Climate change will lead to an increase in earthquakes

Every year, dozens of major earthquakes¹ occur around the world. These events can cause the death² of hun­dreds of thou­sands of people, as was the case recently in 2010 (226,000 deaths) and 2004 (227,000 deaths). Earth­quakes are triggered by an abrupt slip along a fault – known as a frac­ture – in the Earth’s crust, i​.​e. the first few kilo­metres beneath our feet. This slip occurs when the stresses on the fault exceed a rup­ture threshold, par­tic­u­larly due to the move­ments of the tec­ton­ic plates. But in recent years, a research top­ic has emerged in the sci­entif­ic com­munity: could cli­mate change caused by human activ­it­ies influ­ence earthquakes?

“The dis­rup­tion caused by human activ­it­ies is so intense and wide­spread that it is felt right across the plan­et – we even talk about the Anthro­po­cene” stresses Chris­tophe Lar­roque. “We’re won­der­ing about the impact on seis­mi­city.” Marco Bohnhoff com­ments, “This is a new sub­ject and there are very few research groups work­ing on it, but there is grow­ing interest.” In an art­icle pub­lished last May, Marco Bohnhoff and his col­leagues sum­mar­ised what is cur­rently known³: over sev­er­al dec­ades and cen­tur­ies, human activ­it­ies will modi­fy the seis­mic clock on faults, trig­ger­ing an increas­ing num­ber of small and large earth­quakes. In oth­er words, where faults are on the verge of rup­ture, ready to slip and gen­er­ate an earth­quake, cli­mate change could be the trig­ger, accel­er­at­ing the occur­rence of the earthquake.

It is now clear that the weath­er influ­ences earth­quakes. In 2020, France was swept by storm Alex, which gen­er­ated very heavy rain­fall in the south-east of the coun­try, up to 600 mm in less than 24 hours. Over the next 100 days, 188 earth­quakes of up to mag­nitude 2 – a mag­nitude too weak to be felt by the gen­er­al pub­lic – were recor­ded in the Tinée val­ley, in the Mer­can­t­our nation­al park⁴. “This is a sig­ni­fic­ant find­ing: we recor­ded as many earth­quakes in this area in three months as we had in 5 years” points out Chris­tophe Larroque.

The first obser­va­tions of the influ­ence of weath­er on seis­mi­city date back to the early 2000s. In France, in 2002, after rain­fall as intense as dur­ing Storm Alex, an increase in seis­mic activ­ity was recor­ded in the fol­low­ing months in west­ern Provence⁵. In 2005, an unusu­al series of 47 earth­quakes was recor­ded in the space of just 12 hours in cent­ral Switzer­land, fol­low­ing intense rain­fall (300 mm in 3 days)⁶. Anoth­er example: in Nepal, a team recor­ded a lower num­ber of earth­quakes in sum­mer (-37%) than in winter, sug­gest­ing that mon­soons influ­ence seismicity⁷.

Examples also exist for oth­er weath­er events. Cali­for­nia, for example, has altern­at­ing wet and dry sea­sons when snow and water accu­mu­late in moun­tains, lakes and reser­voirs. This sea­son­al pat­tern has been shown to modi­fy seis­mi­city rates in the region⁸. And in Tur­key, oscil­la­tions in the level of the Sea of Mar­mara – asso­ci­ated with sea­son­al vari­ations – also cor­rel­ate with the num­ber of earthquakes⁹. “So far, the earth­quakes linked to met­eor­o­lo­gic­al events have mainly been of low mag­nitude, although this does not rule out the pos­sib­il­ity of lar­ger mag­nitude earth­quakes in the future” explains Chris­tophe Lar­roque. “This poses real chal­lenges in terms of obser­va­tion, since we need to deploy a high-qual­ity seis­mo­lo­gic­al record­ing net­work to accur­ately record these small earth­quakes. In recent years, the improve­ment and mul­ti­plic­a­tion of seis­mo­lo­gic­al net­works has improved our understanding.”

What mech­an­isms explain this cor­rel­a­tion between weath­er and seis­mi­city? “A num­ber of pro­cesses are in play, and it is dif­fi­cult to identi­fy which ones are respons­ible” replies Chris­tophe Lar­roque. Regard­ing storm Alex, Chris­tophe Lar­roque and his col­leagues have just pub­lished a ground-break­ing ana­lys­is that brings togeth­er a num­ber of dif­fer­ent dis­cip­lines. “We pin­pointed the pre­cise loc­a­tion of the earth­quakes, traced their migra­tion over time, and mod­elled vari­ous hypo­theses to under­stand the pro­cesses involved” explains Chris­tophe Lar­roque. For example, the team has shown that the rain gen­er­ated excess flu­id pres­sure that propag­ated at depth – known as a pres­sure front – until it destabil­ised a fault that was already under stress. The fault then began to slip until it triggered small clusters of earthquakes.

It is import­ant to under­stand that small vari­ations in stress can be enough to trig­ger earth­quakes. While the pres­sure front gen­er­ated by storm Alex is at the root of the unusu­al seis­mi­city in the region, met­eor­o­lo­gic­al events can modi­fy stresses loc­ally through oth­er mech­an­isms. One study looked at the effects of typhoon Morakot in Taiwan in 2009¹⁰. The intense rain­fall caused more than 10,000 land­slides and a very large amount of rock and sed­i­ment to be trans­por­ted in rivers. These sur­face mass move­ments increased stresses on faults, explain­ing the sig­ni­fic­ant increase in the fre­quency of sur­face earth­quakes for two and a half years after the typhoon hit. Anoth­er example: in Cali­for­nia, sea­son­al vari­ations in seis­mi­city are mainly linked to stresses gen­er­ated by the weight of water accu­mu­lat­ing on the sur­face dur­ing the wet season¹¹.

Yet all these met­eor­o­lo­gic­al events are influ­enced by cli­mate change linked to human activ­it­ies and will be even more so in the future. Rising tem­per­at­ures at the Earth’s sur­face are caus­ing ice caps to melt rap­idly, increas­ing the intens­ity of extreme pre­cip­it­a­tion and cyc­lones, as well as glob­al sea levels¹². All these sur­face changes are modi­fy­ing stresses on faults locally.

A study of earth­quakes on the Korean pen­in­sula over the last 650,000 years shows a cor­rel­a­tion between seis­mi­city and the end of the vari­ous ice ages: with each degla­ci­ation, the melt­ing of the ice causes a rap­id rise in sea level and an increase in seis­mic activ­ity in the region¹³. With glob­al sea levels already hav­ing ris­en by 0.2m since 1901 and set to rise by a fur­ther 1m by 2100, the con­sequences could be sig­ni­fic­ant. Some estim­ate that a rise of 1m could increase the pres­sure on all the oceans (i​.​e. 70% of the earth’s sur­face) enough to trig­ger earth­quakes on many faults that are already close to rup­ture. “Rising sea levels will in most cases accel­er­ate the onset of earth­quakes, though in oth­er cases it will delay them” con­cludes Marco Bohnhoff. “In this case, more energy will accu­mu­late on the faults and the earth­quakes will be stronger. This will affect the people who live on the coast, and we urgently need to reduce green­house gas emis­sions to lim­it the num­ber of earth­quakes caused by cli­mate change.”

Anaïs Marechal