Earthquake information on the Web 30 Glossary 31 Introduction The Bay Area Is “Earthquake Country” T his handbook provides information about the threat posed by earthquakes in the San Francisco Bay region and explains how you can prepare for, survive, and recover from these inevitable events. If you live or work in the region, you need to know why you should be concerned with earthquakes, what you can expect during and after a quake, and what you need to do beforehand to be safe and reduce damage. Since the Great earthquake of 1906, much has been learned about earthquake hazards and vulnerabilities in the Bay Area: • We know why earthquakes occur here—The Bay Area straddles the boundary where two of the Earth’s largest tectonic plates meet and slowly move past one another. When boundary faults break and the North American and Pacific Plates lurch past each other, quakes occur. • We know large and damaging earthquakes are certain to occur in the future—At least eight faults in the Bay Area are capable of producing earthquakes of magnitude 6.7 or larger. Such quakes can kill and injure many people and cause substantial damage to buildings, roads, bridges, and utilities. • We know how to reduce losses in future large earthquakes—Building codes have been improved, some older buildings strengthened, and bond measures approved to upgrade critical facilities. Some Bay Area residents have secured their homes to better withstand shaking, created emergency plans and disaster supply kits, and held home earthquake drills. BUT we have not done enough to be prepared for the next large earthquake: • Fewer than 10% of households have disaster plans— If an earthquake occurred right now, where would you go to be safe? If you are at work and your children are at school when the earthquake occurs, how will you get back together? • Fewer than 10% of homeowners have taken steps to retrofit their homes—Is your home bolted to its foundation? If you live in an older building, has it been retrofitted? Is your water heater strapped? Could unsecured furniture or objects fall and cause injury or damage? • Fewer than 50% of households have disaster supply kits—You will likely be on your own in the hours and days following an earthquake. Are you prepared with water, food, first aid supplies, and medications? 3 Why Should I Care? All Bay Area Residents Live on an Active Plate Boundary Where Earthquakes Are Frequent Events! W e know that the San Andreas Fault produces large earthquakes and that many other Bay Area faults are also hazardous. However, even knowing this, it can be difficult to understand how to use this information to make us safer in our daily lives. Should we care only if we live near the San Andreas Fault, or is every place in the Bay Area just as dangerous? This eight-page section describes where earthquakes occur in northern California. It also explains how earthquakes will shake the ground and cause damage in other ways, such as liquefaction and landslides (see pages 8 through 11). Technical terms used throughout this book are explained in the Glossary (see page 31). Myth number 1 ADA Plate boundary faults Earthquake rupture tle h i n g t o n t a Se a s W Casc adi Jan. 1700 M9 a S ub du c e Fuca d n a u J Plate ti on Zo Plate Motions Load the Faults Deep beneath California, the Pacific and North American Plates relentlessly grind past one another, straining or “loading” faults in the Earth’s rigid crust above. The horizontal (“strike slip”) movement between these plates along the San Andreas Fault Zone is about 1.7 inches per year (40 mm/yr), about as fast as your fingernails grow. At this rate, Los Angeles will be west of San Francisco in about 12 million years. Don’t be fooled!— 4 CAN “BEACHFRONT PROPERTY”? JANUARY 1700 M 9 The idea or myth of California sliding into the Pacific Ocean in an earthquake and creating new beachfront property to the east appeals to those having a bit of fun at the Golden State’s expense. Although part of the State west of the San Andreas Fault system is very slowly moving northward and in millions of years could become an island, earthquakes caused by this horizontal motion of the Earth’s tectonic plates will not make California disappear into the sea, like fabled Atlantis. In this computer simulation, tsunami waves are radiating outward after a magnitude (M) 9 earthquake that occurred on the Cascadia Subduction Zone offshore of northern California, Oregon, and Washington on January 26, 1700. This view shows the waves 4 hours after the quake. Colors indicate wave heights—red is highest. Along parts of the coast of the Pacific Northwest, 30-foot-high (9 m) waves rushed inland. Within 20 hours the tsunami did damage throughout the Pacific, and it is well documented in written records from Japan. (For more information go to http://pubs.usgs. gov/circ/c1187/. ”Orphan tsunami” Web address: http://pubs.usgs.gov/pp/pp1707/.) ne The Bay Area Is Your Home W yo m in g M on ta na D S ak ot a D N ak ot a JANUARY 1857 M 7.9 ho U ta h n North Amer Oct. 1989 M 6.9 co Sanrancis F zo Jan. 1857 M 7.9 O Apr. 1906 M 7.8 na ican Plate IC li a da M Ca ni for va EX Ne ri go A Ore San Andreas Fau lt Zone Pacific Pla An Lo ge s le s Ida The magnitude (M) 7.9 Fort Tejon earthquake in 1857 caused a horizontal shift of about 30 feet (9 m) on this stretch of the southern San Andreas Fault (red arrows indicate this “strikeslip” movement). Although the quake ruptured nearly 200 miles (300 km) of the fault, it did little damage because southern California’s population was small at that time. (USGS photo) te APRIL 1906 M 7.8 In the Great 1906 magnitude (M) 7.8 earthquake, nearly 300 miles (480 km) of the San Andreas Fault ruptured, producing strong shaking along all of coastal northern California. Shaking was most intense in Santa Rosa and San Francisco. In San Francisco, broken water mains kept firefighters from battling the fires that swept through the city and contributed to the devastation shown in this photograph (looking south) taken from a tethered balloon 5 weeks after the earthquake. San Francisco City Hall For more information go to: http://neic.usgs.gov/neis/plate_tectonics/rift_man.html http://pubs.usgs.gov/gip/earthq3/ 5 Why Should I Care? History Shows That Damaging Earthquakes Have Occurred Throughout the Bay Area… I n the last half of the 1800s, damaging earthquakes (magnitude 6 or greater) occurred in the San Francisco Bay region on average every 4 years. However, this changed in 1906: Fault Rupture Lengths for Historical Quakes in the Bay Area • On April 18, 1906, the San Andreas Fault ruptured violently over a length of 300 miles, causing damage from San Juan Bautista north as far as Eureka. This magnitude 7.8 earthquake—the “Great San Francisco earthquake”—relieved stresses on faults throughout the bay region. • Because fault stresses were reduced, the rate of large quakes in the San Francisco Bay region dropped abruptly after the 1906 earthquake. • The three-quarters of a century following the 1906 quake was a golden age for the bay region, in which urban areas and population expanded rapidly during a time of minimal quake activity. • Although the level of seismic activity has not yet reached that of the late 1800s, since 1906 stresses on Bay Area faults have been building up once again. The area can expect more frequent and stronger earthquakes in the future. )NC O BEMF PLET ORE E R ECOR D ‘REATER THAN ILIITTUYDE B A OB AGN KE PRST ONEATMER QUA T LEA GRE R A TO FOR O TO )NCREASING QUAKE PROBABILITY TO %!24(15!+% -!’.)45$%3 1906 epicenter 4H E FU TU RE The Past and Future of Significant Bay Area Quakes 6 The Bay Area Is Your Home Quakes can cause damage far from the fault rupture… 3ACRAMENTO 3 %2 $’ 2/ . 3! 3ANTA 2OSA 3 %! $2 !. 0ETALUMA 6ALLEJO 3AN 2AFAEL FIC 4 5, &! 4( -4 25 $ 34 )!” &! ,/ 5, 4 (AYWARD EAN (ALF -OON “AY “A Y 3AN *OSE 3!. 4 5, &! -),%3 /2 %’ ‘2 1868—Damage in Hayward )/ 5RBAN AREAS IN 4 &!5, )NCREASING PROBABILITY ALONG FAULT SEGMENTS 3 2! 6% ,! #! 0ALO !LTO . +),/-%4%23 4RACY 4 5, &! 3AN -ATEO O /C 0ACIFICA 3TOCKTON ,IVERMORE 0LEASANTON !NTIOCH ,% ), .6 %% ‘2 $ANVILLE /AKLAND SC NCI &RA CI $ !2 97 (! 3AN 0A 3AN &RANCISCO 7ALNUT #REEK PROBABILITY FOR ONE OR MORE MAGNITUDE OR GREATER EARTHQUAKES FROM TO 4 5, &! 1969—Damage in Santa Rosa .OVATO .OV 1906—Damage in Healdsburg .APA 4 5, &! 3ONOMA 3!. &2!.#)3#/ “!9 2%’)/. %!24(15!+% 02/”!”),)49 4 5, &! %9 ,, %%. 6! 2$n’2 #/.#/ + %% #2 ). %84 ‘IL
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