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EARTHQUAKE PREDICTION

Earthquakes, like hurricanes and other natural phenomena, every year produce numerous losses of both life and property around the world. With this in mind, and given the destructive capacity of an earthquake of great magnitude, one of the greatest challenges of modern science is earthquake prediction. This has been a multidisciplinary effort that has surrounded geologists, physicists, mathematicians, engineers, biologists, and many other profesionals. Earthquake predictions has included methods that go from the creation of Earth models, using geophysics techniques , to studies in animal behavior.

In an effort to obtain a prediction method of seismic events, there are efforts that range from short term predictions to long term predictions. For short and medium term predictions, the goal is to obtain a method that is at least as effective as the prediction of meteorological phenomena. For long term predictions, the time cycles are of the order of 50, 100 or up to 250 years. For both short and long term predictions, the question is not only when, but also where is going to happen an earthquake and what its magnitude and intensity will be.

Many prediction efforts have been based on identifying earthquake foreshock signals. One of the best-known cases of correctly-guessed predictions in this field, was the earthquake of 1975 in the province of Liaoning, China. In 1975, a joint effort of the government and sectors of the Chinese population, managed to identify noticeable changes in the elevation of the ground and the levels of the phreatic mantle. These circumstances were added to an increase in the seismic activity of the area and reports of strange animal behavior in all the province. This made the Chinese gobernment to broadcast an earthquake vigilance, and later an earthquake alert for the zone (short term prediction). This alert, along with an evacuation order for the population, was emitted a day before an earthquake of magnitude 7.3 shook the region. It was estimated that this action from the Chinese government saved the life of 250,000 people. This case is considered an exceptional case, because earthquakes usually do not present/display precursory elements as evident as these. Proof of this situation was the incapacity of the Chinese gobernment to predict the earthquake of magnitude 7.6 that whipped the province of Tangshan in 1976, resulting in an estimate amount of 650.000 deaths, or the case of Kwantung in August of 1976 where the population spent two months waiting for a predicted earthquake that never materialized (Bolt, 1981).

A good example of a effort of a mid term prediction, is the case of Parkfield. In 1984 an impressive network of equipment was installed in a part of San André's Fault, a place that already had a well-defined seismic pattern: an event with magnitude greater to 6.0 every 22 years. It was considered that between 1988 and 1992, this segment of the fault, located near the town of Parkfield, would generate an earthquake. Therefore, a dense network of seismic stations and geodesic measuring equipment with a series of apparatuses to measure levels of the phreatic mantle, electrical resistivity, magnetic fields and geoquímicos changes in the zone. Until today, the prediction of Parkfield has not been fulfilled. Even worse, this effort was not able to warn the population with the earthquake of 1989 in Dark Hill, magnitude 7.1 (the died earthquake of the World-wide Series with 63), or the devastating event of Northridge, magnitude 6.8 and its effects (about 57 died).

Different prediction methodologies exist for long term earthquake predictions. By means of studies of the distribution of the seismic activity at world-wide level, it has been possible to identify those places where the probability of an event of great magnitude is greater, for example in the zones of contact of the tectonic plates, like Puerto Rico. This, nevertheless, must be interpreted as a possibility and not like as an absolute guarantee of the occurrence of an event in a specific area.


For the long term prediction some studies are based on the recurrence of events. For example, in Puerto Rico there have been approximately 1670 earthquakes, of these only four earthquakes are of great intensity: 1670, 1787, 1867 and 1918. This earthquake distribution reflects a cycle of 51 to 117 years or an average of 83 years for destructive earthquakes. Nevertheless, it is necessary to indicate that each one of these events was generated throughout a different fault, therefore, in base of these events exclusively a prediction cannot be done on its recurrence.

For other studies, by far more complete, the geologic maps of the particular zone are examined, putting special attention to active faults and other geologic structures. This is correlated with the distribution of intensity of historical earthquakes, models of seismic attenuation and other statistical data. The result of this, are maps of seismic zones that identify the regions where different degrees of intensity can be anticipated. Often these studies not only consider the direct risk of an earthquake, but also of other phenomena associated as a result of the earthquake, such as tidal waves (tsunamis), liquification, seismic amplification and landslides. The identification of these zones of high risk is necessary to make decisiosn of land use and the preparation of contingency plans.

In Puerto Rico, studies of seismic vulnerability have shown a probability between 33% and 50% of an earthquake of strong intensity (Intensity VII or more in the Modified Mercalli scale) for different parts from the island over a period of 50 years (McCann, 1987). On the other hand, it is recognized that the low coastal zones are more vulnerable to phenomena like tidal waves, liquification and amplification of seismic wave, whereas in the interior the greatest threat are landslides.

Long-term predictions do not offer a 100% warranty of what can happen, and although up until now a prediction method with a strong scientific foundation that may allow the exact prediction of a seismic event of any magnitude does not exist, the time and the effort used with this aim has not been in vain, since great advances in the sismología have arisen as a result of these investigations.

Figure 1: Map that shows the probability of great seismic intensity over a period of 50 years (McCann, 1987).





References:

* Bolt, B., 1981. Terremotos. Editorial Reverté, Barcelona, 266p.

* McCann, W., 1987. Preliminary Seismic Hazard Map.