<P> In July Brady formally withdrew his prediction on the grounds that prerequisite seismic activity had not occurred . Economic losses due to reduced tourism during this episode has been roughly estimated at one hundred million dollars . </P> <P> The "Parkfield earthquake prediction experiment" was the most heralded scientific earthquake prediction ever . It was based on an observation that the Parkfield segment of the San Andreas Fault breaks regularly with a moderate earthquake of about M 6 every several decades: 1857, 1881, 1901, 1922, 1934, and 1966 . More particularly, Bakun & Lindh (1985) pointed out that, if the 1934 quake is excluded, these occur every 22 years, ± 4.3 years . Counting from 1966, they predicted a 95% chance that the next earthquake would hit around 1988, or 1993 at the latest . The National Earthquake Prediction Evaluation Council (NEPEC) evaluated this, and concurred . The U.S. Geological Survey and the State of California therefore established one of the "most sophisticated and densest nets of monitoring instruments in the world", in part to identify any precursors when the quake came . Confidence was high enough that detailed plans were made for alerting emergency authorities if there were signs an earthquake was imminent . In the words of the Economist: "never has an ambush been more carefully laid for such an event ." </P> <P> 1993 came, and passed, without fulfillment . Eventually there was an M 6.0 earthquake on the Parkfield segment of the fault, on 28 September 2004, but without forewarning or obvious precursors . While the experiment in catching an earthquake is considered by many scientists to have been successful, the prediction was unsuccessful in that the eventual event was a decade late . </P> <P> In 1981, the "VAN" group, headed by Panayiotis Varotsos, said that they found a relationship between earthquakes and' seismic electric signals' (SES). In 1984 they presented a table of 23 earthquakes from 19 January 1983 to 19 September 1983, of which they claimed to have successfully predicted 18 earthquakes . Other lists followed, such as their 1991 claim of predicting six out of seven earthquakes with M ≥ 5.5 in the period of 1 April 1987 through 10 August 1989, or five out of seven earthquakes with M ≥ 5.3 in the overlapping period of 15 May 1988 to 10 August 1989, In 1996 they published a "Summary of all Predictions issued from January 1st, 1987 to June 15, 1995", amounting to 94 predictions . Matching this against a list of "All earthquakes with M (ATH)" and within geographical bounds including most of Greece they come up with a list of 14 earthquakes they should have predicted . Here they claim ten successes, for a success rate of 70%, but also a false alarm rate of 89% . </P>

Which parameter was a precursor to the 2004 parkfield earthquake
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