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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.”

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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. 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 the 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, described in the Nov. 4 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 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.”