Earthquakes: San Francisco

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Earthquakes: San Francisco

The prediction of earthquakes may be inexact, but it is vital, especially when large cities such as San Francisco or Los Angeles are threatened. The San Andreas Fault and two other lesser-known faults all have the potential to deliver a massive earthquake to the San Francisco Bay area. In this video segment adapted from NOVA, a seismologist interprets earthquake data and explains how these data are used to predict the location and timing of San Francisco's next big earthquake.

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Background Essay

Earthquake detection has come a long way since the 1906 earthquake hit San Francisco. Improved seismographs pick up vibrations anywhere in the world and enable scientists to identify the location and magnitude of the smallest tremor. Nevertheless, earthquake prediction remains a very inexact science. Data are scarce and difficult to interpret. Often, by the time scientists are able to predict an earthquake, it has already happened. Yet, seismologists continue to try to predict earthquakes. However, rather than trying to predict the exact timing and location of a tremor, they estimate the probability that an earthquake will strike a particular area.

Seismologists base their probability estimates on several indirect indicators. First, they look at earthquake history. Earthquake-prone areas typically experience seismic activity at regular intervals. Based on how long it has been since the last earthquake, scientists estimate the probability that a future tremor will occur within a given time period. Second, they measure the accumulation of strain in the rocks surrounding a fault relative to the amount of strain released in the most recent earthquake. This provides a measure of how much more strain the rocks can withstand before they fracture again. Seismologists also look for precursor events, such as changes in groundwater levels, uplifting or tilting of the landscape, or small tremors called foreshocks. Any of these events increases the probability that an earthquake will occur in the near future.

Because earthquakes are inherently unpredictable, governments in earthquake-prone areas have sought to limit damage and loss of life by strengthening building codes. Collapsed buildings are responsible for many earthquake-related deaths. Not surprisingly, the damage produced by an earthquake is directly related to the strength or quality of the structures involved. In 2003, a 6.6-magnitude earthquake in southeastern Iran damaged or destroyed 85 percent of the buildings in the city of Bam. Forty thousand people died and 30,000 were injured.

Increasingly, at least in developed countries, structures are built to withstand the vibrations caused by earthquakes. Builders often use materials that bend somewhat under stress, so that they sway slightly when an earthquake strikes, instead of crumbling. Many building codes call for buildings to have solid foundations attached to bedrock, which provides strength at a structure's base, even if the soil around it becomes unstable.

Discussion Questions

Why do scientists try to find ways to predict earthquakes?
Discuss the connection between clusters of seismic activity and the occurrence of major earthquakes.
What earthquake pattern do scientists see in California?
Why does the seismologist say that San Franciscans should act as though they are certain a major earthquake is going to happen?
What proactive steps can people in those areas take to prevent death or great property loss in the event of a large earthquake?