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###      Reducing Earthquake Losses Throughout the United States

# When Will the Next Great Quake Strike Northern California?

(Click on image for a full size version - 136K) The upper photograph, taken from a tethered balloon five weeks after thegreat earthquake of April 18, 1906, shows the devastation wrought on the cityof San Francisco by the quake and subsequent fire. In the city andsurrounding region, at least 700 people were killed and losses totaled morethan $400 million (1906 dollars). The lower view of San Francisco, taken froma similar vantage point atop a tall residential apartment building in 1996,shows the development of the densely populated city since the great quake. Ifa similar earthquake occurred in Northern California today, after manydecades of rapid urban growth, thousands of people would likely be killed,and economic losses might be in the hundreds of billions of dollars. 
On April 18, 1906, a powerful earthquake (magnitude 7.8) struck NorthernCalifornia. During the minute of violent ground shaking, San Francisco andmany smaller cities were devastated. At least 700 people were killed, andlosses totaled more than $400 million (1906 dollars). The ground broke alongthe San Andreas Fault over nearly half the length of California (about 300miles). Analysis by U.S. Geological Survey (USGS) scientists shows thatground movement along the fault (slippage) locally exceeded 25 feet near bothPoint Reyes and Shelter Cove, 150 miles apart on the coast north of SanFrancisco. 
The "Great 1906 San Francisco Earthquake" is one of the strongest everrecorded on the North American continent. If a similar earthquake occurred inNorthern California today, after many decades of rapid urban growth,thousands of people would likely be killed and economic losses might be inthe hundreds of billions of dollars. Such an event would easily be the worstnatural disaster in the Nation's history.
(Click on image for a full size version - 161K)In this trench dug across the San Andreas Fault near Loma Prieta, California,U.S. Geological Survey scientists identified soil layers disrupted by thelast giant earthquake before the Great 1906 San Francisco Earthquake.Radiocarbon dating of plant remains from the layers in the trench indicatesthat this quake occurred about A.D. 1600, before written historical recordswere kept in California.
How soon is such an event likely to happen? Recent research offers someanswers by providing new insights into the 1906 quake and the San AndreasFault system. For example, geologists have dug trenches across the SanAndreas Fault to find out how often very powerful temblors occur in NorthernCalifornia. In these trenches, they have identified soil layers disrupted bya great earthquake before 1906. Charcoal in these layers has been shown to beabout 300 years old. These and similar studies in Southern California, aswell as thousands of years of historical records from China and Japan, tellus that giant earthquakes on faults like the San Andreas tend to occur everyfew hundred years. All this evidence suggests that another shock as powerfulas the 1906 earthquake is not likely to strike Northern California soon,probably not for at least 100 years.
(Click on image for a full size version - 48K)SLIPPAGE ALONG THE SAN ANDREAS FAULT IN THE GREAT 1906 EARTHQUAKE In theGreat 1906 San Francisco Earthquake, the ground broke along the San AndreasFault over nearly half the length of California (about 300 miles). Groundmovement along the fault (slippage) locally exceeded 25 feet near both PointReyes and Shelter Cove. Scientists believe that future giant earthquakes willoccur only after the slow and steady (about 3/4 inch per year) relativemotion (red arrows) of the Pacific and North America Plates has built upstress along the fault to 1906 levels. This '"recharge" of stress should takefrom 130 to 400 years at different sites along the fault.  
(Click on image for a full size version - 136K)The photo shows a fence near Bolinas, about 20 miles southeast of Point Reyes, that was offset8 1/2 feet by slippage in the 1906 quake.
Unfortunately, this reassuring result is balanced by other, more somberfindings. Although a great earthquake is not expected soon, stress on the SanAndreas Fault system has built up again after having been relieved byslippage in the 1906 shock. Seven decades of low seismic activity in NorthernCalifornia ended with the occurrence between 1979 and 1984 of three quakeswith magnitudes of about 6 and the occurrence in 1989 of the devastating LomaPrieta earthquake (magnitude 7.0, recently revised from 7.1). In the 70 yearsprior to the great 1906 quake, moderate earthquakes were common in theregion, 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 beunderway.
Because of extensive urban development in Northern California since 1906, thestrong earthquakes expected in the coming decades may be very destructive.For example, a magnitude 7 earthquake occurring today on the Hayward Fault (apart of the San Andreas Fault system, along the densely populated easternside 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% chancethat one or more quakes of about magnitude 7 or larger will occur in the SanFrancisco Bay area before the year 2020.
(Click on image for a full size version - 126K)Between 1836 and 1911, moderate earthquakes were common in NorthernCalifornia, and shocks of magnitude 6.5 to 7 occurred every 10 to 15 years.However, from 1911 to 1979, no temblors of even moderate magnitude occurred.In 1979, a new period of high earthquake activity, similar to the period prior to 1911, appears to have begun. The magnitude and year of each quakeare shown. (Some magnitudes are recent updates based on the latest U.S.Geological Survey data.)
Future strong temblors in Northern California are inevitable, but the damagethey cause can be reduced significantly with adequate preparation. Studies ofearthquake shaking, active faults, and the response of structures to shakinghave already led to improved building codes and a better understanding of howto 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, scientistsand engineers gain the knowledge and understanding necessary to assess therisk from future shocks and to reduce the vulnerability of buildings andother structures to damage in these inevitable and terrifying events. In thisway they help to protect the lives and property of citizens of the UnitedStates.
**Wayne R. Thatcher, Peter L. Ward,David J. Wald, James W. Hendley II, and Peter H. StaufferGraphic design bySusan Mayfield and Sara Boore**
**COOPERATING ORGANIZATIONS**California Institute of TechnologyCalifornia State Division of Mines and GeologyGeomatrix Consultants Inc.Harlan Tait Associates Inc.
For more information contact:Earthquake Information Hotline (415) 329-4085U.S. Geological Survey, MS 977345 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