It’s not an earthquake—it’s an aftershock from long ago

Nov. 4, 2005
Courtesy Nature
and World Science staff

Earth­quakes that oc­cur on land far from the bound­aries of tec­ton­ic plates may ac­tu­ally be af­ter­shocks of large quakes cen­turies ago, a new re­port sug­gests.

Tec­ton­ic plates are dis­tinct seg­ments of the Earth’s crust whose bor­ders tend to un­dergo large amounts of seis­mic, or earth­quake-re­lat­ed, ac­ti­vity. This oc­curs when a build­up in pres­sure along these bound­aries causes the ground to slip sud­den­ly.

U­ni­ver­si­ty stu­dents vis­it­ing a de­form­a­tion in the ground left by the 1959 mag­ni­tude 7.5 Heb­gen Lake, Mon­tana earth­quake. This earth­quake trig­gered an enor­mous land­slide that bur­ied a camp­ground, caus­ing 28 deaths and dammed the Mad­i­son Riv­er, form­ing Quake Lake. Even to­day, af­ter­shocks con­tin­ue. (Cred­it: Seth Stein)

This can al­so oc­cur with­in the plates, along cracks in the crust called fault­lines, but less of­ten. Such an event, though, was the 2008 Wenchuan earth­quake in Chi­na, which killed some 68,000 peo­ple by of­fi­cial es­ti­mates. It came as a sur­prise to many be­cause it oc­curred on a fault­line that had un­der­gone lit­tle re­cent seis­mic ac­ti­vity.

Be­cause of the in­fre­quent seis­mic ac­ti­vity at con­ti­nen­tal in­te­ri­ors like the Wenchuan re­gion, as­sess­ment of earth­quake haz­ard in these ar­eas re­lies on a rel­a­tively short his­tor­i­cal rec­ord, ac­cord­ing to re­search­ers Seth Stein of North­west­ern Un­ivers­ity in Il­li­nois and Mian Liu of the Un­ivers­ity of Mis­souri.

This, they added, makes it hard to dis­tin­guish po­ten­tially long af­ter­shock se­quences from “back­ground” seis­mic ac­ti­vity, which can point to a stress build-up fore­shad­ow­ing a pos­si­ble earth­quake.

In their stu­dy, to be pub­lished in the Nov. 5 is­sue of the re­search jour­nal Na­ture, Stein and Liu de­vel­oped a mod­el com­par­ing the length of af­ter­shock se­quences to the rate at which stress builds up in a fault in a va­ri­e­ty of sce­nar­i­os.

They found that at plate bound­aries, where most large earth­quakes oc­cur, the mo­tion of tec­ton­ic plates rap­idly “reloads” faults with stress that must be re­leased through an earth­quake. How­ev­er, af­ter­shock ac­ti­vity drops off rel­a­tively quickly al­so, af­ter a dec­ade or so.

With­in con­ti­nents, the op­po­site hap­pens. Slower changes in the po­si­tion of the un­der­ly­ing crust means af­ter­shocks can con­tin­ue much long­er.

The sci­en­tists did­n’t spec­u­late as to which past earth­quake the Wenchuan event might be re­lat­ed to. Chi­na has had sev­er­al ma­jor earth­quakes over the coun­try’s his­to­ry. In ad­di­tion, the re­search­ers wrote, oth­er “seis­micity in the ar­eas of past large earth­quakes, in­clud­ing those in New Ma­drid, Mis­souri (1811 1812), Char­le­voix, Que­bec (1663), and Ba­sel, Switz­er­land (1356), may be af­ter­shocks.”