Blast from the past

An early 20th cen­tury US earth­quake teaches lessons in tec­ton­ics

Down to Earth - - CONTENTS - KAUSHIK DAS­GUPTA

The early 20th cen­tury earth­quake in the US that gave the world lessons in tec­tonic

Aon April 18,1906,res­i­dents of San Francisco were T 5.12 AM jolted out of their sleep. Stum­bling from their beds, many were un­able to stand as the floor and their build­ings shook vi­o­lently. The quake lasted only a minute but caused one of the most dev­as­tat­ing dis­as­ters in US his­tory. Fires raged in San Francisco three days after the earth­quake. As wa­ter mains were also bro­ken,the city fire depart­ment had few re­sources with which to fight the fires. Sev­eral fires in the down­town area merged to be­come one gi­ant in­ferno. More than 3,000 peo­ple were killed and 225,000 of the city’s 400,000 res­i­dents were left home­less.When the fires were fi­nally out,more than 28,000 build­ings had been de­stroyed.

At the time of the quake, San Francisco was the wealth­i­est and most im­por­tant city on the US’ Pa­cific Coast. It was a sig­nif­i­cant eco­nomic cen­tre. His­tor­i­cal records show that the sec­ond half of the 19th cen­tury was a pe­riod of seis­mic ac­tiv­ity in the area. Earth­quakes seemed to be ac­cepted as a nui­sance but not some­thing that would dis­turb the re­gion’s daily life.

Two years be­fore the 1906 earth­quake, ge­ol­o­gist An­drew Law­son wrote in The Daily Cal­i­for­nian, “His­tory and records show that earth­quakes in this re­gion have never been of a vi­o­lent na­ture. So far I can judge from the na­ture of re­cent dis­tur­bances and from ac­counts of past oc­cur­rences there is not oc­ca­sion for alarm at present.”

Law­son had to cor­rect him­self when he was com­mis­sioned by Cal­i­for­nia Gov­er­nor—San Francisco is part of Cal­i­for­nia—to head an eight-mem­ber team to in­ves­ti­gate the San Francisco earth­quake. The Law­son com­mis­sion was re­lent­less in sys­tem­at­i­cally re­port­ing data about the earth­quake.They in­cluded over 300 photographs and nu­mer­ous sketches cap­tur­ing cracks on earth and da­m­ages to build­ings. In ad­di­tion to doc­u­ment­ing the earth­quake, the team con­ducted lab­o­ra­tory ex­per­i­ments and math­e­mat­i­cal analy­ses to un­der­stand some puzzling ob­ser­va­tions.In the end,the com­mis­sion’s thought­ful in­ter­pre­ta­tion of data made the re­port a sem­i­nal piece of lit­er­a­ture on earth­quake. Its find­ings make up a two-vol­ume tome, the largest set of seis­mic ef­fects ever com­piled in a sin­gle re­port.

Law­son’s team cov­ered hun­dreds of kilo­me­tres on foot and horse­back, of­ten through hills cov­ered in poi­son oak. No de­tail was too small. Since early seis­mome­ters were un­able to reg­is­ter the strength of such a pow­er­ful quake, the sci­en­tists in­ter­viewed wit­nesses, ex­am­ined da­m­ages to build­ings, took stock of land­slides and trees that snapped and col­lected ev­i­dence of ground cracking.

Sci­en­tists then didn’t know much about plate tec­ton­ics, but the de­tailed in­for­ma­tion proved in­valu­able in un­der­stand­ing links be­tween tec­ton­ics and earth­quakes. The team’s in­ves­ti­ga­tions led them to Cal­i­for­nia’s sleep­ing gi­ant: the San An­dreas Fault. Born 30 mil­lion years ago, the fault is the bound­ary be­tween the Earth’s North Amer­i­can Plate to the east and the Pa­cific Plate to the west. The Pa­cific Plate is mov­ing north, the North Amer­i­can is mov­ing south,the rate of “creep” be­ing about two inches ev­ery year.

Some­times the plates lock, al­low­ing no room for fur­ther move­ment. The Earth, how­ever, is a dy­namic en­tity, which means the plates con­tinue to shift pro­duc­ing in­cred­i­ble strain at the point where the plates have locked. An earth­quake oc­curs when the strain reaches the break­ing point, and the two plates lurch for­ward, of­ten over­lap­ping, re­leas­ing huge amounts of pent-up en­ergy in the process. Ac­cord­ing to the Law­son Com­mis­sion this is ex­actly what hap­pened along 500 kilo­me­tres (km) of the 1,300 km San An­dreas Fault on April 18,1906.

Tech­niques for mea­sur­ing the in­ten­sity of quakes had not been in­vented by 1906,but sci­en­tists who stud­ied the Law­son Com­mis­sion re­port es­ti­mate the San Francisco earth­quake at about 7.9 on the present Richter scale.The ge­ol­o­gists also noted tiny ridges and val­leys near the San An­dreas Fault. One of them, G K Gil­bert, noted,“The sur­face changes as­so­ci­ated with earth­quakes tended to in­crease the dif­fer­en­ti­a­tion of lands into ridges and val­leys.” Some of the ridges and val­leys were prod­ucts of the 1906 earth­quake. But most were cre­ated dur­ing the earth­quakes of the late 19th cen­tury which Law­son had dis­missed as “nui­sance”in his ar­ti­cle in The Daily Cal­i­for­nian in 1904.H H Reid who worked on the re­port’s sec­ond vol­ume de­scribed how nearly half a decade of seis­mic ac­tiv­i­ties had in­ten­si­fied strain,lead­ing to the cat­a­clysmic earth­quake of 1906. This was another of Law­son Com­mis­sion’s sig­nif­i­cant con­tri­bu­tion: strain built up from ear­lier quakes can as­sume cat­a­clysmic proportions.

In the af­ter­math of the earth­quake the dam­age it caused seemed another of na­ture’s caprice. But some build­ings were vir­tu­ally in­tact, while oth­ers were heav­ily dam­aged. The Law­son re­port and later stud­ies showed that con­struc­tion tech­niques, ma­te­ri­als used and above all the makeup of the ground un­der­neath had a big say in the de­struc­tion. Dur­ing the Cal­i­for­nia Gold Rush, parts of the San Francisco Bay had been filled in to cre­ate new real es­tate.Made up of loose earth, old tim­bers, rocks and other de­bris, this hodge­podge lacked co­he­sion and the strong tem­blors trans­formed it into a soft, un­sta­ble “pud­ding”,a process known to sci­ence as liq­ue­fac­tion.

San Francisco’s City Hall, which was par­tic­u­larly hard hit, re­duced to a ruin as the Greco-Ro­man col­umns that ringed the dome fell away with much of the ma­sonry fa­cade in a mat­ter of seconds.The site had once been a marsh,the soft ground mak­ing any large build­ing erected there vul­ner­a­ble to a ma­jor earth­quake. The greed of city of­fi­cials and con­trac­tors dur­ing the nearly 30-year span of the build­ing’s con­struc­tion com­pounded the city’s woes. Shoddy ma­te­ri­als were de­lib­er­ately used. Old news­pa­pers and trash had been in­cor­po­rated into the build­ing ma­te­ri­als. Even be­fore the great quake, City Hall’s in­ter­nal sewage had seeped into its base­ment, col­lect­ing in a stink­ing pool of filth. The stench of sewage was the per­fect metaphor for the stench of cor­rup­tion seep­ing from be­hind the city’s hand­some fa­cade.

The densely pop­u­lated South Mar­ket area was also hit hard. Much of the site had been a marsh in the Gold Rush pe­riod. The four-story Va­len­cia Ho­tel came to sym­bol­ise the dis­as­ter.Three sto­ries had sunk into the marshy soil be­fore the whole build­ing col­lapsed on it­self. Only the fourth storey,its walls crazily askew,re­mained above ground. Heroic res­cue ef­forts man­aged to save about a dozen vic­tims, but nearly 30 per­ished in the ho­tel. Many prob­a­bly drowned, be­cause a nearby wa­ter main had flooded the al­ready mushy soil.

Law­son re­port showed that con­struc­tion tech­niques, ma­te­rial used and makeup of the ground had a big say in the de­struc­tion caused by an

earth­quake

Sacra­mento Street, San Francisco, on April 18, 1906

CHAD­WICK , H. D. San Francisco used to be the rich­est and most im­por­tant city on the US' Pa­cific coast. But the 1906 earth­quake changed that

Newspapers in English

Newspapers from India

© PressReader. All rights reserved.