Many of us breathed a little easier after October 17, 1989. The Loma Prieta earthquake, 7.1 on the Richter scale, meant that the big one, talked about for decades, had finally happened. And, bad as it was, we had survived.

There are two things wrong with that. First, Loma Prieta was not the big one. It was a moderately big one, certainly destructive to some parts of the Bay Area, but nowhere near the size of the great San Francisco earthquake of 1906. Second, having an earthquake like Loma Prieta has little to do with the likelihood of having another one on a different fault, somewhere else in the area.

The inevitability of a damaging earthquake still confronts everybody in the Bay Area, and we still risk substantial damage. A new study, released in 2003 by the United States Geological Survey, says that there is a 62 percent chance of a M>=6.7 earthquake during the next 30 years and that the quake could strike at any time, including today. In other words, scientists think that a magnitude 6.7 or larger earthquake is nearly twice as likely to happen as not to happen. This is a substantial increase, since in 1988, scientists thought the chance for such an earthquake was 50 percent (just as likely to occur as not to occur) within 30 years.

The new report also says that the next one will most likely strike farther north than Loma Prieta, somewhere between San Jose and Santa Rosa on either side of the Bay. The epicenter of the October 1989 quake was in a sparsely populated area. The next one, according to the study, will likely be centered in a more populated area. During the Loma Prieta earthquake, shaking was so severe in the Santa Cruz Mountains that a van overturned, treetops snapped off, and many people were thrown to the ground. Because the next one is expected to strike closer to an urban area, it will cause much more damage.

Fortunately, there is something we can do about it. By taking actions, such as those described in this booklet, we can drastically reduce the losses and we can make the Bay Area a safer place to live.

Earthquake damage is particularly great in certain locations and in certain buildings. Most locations and most modern buildings are relatively safe. By identifying the greatest hazards, we can set priorities for using our limited resources most effectively to reduce them. The choice is ours.

1. Strength of shaking. Strength decreases rapidly with distance from the earthquake. The strong shaking along the fault segment that slips during an earthquake becomes half as strong at a distance of 8 miles, a quarter as strong at a distance of 17 miles, an eighth as strong at a distance of 30 miles, and a sixteenth as strong at a distance of 50 miles.
2. Length of shaking. Length depends on how the fault breaks during the earthquake. The maximum shaking during the Loma Prieta earthquake lasted only 10 to 15 seconds. During other magnitude 7 earthquakes in the Bay Area, the shaking may last 30 to 40 seconds. The longer buildings shake, the greater the damage.
3. Type of soil. Shaking is increased in soft, thick, wet soils. In certain soils the ground surface may settle or slide.
4. Type of building. Certain types of buildings, discussed in the reducing earthquake damage section, are not resistant enough to the side-to-side shaking common during earthquakes.