Editing Earthquake prediction (section)

==== Changes in V<sub>p</sub>/V<sub>s</sub> ====
''V''<sub>p</sub> is the symbol for the velocity of a seismic "P" (primary or pressure) wave passing through rock, while ''V''<sub>s</sub> is the symbol for the velocity of the "S" (secondary or shear) wave. Small-scale laboratory experiments have shown that the ratio of these two velocities – represented as ''V''<sub>p</sub>/''V''<sub>s</sub> – changes when rock is near the point of fracturing. In the 1970s it was considered a likely breakthrough when Russian seismologists reported observing such changes (later discounted.<ref name=":3">{{Harvnb|Hough|2010b}}.</ref>) in the region of a subsequent earthquake.<ref>{{Harvnb|Hammond|1973}}. Additional references in {{Harvnb|Geller|1997|loc=§2.4}}.</ref> This effect, as well as other possible precursors, has been attributed to dilatancy, where rock stressed to near its breaking point expands (dilates) slightly.<ref name=":4">{{Harvnb|Scholz|Sykes|Aggarwal|1973}}.</ref>

Study of this phenomenon near [[Blue Mountain Lake (New York lake)|Blue Mountain Lake]] in [[New York State]] led to a successful albeit informal prediction in 1973,<ref>{{Harvnb|Aggarwal|Sykes|Simpson|Richards|1975}}.</ref> and it was credited for predicting the 1974 Riverside (CA) quake.<ref name=":2"/> However, additional successes have not followed, and it has been suggested that these predictions were a fluke.<ref>{{Harvnb|Hough|2010b|p=110}}.</ref> A ''V''<sub>p</sub>/''V''<sub>s</sub> anomaly was the basis of a 1976 prediction of a M 5.5 to 6.5 earthquake near Los Angeles, which failed to occur.<ref>{{Harvnb|Allen|1983|p=79}}; {{Harvnb|Whitcomb|1977}}.</ref> Other studies relying on quarry blasts (more precise, and repeatable) found no such variations,<ref>{{Harvnb|McEvilly|Johnson|1974}}.</ref> while an analysis of two earthquakes in California found that the variations reported were more likely caused by other factors, including retrospective selection of data.<ref>{{Harvnb|Lindh|Lockner|Lee|1978}}.</ref> {{Harvtxt|Geller|1997}} noted that reports of significant velocity changes have ceased since about 1980.