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California ’s rock and roll - and - roll reputation was set more than a century ago , when a devastating quake flatten out San Francisco in 1906 . Afterward , the northern San Andreas Fault , the state ’s monumental temblor - maker , lie quiet for eight decades — until 1989 ’s Loma Prieta temblor , which shook up the 1989 World Series game at Candlestick Park .
But it turns out that Northern California ’s earthquake lull may be an anomaly .
UAVSAR image of the San Andreas fault in the San Francisco Bay area just west of San Mateo and Foster City. The fault runs diagonally from upper left to lower right. The body of water along the fault line is Crystal Springs Reservoir.
In the 70 years before the 1906 seism , theSan Andreas Faultunleashed three earthquakes cock-a-hoop than magnitude-6 in the Santa Cruz Mountains south of San Francisco , researcher report in this month ’s offspring of the Bulletin of the Seismological Society of America . [ Album : The Great San Francisco Earthquake ]
Geologists show that two quakes bragging than magnitude-6 , centered near Corralitos , rattled former colonist , one in 1838 and one in 1890 . The team also found sign of even more early quake , including one in 1865 . That means that the northern San Andreas Fault’searthquakepattern may be different than previously conceive .
" The manikin for the fault was always fairly prominent but fairly infrequent earthquakes , " said lead study writer Ashley Streig , a geologist at the University of Oregon . " What we ’re seeing is the Santa Cruz Mountains section rupturing more frequently , " Streig told Live Science ’s Our Amazing Planet . The findings are another nail in the coffin for the whimsy that earthquakes strike like clockwork in California . In the early decades of earthquake science , researchers guess all seism go back at veritable intervals , but in recent eld , detailed studies are prove that theoretical account wrong in many cases . For example , earthquakes may cluster in time , or faults may go smooth after giant quake , like the deadly1906 San Francisco shocker .
UAVSAR image of the San Andreas fault in the San Francisco Bay area just west of San Mateo and Foster City. The fault runs diagonally from upper left to lower right. The body of water along the fault line is Crystal Springs Reservoir.
" Earthquake return is very variable , and that ’s what we ’re seeing , " Streig said .
earthquake chaos
Before Streig ’s study , scientist had suspected the San Andreas have the nineteenth - century earthquakes , based on diachronic records . But because northerly California is pervade with error , there were many other possible culprits . ( California has a lot of seism faults because it is the bounds between two tectonic plates , the North America and Pacific . The San Andreas Fault punctuate this boundary . )
Carbon-dating wood chips and charcoal revealed evidence of three earthquakes on the northern San Andreas Fault in 1838, 1890 and 1906.
Streig and her co - authors pinned the preceding quake on the San Andreas by examining sediment and Sir Henry Wood in 16 trenches across the fault , all in the key Santa Cruz Mountains . Within the trenches , they chance redwood chip and stumps from lumber by Spanish settler , and charcoal that could be analyzed with high - resolutionradiocarbon dating . The wood long time , combine with sediment depth psychology , confirmed that the basis airfoil suffered crevice and fractures during the earthquake .
The researchers also found touch of two more ancient quake before 1300 , in summation to the nineteenth - century quakes .
By excavating at several website , the squad could also gauge how far the faulting broke , or ruptured , during each earthquake . Combined with the historic hurt accounts , this aloofness provides an estimate of each quake ’s size .
Geologists mapped the San Andreas Fault in the Santa Cruz Mountains using a combination of on-the-ground observations and lidar, a remote-sensing technology.
The 1838 earthquake was between magnitude 6.8 and 7.2 and the 1890 quake was between order of magnitude 6.2 and 6.4 , the team estimates . For comparison , the 1989 Loma Prieta quake , also in the Santa Cruz Mountains , was a magnitude 6.9 . Each of the 19th - century temblor rupture a short segment of the San Andreas , some 48 to 62 miles ( 62 to 100 kilometers ) long . The 1906 quake , estimated at magnitudes from 7.7 to 7.9 , ruptured the ground for 296 mile ( 476 km ) .
Quiet in the west
The study fills a opening in the fault ’s history — thefrequency of past quake on the San Andreas Faulthad been detailed to the N and Dixieland of Streig ’s study situation , but never in this stretchability of contorted crust . Streig say the result suggest the Santa Cruz Mountains segment of the San Andreas may be a transition zone , shaking more often than other percentage of the fault . To the south , thefault creep , sliding without locking and touch off earthquakes . To the Union , along the San Francisco Peninsula , earthquake seem less frequent , take up every 300 year or more .
Streig thinks the giant 1906 earthquake temporarily keep out down the repeat rattling in the Santa Cruz Mountains . A similar quiet geological period may exist in old Santa Cruz trench earthquakes , but necessitate effective examination , the researchers sound out . Streig plan to look for a longer earthquake record to prove her model . " you may have a period like the early settler see , when there was really sharpen natural process , or you may have things exclude down , which is what we ’ve seen since 1906 , " she say .
" We ’re really attend at a snap of a short period in prison term , " Streig said . " A prospicient phonograph record would be really nice to further quiz that transition zone model . "
