An earthquake prediction is a prediction that an earthquake in a specific magnitude range will occur in a specific region and time window. Predictions are considered as such to the extent that they are reliable for practical, as well as scientific, purposes. Although there is evidence that at least some earthquakes in some tectonic regimes are predictable with useful accuracy of time and space, the reliability and reproducibility of prediction techniques have not been established and are therefore generally not accepted by seismologists. For practical purposes, seismologists bring forth seismic hazard assessment programs by estimating the probabilities that a given earthquake or suite of earthquakes will occur.
In the effort to predict earthquakes, people have tried to associate an impending earthquake with such varied phenomena as seismicity patterns, electromagnetic fields, weather conditions and unusual clouds, radon or hydrogen gas content of soil or ground water, water level in wells, and animal behavior.
Thus far, earthquake prediction is controversial because data are sparse and there is little evidence or verified physical theory to link observable phenomena to subsequent seismicity. The frequent practice of publishing predictions after the fact further complicates matters. Also, given enough predictions, it is virtually inevitable that some will succeed "by chance." Assessing whether a successful prediction is a fluke is challenging. Most assessments rely on chance models for earthquake occurrence, models that are difficult to test or validate, because large earthquakes are so rare, and because earthquake activity is naturally clustered in space and time.
Earthquake pre-detection is more reliable than earthquake prediction since it is based upon detecting the non-destructive primary waves that travel more quickly through the earth's crust than do the destructive secondary and Rayleigh waves, in the same way that lightning flashes reaches our eyes before we hear the thunder during a storm. The advance warning time available using this approach is only on the order of seconds (or tens of seconds for deep and distant large quakes), however there exists a technology currently in use known as the QuakeGuard™, Quake Alarm™ systems that employs this technique to automate emergency response procedures that protect against loss of life and reduce property damage.
A meaningful earthquake prediction must have all the following elements:
A meaningful 'forecast' does not require the same accuracy, rather the term refers to prognostications on a longer time scale that are usually probabilistic.
Note that a physical basis is most critical for a prediction to have meaning to the geological community. Any method to make accurate predictions would be welcome and meaningful to many people even if no mechanism were known (much like doctors are now investigating diagnostics through statistical analysis of gene expression microarrays, even when the mechanisms linking particular genes to conditions remain unknown), as long as it was repeatable and the reliability could be quantified.
There are two flavors of tidal stressing that have been claimed to generate enhanced rates of earthquakes - diurnal and biweekly tides. The diurnal correlations would arise from more earthquakes only during the hours when the tidal stress is pushing in an encouraging direction, in contrast, biweekly effects would be based on earthquakes occurring during the days when the sinusoidal stressing oscillations are largest. The former, as most easily observed in the twice-daily rise and fall of the ocean tides, have occasionally been shown to influence tides (e.g.,, this paper shows there may be some weak tidal triggering of shallow, oceanic thrust-faulting earthquakes). The latter, which arises from the periodic alignment of the Sun and Moon, has often been claimed in the popular press to incubate earthquakes (sometimes termed the "syzygy" effect) and occasionally for small datasets in the scientific literature (e.g.,), but generally such effects do not appear in careful studies of large datasets.
Syzygy, which is not given much credence in the scientific community, is motivated by the observation that, historically, there have been some great earthquakes whose timing with when the tidal forces are near their maximum. For maximum tidal force, three factors must coincide: First, when the moon (in its elliptical orbit) is closest to the earth; second, when it is within a day or two of a new moon (so that the tidal forces of the moon and sun are acting in concert); and third, when the earth (in its elliptical orbit) is at or near its closest distance to the sun.
Shallow earthquakes near mid-ocean ridges, volcanic earthquakes, and episodic tremor and slip have also been observed to sometimes correlate with the diurnal tides, with enhanced activity correlating with times that faults are unclamped.
It is claimed that animals can detect the P-wave or ultrasonic wave generated by a big underground explosion or the rupture of an earthquake, even if the waves are too small for humans' senses. These waves travel faster than the S-wave and Rayleigh earthquake waves that most strongly shake the ground and causes the most damage; when this happens, animals can detect the incoming earthquake wave, and start behaving agitatedly or nervously.
Others postulate that the animal behavior is simply their response to an increase in low-frequency electromagnetic signals. The University of Colorado has demonstrated that electromagnetic activity can be generated by the fracturing of crystalline rock. Such activity occurs in fault lines before earthquakes. According to recent research, electromagnetic sensors yield statistically valid results in predicting earthquakes.
Some people believe that in these ways, animals sense the immediate onset of earthquakes. In support of this claim, instances are cited when people have witnessed flight of animals just before an earthquake disaster. In fact, according to the Chief conservator of forests for the Indian state of Tamil Nadu, a few minutes before the killer tsunami waves generated by an underwater earthquake hit the Indian coastline in December 2004, a 500-strong herd of blackbucks rushed away from the coastal areas to the safety of a nearby hilltop. Since the beginning of recorded history, observations of unusual animal behavior before earthquakes have been recorded by people from almost all civilizations. The Chinese began a systematic study of this unusual animal behavior and in December 1974 predicted a major earthquake that did, in fact, occur in February 1975. But skeptics claim to debunk nearly all such observations. In fact, the 1975 prediction relied most heavily on a series of strong foreshocks. The animal behavior reports are often ambiguous and not consistently observed. There is little evidence for animals being able to sense impending earthquakes, although it is likely they can sense the initial, weaker P-waves before people. Seismometers remain much more sensitive than even the animals, however.
In folklore, some animals have had more reports of being able to predict earthquakes than others, especially dogs, cats, chickens, horses, and other smaller animals. There have been reports with elephants, as well. Goats, cows, and most larger animals are generally reported as being less able to predict earthquakes.
Japan has a long tradition associating catfish with earthquake prediction. From this idea emerged a long university research programme concluding in 2004 in which it was proposed that the (established) high sensitivity of catfish to electric fields was involved in detecting fields of a few hertz because of piezoelectric effects on deeply buried quartz crystals. Actual monitoring of catfish and correlation with earthquakes gave results that are not promising.
In April 2004, the California Earthquake Prediction Evaluation Council (CEPEC) evaluated Keilis-Borok's prediction and reported to the California State Office of Emergency Services. CEPEC concluded that the "uncertainty along with the large geographic area included in the prediction (about 12,400 square miles) leads (us) to conclude that the results do not at this time warrant any special policy actions in California.” The predicted time window came and went with no significant earthquake.
Based on the historic record of the various published efforts to predict a quake, one might conclude that earthquake prediction is usually imprecise, but remains an art that is scientifically and socially useful.
According to new research to be published by Prof. Shlomo Havlin, of Bar-Ilan University's Department of Physics, earthquakes form patterns which can improve the ability to predict the timing of their recurrence. In November 2005 (November 11 issue) the journal Physical Review Letters, published by the American Physical Society, published an article by researchers from Israel and Germany that say that there is a way to predict when the next earthquake will hit.
Prof. Shlomo Havlin's from Bar-Ilan University in Israel, in collaboration with Prof. Armin Bunde, of the Justus-Liebig University in Giessen, Germany, and Bar-Ilan University graduate student Valerie Livina used the "scaling" approach from physics to develop a mathematical function to characterize earthquakes of a wide range of magnitudes in order to learn from smaller magnitude earthquakes about larger magnitude earthquakes. The team's findings reveal that the recurrence of earthquakes is strongly dependent on the recurrence times of previous earthquakes.
This memory effect not only provides a clue to understanding the observed clustering of earthquakes, but also suggests that delays in earthquake occurrences, as seen today in Tokyo and in San Francisco, are a natural phenomenon.
In another paper in the journal Nature, Richard Allen of the University of California claims that the distinction between small and large earthquakes can be made from the very first seconds of seismic energy recorded by seismometers, though other scientists are not convinced. If correct this may make earthquake early warning (as distinct from prediction) more powerful. Earthquake early warning provides an alarm that strong shaking is due soon to arrive, and the more quickly that the magnitude of an earthquake can be estimated, the more useful is the early warning. However, earthquake early warning can still be effective without the ability to infer the magnitude of an earthquake in its initial second or two.
From 1968 to 1988 scientists in California mapped seismic activity on a cross section of the fault lines. They identified a "seismic gap" in the Loma Prieta area from various features of the regional seismicity. They therefore concluded that Loma Prieta was due for an earthquake. On 17 October 1989 the Loma Prieta earthquake occurred, initially reported as measuring 7.1 on the Richter scale but later more accurately recorded as moment magnitude 6.9, causing 63 deaths. This prediction, however was not very useful as it could not predict the exact date.
Earthquake prediction by geometric relationship of planets, moon, and sun.