The mystery behind earthquakes happening in unexpected regions

The sur­face of the Earth is divid­ed into dif­fer­ent tec­ton­ic plates, upon which the oceans and con­ti­nents rest. Over time these plates move, slide, col­lide or over­lap with on anoth­er. Where the bound­aries of the plates become deformed, pres­sure often builds up to the point that is sud­den­ly released by a sud­den slip along the faults – this is the well under­stood ori­gin of earth­quakes¹. How­ev­er, towards the inte­ri­or of tec­ton­ic plates, far from the agi­ta­tion of plate bound­aries, in areas that are sup­posed to be calm and free of defor­ma­tion, some unex­pect­ed earth­quakes have been occur­ring. They have raised ques­tions in the field seis­mol­o­gy, where sci­en­tists have pro­posed new the­o­ries to explain the ori­gin of these earth­quakes. What could be caus­ing this, if not tra­di­tion­al plate tec­ton­ics? Ero­sion, flu­id cir­cu­la­tion or melt­ing glac­i­ers are thought to be triggers…

What exact­ly are we talk­ing about here? These earth­quakes – record­ed far from the bound­aries of tec­ton­ic plates – are called intraplate earth­quakes. “Although they are known on all con­ti­nents, they are quite dis­parate and we have only record­ed them in a few areas of the globe,” describes Éric Calais. In 2017, an earth­quake of mag­ni­tude 6.5 occurred in Botswana, in an area with no tec­ton­ic move­ment as far as we know. In 1811–1812, four major intraplate earth­quakes (mag­ni­tude 7 or more) shook the New Madrid region along the Mis­sis­sip­pi Riv­er in the Unit­ed States. Intraplate earth­quakes have also occurred in cen­tral Aus­tralia, north­ern Swe­den and French Guiana… but also in main­land France, an area con­sid­ered as “intraplate”². 

In 2019, an earth­quake of mag­ni­tude 4.9 in the Rhone val­ley sur­prised the sci­en­tif­ic com­mu­ni­ty. “Before this earth­quake, the seis­mic­i­ty record­ed was rel­a­tive­ly low: 39 earth­quakes were mea­sured between 1962 and 2018 in this area, all of low mag­ni­tude (less than 3),” explains Jean-François Ritz. The fault at the ori­gin of this earth­quake was not iden­ti­fied as an active fault either – mean­ing one with the poten­tial to gen­er­ate earth­quakes. It seemed to have been qui­et for 20 mil­lion years!

“Intraplate earth­quakes can reach sig­nif­i­cant mag­ni­tudes, but they are much rar­er than those locat­ed at plate bound­aries,” explains Éric Calais. Since the first seis­mic record­ings were made in 1974, only about twen­ty major intraplate earth­quakes (of mag­ni­tude 6 or more) have been record­ed through­out the world³. Old­er earth­quakes are known from his­tor­i­cal archives or from traces left in the geo­log­i­cal land­scape. “Anoth­er fea­ture is their fre­quen­cy. At the plate bound­ary, the same earth­quake tends to be repeat­ed at a vari­able fre­quen­cy, for exam­ple a few hun­dred years in Cal­i­for­nia,” con­tin­ues Éric Calais. This repet­i­tive behav­iour is not observed for intraplate earth­quakes: some seem to be ‘orphan’ earth­quakes on faults that have been inac­tive for mil­lions of years.

A bet­ter under­stand­ing of these earth­quakes is essen­tial: their mod­er­ate to high mag­ni­tude makes them poten­tial­ly very destruc­tive. They affect regions that are often poor­ly pre­pared and can be trig­gered at very shal­low depths. The Le Teil earth­quake – which occurred less than 20 kilo­me­tres from two nuclear pow­er plants – was trig­gered only 1 km under­ground⁴. It seri­ous­ly dam­aged 900 build­ings, injured 3 peo­ple, and caused an eco­nom­ic loss of sev­er­al mil­lion euros. 

In France, for exam­ple, a reg­u­la­to­ry zon­ing sys­tem impos­es earth­quake-resis­tant con­struc­tion stan­dards for build­ings. But it is based on the nation­al seis­mic haz­ard map in which unex­pect­ed earth­quakes like the Le Teil earth­quake are not iden­ti­fied. “In recent years, the sci­en­tif­ic com­mu­ni­ty has become aware that intraplate earth­quakes can occur in unex­pect­ed places and that the tec­ton­ic mod­els on which con­ven­tion­al seis­mic haz­ard cal­cu­la­tions are based do not allow this risk to be reflect­ed,” says Éric Calais.

Intraplate earth­quakes can occur in unex­pect­ed places and clas­si­cal tec­ton­ic mod­els do not allow this risk to be reflected.

Research is active in this area because the impli­ca­tions are sig­nif­i­cant, as a group of French sci­en­tists write: “This lack of con­sen­sus […] leads to sig­nif­i­cant uncer­tain­ties in the char­ac­ter­i­sa­tion of seis­mic­i­ty and the asso­ci­at­ed seis­mic haz­ard. For a long time, the sci­en­tif­ic com­mu­ni­ty thought that plate tec­ton­ics alone explained these earth­quakes. The inte­ri­or of the plates would deform very slight­ly, at such low rates that it would be impos­si­ble to actu­al­ly mea­sure them⁵.”

But today a com­plete­ly new expla­na­tion is being put for­ward. Small phe­nom­e­na, very rapid on the geo­log­i­cal time scale (a few thou­sand years or even tens of years), are thought to be the cause of these earth­quakes⁶. These may be the cir­cu­la­tion of flu­ids: rain that seeps into the sur­face; or gas­es or liq­uids that come from the man­tle, sev­er­al tens of kilo­me­tres down, and rise through the tec­ton­ic plate. These increase the pres­sure through the rock to the point of trig­ger­ing intraplate earth­quakes, as sug­gest­ed for the Botswana earth­quake in 2017⁷. 

Oth­er trig­gers include melt­ing glac­i­ers and ero­sion. By light­en­ing the Earth’s sur­face, these phe­nom­e­na cause a slight move­ment of the tec­ton­ic plate, sim­i­lar to a rebound. For the Le Teil earth­quake, the melt­ing of glac­i­ers at the end of the last major ice age (12,000 years ago) is pro­posed as one hypoth­e­sis⁸. “The expert report on the Le Teil earth­quake⁹ con­cludes that it is pos­si­ble that the extrac­tion of rocks – also light­en­ing the earth’s crust – at a near­by quar­ry con­tributed to the trig­ger­ing of the earth­quake,” adds Jean-François Ritz. How­ev­er, it is clear that its mag­ni­tude is explained by tec­ton­ic forces.

“While these small phe­nom­e­na are the key to the new the­o­ry, they are not the only ones at play. Although they are trig­gers of the earth­quake, it is impor­tant to under­stand that it is the weak stress­es accu­mu­lat­ed – some­times over sev­er­al mil­lion years – that dri­ve it,” adds Éric Calais. With­out a trig­ger, these stress­es remain pre­served and no earth­quake would occur. On a scale of mil­lions or even tens of mil­lions of years, the move­ments of tec­ton­ic plates fluc­tu­ate: some plates change direc­tion, oth­ers break up, or sta­bilise… These ancient defor­ma­tions that they under­go con­sti­tute this ‘reser­voir of stress­es’ dis­trib­uted across the tec­ton­ic plate. “Our obser­va­tions on the fault that gen­er­at­ed the Le Teil earth­quake sug­gest that melt­ing ice or ero­sion can also gen­er­ate reg­u­lar earth­quakes, but at very long fre­quen­cies of the order of 10,000 years¹⁰,” also points out Jean-François Ritz.

The fact remains that intraplate earth­quakes are few, so it is dif­fi­cult to quan­ti­fy their behav­iour. Researchers repro­duce them in numer­i­cal sim­u­la­tions, but it is com­pli­cat­ed to clear­ly iden­ti­fy the role of each phe­nom­e­non (plate tec­ton­ics, ero­sion, flu­id cir­cu­la­tion, etc.). The fall­out is sig­nif­i­cant, as Éric Calais describes, “with these spe­cial earth­quakes, it is very com­pli­cat­ed to cal­cu­late future risks, espe­cial­ly as they can some­times only occur once in a giv­en loca­tion. We lack objec­tive indi­ca­tors to assess future intraplate seismicity. 

In France, work is con­tin­u­ing around the Le Teil earth­quake in search of traces of past earth­quakes. “We can rea­son­ably assume that the fault where this earth­quake was trig­gered will remain calm for some time, but oth­er faults exist in the area,” con­cludes Jean-François Ritz. “I have no doubt that a reassess­ment of the region­al seis­mic haz­ard will be car­ried out in a few years’ time, we need to launch a lot more research and obser­va­tions in this area and review our ideas!”

Anaïs Maréchal