Blind faults, miss­ing links and ever-build­ing stress – KATE RAVILIOUS finds out what keeps seis­mol­o­gists in Cal­i­for­nia up at night.

Cosmos - - Front Page - KATE RAVILIOUS is a free­lance sci­ence jour­nal­ist, based in York, UK IMAGES 01 Den­nis­saxer Pho­tog­ra­phy / Getty Images

A SE­RIES OF SMALL EARTHQUAKES up to mag­ni­tude 4 started pop­ping off right next to the San An­dreas fault last year, giv­ing Cal­i­for­nian seis­mol­o­gists the jit­ters. The swarm of more than 200 mini-quakes ra­di­ated from faults un­der the Salton Sea, right down at the south­ern end of the fault. With more than 300 years of ac­cu­mu­lated strain, it is this seg­ment that sci­en­tists view as the most haz­ardous.

“It looks like it is locked, loaded and ready to roll,” Thomas Jor­dan, di­rec­tor of the South­ern Cal­i­for­nia Earth­quake Cen­tre in Los An­ge­les an­nounced at the Na­tional Earth­quake Con­fer­ence in Long Beach.

And al­though the small quakes only re­leased tiny amounts of en­ergy, the fear was that this fid­get­ing could be enough to trig­ger an earth­quake on the big fault. “Any time there is sig­nif­i­cant seis­mic ac­tiv­ity in the vicin­ity of the San An­dreas fault, we seis­mol­o­gists get ner­vous,” said Jor­dan.

Be­cause de­spite a plethora of sen­si­tive in­stru­ments, satel­lite mea­sure­ments and pow­er­ful com­puter mod­els, no one can pre­dict when the next big one will rat­tle the Golden State.

Slic­ing through 1,300 kilo­me­tres of Cal­i­for­nian land­scape from Cape Men­do­cino in the north-west all the way to the Mex­i­can bor­der in the south-east, the San An­dreas fault makes it­self known. Rivers and moun­tain ranges – and even fences and roads – are off­set by the hor­i­zon­tal move­ment of this “trans­form” fault, where the Pa­cific Ocean plate to the west meets the North Amer­i­can plate to the east. The fault moves an av­er­age of around 3.5 cen­time­tres each year, but the move­ment comes in fits and starts. Large earthquakes do­ing most of the work, punc­tu­at­ing long pe­ri­ods of build­ing pres­sure.

The fault di­vides roughly into three seg­ments, each of which tends to pro­duce a big quake ev­ery 150 to 200 years. The last time the north­ern seg­ment (from Cape Men­do­cino to Juan Bautista, south of San Fran­cisco) re­leased stress was dur­ing the dev­as­tat­ing mag­ni­tude-7.8 San Fran­cisco Bay quake in 1906, which killed thou­sands and de­stroyed around 80% of San Fran­cisco.

Mean­while, the cen­tral sec­tion, from Park­field to San Bernardino, has been quiet for longer still, with its last sig­nif­i­cant quake in 1857, when a mag­ni­tude-7.9 erupted un­der­neath Fort Te­jon.

But most wor­ry­ing of all is the south­ern por­tion (from San Bernardino south­wards through the Coachella Val­ley), which last rup­tured in the late 1600s. This ex­plains why the re­cent earth­quake swarm was con­sid­ered se­ri­ous enough for the United States Ge­o­log­i­cal Sur­vey to is­sue a state­ment: that the risk of a mag­ni­tude-7 quake in South­ern Cal­i­for­nia was tem­po­rar­ily el­e­vated from a one in 10,000 chance to as much as a one in one in 100.

“We think that such swarms of small earthquakes in­di­cate ei­ther that flu­ids are mov­ing through the crust or that faults have started to slip slowly,” says Roland Bürgmann, a seis­mol­o­gist at Univer­sity of Cal­i­for­nia, Berke­ley. “There is a prece­dent for such events hav­ing the po­ten­tial to trig­ger earthquakes.”

And last year he showed it’s not just the San An­dreas fault we need to worry about. Work­ing near the north­ern­most seg­ment of the fault, Bürgmann and his col­leagues used satel­lite mea­sure­ments and data from in­stru­ments buried deep un­der­ground to map out the un­der­ground shape of two smaller faults – the Hay­ward and Calav­eras – which veer off to the east of San Fran­cisco. These two smaller faults, which are known to be ca­pa­ble of pro­duc­ing their own size­able earthquakes (up to mag­ni­tude 7), turned out to be con­nected. Un­til now, sed­i­ments smoth­ered the link.

In an­other study pub­lished in Sci­ence Ad­vances showed that the Hay­ward fault is con­nected by a sim­i­larly di­rect link to a third fault to the north – the Rodgers Creek fault. “This opens up the pos­si­bil­ity of an earth­quake that could rup­ture through this con­nec­tion, cov­er­ing a dis­tance of up to 160 kilo­me­tres and pro­duc­ing an earth­quake with mag­ni­tude much greater than 7,” Bürgmann says.

“It doesn’t mean that this will hap­pen, but it is a sce­nario we shouldn’t rule out.”

Down the other end of the San An­dreas fault, Ju­lian Lo­zos from the Cal­i­for­nia State Univer­sity in Los An­ge­les has been test­ing var­i­ous earth­quake sce­nar­ios us­ing a de­tailed com­puter model of the fault

sys­tem. He too has shown that a seem­ingly mi­nor side-fault – known as the San Jac­into – is more of a worry than pre­vi­ously thought. In this case, the San Jac­into falls short of in­ter­sect­ing the San An­dreas by around 1.5 kilo­me­tres, but Lo­zos’ model sug­gests large earthquakes can leap this gap.

“We al­ready know that the San An­dreas is ca­pa­ble of pro­duc­ing a mag­ni­tude-7.5 on its own, but the new pos­si­bil­ity of a joint rup­ture with the San Jac­into means there are now more ways of mak­ing a mag­ni­tude-7.5,” says Lo­zos, whose find­ings were pub­lished in Sci­ence Ad­vances in March last year.

By feed­ing historic earth­quake data into his model, he showed that the mag­ni­tude-7.5 earth­quake that shook the re­gion on 8 De­cem­ber 1812 is best ex­plained by a quake that started on the San Jac­into but hopped across onto the San An­dreas and pro­ceeded to rup­ture around 50 kilo­me­tres north and south­wards.

If such a quake were to strike again to­day, the con­se­quences could be dev­as­tat­ing, de­pend­ing on the rup­ture di­rec­tion.

“The shak­ing is stronger in the di­rec­tion of un­zip­ping,” ex­plains Lo­zos. And in this case, the big worry is a north­ward un­zip­ping, which would fun­nel en­ergy into the Los An­ge­les basin.

In 2008, the United States Ge­o­log­i­cal Sur­vey pro­duced the Shake­out Sce­nario: a model of a mag­ni­tude-7.8 earth­quake, with be­tween two and seven me­tres of slip­page, on the south­ern por­tion of the San An­dreas fault.

Mod­ern build­ings could gen­er­ally with­stand the quake, thanks to strict mod­ern build­ing codes, but older build­ings and any build­ings strad­dling the fault would likely be se­verely dam­aged.

But the great­est con­cern was the ef­fect the move­ment would have on in­fra­struc­ture – slic­ing through 966 roads, 90 fi­bre op­tic ca­bles, 39 gas pipes and 141 power lines. Smashed gas and wa­ter mains would en­able fires to rage, caus­ing more dam­age than the ini­tial shak­ing of the quake. The over­all death toll was es­ti­mated at 1,800, and the long-term con­se­quences ex­pected to be se­vere, with peo­ple liv­ing with a se­quence of pow­er­ful af­ter­shocks, and a long slow road to re­cov­ery. Sim­ply re­pair­ing wa­ter mains, for in­stance, could take up to six months.

In this sim­u­la­tion, the city of Los An­ge­les doesn’t take a di­rect hit, since it lies some way from the San An­dreas fault. But there is an­other sce­nario which keeps Jor­dan awake at night.

Back in 1994, a mag­ni­tude-6.7 “Northridge” earth­quake struck the San Fer­nando val­ley, about 30 kilo­me­tres north-west of down­town Los An­ge­les, killing 57 peo­ple and caus­ing be­tween US$13 and $40 bil­lion of dam­age – the costli­est nat­u­ral disas­ter in the US at that time.

“This was a com­plete eye-opener for us all, as it oc­curred on a blind thrust fault that no-one knew ex­isted,” says Jor­dan. Ge­ol­o­gists have since worked over­time to dis­cover these hid­den faults, and in 1999 they found that Los An­ge­les it­self sits atop the Puente Hills fault – a steeply an­gled “thrust” fault that is thought to pro­duce earthquakes of greater than mag­ni­tude 7 ev­ery few thou­sand years. “We are more likely to see a large earth­quake on the San An­dreas fault in the short to medium term, but we still have to ac­cept that this thrust fault could move at any time, and be­cause of its lo­ca­tion un­der­neath Los An­ge­les, the con­se­quences would be very se­vere,” says Jor­dan.

Much of Los An­ge­les is un­der­lain by soft sed­i­ments, which wob­ble fu­ri­ously when rat­tled by a quake, and it is these ar­eas that would likely sus­tain the most dam­age. Thank­fully, the Los An­ge­les city coun­cil is tak­ing the risk se­ri­ously. Mod­els such as Shake­out Sce­nario mo­ti­vated the city to pro­duce emer­gency plans and retro­fit dan­ger­ous build­ings. Seis­mol­o­gists such as Jor­dan and Lo­zos live in Los An­ge­les, but con­fess that the risk does af­fect their ev­ery­day life.

“It crosses my mind when I drive over the free­way that col­lapsed in 1994, or when I’m de­cid­ing what kind of house to live in,” says Lo­zos. “Oth­ers mock me for wor­ry­ing, but as a seis­mol­o­gist, I know that the longer you go with­out a quake the greater the chances of a quake are.”

Mean­while, Jor­dan, who lives in a house un­der­lain by solid gran­ite bedrock, jus­ti­fies his de­ci­sion to live in this pre­car­i­ous part of the world: “If you want to hunt ele­phants, you have to go to ele­phant coun­try.”

‘ Oth­ers mock me for wor­ry­ing, but ... the longer you go with­out a quake the greater the chances of a quake are.’

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