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1989: Loma Prieta, U.S.

The 1989 Loma Prieta earthquake (epicenter in the Santa Cruz Mountains northwest of San Juan Bautista, California) caused significant damage in the San Francisco Bay Area of California.[194] The U.S. Geological Survey (USGS) reportedly claimed, twelve hours after the event, that it had "forecast" this earthquake in a report the previous year.[195] USGS staff subsequently claimed this quake had been "anticipated";[196] various other claims of prediction have also been made.[197]

Harris (1998) reviewed 18 papers (with 26 forecasts) dating from 1910 "that variously offer or relate to scientific forecasts of the 1989 Loma Prieta earthquake." (In this case no distinction is made between a forecast, which is limited to a probabilistic estimate of an earthquake happening over some time period, and a more specific prediction.[198]) None of these forecasts can be rigorously tested due to lack of specificity,[199] and where a forecast does bracket the correct time and location, the window was so broad (e.g., covering the greater part of California for five years) as to lose any value as a prediction. Predictions that came close (but given a probability of only 30%) had ten- or twenty-year windows.[200]

One debated prediction came from the M8 algorithm used by Keilis-Borok and associates in four forecasts.[201] The first of these forecasts missed both magnitude (M 7.5) and time (a five-year window from 1 January 1984, to 31 December 1988). They did get the location, by including most of California and half of Nevada.[202] A subsequent revision, presented to the NEPEC, extended the time window to 1 July 1992, and reduced the location to only central California; the magnitude remained the same. A figure they presented had two more revisions, for M ≥ 7.0 quakes in central California. The five-year time window for one ended in July 1989, and so missed the Loma Prieta event; the second revision extended to 1990, and so included Loma Prieta.[203]

When discussing success or failure of prediction for the Loma Prieta earthquake, some scientists argue that it did not occur on the San Andreas fault (the focus of most of the forecasts), and involved dip-slip (vertical) movement rather than strike-slip (horizontal) movement, and so was not predicted.[204]. The map showing the places of future strong earthquales was published in 1976 (Gelfand, Keilis-Borok, Knopoff, and Press) indicates that seismicity of Loma Prieta area is controlled by the intersection of faults located at Calaveras. The reliability of this map was proved during 20 years preceding the Loma Prieta event: in 1975 – 1994 eleven earthquakes with M>= 6.0 occurred in California, and ten of them occurred in predicted places. Because the total predicted danger area covers less than 18% of California, the probability of such a coincidence occurring by chance is negligibly small.

Other scientists argue that it did occur in the San Andreas fault zone, and released much of the strain accumulated since the 1906 San Francisco earthquake; therefore several of the forecasts were correct.[205] Hough states that "most seismologists" do not believe this quake was predicted "per se".[206] In a strict sense there were no predictions, only forecasts, which were only partially successful. Iben Browning claimed to have predicted the Loma Prieta event, but (as will be seen in the next section) this claim has been rejected.