Predicting quakes

Originally published 17 February 1986

The recent earth­quake near Cleve­land, Ohio, and New York City are reminders that no part of the Earth­’s sur­face is free from the dan­ger of quakes.

We have become used to the idea that earth­quakes occur on plate bound­aries, those cracks in the Earth­’s rigid crust where one part of the crust slips vio­lent­ly past anoth­er. The San Andreas Fault in Cal­i­for­nia is an active plate bound­ary, and earth­quakes occur there with a reg­u­lar and ter­ri­fy­ing frequency.

But in oth­er parts of the coun­try, old, most­ly inac­tive faults lie deeply buried beneath the sur­face. Now and then, resid­ual stress in these faults is released by earth move­ments. These “mid-plate” quakes are usu­al­ly only tremors, but they need not be small. The largest earth­quake in North Amer­i­ca in his­toric times occurred in 1821 near New Madrid, Mo., far from any present plate boundary.

The Cleve­land earth­quake was asso­ci­at­ed with move­ments on minor hid­den faults that are only poor­ly under­stood. The New York quake took place along the crum­pled and bro­ken part of crust where Africa and North Amer­i­ca col­lid­ed 400 mil­lion years ago.

Eastern quakes hard to predict

There is lit­tle like­li­hood that major earth­quakes in the East­ern Unit­ed States can be pre­dict­ed. Seis­mic activ­i­ty in the East is so infre­quent and irreg­u­lar that his­tor­i­cal records pro­vide scant basis for pre­dict­ing future quakes.

The sit­u­a­tion is dif­fer­ent along the San Andreas Fault in Cal­i­for­nia. There, geol­o­gists are busi­ly engaged in activ­i­ties aimed at pre­dict­ing future quakes.

The most intense­ly mon­i­tored site in the Unit­ed States is a sec­tion of the San Andreas Fault at Park­field. Sophis­ti­cat­ed instru­men­ta­tion enables geol­o­gists to mea­sure even minor strains along the fault. Records of past earth­quake activ­i­ty near Park­field have revealed reg­u­lar pat­terns for the build­ing and release of stress on the fault. Some time ago it was pre­dict­ed that the Park­field seg­ment of the fault will expe­ri­ence an earth­quake of mag­ni­tude 5.6 in Jan­u­ary 1988 — plus or minus five years. Accord­ing to recent reports, the Park­field quake is “on schedule.”

Three oth­er parts of the San Andreas Fault sys­tem have been tar­get­ed as like­ly sites for big quakes in the rel­a­tive­ly near future. The Mojave seg­ment of the fault to the north of Los Ange­les pos­es a con­tin­u­ing threat to that city. The sec­tion of the fault between Palm Springs and the Salton Sea is “ripe” for a break. And a side branch of the San Andreas Fault west of Palm Springs, called the San Jac­in­to Fault, is a can­di­date for activ­i­ty. All three sec­tions of the fault sys­tem are being close­ly watched by seismologists.

Any­one who live on a plate bound­ary must expect a fre­quent shak­ing. Earth­quakes are as much a part of “the Cal­i­for­nia expe­ri­ence” as hot tubs. In the East, it is easy to be lulled into the con­vic­tion that it can’t hap­pen here.

But it can hap­pen here, and with­out warning.

The Boston Basin and south­east­ern New Hamp­shire is one of the four most active earth­quake regions in the East­ern Unit­ed States. Oth­er high risk regions are near New Madrid, Mo., Charleston, S.C., and the St. Lawrence Val­ley in upper New York State. It is pos­si­ble (how­ev­er improb­a­ble) that an earth­quake of rel­a­tive­ly large mag­ni­tude could occur in any of these regions at any time.

Big Boston quake in 1755

Boston was jolt­ed awake by a very pow­er­ful quake in the ear­ly morn­ing hours of Nov. 18, 1755. The quake was cen­tered just to the east of Cape Ann but it was felt from Nova Sco­tia to South Car­oli­na. The shock made the ground roll in waves like the sea, and it was nec­es­sary to hold onto some­thing to avoid being thrown to the ground. In Boston, chim­neys were top­pled, beams cracked, and gable ends fell from brick hous­es. The gild­ed crick­et atop the Faneuil Hall weath­er vane was snapped off. At Pem­broke and Sci­t­u­ate, cracks opened in the earth and spewed out fine sand. A fis­sure two feet wide and 1000 feet long opened at New­ing­ton, N.H. After­shocks con­tin­ued into December.

No one was report­ed killed in the great Boston earth­quake of 1755. But an earth­quake of the same mag­ni­tude could be more seri­ous for a mod­ern city of skyscrapers.

In Cal­i­for­nia, geol­o­gists sit along the high­ly vis­i­ble San Andreas Fault and mea­sure earth move­ments of mil­lime­ters. New Eng­land’s faults are hid­den. If stress is build­ing toward a cli­max some­where below our feet, there is no way for us to know it.

---

The earth­quake pre­dict­ed in Park­field for 1988 did not arrive until 2004. A siz­able and unex­pect­ed earth­quake shook the East­ern Unit­ed States in 2011. ‑Ed.