Earthquake is not as damaging as its After-Shock

Courtesy Nature
and World Science staff & World Science


Earth quakes that occur on land far from the boundaries of tectonic plates may actually be after shocks of large quakes cen turies ago, a new re port sug gests.

Tec tonic plates are distinct segments of the Earth’s crust whose borders tend to un­dergo large amounts of seismic, or earth quake-related, activity. This occurs when a build up in pres sure along these bound aries causes the ground to slip suddenly.

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

This can also occur with in the plates, along cracks in the crust called fault lines, but less of ten. Such an event, though, was the 2008 Wenchuan earth quake in China, which killed some 68,000 people by official estimates. It came as a surprise to many because it occurred on a fault line that had undergone little re­cent seismic activity.

Be cause of the in frequent seismic activity at conti­nental interiors like the Wenchuan region, assessment of earth quake hazard in these areas relies on a relatively short historical record, according to re­searchers Seth Stein of North western University in Il­linois and Mian Liu of the University of Missouri.

This, they added, makes it hard to distinguish poten­tially long after shock sequences from “back ground” seismic activity, which can point to a stress build-up foreshadowing a possible earth quake.

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 after shock se­quences to the rate at which stress builds up in a fault in a variety of scenarios.

They found that at plate bound aries, where most large earth quakes oc cur, the mo tion of tec ton ic plates rap idly “reloads” faults with stress that must be re leased through an earth quake. How ev er, af ter shock ac ti vity drops off rel a tively quickly al so, af ter a dec­ade or so.

With in con ti nents, the op po site hap pens. Slower changes in the po si tion of the un der­ly ing crust means af ter shocks can con tin ue much long er.

The sci en tists did n’t spec u late as to which past earth quake the Wenchuan event might be re lat ed to. Chi na has had sev er al ma jor earth quakes over the coun try’s his to ry. In ad di tion, the re search ers wrote, oth er “seis micity in the ar eas of past large earth­quakes, in clud ing those in New Ma drid, Mis souri (1811 1812), Char le voix, Que bec (1663), and Ba sel, Switz er land (1356), may be af ter shocks.”