The 2004 Sumatra-Andaman earth­quake and re­sult­ing tsu­na­mi are in­fa­mous for the de­struc­tion they wrought. But at the time, many sci­en­tists thought the ar­ea un­like­ly to wit­ness a quake of such size.

Now, a geophys­i­cist is urg­ing the pub­lic and pol­i­cy­mak­ers to treat as “locked, load­ed, and dan­ger­ous” all ge­o­log­i­cal struc­tures like the one that spawned that quake.

These dan­ger zones are at tec­ton­ic bound­aries, bor­ders be­tween seg­ments of Earth’s crust that grad­u­al­ly move rel­a­tive to each oth­er, ac­cord­ing to the sci­ent­ist, Rob­ert Mc­Caf­frey of Rens­se­laer Pol­y­tech­nic In­sti­tute in Troy, N.Y.

A tsu­na­mi can oc­cur when one of the many tec­ton­ic plates that make up Earth’s out­er shell de­scends, or “sub­ducts,” un­der an ad­ja­cent plate. Above, in the first step, the over­rid­ing plate, stuck to the sub­duct­ing plate, gets squeezed. Its lead­ing edge is dragged down, while an ar­ea be­hind bulges up­ward. This move­ment goes on for dec­ades or cen­turies, slow­ly build­ing up stress. An earth­quake re­sults when the lead­ing edge of the over­rid­ing plate breaks free and springs up­ward, rais­ing the sea floor and the wa­ter above it. (Cour­te­sy U.S. Ge­o­log­i­cal Sur­vey)

Mc­Caf­frey issued his warning in an ar­t­i­cle in the March 23 is­sue of the re­search jour­nal Sci­ence.

Cul­prits in the most pow­er­ful quakes are typ­i­cal­ly “sub­duc­tion-type” bound­aries, he ex­plained. These are where one plate gen­tly slips un­der­neath anoth­er, caus­ing fric­tion, crack­ing, and lift­ing of plates. Such zones are typ­i­cal­ly re­spon­si­ble for the mag­ni­tude 9 quakes, the most pow­er­ful on rec­ord, Mc­Caf­frey said.

“Seis­mol­o­gists have long tried to de­ter­mine which sub­duc­tion bound­aries are more like­ly than oth­ers to break,” he wrote. “Yet, the great earth­quake of 2004 rup­tured a seg­ment that was thought to be among the least like­ly to go.”

On Dec. 26 of that year, the earth un­der the In­di­an Ocean buck­led and broke, un­leash­ing one of his­to­ry’s worst quakes, logged at mag­ni­tude 9.2. Its shock waves cre­at­ed a wall of rush­ing wa­ter that shat­tered com­mu­ni­ties up to 1,000 miles away.

A mag­ni­tude 9 event can re­sult from just 20 me­ters (22 yards) of slip be­tween two con­verg­ing plates, less then the length of an 18-wheeler truck—but its im­pact can be glob­al, Mc­Caf­frey said. Slips of this length on­ly oc­cur eve­ry 200 to 1,000 years or more at a giv­en bound­a­ry, he added; com­plete rec­ords of such events are avail­a­ble for on­ly the past cen­tury. 

Sci­en­tists had wide­ly ac­cept­ed that the age and speed of the sub­duct­ing plate is im­por­tant in cre­at­ing mag­ni­tude 9 quakes, based pri­mar­i­ly on this 100-year rec­ord. But this nar­row un­derstanding put the Su­ma­tran sub­duc­tion zone in a very low risk cat­e­go­ry, Mc­Caf­frey noted. Such lim­it­ed rec­ords can’t map a trend in events that could be sev­er­al cen­turies or more apart, he added.

In 2004, “Earth gave us a stark re­minder of the im­por­tant dif­fer­ence be­tween im­prob­a­bil­ity and im­pos­si­bil­ity,” Mc­Caf­frey wrote. “Our un­derstanding of where and when the next great earth­quake will hap­pen is in its in­fan­cy at best.” Sev­er­al sub­duc­tion zones “are near dense­ly pop­u­lated land ar­e­as, and the po­ten­tial im­pacts of shak­ing and tsu­na­mis can­not be over­stat­ed,” he added. 

Warn­ing sys­tems must be cre­at­ed with in­put and sup­port from many coun­tries, in ad­di­tion to ed­u­ca­tional out­reach to coast­al com­mu­ni­ties, Mc­Caf­frey said. “These sys­tems need to be strong and they need to be main­tained over the long term, be­cause we have no way of know­ing when the next great earth­quake will hit.”

Courtesy Rensselaer Polytechnic Institute
and World Science staff