Latest Quake Info | General Quake Info | Hazards & Preparedness | Earthquake Research | Special Features | Additional Resources |
On April 18, 1906, a powerful earthquake (magnitude 7.8) struck Northern California. During the minute of violent ground shaking, San Francisco and many smaller cities were devastated. At least 700 people were killed, and losses totaled more than $400 million (1906 dollars). The ground broke along the San Andreas Fault over nearly half the length of California (about 300 miles). Analysis by U.S. Geological Survey (USGS) scientists shows that ground movement along the fault (slippage) locally exceeded 25 feet near both Point Reyes and Shelter Cove, 150 miles apart on the coast north of San Francisco.
The "Great 1906 San Francisco Earthquake" is one of the strongest ever recorded on the North American continent. If a similar earthquake occurred in Northern California today, after many decades of rapid urban growth, thousands of people would likely be killed and economic losses might be in the hundreds of billions of dollars. Such an event would easily be the worst natural disaster in the Nation's history.
How soon is such an event likely to happen? Recent research offers some answers by providing new insights into the 1906 quake and the San Andreas Fault system. For example, geologists have dug trenches across the San Andreas Fault to find out how often very powerful temblors occur in Northern California. In these trenches, they have identified soil layers disrupted by a great earthquake before 1906. Charcoal in these layers has been shown to be about 300 years old. These and similar studies in Southern California, as well as thousands of years of historical records from China and Japan, tell us that giant earthquakes on faults like the San Andreas tend to occur every few hundred years. All this evidence suggests that another shock as powerful as the 1906 earthquake is not likely to strike Northern California soon, probably not for at least 100 years.
Unfortunately, this reassuring result is balanced by other, more somber findings. Although a great earthquake is not expected soon, stress on the San Andreas Fault system has built up again after having been relieved by slippage in the 1906 shock. Seven decades of low seismic activity in Northern California ended with the occurrence between 1979 and 1984 of three quakes with magnitudes of about 6 and the occurrence in 1989 of the devastating Loma Prieta earthquake (magnitude 7.0, recently revised from 7.1). In the 70 years prior to the great 1906 quake, moderate earthquakes were common in the region, and shocks of magnitude 6 to 7 occurred every 10 to 15 years. Scientists believe that a similar period of strong quake activity may now be underway.
Because of extensive urban development in Northern California since 1906, the strong earthquakes expected in the coming decades may be very destructive. For example, a magnitude 7 earthquake occurring today on the Hayward Fault (a part of the San Andreas Fault system, along the densely populated eastern side of San Francisco Bay) would likely cause hundreds of deaths and almost $100 billion of damage. In 1990, the USGS reported that there is a 67% chance that one or more quakes of about magnitude 7 or larger will occur in the San Francisco Bay area before the year 2020.
Future strong temblors in Northern California are inevitable, but the damage they cause can be reduced significantly with adequate preparation. Studies of earthquake shaking, active faults, and the response of structures to shaking have already led to improved building codes and a better understanding of how to reduce the threat posed by earthquakes.
In 1990, the USGS, in close cooperation with the Red Cross, United Way, and many other contributors, created a magazine titled The Next Big Earthquake in the Bay Area May Come Sooner Than You Think-Are You Prepared? This magazine was distributed as a public service by 41 San Francisco Bay area newspapers to more than 2.5 million households. In addition to describing the earthquake risk, the publication advised residents on how to strengthen their homes and protect their families against earthquake shaking.
By studying quakes like the Great 1906 San Francisco Earthquake, scientists and engineers gain the knowledge and understanding necessary to assess the risk from future shocks and to reduce the vulnerability of buildings and other structures to damage in these inevitable and terrifying events. In this way they help to protect the lives and property of citizens of the United States.
Wayne R. Thatcher, Peter L. Ward,
David J. Wald, James W. Hendley II,
and Peter H. Stauffer
Graphic design by
Susan Mayfield and Sara Boore
COOPERATING ORGANIZATIONS
California Institute of Technology
California State Division of Mines and Geology
Geomatrix Consultants Inc.
Harlan Tait Associates Inc.
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
U.S. Geological Survey Fact Sheet-094-96
Web page by Will Prescott - 1996 April 9