuo and K. Y. Wu, J. Geophys. Res. 102, 11775 (1997).
27. For P-velocity models with slower than average D″ velocities beneath the northernmost Pacific and Alaska, see (3); H. Inoue, Y. Fukao, K. Tanabe, Y. Ogata, Phys. Earth Planet. Inter. 59, 294 (1990); R. J. Pulliam, D. W. Vasco, L. R. Johnson, J. Geophys. Res. 98, 699 (1993); M. E. Wysession, Nature 382, 244 (1996). For S-velocity models with faster than average D″ velocities beneath the northernmost Pacific and Alaska, see (4); T. Tanimoto, Geophys. J. Int. 100, 327 (1990); W. Su, R. L. Woodward, A. M. Dziewonski J. Geophys. Res. 99, 6945 (1994); G. Masters, S. Johnson, G. Laske, H. Bolton, Philos. Trans. R. Soc. London Ser. A 354, 1385 (1996); X.-D. Li and B. Romanowicz, J. Geophys. Res. 101, 22245 (1996); M. Sylvander and A. Souriau, Phys. Earth Planet, Inter. 94, 1 (1996); A. M. Dziewonski, G. Ekström, X.-F. Liu, in Monitoring a Comprehensive Test Ban Treaty (Kluwer Academic, Dordrecht, Netherlands, 1996), pp. 521-550; B. Y. Kuo and K. Y. Wu, J. Geophys. Res. 102, 11775 (1997).
28. For P-velocity models with slower than average D″ velocities beneath the northernmost Pacific and Alaska, see (3); H. Inoue, Y. Fukao, K. Tanabe, Y. Ogata, Phys. Earth Planet. Inter. 59, 294 (1990); R. J. Pulliam, D. W. Vasco, L. R. Johnson, J. Geophys. Res. 98, 699 (1993); M. E. Wysession, Nature 382, 244 (1996). For S-velocity models with faster than average D″ velocities beneath the northernmost Pacific and Alaska, see (4); T. Tanimoto, Geophys. J. Int. 100, 327 (1990); W. Su, R. L. Woodward, A. M. Dziewonski J. Geophys. Res. 99, 6945 (1994); G. Masters, S. Johnson, G. Laske, H. Bolton, Philos. Trans. R. Soc. London Ser. A 354, 1385 (1996); X.-D. Li and B. Romanowicz, J. Geophys. Res. 101, 22245 (1996); M. Sylvander and A. Souriau, Phys. Earth Planet, Inter. 94, 1 (1996); A. M. Dziewonski, G. Ekström, X.-F. Liu, in Monitoring a Comprehensive Test Ban Treaty (Kluwer Academic, Dordrecht, Netherlands, 1996), pp. 521-550; B. Y. Kuo and K. Y. Wu, J. Geophys. Res. 102, 11775 (1997).
29. For P-velocity models with slower than average D″ velocities beneath the northernmost Pacific and Alaska, see (3); H. Inoue, Y. Fukao, K. Tanabe, Y. Ogata, Phys. Earth Planet. Inter. 59, 294 (1990); R. J. Pulliam, D. W. Vasco, L. R. Johnson, J. Geophys. Res. 98, 699 (1993); M. E. Wysession, Nature 382, 244 (1996). For S-velocity models with faster than average D″ velocities beneath the northernmost Pacific and Alaska, see (4); T. Tanimoto, Geophys. J. Int. 100, 327 (1990); W. Su, R. L. Woodward, A. M. Dziewonski J. Geophys. Res. 99, 6945 (1994); G. Masters, S. Johnson, G. Laske, H. Bolton, Philos. Trans. R. Soc. London Ser. A 354, 1385 (1996); X.-D. Li and B. Romanowicz, J. Geophys. Res. 101, 22245 (1996); M. Sylvander and A. Souriau, Phys. Earth Planet, Inter. 94, 1 (1996); A. M. Dziewonski, G. Ekström, X.-F. Liu, in Monitoring a Comprehensive Test Ban Treaty (Kluwer Academic, Dordrecht, Netherlands, 1996), pp. 521-550; B. Y. Kuo and K. Y. Wu, J. Geophys. Res. 102, 11775 (1997).
30. For P-velocity models with slower than average D″ velocities beneath the northernmost Pacific and Alaska, see (3); H. Inoue, Y. Fukao, K. Tanabe, Y. Ogata, Phys. Earth Planet. Inter. 59, 294 (1990); R. J. Pulliam, D. W. Vasco, L. R. Johnson, J. Geophys. Res. 98, 699 (1993); M. E. Wysession, Nature 382, 244 (1996). For S-velocity models with faster than average D″ velocities beneath the northernmost Pacific and Alaska, see (4); T. Tanimoto, Geophys. J. Int. 100, 327 (1990); W. Su, R. L. Woodward, A. M. Dziewonski J. Geophys. Res. 99, 6945 (1994); G. Masters, S. Johnson, G. Laske, H. Bolton, Philos. Trans. R. Soc. London Ser. A 354, 1385 (1996); X.-D. Li and B. Romanowicz, J. Geophys. Res. 101, 22245 (1996); M. Sylvander and A. Souriau, Phys. Earth Planet, Inter. 94, 1 (1996); A. M. Dziewonski, G. Ekström, X.-F. Liu, in Monitoring a Comprehensive Test Ban Treaty (Kluwer Academic, Dordrecht, Netherlands, 1996), pp. 521-550; B. Y. Kuo and K. Y. Wu, J. Geophys. Res. 102, 11775 (1997).
31. For P-velocity models with slower than average D″ velocities beneath the northernmost Pacific and Alaska, see (3); H. Inoue, Y. Fukao, K. Tanabe, Y. Ogata, Phys. Earth Planet. Inter. 59, 294 (1990); R. J. Pulliam, D. W. Vasco, L. R. Johnson, J. Geophys. Res. 98, 699 (1993); M. E. Wysession, Nature 382, 244 (1996). For S-velocity models with faster than average D″ velocities beneath the northernmost Pacific and Alaska, see (4); T. Tanimoto, Geophys. J. Int. 100, 327 (1990); W. Su, R. L. Woodward, A. M. Dziewonski J. Geophys. Res. 99, 6945 (1994); G. Masters, S. Johnson, G. Laske, H. Bolton, Philos. Trans. R. Soc. London Ser. A 354, 1385 (1996); X.-D. Li and B. Romanowicz, J. Geophys. Res. 101, 22245 (1996); M. Sylvander and A. Souriau, Phys. Earth Planet, Inter. 94, 1 (1996); A. M. Dziewonski, G. Ekström, X.-F. Liu, in Monitoring a Comprehensive Test Ban Treaty (Kluwer Academic, Dordrecht, Netherlands, 1996), pp. 521-550; B. Y. Kuo and K. Y. Wu, J. Geophys. Res. 102, 11775 (1997).
32. For P-velocity models with slower than average D″ velocities beneath the northernmost Pacific and Alaska, see (3); H. Inoue, Y. Fukao, K. Tanabe, Y. Ogata, Phys. Earth Planet. Inter. 59, 294 (1990); R. J. Pulliam, D. W. Vasco, L. R. Johnson, J. Geophys. Res. 98, 699 (1993); M. E. Wysession, Nature 382, 244 (1996). For S-velocity models with faster than average D″ velocities beneath the northernmost Pacific and Alaska, see (4); T. Tanimoto, Geophys. J. Int. 100, 327 (1990); W. Su, R. L. Woodward, A. M. Dziewonski J. Geophys. Res. 99, 6945 (1994); G. Masters, S. Johnson, G. Laske, H. Bolton, Philos. Trans. R. Soc. London Ser. A 354, 1385 (1996); X.-D. Li and B. Romanowicz, J. Geophys. Res. 101, 22245 (1996); M. Sylvander and A. Souriau, Phys. Earth Planet, Inter. 94, 1 (1996); A. M. Dziewonski, G. Ekström, X.-F. Liu, in Monitoring a Comprehensive Test Ban Treaty (Kluwer Academic, Dordrecht, Netherlands, 1996), pp. 521-550; B. Y. Kuo and K. Y. Wu, J. Geophys. Res. 102, 11775 (1997).
33. Y. Ricard, M. A. Richards, C. Lithgow-Berteltoni, Y. Le Stunff, J. Geophys. Res. 98, 21895 (1993); C. Lithgow-Bertelloni and M. A. Richards, Rev. Geophys. 36, 27 (1998).
34. Y. Ricard, M. A. Richards, C. Lithgow-Berteltoni, Y. Le Stunff, J. Geophys. Res. 98, 21895 (1993); C. Lithgow-Bertelloni and M. A. Richards, Rev. Geophys. 36, 27 (1998).
35. Melt may accumulate at the base of the mantle because it may have a greater density than surrounding solids [S. M. Rigden, T. J. Ahrens, E. M. Stolper J. Geophys Res. 94, 9508 (1989)], especially if iron partitions preferentially into the liquid [E. McFarlane, M. J. Drake, D. C. Rubie, Geochim. Cosmochim. Acta 58, 5161 (1994); E. Ohtani, T. Kato, E. Ito, Geophys. Res. Lett. 18, 85 (1991)].
36. Melt may accumulate at the base of the mantle because it may have a greater density than surrounding solids [S. M. Rigden, T. J. Ahrens, E. M. Stolper J. Geophys Res. 94, 9508 (1989)], especially if iron partitions preferen tially into the liquid [E. McFarlane, M. J. Drake, D. C. Rubie, Geochim. Cosmochim. Acta 58, 5161 (1994); E. Ohtani, T. Kato, E. Ito, Geophys. Res. Lett. 18, 85 (1991)].
37. Melt may accumulate at the base of the mantle because it may have a greater density than surrounding solids [S. M. Rigden, T. J. Ahrens, E. M. Stolper J. Geophys Res. 94, 9508 (1989)], especially if iron partitions preferentially into the liquid [E. McFarlane, M. J. Drake, D. C. Rubie, Geochim. Cosmochim. Acta 58, 5161 (1994); E. Ohtani, T. Kato, E. Ito, Geophys. Res. Lett. 18, 85 (1991)].
38. M. E. Wysession, C. R. Bina, E. A. Okal, in Dynamics of the Earth's Deep Interior and Earth Rotation, J.-L LeMoüel et al., Eds. (American Geophysical Union, Washington, DC, 1993), pp. 181-190.
39. J.-M. Kendall and P. G. Silver, Nature 381, 409 (1996).
40. P = -30% with a volume of 24%. Elongated anomalies might result from the lateral shearing of D″ beneath the more viscous lower mantle, though the inclusion volumes and velocity differences are extremely large. With the fast-velocity tubules, all velocities are increased, and a mechanism (most likely chemical) is required to lower them all so that the P velocities are slower than the global average. With the slow-velocity tubules, all velocities are decreased, so a mechanism (most likely thermal) is required to make the S velocities faster than the global average.
41. T. Lay, E. J. Garnero, V. Maupin, Eos 79, F607 (1998).
42. L. Stixrude, in (5), pp. 83-96; S. I. Karato, S. Zhang, H. R. Wenk, Science 270, 458 (1995).
43. L. Stixrude, in (5), pp. 83-96; S. I. Karato, S. Zhang, H. R. Wenk, Science 270, 458 (1995).
44. S. Karato et al., Eos 79, F607 (1998).
45. M. J. Fouch, K. M. Fischer, M. E. Wysession, ibid., p. F617.
46. A. M. Dziewonski and D. L. Anderson, Phys. Earth Planet. Inter 25, 297 (1981).
47. M. J. Fouch, K. M. Fischer, M. E. Wysession, T. J. Clarke, in preparation.
48. A. H. Mula and G. Müller, Pure Appl. Geophys. 118. 1270 (1980).
49. E. A. Okal and R. J. Geller, Bull. Seismol. Soc. Am. 69, 1039 (1979).
50. diff phases due to receiver-side mantle anisotropy above D″ are small, and time corrections for this effect are not applied to the data.
51. diff phases due to receiver-side mantle anisotropy above D″ are small, and time corrections for this effect are not applied to the data.
52. Selecting onset arrival times, common for most other seismic phases, is not possible for core-diffracted waves because of the rapid loss of high frequencies during diffraction. A linear regression through the times of the wave peak maxima is the common means of determining the apparent slowness (18, 27) [J. C. Mondt, Phys. Earth Planet. Inter. 15, 46 (1977); A. Souriau and G. Poupinet, ibid. 84, 227 (1994)], and this has been shown to be as reliable as using a multiwaveform cross-correlation [M. E. Wysession and E. A. Okal, Geophys. Res. Lett. 16, 1417 (1989)].
53. Selecting onset arrival times, common for most other seismic phases, is not possible for core-diffracted waves because of the rapid loss of high frequencies during diffraction. A linear regression through the times of the wave peak maxima is the common means of determining the apparent slowness (18, 27) [J. C. Mondt, Phys. Earth Planet. Inter. 15, 46 (1977); A. Souriau and G. Poupinet, ibid. 84, 227 (1994)], and this has been shown to be as reliable as using a multiwaveform cross-correlation [M. E. Wysession and E. A. Okal, Geophys. Res. Lett. 16, 1417 (1989)].
54. Selecting onset arrival times, common for most other seismic phases, is not possible for core-diffracted waves because of the rapid loss of high frequencies during diffraction. A linear regression through the times of the wave peak maxima is the common means of determining the apparent slowness (18, 27) [J. C. Mondt, Phys. Earth Planet. Inter. 15, 46 (1977); A. Souriau and G. Poupinet, ibid. 84, 227 (1994)], and this has been shown to be as reliable as using a multiwaveform cross-correlation [M. E. Wysession and E. A. Okal, Geophys. Res. Lett. 16, 1417 (1989)].
55. M. E. Wysession, L. Bartkó, J. Wilson, J Geophys. Res. 99, 13667 (1994).
56. We thank the many people with the IRIS PASSCAL and DMC programs who helped with the MOMA experiment and K. Koper, A. Li, E. Roth, L. Salvati, R. Valenzuela, and J. Zaslow for additional help with the MOMA deployment. Supported by NSF grants EAR-9319324, EAR-9315971, and EAR-9315925 and by the David and Lucile Packard Foundation.