Once the tremors subside, the most immediate proof of an earthquake is seeing the altered landscape it left behind. But much less obvious markers of seismic shifts do exist, and not in the geological record.
In a new study, scientists discovered a surprising consequence of earthquakes, one that lingers beyond when the aftershocks have faded: a detectable increase in tree growth, prompted by earthquake-triggered shifts in groundwater availability.
It's long been known that earthquakes can change trees' fortunes, but there is still much we don't know about how tree growth is affected by earthquakes, and about how much is being recorded in the living, biological archive of their trunks.
In simple terms, the thinking is basically this.
"Large earthquakes can increase the amount of water feeding stream flows, raise groundwater levels, and thus grant plant roots more access to water in water-limited environments," explains a team of researchers led by first author and hydrologist Christian Mohr from the University of Potsdam in Germany.
"If tree growth is limited mainly by water, trees should in theory record hydrological responses to earthquakes by changing their growth rates."
To explore these ideas and test their hypothesis that earthquake alterations to groundwater supply would promote tree growth when trees are close to valley streams – but hinder their growth if they're higher up on hillsides – the researchers studied Pinus radiata pine trees in Chile, seeking to measure effects of the powerful 8.8-magnitude earthquake that severely affected the Maule region in 2010.
Analysis of tree cores taken in 2014, extracted from trees on the valley floor as well as hillslope ridges, showed that some of the trees in the valley did experience temporary increased growth after the quake, on the basis of both tree ring evidence (increased lumen area), and in the ratio of carbon isotopes in the trees' cells, giving a cellular-level perspective on aspects of tree health, growth, and water availability.
By contrast, some of the trees on the slopes didn't fare so well in the same period, offering some support to the researchers' hypothesis, although the team acknowledges the overall effect of the earthquake was slight, and only seemed to last temporarily – for a period of weeks.
Still, as a case study demonstrating how these techniques can be used in the field, the researchers say their findings show that postseismic changes in lumen area and carbon isotope ratios can be used to study tree growth and photosynthetic responses to earthquakes.
And with that, we might have a new tool to study earthquakes of the past.
"Details in wood anatomy and isotopes might offer a tree-based approach for paleoseismology beyond simply considering width," the researchers write.
The findings are reported in JGR Biogeosciences.