Can Earthquakes be Predicted?
No. Earthquakes cannot be predicted. Earthquake prediction can be considered the "holy grail" of earthquake science. Despite efforts of many scientists including geologists, physicist, mathematicians, engineers, biologists, amongst others, as of today is not possible to predict location, time, or the magnitude (size) of an earthquake in a specific region (e.g. Puerto Rico).
A Classic Example of an Attempt at Earthquake Prediction
One classic example of an attempt at earthquake prediction occurred in what is dubbed the earthquake capital of the world, Parkfield, CA. Scientists observed that in Parkfield an event with a magnitude greater than 6.0 occurred approximately every 22 years. Based on this observation it was predicted that the next event would occur between 1988 and 1992. Therefore, a dense network of seismographic stations was installed and equipment for geodesic measurements assembled with a series of devices to measure the levels of phreatic mantle electric resistance, magnetic fields and geochemical changes in the zone. The awaited earthquake with a magnitude of 6.0, did not occur until September 28 of 2004 over a decade later than the scientists predicted. Needless to say, this example shows that currently the only thing predictable about earthquakes is that they are unpredictable.
Forecasting is Different than Prediction
Earthquake forecasting is different from earthquake prediction. Like weather forecasting, earthquake forecasting attempts to provide the probability of a significant earthquake in a specific region over a specific time range, usually on the order of 20 or 50 years. These forecasts are based on multiple datasets including historic seismicity, mapping of faults, and measurements of tectonic plate velocities.
Earthquake Forecasting for Puerto Rico
In Puerto Rico, between 1670 and today, 4 significant, destructive earthquakes have occurred in 1670, 1787, 1867, and 1918. This historical earthquake record reflects a 51 to 117-year cycle or an average of 83 years for destructive earthquakes. However, it should be noted that each one of these events were generated along a different fault, therefore, a calculation of their re-occurrence rate cannot be based exclusively from these events.