A recent Eos article [Walker, 1999] describes a correlation between El Niño occurrence and seismicity along the most rapidly spreading portion of the East Pacific Rise, located below the eastern Pacific atmospheric high-pressure zone most strongly influenced by El Niño.The correlation between seismicity and El Niño is accentuated by seismic precursors to the development of an El Niño. Walker suggests that a trigger for El Niño has not been unambiguously identified. Walker concludes that increased seismicity is likely to be associated with enhanced heat flow, which might ultimately trigger, or be connected with a trigger for, El Niño.
Assuming that a correlation between seismicity and El Niño does exist, other explanations might be found, and one is suggested here to be at least more likely, if also improbable. Changes of sea level resulting from El Niño can induce small crustal stresses, which in an area of intense seismicity may have a small effect on earthquake statistics or trigger an earthquake that was about to happen anyway While I suspect that the counter-proposal is, by itself, insufficient cause to discuss the correlation further, this argument is strengthened by the known correlation between reservoir filling and increased seismicity [Gupta and Chandha, 1995].The main problem with my counter-argument is that the changes in sea level resulting from El Niño are only tens of centimeters, resulting in stresses of less than .lbar. Probably no seismologist would be comfortable with the suggestion that such a small stress could materially affect earthquake frequencies, even in an area with exceptionally high seismicity.The counterproposal is based on the statistics of global temperature and understanding the role of El Niño therein.