Seismology refutes global clustering of M9 earthquakes. Really?
Two recent papers (Shearer and Stark, 2011; Michael 2011) unfortunately perpetuate a long-standing notion that statistical analysis of the ~ 100 year instrumental earthquake catalog informs us about clustering of M~9 subduction earthquakes. This fallacy, perpetuated now for decades, has misled us into categorizing subduction zones by seismic capacity based on plate age and convergence rate. Shearer and Stark (2011) make the same error committed in the 1970’s by turning to the instrumental catalog to “test” the possibility of clustering of M9 earthquakes. There are a number of serious flaws in this approach. With regard to the greatest earthquakes, recurrence times can be many hundreds of years. Cascadia varies from 240-500 years, with gaps as long as 1200 years. Cascadia has likely had two superquakes in 10,000 years, with long term cycling and clustering of events that is now becoming apparent (Goldfinger et al., 2011;submitted). NE Japan likely had its penultimate M~9 event in the year 869, (Minoura et al. 2001). During the intervening ~1000 years, numerous smaller earthquakes in the 8.2-8.4 range used only a small fraction of the accumulated strain, requiring the eventual superquake of March 2011 (forecast by Ikeda, 2005). The Sumatran subduction zone (Sieh et al., 2008), Cascadia and NE Japan apparently each have long term energy cycling, with groups of smaller events punctuated by larger events and long time gaps in their histories. These three zones are the only three with paleoseismic records long enough to make these observations.
A second problem is the authors, who are well aware of the problems with short records, attempt to circumvent what we consider to be a brick wall by using smaller earthquakes with higher frequencies. This approach simply does not address the question of whether clustering of M9 earthquakes exists, but instead answers an entirely different question, that is have global rates of M> 7 earthquakes clustered in the 20th century. We see no direct connection between these two distinct questions. One need look no further than Cascadia, which has a b value of near zero, to see the fallacy of this assumption.
A third problem is that the basic observation that M9 events have clustered twice in the 1957-1965 period, and again 2004-2011 is questioned because a mechanism is not known. Numerous arguments have been made against both static and dynamic triggering, the only two obvious options, and tests of both of them have been made as well using, again, smaller earthquakes. Geology has a rather sordid history of throwing out observations for lack of a good mechanism. Plate Tectonics and the Missoula Floods come to mind. Lack of a mechanism is not evidence, it’s simply a neutral observation that may or not be relevant.
To evaluate global clustering of M9 earthquakes, long paleoseismic records from more subduction zones are required, and it is unlikely that seismology can address this question. Statistical tests do not address this problem because they use a much larger range of earthquake magnitudes, addressing a different question. As with plate tectonics, the current absence of evidence for a mechanism, is not evidence of absence of global clustering.
Goldfinger, C., Nelson, C.H., Morey, A., Johnson, J.E., Gutierrez-Pastor, J., Eriksson, A.T., Karabanov, E., Patton, J., Gracia, E., Enkin, R., Dallimore, A., Dunhill, G., and Vallier, T., 2011, Turbidite Event History: Methods and Implications for Holocene Paleoseismicity of the Cascadia Subduction Zone, USGS Professional Paper 1661-F, Reston, VA, U.S. Geological Survey, 332 p, 64 Figures.
Goldfinger, C., Ikeda, Y., and Yeats, R.S., 2011, Superquakes and Supercycles, AGU fall meeting and submitted paper.
Ikeda, Y., 2005, Long-term and short-term rates of horizontal shortening over the Northeast Japan arc, Hokudan International Symposium on Active Faulting: January 17-24 2005, Hokudan City, Japan.
Michael, A. J., 2011, Random variability explains apparent global clustering of large earthquakes, Geophys. Res. Lett., 38, L21301, doi:10.1029/2011GL049443.
Minoura, K., Imamura, F., Sugawara, D., Kono, Y., and Iwashita, T., 2001, The 869 Jogan tsunami deposit and recurrence interval of large-scale tsunami on the Pacific coast of northeast Japan: Journal of Natural Disaster Science, v. 23, p. 83-88.
Shearer, P.M., and Stark, P.B., 2011, Global risk of big earthquakes has not recently increased: Proceedings of the National Academy of Sciences, published ahead of print December 19, 2011, doi:10.1073/pnas.1118525109.
Shishikura, M., Sawai, Y., Okamura, Y., Komatsubara, J., Tin Aung, T., Ishiyama, T., Fujiwara, O., and Fujino, S., 2007, Age and distribution of tsunami deposit in the Ishinomaki plain, Northeast Japan: Annual Report on Active Fault and Paleoearthquake Researches, p. 31-46.
Sieh, K., Natawidjaja, D.H., Meltzner, A.J., Shen, C.-C., Cheng, H., Li, K.-S., Suwargadi, B.W., Galetzka, J., Philibosian, B., and Edwards, R.L., 2008, Earthquake Supercycles Inferred from Sea-Level Changes Recorded in the Corals of West Sumatra: Science, v. 322, p. 1674-1678.