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Big quakes spark jolts worldwide, study finds

May 27, 2008
Courtesy University of Utah
and World Science staff

Large earth­quakes rou­tinely trig­ger smaller jolts world­wide, in­clud­ing on the op­po­site side of the globe and in places not prone to quakes, a study has found.

Sci­en­tists did­n’t think such dis­tant tremors oc­curred be­fore Cal­i­for­nia’s mag­ni­tude-7.3 Lan­ders earth­quake in 1992 set off small jolts as far as 800 miles (1,300 km) away.

This world map shows seis­mic sta­tions that de­tected more than twice the nor­mal num­ber of small, near­by earth­quakes af­ter the pas­sage of what are known as "sur­face waves" from ma­jor quakes cen­tered hun­dreds to thou­sands of miles away, from 1992 through 2006. A study has found that at least 12 of the 15 ma­jor earth­quakes in this period trig­gered small quakes in dis­tant parts of the world. (Cred­it: Aar­on Ve­lasco, U. of Tex­as at El Pa­so)


In the stu­dy, re­search­ers an­a­lyzed 15 ma­jor quakes stronger than mag­ni­tude-7 since and in­clud­ing the Lan­ders event. They found that at least 12 trig­gered small quakes hun­dreds and even thou­sands of miles away.

This pro­cess, called dy­nam­ic trig­gering, is “a ubiq­ui­tous phe­nomenon,” they wrote, in find­ings pub­lished on­line May 25 in the re­search jour­nal Na­ture Ge­o­sci­ence

They re­viewed mea­sure­ments from more than 500 seis­mic re­cord­ing sta­t­ions five hours be­fore and af­ter the quakes. The da­ta came from the In­cor­po­rat­ed Re­search In­sti­tu­tions for Seis­mol­ogy, a con­sor­ti­um of uni­ver­s­i­ties.

The dis­tant tremors dif­fer from af­tershocks, which oc­cur fairly close to the main quake. Af­ter the dev­as­tat­ing 2004 Su­ma­tra earth­quake, trig­gered quakes even oc­curred in Ec­ua­dor, on the op­po­site side of the Earth, re­search­ers said.

An earth­quake first re­leases en­er­gy in the form of shock waves that move through the ground. The first waves are called pres­sure waves, which tra­vel quickly with an up-and-down mo­tion. Next come shear waves, which move from side to side, caus­ing much dam­age from an earth­quake. The next waves are two types that move along the sur­face: Love waves in a side-to-side fash­ion, fol­lowed by Ray­leigh waves, which have a roll­ing mo­tion.

The re­search­ers found that mag­ni­tude-4 or smaller events of­ten are trig­gered when ei­ther Love or Ray­leigh waves from a ma­jor quake pass a giv­en point. “We can rec­og­nize the dif­ferent kinds of waves as they pass and can fil­ter out eve­ry­thing ex­cept the small seis­mic events, which are pre­sumed to be lo­cal small earth­quakes,” said co-author Kris Pankow, a seis­molo­g­ist at the Uni­ver­s­ity of Utah.

About 600 small seis­mic events oc­cur world­wide eve­ry five min­utes. For five hours af­ter the ar­ri­val of Love waves from a ma­jor quake, the re­search­ers saw a 37 per­cent in­crease in the num­ber of small quakes world­wide. And af­ter Ray­leigh waves from the same large quake fol­lowed the Love waves, the num­ber of small quakes world­wide shot up by 60 per­cent, the in­vest­i­ga­t­ors found.

“Ray­leigh and Love waves, two very dif­ferent types of sur­face waves, are both able to trig­ger these events,” said Pankow. The re­search­ers measured quake strength using the so-called Mo­ment Mag­ni­tude scale, which es­ti­mates the en­ergy re­leased and is a new­er sys­tem than the oft-cited Rich­ter scale. Both, how­ev­er, give si­m­i­lar nu­mer­i­cal re­sults in most cases. 


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Large earthquakes routinely trigger smaller jolts worldwide, including on the opposite side of the globe and in places not prone to quakes, a study has found. Scientists didn't think such distant tremors occurred before California's magnitude-7.3 Landers earthquake in 1992 set off small jolts as far as 800 miles (1,300 km) away. In the study, researchers analyzed 15 major quakes stronger than magnitude-7 since and including the Landers event. They found that at least 12 triggered small quakes hundreds and even thousands of miles away. This process, called dynamic triggering, is “a ubiquitous phenomenon,“ they wrote, in findings published online May 25 in the research journal Nature Geoscience. They reviewed measurements from more than 500 seismic recording stations five hours before and after the quakes. The data came from the Incorporated Research Institutions for Seismology, a consortium of universities. The distant tremors differ from aftershocks, which occur fairly close to the main quake. After the devastating 2004 Sumatra earthquake, triggered quakes even occurred in Ecuador, on the opposite side of the Earth. An earthquake releases energy in the form of shock waves that move through the ground. The first waves are called P or pressure waves, which move at high speed with an up-and-down motion. The next waves are S or shear waves. These move from side to side, causing much damage from an earthquake. The next waves are two types of surface waves: Love waves move in a shearing fashion, followed by Rayleigh waves, which have a rolling motion. The researchers found that magnitude-4 or smaller events often are triggered when either Love or Rayleigh waves from a major quake pass a given point. “We can recognize the different kinds of waves as they pass and can filter out everything except the small seismic events, which are presumed to be local small earthquakes,“ said co-author Kris Pankow, a seismologist at the University of Utah. About 600 small seismic events occur worldwide every five minutes. For five hours after the arrival of Love waves from a major quake, the researchers saw a 37 percent increase in the number of small quakes worldwide. And after Rayleigh waves from the same large quake followed the Love waves, the number of small quakes worldwide shot up by 60 percent during the five hours after the major quake. “Rayleigh and Love waves, two very different types of surface waves, are both able to trigger these events,“ said Pankow. The magnitude scale used by the researchers was the so-called Moment Magnitude scale, a newer measuring system than the older Richter scale, which however gives similar numerical results in most cases.