`Hi All. There have been talks about foreshocks to large earthquakes and if there was a way to tell if the earthquake is a foreshock to larger event. As of right now there doesn’t appear to be anyway to tell a foreshock from just another earthquake.

There were foreshocks to the Loma Prieta and Hector Mine quakes. While one set of earthquakes were recognized as foreshocks in the classic sense the others weren’t. Earthquake swarms, or cluster declare that a temblor may occur within a given time, not that one will definitely strike. The largest quake in a cluster is called the mainshock, those before it are called foreshocks, and those after it are called aftershocks.
While some large earthquakes are preceded by foreshocks there are those, which do not have any foreshocks. Knowledge of past earthquake patterns allows scientists to estimate the odds that an earthquake striking today is a foreshock and will soon be followed by a larger mainshock in the same area. These odds depend on the earthquake's magnitude and the seismic history of the fault on which it occurred. When a moderate earthquake hits California, scientists immediately estimate the probability that a damaging mainshock will follow. If the threat is significant, a warning is issued.
This warning process was put into action in June 1988 when a magnitude 5.1 shock--one of the largest in the San Francisco Bay region since the great 1906 earthquake--struck 60 miles south of San Francisco. Alerted by the USGS that there was a 1 in 20 chance of a larger earthquake in the next 5 days, the California OES issued an advisory to warn the public. (The usual daily odds of a large quake in the Bay region are 1 in 15,000.) The warning period passed without further activity. In August 1989, another earthquake hit the same area and a similar advisory was issued. Again nothing happened in the specified warning period. However, 69 days later, the area was rocked by the magnitude 7.1 Loma Prieta earthquake, which killed 63 people and caused $6 billion damage in the San Francisco Bay region.
It can be seen that there were foreshocks to both earthquakes. In the Hector Mine quake there was a very distinct luster of earthquakes hours before the main event. In the Loma Prieta quake there were some foreshocks, but not at the same magnitude, or frequency as the Loma Prieta earthquake.
Still all said and done, foreshocks appear to have the best chance in forecasting the location and time of a large earthquake. The problem however still remains the same. There isn’t a way to tell a common earthquake from a foreshock. Take Care…Don in creepy town.