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The next great quake

March 22, 2007
Courtesy Rensselaer Polytechnic Institute
and World Science staff

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.

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)
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.

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.”

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The 2004 Sumatra-Andaman earthquake and resulting tsunami are infamous for the destruction they wrought. But at the time, many scientists believed the area was unlikely to witness a quake of such size. In a new paper, a geo physicist urges the public and policymakers to consider as “locked, loaded, and dangerous” all geological structures like the one that spawned that quake. The danger zones are at tectonic boundaries, borders between segments of the earth’s crust that gradually move relative to each other, wrote the scientist, Robert McCaffrey of Rensselaer Polytechnic Institute in Troy, N.Y. Culprits in the most powerful quakes are typically “subduction-type” boundaries, he explained. Those are where one plate gently slips underneath another, causing friction, cracking, and lifting of plates. These zones are typically responsible for the magnitude 9 quakes, the most powerful on record, added McCaffrey, whose paper appears in the March 23 issue of the research journal Science. “Seismologists have long tried to determine which subduction boundaries are more likely than others to break,” he said. “Yet, the great earthquake of 2004 ruptured a segment that was thought to be among the least likely to go.” On Dec. 26, 2004, the earth under the Indian Ocean buckled and broke, unleashing one of history’s worst quakes, logged at magnitude 9.2. Shockwaves 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 100 years. Scientists had widely accepted that the age and speed of the subducting plate is important in creating magnitude 9 quakes, based primarily on support from this 100-year record. But this narrow understanding put the Sumatran subduction zone in a very low risk category. McCaffrey suggests that such limited records can’t map a trend in events that could be several centuries or more apart. In 2004, “Earth gave us a stark reminder of the important difference between improbability and impossibility,” McCaffrey said. “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.”