Hi Mary. I think this is the answer you’re looking for in regards to why if the quake had been M>5.0 or larger the people at USGS would have been concerned.

Some large earthquakes are preceded by 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.

Some good came out of the Loma Prieta earthquake. In any cluster, most quakes are aftershocks. Most aftershocks are too small to cause damage, but following a large mainshock one or more may be powerful. Such strong aftershocks can cause additional damage and casualties in areas already devastated by a mainshock and also threaten the lives of rescuers searching for the injured. In the first few weeks after the 1994 magnitude 6.7 Northridge, California, earthquake, more than 3,000 aftershocks occurred. One magnitude 5.2 aftershock caused $7 million in damage just to electric utility equipment in the Los Angeles area.

The U.S. Geological Survey (USGS) first began forecasting aftershocks following the 1989 magnitude 7.1 Loma Prieta, California, earthquake. By studying previous earthquakes, scientists had detected patterns in the way aftershocks decrease in number and magnitude with time. With such knowledge, scientists can estimate the daily odds for the occurrence of damaging aftershocks following large California temblors. These forecasts are relayed directly to the California Office of Emergency Services (OES) and to the public. In this case it was done during the rescues on the Cypress Freeway. When the “P” wave of a strong aftershock was detected they would advise the scene commander who would in turn warn the people working on the Cypress Freeway. The time period between the arrival of the “P” and the “S” gave the rescuers sufficient time to clear the area. Once the “S” wave passed though the area they went back to work. Compressional waves, also known as primary or P waves, travel the fastest, at speeds between 1.5 and 8 kilometers per second in the Earth's crust. Shear waves, also known as secondary or S waves, travel more slowly, usually at 60% to 70% of the speed of P waves. Take Care…Don in creepy town
Reference:
Andrew Michael, Paul Reasenberg, Peter H. Stauffer, and James W. Hendley II

COOPERATING ORGANIZATIONS
California Office of Emergency Services
California Division of Mines and Geology

For more information contact:
Earthquake Information Hotline (415) 329-4085
U.S. Geological Survey, MS 977
345 Middlefield Road, Menlo Park, CA 94025
USGS Menlo Park Earthquakes Home Page